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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
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carbon dioxide (CO2) and nitrous oxide (N2O) requires a reassessment of management
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practices (i.e., cropping, tillage, fertilization) to retain organic carbon (Corg) and nitrogen in
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soils. The conversion of intensive to conservation tillage systems, for example, was shown to
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increase the Corg and total nitrogen (Nt) contents of soils (Salinas-Garcia et al, 1997; Watts et
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al., 2010) by lowering the decomposition rates of organic matter (OM) (Kladivko, 2001;
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Zibilske et al., 2002). In general, compared to intensive tillage, the lower physical impact of
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reduced or no tillage systems leads to less mechanical disruption of soil aggregates and,
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therefore, improved physical protection of OM against microbial decomposition (Balesdent et
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al., 2000; Cambardella and Elliott, 1993; Mikha and Rice, 2004; Six et al., 2000a; Tisdall and
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Oades, 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., 2007b). Crop residues may accumulate on the soil surface of conservation tillage
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systems due to a reduced contact to the soil microbial community. In contrast, the litter
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distribution in ploughed soil layers is relatively uniform and OM decomposition rates are
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higher (Lenz and Eisenbeis, 1998; Oorts et al., 2007b). Furthermore, changes in the micro-
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climatic conditions due to drying and rewetting are much more severe on the soil surface than
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deeper in the soil. It is well known that drying as well as rewetting induce stress on
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microorganisms and their metabolism (Fierer and Schimel, 2002) decreasing the microbial
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activity and OM 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
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(Heitkamp et al., 2009). Additional information about the fluxes related to the turnover of the
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labile OM pool can be gained from measurements of mineralized N (Nmin) concentrations
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within laboratory incubation experiments (Kader et al., 2010). A highly important sub
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compartment of the labile OM pool represents the microbial biomass (von Lützow et al. 2007)
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because this parameter quantifies on the one hand the potentially active decomposer
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community responsible for the OM decomposition and on the other hand a readily available
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energy source (e.g., dead microorganisms, extra- and intracellular compounds) for
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microorganisms. The microbial biomass activity, and community structure and, consequently,
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the microbially induced evolution of CO2 and N2O are highly sensitive against changes in the
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soil 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 distinctly larger than 100
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years. It is known that the stable OM pool is heavily affected by site conditions such as soil
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mineral characteristics and pH (Mikutta et al., 2006; Kaiser et al., 2012). In contrast, the effect
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of 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 from soils of four different long-term field experiments (LFE) in
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Germany initiated by the Institute of Sugar Beet Research (Göttingen, Germany) in
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cooperation with the agricultural division of Südzucker AG (Mannheim, Germany). The
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annual mean air temperature range from 8.0 to 9.3 °C and the annual precipitation from 512 to
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776 mm (data were provided by Deutscher Wetterdienst) (Table 1). Soil texture varies
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between the four loess sites with, for example, clay contents ranging from 16% (Zschortau) to
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31% (Friemar) (Table 1). At all sites, the crop rotation consists of two growing seasons of
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winter wheat (Triticum aestivum L.), followed by sugar beet (Beta vulgaris ssp. vulgaris var.
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altissima DÖLL). Crop residues are left in the field. At Friemar and Grombach, winter wheat
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was sown in fall 2010 after previous wheat and harvested before soil sampling in fall 2011,
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while at Lüttewitz and Zschortau, sugar beets were sown in spring 2011, after a seedbed
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preparation down to 5 cm. At soil sampling in fall 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 ranging 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. Soil samples were taken in the time from the 13.09. to the 16.09.2011,
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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:
CO3-C = (a x p x (1 x R-1)) x ((273 + t) x w)-1
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(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).
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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 experiment 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 correct 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 organic particles were removed from the soil
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samples following an approach of Balesdent et al. (1991). This was done to avoid the mixing
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of C and N derived from labile pools with C and N of the passive OM pool (Jagadamma et al.,
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2010). Free as well as aggregate occluded organic particles contribute to the easily
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decomposable amount of OM in topsoils (Kaiser et al., 2010). For the present study, we used
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a sodium polytungstate (SPT) solution (Sometu, Berlin, Germany) with a density of 1.8 g cm-
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3
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beads (5 mm diameter) to the suspension, which was shaken for 18 h at 175 rpm on a
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reciprocal shaker. Then, the suspension was centrifuged for 30 min. by 2000 x g and the
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supernatant was filtered through a polyamide filter (0.45 µm). The material on the filter (light
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fraction: <1.8 g cm-3; LF) was washed with 2 l of DI water. The glass beads were removed
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and the soil pellet (heavy fraction: >1.8 g cm-3; HF) was also filtered and washed with 2 l of
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DI water. The light and heavy fractions were dried for 48 h at 40 °C and the C and N
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concentrations of the HF fraction were determined via dry combustion.
. We added 40 ml of SPT solution to 10 g field moist soil (<5 mm) and applied 10 glass
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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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The stocks of the soil C and N pools, microbial biomass C and N (Cmic, Nmic),
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mineralizable N (Nmin), the light and heavy fractions (LF, HF), and the oxalate soluble Fe and
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Al (Feox, Alox) were calculated for 0-5 cm and 5-25 cm depth using the measured depth
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specific concentration of the respective parameter and the depth specific bulk density. For the
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calculation of the respective clay stocks, we used the clay concentration from mixed samples
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encompassing 0-25 cm soil depth and the depth specific bulk density.
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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 and modeling
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 = 0.0027 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
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the three tillage systems are given in Table 3. The stocks of Cmic were higher under NT (340
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kg ha-1) and MT (277 kg ha-1) compared to CT (160 kg ha-1) (Table 4). In 5-25 cm soil depth,
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the Cmic stocks ranged from 625 to 675 kg ha-1 and were similar under the three tillage
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treatments (Table 4). The Nmic stocks were significantly higher under NT (79 kg ha-1) and MT
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(61 kg ha-1) compared to CT (38 kg ha-1) in 0-5 cm soil depth but showed no significant
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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
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0-5 cm and 5-25 cm depth of the three tillage systems are given in Table 3. The stocks of
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cumulative emitted CO2-C after 341 days ranged from 343 to 1698 kg ha-1 and with higher
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CO2-C emissions from soils under NT and MT (1267 and 829 kg ha-1 341 days-1,
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respectively) compared to CT (343 kg ha-1 341 days-1) (Figure 1 a). The CO2-C emissions for
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5-25 cm soil depth were similar under all treatments with 1698, 1381 and 1239 kg ha-1 341
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days-1 under CT, NT, and 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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intensity in 0-5 cm as well as in 5-25 cm soil depth. Under CT, the Nmin stock was lower in 0-
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5 cm soil depth (20 kg ha-1 341 days-1) compared to MT and NT (44 kg ha-1 341 days-1 and 59
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kg ha-1 341 days-1, respectively) (Figure 1 b). In 5-25 cm soil depth, the cumulative
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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). The experimentally determined passive C pool stored between
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0.52 and 4.54 t C ha-1 (Table 3; 5-14% of Corg, C/N ratio: 14.8). The cumulative net N
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mineralization was well described by the applied one-pool model (R2 >0.99). The modeled N
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storage in the labile N pool ranged from 43 to 359 kg ha-1 (Table 3; