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Impact of tillage intensity on carbon and nitrogen pools in surface
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and subsurface soils of four long-term field experiments
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Christiane Piegholdt*1, Rouven Andruschkewitsch1, Michael Kaiser1, Bernard Ludwig1
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Witzenhausen, Germany
Department of Environmental Chemistry, University of Kassel, Nordbahnhofstr. 1a, 37213
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*
piegholdt@uni-kassel.de
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Key words: long-term field experiment, labile C and N pools, intermediate C and N pools,
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passive C and N pools, conventional tillage, reduced tillage, no tillage
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Abstract
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1
Introduction
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The global increase in concentrations of atmospheric greenhouse gases (GHG) such as carbon
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dioxide (CO2) and nitrous oxide (N2O) requires a reassessment of management practices (i.e.,
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cropping, tillage, fertilization) to retain organic carbon (Corg) and nitrogen in soils. The
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conversion of intensive to conservation tillage systems, for example, was shown to increase
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the Corg and total nitrogen (Nt) contents of soils (Salinas-Garcia et al, 1997; Watts et al., 2010)
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by lowering the decomposition rates of organic matter (OM) (Kladivko, 2001; Zibilske et al.,
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2002). In general, compared to intensive tillage, the lower physical impact of reduced or no
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tillage systems leads to less mechanical disruption of soil aggregates and, therefore, improved
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physical protection of OM against microbial decomposition (Balesdent et al., 2000;
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Cambardella and Elliott, 1993; Mikha and Rice, 2004; Six et al., 2000; Tisdall and Oades,
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1982; Zotarelli et al., 2007). The tillage intensity also affects litter placement and thus
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decomposition dynamics (Coppens et al., 2006; Hermle et al., 2008; Jacobs et al., 2010; Oorts
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et al., 2007). Crop residues may accumulate on the soil surface of conservation tillage systems
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due to a reduced contact to the soil microbial community. In contrast, the litter distribution in
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ploughed soil layers is relatively uniform and OM decomposition rates are higher (Lenz and
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Eisenbeis, 1998; Oorts et alFurthermore, changes in the micro-climatic conditions due to
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drying and rewetting are much more severe on the soil surface than deeper in the soil. It is
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well known that drying as well as rewetting induce stress on microorganisms and their
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metabolism (Fierer and Schimel, 2002) decreasing the microbial activity and OM
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decomposition.
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The contents of Nt and Corg comprise OM pools of different mean residence times
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(MRT) resulting from differences in their stabilization against microbial decomposition.
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Based on their different turnover dynamics, such pools have different ecological functions.
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The labile OM pool (i.e., MRT <10 years) is highly important for the nutrient cycling and the
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productivity of agricultural ecosystems (Janzen, 2004). The stable or passive OM pool (i.e.,
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MRT >100 years) is crucial for the long-term sequestration of organic C in agricultural soils
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and mitigating climate change (Janzen, 2004). The transitional, intermediate OM pool (i.e.,
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MRT 10-100 years) is probably important for both soil productivity and long-term C storage.
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Changes in OM pools of different ecological functions resulting from changes in tillage
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intensity will have specific consequences for the services of agricultural ecosystems. A
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management induced increase of the labile OM compartment, for example, may lead to
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increased CO2 and N2O emissions and/or higher productivity. However, for now, there is less
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cohesive information about the influence of tillage intensity on the labile, intermediate and
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stable/passive soil OM pool.
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The labile soil OM pool representing highly bio available organic compounds, which
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are readily decomposed, can be quantified, for example, within laboratory incubation
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experiments by measuring the CO2 and N2O emissions over a certain amount of time (<10
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years) (Heitkamp et al., 2009). Additional information about the fluxes related to the turnover
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of the labile OM pool can be gained from measurements of mineralized N (Nmin)
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concentrations within laboratory incubation experiments (Kader et al., 2010). A highly
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important sub compartment of the labile OM pool represents the microbial biomass (von
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Lützow et al. 2007) because this parameter quantifies on the one hand the potentially active
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decomposer community responsible for the OM decomposition and on the other hand a
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readily available energy source (e.g., dead microorganisms, extra- and intracellular
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compounds) for microorganisms. The microbial community structure, activity, and biomass
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and the related evolution of CO2 and N2O are highly sensitive against changes in the soil
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environment. Such changes can be induced due to climatically driven processes (e.g., soil
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drying and rewetting) as well as management practices by additions of nutrients and OM (i.e.,
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fertilization), modifying the litter input (i.e., cultivated crops), or disrupting the soil structure
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and redistribution of soil material (i.e., tillage intensity).
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Stabilization of soil OM (i.e., MRT >100 years) results in temperate aerated topsoils
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from interactions between organic molecules and mineral constituents (e.g., oxides,
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polyvalent cations, Al-Silicates) and the occlusion of OM in aggregates (von Lützow et al.,
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2006; Kögel-Knabner et al., 2008; Schmidt et al., 2011). To quantify the stabilized OM
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compartment the intermediate and labile compartments have to be removed from the sample.
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One way to achieve this is to disperse in the first step sand-size aggregates (>53 µm) by
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shaking the sample with glass beads. Thereafter, the free and aggregate occluded organic
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particles, which generally show MRT´s <100 years (Six et al., 2002), can be removed by
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density separation (>1.8 g cm-3). The remaining heavy fraction (<1.8 g cm-3) can be further
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treated with Na2S2O8 (Helfrich et al., 2007), which mimics biological oxidation and removes
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potentially bio available OM from the heavy fraction as complete as possible. The OM left
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behind in the residue should represent the OM pool stabilized by strong organo-mineral
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interactions and/or occlusion in highly stable aggregates <53 µm. Helfrich et al. (2007)
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showed for this OM fraction separated from several soils MRT´s larger than 100 years. It is
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known that the stable OM pool is heavily affected by site conditions such as soil mineral
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characteristics and pH (Mikutta et al., 2006; Kaiser et al., 2012). In contrast, the effect of
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tillage intensity which, for example, largely affects the aggregate turnover, on the stabilized
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OM pool is still uncertain. The intermediate OM pool with MRT´s between 10 and 100 years
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can be regarded quantity-wise as the difference of the total SOM, the labile OM, and the
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stable OM pool. Hermle et al. (2008) showed tillage effects on the intermediate OM pool, but
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only under moist and cold-temperate conditions in Switzerland. Respective information for
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dryer and warmer climatic conditions prevailing in large agricultural areas worldwide is
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scarce.
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In this study, we aimed to analyze the impact of tillage intensity on the labile,
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intermediate, and stable OM pools. We took samples from 0-5 cm and 5-25 cm depth from
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soils of four long-term field experiments showing differences in soil texture and as well as in
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concentrations of Fe and Al oxides. Each long-term field experiment included 3 tillage
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systems of conventional tillage (CT), reduced tillage (MT), no tillage (NT). We quantified the
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labile OM pool of these samples within a one year incubation experiment (cumulative CO2
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and N2O emissions). The labile pool was further characterized by the Nmin rates and microbial
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biomass C and N amounts. Furthermore, we quantified the intermediate and stable OM pools
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as described above. We hypothesized that:
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(i)
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NT increases the labile and intermediate soil OM pool compared to conventional
and reduced tillage systems independent from site characteristics;
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(ii)
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the labile and intermediate soil OM pool is highest under no-till, followed by
reduced and conventional tillage;
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(iii)
the passive OM pool is not affected by tillage intensity;
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(iv)
there is a site specific effect on the passive C pool through different soil mineral
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characteristics.
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2
Materials and Methods
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2.1
Study sites
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Samples were taken in the time from the 13.09. to the 16.09.2011 from soils of four different
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long-term field experiments (LFE) in Germany initiated by the Institute of Sugar Beet
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Research (Göttingen, Germany) in cooperation with the agricultural division of Südzucker
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AG (Mannheim, Germany). The annual mean air temperature range from 8.0 to 9.3 °C and
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the annual precipitation from 512 to 776 mm (data were provided by Deutscher Wetterdienst)
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(Table 1). Soil texture varies between the four loess sites with, for example, clay contents
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ranging from 16% (Zschortau) to 31% (Friemar) (Table 1). At all sites, the crop rotation
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consists of two growing seasons of winter wheat (Triticum aestivum L.), followed by sugar
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beet (Beta vulgaris ssp. vulgaris var. altissima DÖLL). Crop residues are left in the field. At
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Friemar and Grombach, winter wheat was sown in fall 2010 after previous wheat and
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harvested before soil sampling in fall 2011, while at Lüttewitz and Zschortau, sugar beets
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were sown in spring 2011, after a seedbed preparation down to 5 cm. At soil sampling in fall
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2011, sugar beets were still in the field.
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Each LFE consisted of one field, which was divided into three stripes of different
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tillage intensity raging from 2.5 to 8 ha, respectively: i) conventional tillage (CT) managed
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with a moldboard plough down to 25-30 cm depth, ii) reduced tillage (MT) managed with a
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rigid-tine cultivator down to 15 cm depth, and iii) no-tillage (NT) without tillage, except for
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seedbed preparation with a rigid-tine cultivator or disc harrow to a depth of 5 cm before the
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sugar beets are sown. For the present study, we took soil samples from three pseudo field
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replicates of the CT, MT and NT treatments of each of the four LFE’s. In each case composite
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soil samples consisting of five cores (core sampler, 4 cm diameter) were taken from 0-5 cm
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and 5-25 cm depth. The samples were sieved (≤ 5 mm) and stored field moist at 4 °C.
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2.2
Soil analyses
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Field moist soil samples were analyzed for pH by extraction with CaCl2 (20 g soil/50 ml 0.01
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M CaCl2). Dry samples were used to determine texture using the pipet method (DIN ISO
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11277, 2002). Gravimetric soil moisture content was determined by drying samples at 105 °C
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for 24 h. Bulk density was determined according to DIN ISO 11272 (1998).
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determination of oxalate extractable iron (Feox) and aluminum (Alox), following DIN 19684-6
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(1997), a 5 g sample was shaken for 2 hours with 50 ml extraction solution (0.1 M
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ammoniumoxalate and 0.1 M oxalic acid). After filtration through a fiberglass filter, the Alox
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and Feox concentrations in the filtrates were measured by using an atomic absorption
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spectrometer (for Alox: Model GBC 906 AAS, GBC Scientific Equipment, Braeside,
For the
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Australia; for Feox: Unicam 939 AAS with a Gilson 222 Rack 22 autosampler, Villiers,
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France).
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Total C and N content of dry soil was determined by dry combustion (Elementar Vario El,
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Heraeus, Hanau, Germany). Carbonate-C (CO3-C) in soil was determined to calculate the Corg
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content as the difference between total C and CO3-C. For the CO3-C determination, following
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DIN 19682-13 (2009), we used a Scheibler equipment, 5-10 g sample, and 20 ml of 10% HCl.
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By gasometric determination of the released CO2 and with regard to the temperature and
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atmospheric pressure, we calculated the CO3-C content according to:
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CO3-C = (a x p x (1 x R-1)) x ((273 + t) x w)-1
(1)
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where CO3-C is the soil carbonate-C content (%), a is the gleaned CO2 volume (ml), p is the
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atmospheric pressure (hPa), R is the ideal gas constant (8.314 J mol-1 K-1), t is the room
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temperature (°C) and w is the initial soil weight (g).
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Microbial biomass C (Cmic) and N (Nmic) was determined before and after the
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incubation experiment by chloroform fumigation extraction (Vance et al., 1987). Briefly, two
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portions of soil (5 g) were taken from each soil sample. One portion was directly extracted
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with 20 ml of 0.5 M K2SO4, the other subsample was extracted after fumigation with CHCl3
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for 24 h at 25 °C. After filtration of the suspensions (Whatman No. 595 ½), the extracts were
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frozen until measurement of C and N with a C/N analyzer (analytikjena multi N/C 2100S,
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Jena, Germany). Microbial biomass C and N was calculated as the difference between
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fumigated and unfumigated samples (conversion factors of 0.45 and 0.54 for Cmic and Nmic,
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respectively) (Brookes et al., 1985). The stocks of the soil C and N pools, microbial biomass
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C and N, mineralizable N, the light and heavy fractions, and Feox, Alox, and clay were
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calculated for 0-5 cm and 5-25 cm depth using the measured concentration of the respective
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parameter and the site and depth specific bulk densitiy.
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2.3
Mineralization experiment
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Net C and N mineralization was determined following the method developed by
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Stanford and Smith (1972). Briefly, duplicates of 200 g dry matter equivalent fresh soil
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sample (sieved <5 mm) were filled into plastic bottles with a volume of 250 ml. To get
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representative samples, we mixed the samples from 0-5 cm (and also from 5-25 cm) of the
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three pseudo field replicates of each tillage treatment and study site. The soil samples were
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brought to 60% of water holding capacity (WHC) with deionisized (DI) water, then covered
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with a net to allocate the irrigation and placed in jars, which were connected over flexible
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tubes with a gas chromatograph. The samples were incubated in a climate chamber at 10 °C,
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which was about the annual mean temperature of the study sites.
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After a pre-incubation for one week at 10 °C, every 4.5 h a gas sample was
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automatically taken with a P64 (Loftfields Analytische Lösungen, Neu Eichenberg, Germany)
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and analyzed for CO2 with a gas chromatograph (Shimadzu Gas Chromatograph GC-14A,
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Duisburg, Germany; flow 2 ml min-1). To determine the Nmin production (i.e., NO3- and
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NH4+), the soil was irrigated with 400 ml 0.01 M CaCl2 at first to remove all mineral N before
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the incubation started to make sure that only N will be measured, which was mineralized
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during the incubation period. A vacuum was applied to the bottles with flexible tubes and a
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pump to suck off the leachates and collect them in polyethylene bottles. The leachates were
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frozen until measurement of NO3- and NH4+ with a continuous flow analyzer (Evolution II
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auto-analyzer, Alliance Instruments, Cergy-Pontoise, France). Subsequent to the sampling of
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the leachate, we added of 25 ml N-free nutrient solution to avoid a suppression of the
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microbial activity by the limitation of nutrients and to recover the 60% of WHC. Leachates
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were sampled at first in a two-week interval and after 2 months in a 6-week interval. The
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greater intervals were chosen to provide a sufficient NO3- and NH4+ concentration for
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measurement in the leachates. During the decomposition experiment, we irrigated each soil
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sample 12 times. We finished the mineralization ecperiment after 341 days because the
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cumulative mineralization of C and N was well described by the applied one-pool model with
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R2 >0.99 ensuring the right estimation of the decay constant k.
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2.4
Chemical fractionation
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At first, the free and aggregate occluded OM were removed from the soil samples
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following an approach of Balesdent et al. (1991). This was done to avoid the mixing the
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passive C and N with C and N derived from labile OM pools (Jagadamma and Lal, 2010).
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Free as well as aggregate occluded organic particles contribute to the easily decomposable
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amount of OM in topsoils (Kaiser et al., 2010). For the present study, we used a sodium
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polytungstate (SPT) solution (Sometu, Berlin, Germany) with a density of 1.8 g cm-3. We
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added 40 ml of SPT solution to 10 g field moist soil (<5 mm) and applied 10 glass beads (5
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mm diameter) to the suspension, which was shaken for 18 h at 175 rpm on a reciprocal
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shaker. Then, the suspension was centrifuged for 30 min. by 2000 x g and the supernatant was
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filtered through a polyamide filter (0.45 µm). The material on the filter (light fraction: <1.8 g
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cm-3; LF) was washed with 2 l of DI water. The glass beads were removed and the soil pellet
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(heavy fraction: >1.8 g cm-3; HF) was also filtered and washed with 2 l of DI water. The light
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and heavy fractions were dried for 48 h at 40 °C and the C and N concentrations of the HF
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fraction were determined via dry combustion.
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After the separation of organic particles, we mixed 0.5 g of the HF fraction with 250
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ml of DI water and added 20 g of Na2S2O8. . The suspension was buffered with 22 g of
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NaHCO3 and heated to 80 °C in a water bath with shaker function for 48 h. To provide a
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constant homogeneous sample distribution facilitating optimal oxidation conditions, we
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applied 80 glass beads (5 mm diameter) to the suspension. After oxidation, the glass beads
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were removed and each sample was washed two times with 40 ml of DI water, once with 40
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ml of 0.01 M hydrochloric acid (HCl) to remove remaining carbonates from the NaHCO3
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buffer, and again twice with 40 ml DI water until a neutral pH was reached. After each
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washing, the suspension was centrifuged at 4000 x g for 20 min. and the supernatant was
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decanted. The cleaned extraction residue was dried at 40 °C and analyzed for C and N
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(passive C and N) concentration by dry combustion as described above.
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2.5
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The data were analyzed with GNU R (Version 2.11.1) by Shapiro-Wilk normality test,
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analysis of variance (ANOVA) and correlation analysis. The data were analyzed as a split-
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plot design with tillage treatment as the main factor and soil depth as sub-factor. Because
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some data sets were not normally distributed, we conducted a logarithmic data transformation
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(boxcox transformation) to provide the preconditions (normal distribution and homogeneity of
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variance of the data sets) for a two-way ANOVA. Analysis of variance was performed on the
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averaged values of two subsamples. The four sites served as field replicates. For correlation
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analyses, we used Spearman rank correlation to detect relationships between the different C
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and N pools and density fractions, microbial biomass as well as soil mineral characteristics.
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Effects were considered to be significant at p ≤ 0.1.
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Modeling of C and N mineralization to estimate the size of the labile C and N pool was
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conducted with a one-pool model using GNU R (Version 2.11.1). For the estimation of the
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decay constants we used a non-linear least square (nls) model with first-order compartment:
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Statistics
Ymin (t) = Yl x (1-exp(-k x t))
(2)
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where Ymin (t) is C or N mineralized (kg ha-1) at time t (days), Yl is the labile C or N pool (kg
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ha-1), k is the decay constant (day-1). To provide an unequivocal measure of soil C and N
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mineralization capacity, we followed the recommendation of Wang et al. (2003) and fitted Eq.
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(2) to the obtained data set of all tillage treatments and soil depths and fixed the decay
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constants as the average of the single decay constants of tillage treatments and soil depths (n =
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6) to the obtained values. The obtained decay constants were: k = 0.0025 d-1 for C
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mineralization and k = 0.0011 d-1 for net N mineralization.
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3
Results
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3.1
Stocks of Corg and Nt
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The site specific stocks of Corg and Nt of the soils in 0-5 cm and 5-25 cm depth of the three
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tillage treatments (CT, MT, NT) are given in Table 2. The Corg and Nt stocks showed
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significant higher stocks under NT (10.5 t Corg ha-1, 0.98 t Nt ha-1) than under CT (7.2 t Corg
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ha-1, 0.68 t Nt ha-1) in 0-5 cm soil depth (Table 4). The Corg and Nt stocks in 5-25 cm soil
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depth ranged from 29.1 to 32.5 t Corg ha-1, and from 2.77 to 3.07 t Nt ha-1 and showed no
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significant differences between the tillage systems.
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3.2
Microbial biomass C and N
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The site specific stocks of Cmic and Nmic of the soils in 0-5 cm and 5-25 cm depth of the three
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tillage treatments (CT, MT, NT) are given in Table 3. The tillage treatment had a significant
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effect on the stocks of Cmic and Nmic in 0-5 cm soil depth (Table 4). The stocks of Cmic were
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higher under NT (340 kg ha-1) and MT (277 kg ha-1) compared to CT (160 kg ha-1). In 5-25
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cm soil depth, the Cmic stocks ranged from 625 to 675 kg ha-1 and were similar under the three
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tillage treatments (Table 4). The Nmic stocks were significantly higher under NT (79 kg ha-1)
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and MT (61 kg ha-1) compared to CT (38 kg ha-1) in 0-5 cm soil depth but showed no
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significant differences in 5-25 cm soil depth (144-164 kg ha-1).
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3.3
CO2 emission and net N mineralization
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Site specific stocks of mineralized C (CO2-C) and mineralized N (Nmin) of the soils in 0-5 cm
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and 5-25 cm depth of the three tillage treatments (CT, MT, NT) are given in Table 3. The
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stocks of cumulative emitted CO2-C after 341 days ranged from 343 to 1698 kg ha-1 and were
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affected by the tillage intensity in the soil from 0-5 cm depth with higher CO2-C emissions
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from soils under NT and MT (1267 and 829 kg ha-1 341 days-1, respectively) compared to CT
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(343 kg ha-1 341 days-1) (Figure 1 a). The CO2-C emissions for 5-25 cm soil depth were
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similar under all treatments with 1698, 1381 and 1239 kg ha-1 341 days-1 under CT, NT, and
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MT, respectively (Figure 1 c).
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The cumulated net N mineralization was not significantly affected by the tillage
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systems in 0-5 cm as well as in 5-25 cm soil depth. Under CT, the cumulative N
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mineralization was lower in 0-5 cm soil depth (20 kg ha-1 341 days-1) compared to MT and
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NT (44 and 59 kg ha-1 341 days-1, respectively) (Figure 1 b). In 5-25 cm soil depth, the
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cumulative mineralization ranged from 70 to 99 kg ha-1 341 days-1 (Figure 1 d).
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3.4
C and N pools
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The cumulative CO2-C emissions were well described by the applied one-pool model
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(R2 >0.99). The modeled C storage in the labile C pool with the decay constant of k = 0.0025
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day-1 ranged from 0.53 to 3.31 t ha-1 (Table 3; 5-26% of Corg, C/N ratio: 12.1) and was
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significantly higher under NT (2.19 t ha-1) and MT (1.44 t ha-1) compared to CT (0.59 t ha-1)
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in 0-5 cm soil depth (Figure 2 a). In 5-25 cm soil depth, the labile C pool was significantly
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higher under CT (2.94 t ha-1) compared to NT (2.39 t ha-1) and MT (2.15 t ha-1) (Figure 2 c).
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Calculated stocks of the intermediate C pool ranged from 3.96 to 32.87 t ha-1 (Table 3; 68-
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88% of Corg, C/N ratio: 10.4) and were influenced, but not significantly, by the tillage
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intensity in 0-5 cm soil depth with higher stocks under NT (7.63 t ha-1) and MT (7.02 t ha-1)
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compared to CT (5.87 t ha-1) (Figure 2 a). The experimentally determined passive C pool
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stored between 0.52 and 4.54 t C ha-1 (Table 3; 5-14% of Corg, C/N ratio: 14.8). This pool was
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unaffected by tillage intensity in both soil depths (Figure 2 a and c). Large differences
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between the study sites were observed for the passive C/passive N ratios, which ranged from
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21 to 38 for the samples from Lüttewitz and Zschortau and from 9 to 16 for the sample from
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Friemar and Grombach (data not shown).
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The cumulative net N mineralization was well described by the applied one-pool
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model (R2 >0.99). The modeled N storage in the labile N pool ranged from 43 to 359 kg ha-1
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(Table 3; 6-24% of Nt; k = 0.0011 day-1) and was higher in 0-5 cm soil depth under NT (165
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kg ha-1) and MT (124 kg ha-1) compared to CT (55 kg ha-1) (Figure 2 b). The stocks of labile
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N in 5-25 cm soil depth ranged from 209 to 286 kg ha-1 and were unaffected by tillage
300
intensity (Figure 2 d). The stocks of the calculated intermediate N pool ranged from 429 to
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2943 kg ha-1 (Table 3; 72-91% of Nt) and were influenced, but not significantly, by tillage
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treatment with higher intermediate N stocks under NT (770 kg ha-1) and MT (666 kg ha-1)
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compared to CT (579 t ha-1) (Figure 2 b). The passive N pool ranged from 14 to 354 kg ha-1
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(Table 3; 2-13% of Nt) and was not influenced by tillage intensity (Figure 2 b and d).
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3.5
Site specific soil mineral characteristics and density fractions
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The stocks of the mineral components and both density fractions (LF and HF) of the soil
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samples from 0-5 cm and 5-25 cm depth of the three tillage treatments (CT, MT, NT) are
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given for each study site separately in Table 2. The stocks of the LF fractionwere significant
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higher under NT (5.7 t ha-1) and MT (4.4 t ha-1) compared to CT (2.9 t ha-1) in 0-5 cm soil
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depth. For the stocks of the HF fraction, significant differences were observed between all
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tillage treatments in 0-5 cm soil depth in the order of CT > NT > MT (Table 4). In 5-25 cm
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soil depth, LF and HF stocks were not significantly affected by the tillage intensity.
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3.6
Correlation analyses
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The coefficients of the Spearman’s rank correlation revealed significant relationships between
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C and N pools and soil mineral characteristics (i.e., clay, Alox), Cmic and Nmic, density
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fractions (LF, HF) as well as Nmin (Table 5). For p < 0.01, the stocks of labile C and N pools
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correlated with stocks of LF, Cmic, and Nmic (Table 5, Figure 3 a-c and 4 a-c). The stocks of
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the intermediate C and N pools correlated with stocks of Nmin, Cmic, and LF (Table 5).
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Furthermore, the stocks of passive C and N pools correlated with stocks of Alox, clay (Figure
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3 d and e, 4 d and e), and HF (Figure 3 f).
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4
Discussion
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4.1
Stocks of Corg and Nt
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The stocks of Corg and Nt in soils results from input and output of C and N. Soil
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tillage of different intensity resulted in the surface soil (0-5 cm) in significantly higher stocks
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of Corg and Nt under NT compared to CT (Table 4)) over a time span of 14 to 21 years.
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Compared to CT, our results also revealed a positive but not significant effect of reduced
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tillage (MT) on the stocks of Corg and Nt. Similar findings were described for silty loam soils
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by Mikha and Rice (2004) and Cosentino et al. (1998). A significant increase in Corg and Nt
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with decreasing tillage intensity was not observed for the subsurface soil (5 - 25 cm). This
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indicates that the effect of the tillage intensity on the stocks of Corg and Nt is influenced by the
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soil depth. A possible explanation for this are differences in the incorporation depth of the C
335
and N inputs in form of crop residues. Even if this input is equal under the three tillage
336
systems the crop residues are incorporated homogeneously into 0-25/30 cm under CT whereas
337
such residues are incorporated in 0-10/15 cm under MT and in 0-5 cm depth under NT (i.e.,
338
seedbed preparation for sugar beets). Consequently, the added C and N amounts are
339
distributed in a larger soil volume under CT compared to MT and NT leading to the largest
340
accumulation under NT in 0-5 cm depth. Beside this dilution effect, the lower physical impact
341
of reduced or no tillage systems compared to CT may lead to an improved physical protection
342
of OM against microbial decomposition due to the occlusion in aggregates and to increased
343
Corg and Nt stocks. Accordingly, Andruschkewitsch et al. (2013) found for the same four long-
344
term field experiments significant higher macroaggregate contents in soils under NT and MT
345
compared to CT in 0-5 cm depth.
346
Gibt es einen Einfluss der mineralischen N-Düngung auf die Nt Werte???  r = 0.73, p <0.01
347
(stocks kg Nt ha-1, N-Dünger kg ha-1)
348
Gibt es Daten zum Ertrag der Flächen  wäre wichtig um den Eintrag an crop residues
349
abschätzen zu können
350
4.2
Microbial biomass C and N
351
The stocks of Cmic and Nmic were significantly higher under MT and NT compared to
352
CT in 0-5 cm soil depth. This is in line with Green et al. (2007) and Balota et al. (2004) who
353
also reported higher Cmic contents under NT compared to CT. Similar to the stocks of Corg and
354
Nt no signifcant differences in Cmic and Nmic between the tillage systems were detected in 5-25
355
cm soil depth. Our data indicate, that the increase in Corg and Nt in 0-5 cm with decreasing
356
tillage intensity is accompanied by an increase in easily decomposable OM leading to higher
357
stocks of Cmic and Nmic. This is supported by positive correlations between stocks of Corg and
358
LF (r = 0.62, p <0.01) as well as between stocks of Cmic and Nmic and LF (r = 0.86 and r =
359
0.70, p <0.01). The LF fraction as separated here represents highly management sensitive free
360
and aggregate occluded organic particles (Six et al., 2002), which were shown to contribute to
361
the easily decomposable OM in topsoils (Kaiser et al., 2010; Strosser, 2010; Virto et al.,
362
2010). Similar findings were reported by Alvarez et al. (1995), who detected a higher
363
biological activity of microorganisms (between 0.035 and 0.078 µg CO2-C µg Biomass-C-1 d-
364
1
365
0.24). Additional to the larger amount of mineralizable OM, Balota et al. (2004) assumed a
366
more favorable conditions for microorganisms in soils without tillage because of improved
367
water supply due to enhanced soil aggregation compared to intensively tillaged soils.
368
Furthermore, especially macro-aggregates in topsoils act as habitats for soil microorganisms
369
and can promote decomposition processes due to the close proximity of decomposers and
370
potential energy sources (i.e., occluded OM).
371
372
) at higher contents of C in the soil LF (between 1.07 and 1.46 µg C g-1 LF) (r = 0.65, p =
373
374
375
4.3
CO2-C emission and mineralized N
376
The stocks of released CO2-C and mineralized N were significantly higher under NT
377
compared to CT in 0-5 cm soil depth (Figure 1a and b). For samples from 5-25 cm soil depth,
378
the cumulative CO2-C release was higher under CT compared to NT. Our findings are in
379
accordance with those of Balesdent et al. (2000), who investigated the influence of the tillage
380
intensity on the amount of mineralizable N. The higher Nmin content under NT compared to
381
CT seems to result from an accumulation of the directly bio-available N in the NT soils
382
compared to CT soils. Also Alvarez et al. (1995) reported a effect of the tillage system on the
383
CO2-C production. In contrast to our findings, they described 88-92% greater CO2-C
384
evolution in soils under conventional tillage compared to reduced tillage and concluded an
385
increase in carbon availability and mineralization in CT through the incorporation of plant
386
residues. The contrary results from from Alvarez et al. (1995) may be in part a result from
387
their method of the CO2-C measurement. They used 0.01 M NaOH traps in a field experiment
388
and measured the CO2 as emitted from the whole soil profile while in our study the CO2
389
emission was measured during an incubation experiment separately for soil samples from 0-5
390
cm and 5-25 cm soil depth. Our soil samples from the CT treatments may not contain all
391
incorporated OM because the incorporation depth ranged between 25 and 30 cm under field
392
conditions, which may have caused an underestimation of the potenial CO2 evolution under
393
CT.
394
395
396
Furthermore, the higher cumulative CO2-C release and net N mineralization in soils
397
with reduced tillage systems compared to conventional tillage resulted presumably from the
398
increased microbial activity through higher Cmic and Nmic stocks under MT and NT compared
399
to CT. This was supported by the significant correlation of Cmic and Nmic with cummulative
400
mineralized C (r = 0.72, p <0.01 and r = 0.50, p < 0.05) and mineralized net N (r = 0.79 and r
401
= 0.60, p <0.01, respectively).
402
Also the aggregate content in soil affects the mineralizable OM content. As aggregates
403
are known to be build up by adhersion of mineral soil components to OM particles (Mikha
404
and Rice, 2004), the C and N mineralization may increase by a decrease of microaggregate
405
and macroaggregate contents, because aggregates are break up by intensive tillage
406
management and release within occluded OM. To quantify the stability of OM occluded
407
within aggregates, further research is required by investigation of the turnover of OM
408
occluded within macro and microaggregates, possibly by a separate incubation of different
409
aggregate sizes.
410
411
4.4
Labile and intermediate C and N pools
412
We proposed to separate the bulk SOM into pools of C and N with different turnover
413
dynamics and ecological function and to reveal the influence of the tillage intensity on these
414
pools. The suggestion, that microbial biomass decompose mainly labile OM was not fully
415
supported by our correlation analyses because next to the positive correlation of C mic and Nmic
416
with the modeled labile C and N pools, Cmic and Nmic correlated also with the intermediate C
417
and N pools (Table 5). The size of the modeled labile C and N and calculated intermediate C
418
and N was higher in 0-5 cm soil depth under NT and MT compared to CT (Figure 2),
419
however, only the differences in the C pools between the tillage treatments were significant.
420
In the soil samples from 5-25 cm depth we found slightly higher labile C pools under CT
421
compared to MT and NT, and higher intermediate C pools under MT, followed by NT and
422
CT. Hermle et al. (2008) also investigated a possible effect of tillage on carbon pools in a
423
sandy loam soil under a wheat-maize-wheat-canola rotation. They found a significantly higher
424
carbon storage under no tillage and reduced tillage compared to conventional tillage only in
425
the labile C pool. The intermediate C pools was unaffected by tillage in their study. They have
426
adjudged only a minor influence of tillage on soil C dynamics and C sequestration. Our
427
results also indicate a tillage effect on the intermediate OM pool and refute the statement of
428
Hermle et al. (2008). Correlation analyses showed the labile C and N pool to be heavily
429
influenced by the light fraction (LF) (Table 5). An association of the labile C pool to C in the
430
LF (r=0.73) was also found by Alvarez et al. (1995). But apparently, the LF partially also
431
contributes to the intermediate C pool as described by a coefficient of r=0.39 (Table 5). The
432
LF fraction comprises free and aggregate occluded organic particles. It is known, that
433
aggregate occluded organic particles are stronger protected against microbial decomposition
434
than free organic particles. Therefore, we assume that the amount of aggregate occluded
435
particles contributes stronger to the intermediate C and N pool and that the amount of free
436
organic particles contribute stronger to the labile C and N Pool. However, because we did not
437
separated free and aggregate occluded organic particles selectively are more precise
438
differentiation is not possible. Probably, after a separation of LF into free and occluded LF,
439
the free LF relates to the labile OM pool, while the occluded LF (physical protected OM by
440
inclusion in aggregates) contributes to the intermediate soil OM pool. For this protected OM,
441
Six et al. (2002) described a tillage effect with a loss of soil OM by breaking up the
442
aggregates at increased cultivation.
443
444
4.5
Passive C and N pools
445
The experimentally determined passive C and N pool was not influenced by tillage system,
446
which was in line with Helfrich et al. (2007) and Hermle et al. (2008). Because the passive
447
OM pool has a MRT of more than 100 years, the fourteen to twenty-one years of continuous
448
tillage intensity in our long-term field experiment should not affect the passive C and N pool,
449
as passive OM is independent by any management (i.e. fertilization, tillage, cropping).
450
Passive (or stable) OM pools are related to soil mineral characteristics (Alox, clay,
451
HF), which were unaffected by tillage treatment. The highly positive correlation of passive C
452
and N pools with Alox and clay indicated a strong binding affinity of passive OM to soil
453
mineral components. Also the aggregate formation through binding of OM to clay particles
454
results in a spatial inaccessibility of OM for microbial decomposition. This OM occluded
455
within aggregates and clay structures < 20 µm is long-term stabilized (von Lützow et al.,
456
2006), as tillage do not destruct this small aggregates.
457
458
5
Conclusion
459
The fractionation of soil organic matter with chemical and biological methods and the
460
estimation of the size of pools with different stabilities by modeling lead to a comprehension
461
of soil organic matter dynamics in soils dependent on tillage intensity.
462
As expected, no-till increases the labile and intermediate organic matter pool
463
compared to conventional tillage but also reduced tillage leads to a build up of more than less
464
easy decomposable organic matter stocks. This larger stocks of labile and intermediate C and
465
N were unaffected by site characteristics. By modeling the labile C and N pools and
466
calculation of intermediate C and N pools, we could show, that tillage intensity affected both
467
of these pools. The increase of intermediate C and N stocks under reduced tillage and no-till
468
compared to conventional tillage is of main interest for global warming, as Corg and Nt storage
469
in soils leads to decreased C and N losses through smaller CO2 emissions and mineralized N
470
losses. Due to the high mean residence time of stable organic matter in soil, the passive C and
471
N pools are not affected by the comparatively short times of tillage practices, as expected, but
472
we observed a study site effect on passive C and N stocks with larger stocks at high clay
473
contents.
474
A shift from intensive to reduced tillage systems gains to a short-term and medium-
475
term storage of soil C and N with turnover times of less than 10 years and 10 to 100 years,
476
respectively for short and medium-term storage. But one has to keep in mind, that a
477
conversion of long-term reduced or no tillage systems to more intensive tillage systems will
478
lead to an escalate rise of CO2 and mineralized N release by mineralization of the previous
479
stored organic matter.
480
481
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Table 1: Site designations, year of establishment, altitude, and climatic conditions of the long-term field experiments, and mean values of the soil
sand, silt and clay concentrations, and the pH values. Values in parenthesis are standard errors (n=3, pseudo replicates).
Site
599
600
a
Site characteristics
Alti- Tempe- PrecipiYear estabtude
rature tation
lished
/m
/ °Ca
/ mma
Soil properties
Sand
Silt
Clay
Soil type (WRB, 2006)
/ g kg-1
pH
Friemar
(Thuringia)
1992
/1993
310
8.0
554
46 (6)
648 (20)
306 (17)
Haplic Phaeozem
7.13 (0.06)
Grombach
(Baden-Wuerttemberg )
1990
/1991
270
9.3
776
20 (2)
721 (20)
259 (21)
Luvisol/ Phaeozem
6.34 (0.14)
Lüttewitz
(Saxony)
1992
/1993
290
8.6
572
18 (3)
778 (9)
204 (9)
Luvisol
6.72 (0.10)
Zschortau
(Saxony)
1997
/1998
110
8.8
512
280 (10)
562 (6)
158 (8)
Haplic Luvisol/Haplic Planosol
7.07 (0.05)
long-term annual means as provided by Deutscher Wetterdienst
601
602
603
604
Table 2: Stocks of soil clay, the oxalate soluble Fe and Al (Feox, Alox), the light and heavy
fractions (LF, HF), organic C (Corg), and total N (Nt) in 0-5 cm as well as 5-25 cm soil depth
of the three tillage treatments. The data shown are mean values of three pseudo replicates and
the and the standard errors are given in parenthesis (n=3).
Site
Tillage
system
Friemar
CTa
MTb
NTc
Grombach
CT
MT
NT
Lüttewitz
CT
MT
NT
Zschortau
CT
MT
NT
605
606
607
608
a
Soil
depth
Corg
Nt
Clayd
Feox
Alox
HF
LF
(cm)
(t ha-1)
(t ha-1)
(t ha-1)
(t ha-1)
(t ha-1)
(t ha-1)
(t ha-1)
0-5
8.4 (0.4) 0.77 (0.03)
172 (23)
0.63 (0.03)
0.45 (0.04)
571 (0)
3.1 (0.2)
5-25
37.5 (1.7) 3.45 (0.13)
790 (104)
3 (0.08)
2.12 (0.24)
2621 (6)
12.3 (5.9)
0-5
10.2 (0.5) 0.92 (0.03)
136 (3)
0.8 (0.01)
0.43 (0.01)
496 (1)
3.9 (1.2)
5-25
38.7 (1.6)
3.48 (0.1)
715 (17)
4.48 (0.02)
2.29 (0.02)
2630 (7)
10.9 (6.8)
0-5
11.6 (0.3) 1.08 (0.02)
190 (24)
0.66 (0.05)
0.43 (0.05)
566 (1)
8.3 (1)
5-25
37.3 (0.3) 3.47 (0.24)
922 (116)
3.71 (0.34)
2.41 (0.23)
2768 (0)
14.8 (0.2)
0-5
5.1 (0.2) 0.52 (0.03)
142 (9)
0.72 (0.18)
0.37 (0.07)
587 (0)
2.1 (0)
5-25
20.9 (0.4) 2.07 (0.02)
617 (40)
3.41 (0.93)
1.6 (0.29)
2554 (4)
5.8 (4.3)
0-5
8 (0.5) 0.75 (0.03)
111 (18)
0.65 (0.01)
0.29 (0)
502 (0)
4.2 (0.2)
5-25
33.2 (1.6) 3.27 (0.13)
619 (102)
4.6 (0.09)
1.88 (0.02)
2810 (1)
7.4 (0.7)
0-5
11.1 (0.3) 1.02 (0.03)
179 (21)
1.16 (0.02)
0.4 (0.02)
568 (1)
3.8 (1.1)
5-25
30.8 (2.1) 3.11 (0.14)
910 (106)
6.01 (0.23)
2.25 (0.03)
2897 (3)
8.5 (2.6)
0-5
7.5 (0.2) 0.75 (0.02)
137 (7)
1.42 (0.05)
0.38 (0.03)
669 (0)
2.7 (0.3)
5-25
28.3 (0.7) 2.85 (0.08)
510 (25)
5.43 (0.12)
1.43 (0.14)
2488 (0)
12.5 (0.2)
0-5
9.1 (0.3) 0.84 (0.04)
113 (11)
0.95 (0)
0.23 (0)
538 (1)
5.6 (1.2)
5-25
27.3 (0.8) 2.71 (0.04)
556 (56)
5.76 (0.06)
1.34 (0.03)
2662 (1)
12.7 (1)
0-5
10.6 (0.2) 1.02 (0.02)
121 (10)
1.19 (0.07)
0.3 (0.03)
612 (1)
4.2 (0.9)
5-25
30 (0.3) 2.92 (0.06)
550 (45)
5.71 (0.31)
1.57 (0.11)
2780 (9)
11.5 (9.4)
0-5
7.7 (0.6) 0.69 (0.04)
113 (7)
1.15 (0.21)
0.41 (0.06)
669 (0)
3.6 (0.1)
5-25
29.8 (1.9) 2.68 (0.15)
430 (25)
4.14 (0.76)
1.63 (0.23)
2548 (1)
13.9 (1.5)
0.8 (0.01)
84 (7)
0.8 (0.01)
0.24 (0)
585 (0)
4 (0.1)
5-25
0-5
30.9 (1.5) 2.83 (0.12)
365 (31)
3.65 (0.02)
1 (0.03)
2542 (2)
8.7 (2.2)
0-5
8.5 (0.2) 0.78 (0.01)
87 (12)
0.88 (0.13)
0.3 (0.03)
561 (1)
6.4 (0.9)
5-25
27.9 (1.3) 2.55 (0.11)
414 (56)
4.44 (0.6)
1.58 (0.14)
2702 (1)
10.7 (1.2)
8.9 (0.1)
b
c
CT: conventional tillage; MT: reduced tillage; NT: no-till
stocks of clay were calculated for 0-5 cm and 5-25 cm soil depth using the measured clay
concentration in soil samples of 0-25 cm depth and the depth specific bulk densitiy.
d
609
610
611
612
613
Table 3: Site specific stocks of soil organic C (Corg), total N (Nt), labile C and N, intermediate C and N, passive C and N, microbial biomass C and
N (Cmic, Nmic), and mineralized C (CO2-C) and N (NO3--N, NH4+-N) (Nmin) in 0-5 cm as well as 5-25 cm soil depth of the three tillage treatments.
The data shown for passive C, passive N, Cmic, and Nmic are mean values of three pseudo replicates and the standard errors are given in parenthesis.
The data shown for labile C and N, intermediate C and N, CO2-C, and Nmin are mean values of the two lab replicates and the standard errors are
given in parenthesis (three pseudo replicates per site, treat and depth were mixed for each lab replicate).
Site
Tillage
system
Friemar
CTa
MTb
NTc
Grombach
CT
MT
NT
Lüttewitz
CT
MT
NT
Zschortau
CT
614
a
MT
NT
Soil
depth
Labile C
Intermediate C
Passive C
Cmic
CO2-C
Labile N
Intermediate N
Passive N
Nmic
Nmin
(cm)
(t ha-1)
(t ha-1)
(t ha-1)
(t ha-1)
(t ha-1)
(t ha-1)
(t ha-1)
(t ha-1)
(kg ha-1)
(kg ha-1)
0-5
0.61 (0.11)
6.92 (0.2)
0.87 (0.08)
0.18 (0.03)
0.35 (0.07)
0.07 (0)
0.6 (0)
0.1 (0)
40 (11)
22 (1)
5-25
3.31 (0.45)
30.92 (1.06)
3.26 (0.23)
0.79 (0.1)
1.91 (0.26)
0.33 (0)
2.83 (0)
0.29 (0)
128 (32)
103 (1)
0-5
1.21 (0.19)
8.45 (0.08)
0.53 (0.18)
0.2 (0.03)
0.7 (0.11)
0.11 (0)
0.76 (0)
0.05 (0)
39 (12)
33 (1)
5-25
2.44 (0.51)
32.87 (1.03)
3.4 (0.05)
0.71 (0.07)
1.41 (0.3)
0.28 (0.04)
2.84 (0.04)
0.35 (0)
142 (40)
87 (26)
1.52 (0.12)
0.21 (0.01)
0.81 (0.01)
0.07 (0)
82 (6)
64 (3)
173 (66)
77 (7)
0-5
2.64 (0.22)
8.27 (0.09)
0.72 (0.17)
0.33 (0)
5-25
2.34 (0.45)
30.44 (0.64)
4.54 (0.54)
0.75 (0.11)
1.35 (0.26)
0.25 (0.01)
2.93 (0.01)
0.29 (0)
0-5
0.53 (0.12)
3.96 (0.04)
0.6 (0.09)
0.13 (0.02)
0.31 (0.07)
0.05 (0.01)
0.42 (0.01)
0.05 (0)
31 (9)
15 (4)
5-25
2.54 (1.16)
15.35 (0.92)
2.96 (0.17)
0.56 (0.1)
1.47 (0.67)
0.24 (0.02)
1.57 (0.02)
0.27 (0)
150 (47)
74 (10)
0-5
1.4 (0.18)
5.94 (0.15)
0.63 (0.18)
0.31 (0.03)
0.81 (0.1)
0.12 (0.01)
0.58 (0.01)
0.05 (0)
70 (9)
36 (8)
5-25
2.39 (0.48)
27.51 (3.77)
3.32 (0.06)
0.71 (0.09)
1.38 (0.28)
0.25 (0.02)
2.72 (0.02)
0.3 (0)
180 (50)
78 (13)
0-5
1.96 (0.26)
8.45 (0.46)
0.69 (0.15)
0.34 (0.02)
1.13 (0.15)
0.14 (0.02)
0.82 (0.02)
0.06 (0)
82 (12)
42 (13)
5-25
2.51 (0.64)
24.83 (5.01)
3.47 (0.13)
0.72 (0.1)
1.45 (0.37)
0.26 (0.01)
2.5 (0.01)
0.35 (0)
176 (49)
81 (6)
0.36 (0.08)
0.06 (0.01)
0.66 (0.01)
0.03 (0)
37 (19)
18 (4)
2.4 (0)
0.09 (0)
116 (94)
110 (1)
0-5
0.63 (0.13)
6.15 (0.04)
0.72 (0.15)
0.14 (0.05)
5-25
3.14 (0.75)
22.69 (0.22)
2.44 (0.13)
0.56 (0.22)
1.81 (0.43)
0.36 (0)
0-5
1.88 (0.25)
6.63 (0.02)
0.54 (0.07)
0.38 (0.03)
1.09 (0.15)
0.22 (0.01)
0.6 (0.01)
0.03 (0)
76 (9)
67 (6)
5-25
2.14 (1.3)
22.6 (0.59)
2.57 (0.11)
0.75 (0.03)
1.24 (0.75)
0.2 (0.01)
2.41 (0.01)
0.1 (0)
192 (33)
61 (8)
0-5
1.98 (0.32)
7.98 (0.12)
0.61 (0.01)
0.37 (0.01)
1.15 (0.18)
0.22 (0)
0.78 (0)
0.03 (0)
85 (7)
68 (3)
1.32 (0.23)
0.26 (0.02)
2.55 (0.02)
0.11 (0)
162 (48)
81 (14)
0.35 (0.07)
0.06 (0)
0.61 (0)
0.02 (0)
43 (9)
18 (1)
5-25
2.28 (0.4)
25.26 (0.14)
2.45 (0.05)
0.51 (0.01)
0-5
0.6 (0.12)
6.43 (0.33)
0.65 (0.13)
0.19 (0.01)
5-25
2.77 (0.47)
24.45 (1.4)
2.55 (0.05)
0.77 (0.09)
1.6 (0.27)
0.26 (0.03)
2.33 (0.03)
0.09 (0)
182 (48)
79 (17)
0.72 (0.07)
0.09 (0.02)
0.69 (0.02)
0.02 (0)
58 (4)
29 (10)
143 (52)
32 (48)
0-5
1.24 (0.11)
7.04 (0.01)
0.57 (0.04)
0.23 (0.02)
5-25
1.61 (0.61)
27.09 (0.74)
2.25 (0.11)
0.52 (0.13)
0.93 (0.35)
0.1 (0.08)
2.66 (0.08)
0.07 (0)
0-5
2.19 (0.2)
5.82 (0.04)
0.52 (0)
0.31 (0.08)
1.27 (0.11)
0.15 (0.03)
0.61 (0.03)
0.01 (0)
67 (18)
47 (16)
5-25
2.43 (0.66)
22.96 (0.55)
2.46 (0.06)
0.52 (0.03)
1.41 (0.38)
0.21 (0.02)
2.23 (0.02)
0.11 (0)
141 (26)
66 (11)
b
c
CT: conventional tillage; MT: reduced tillage; NT: no-till
615
616
617
618
619
Table 4: Stocks of the soil organic C (Corg) and total N (Nt), the light (LF) and heavy fractions
(HF) (dry mass), the microbial biomass C and N (Cmic, Nmic) in 0-5 cm as well as 5-25 cm soil
depth of the three tillage treatments. The data shown are mean values of the four study sites, the
standard errors are given in parenthesis. Values followed by different letters are significantly
different (p ≤ 0.1). Letters refer to the comparison of tillage treatments within one depth.
Tillage
system
CTa
MTb
NTc
620
a
Soil
depth
Corg
Nt
LF
HF
Cmic
Nmic
(cm)
(t ha-1)
(kg ha-1)
(t ha-1)
(t ha-1)
(kg ha-1)
(kg ha-1)
0-5
5-25
0-5
5-25
0-5
5-25
7.2 (0.7) b
29.1 (3.4)
9.0 (0.5) ab
32.5 (2.4)
10.5 (0.7) a
31.5 (2.0)
2.9 (0.3) b
11.1 (1.8)
4.4 (0.4) ab
9.9 (1.2)
5.7 (1.0) a
11.4 (1.3)
624 (26) a
2553 (27)
531 (20) b
2661 (56)
577 (12) ab
2787 (41)
160 (15) b
673 (63)
277 (40) a
675 (51)
340 (11) a
625 (64)
684 (56) b
2765 (284)
827 (36) ab
3072 (181)
977 (66) a
3012 (192)
CT: conventional tillage; bMT: reduced tillage; cNT: no-till
38 (3) b
144 (14)
61 (8) a
164 (13)
79 (4) a
163 (8)
621
622
623
Table 5: Coefficient of determination (r) and significance level (p) of the Spearman rank correlations (n = 24) between stocks of the labile,
intermediate, and passive C and N pools and the stocks of oxalat soluble Al (Alox) and clay, the light (LF) and heavy fractions (HF), the
mineralizable N (Nmin), and the stocks of microbial biomass C and N (Cmic, Nmic). Coefficients of determination are significant at p ≤ 0.1.
labile C
624
625
a
Alox
clay
LF
HF
Nmin
Cmic
Nmic
n.c.a
n.c.
r = 0.73, p < 0.01
n.c.
r = 0.89, p < 0.01
r = 0.80, p < 0.01
r = 0.57, p < 0.01
labile N
n.c.
n.c.
r = 0.75, p < 0.01
n.c.
n.a.b
r = 0.79, p < 0.01
r = 0.59, p < 0.01
intermediate C
n.c.
n.c.
r = 0.39, p < 0.1
n.c.
r = 0.50, p < 0.05
r = 0.44, p < 0.05
n.c.
intermediate N
n.c.
n.c.
r = 0.36, p < 0.1
n.c.
r = 0.41, p < 0.05
r = 0.50, p < 0.05
n.c.
passive C
r = 0.79, p < 0.01
r = 0.78, p < 0.01
n.c.
r = 0.54, p < 0.01
n.c.
n.c.
n.c.
passive N
r = 0.76, p < 0.01
r = 0.83, p < 0.01
n.c.
n.c.
n.c.
n.c.
n.c.
b
n.c.: no correlation; n.a.: not analyzed (modeled labile N pool stocks based on mineralized N stocks)
626
627
628
629
630
631
632
633
Figure 1: Cummulated stocks of emitted CO2-C and mineralized net N (on a kg ha-1 basis) of
soils from 0-5 cm as well as 5-25 cm depth of the three tillage treatments (CT: conventional
tillage, MT: reduced tillage, NT: no-till). Points plotted are means of the four study sites, error
bars refer to standard errors of the means. Values followed by different letters are
significantly different (p ≤ 0.1). Letters refer to the comparison of tillage treatments within
one depth.
634
635
636
637
638
639
Figure 2: Stocks of the labile, intermediate, and passive C (t ha-1) and N pools (kg ha-1) of soil samples from 0-5 cm and 5-25 cm depth of the three
tillage treatments (CT: conventional tillage; MT: reduced tillage; NT: no-till). Columns show the means values of the four study sites, error bars
refer to standard errors of the means. Values followed by different letters are significantly different (p ≤ 0.1). Letters refer to the comparison tillage
treatments within one depth.
640
641
642
643
Figure 3: Stocks of labile C pool versus (a) light fraction (LF) stocks (δ ≤ 1.8 g cm-3), versus (b) microbial C (Cmic) stocks, and versus (c) microbial
N (Nmic) stocks, and stocks of passive C pool versus (d) oxalate soluble Al (Alox) stocks, versus (e) clay stocks, and versus heavy fraction stocks (δ
> 1.8 g cm-3) (on a t ha-1 cm-1 basis).
644
645
646
647
Figure 4: Stocks of labile C pool versus (a) light fraction (LF) stocks (δ ≤ 1.8 g cm-3), versus (b) microbial C (Cmic) stocks, and versus (c) microbial
N (Nmic) stocks, and stocks of passive C pool versus (d) oxalate soluble Al (Alox) stocks, and versus (e) clay stocks (on a t ha-1 cm-1 basis).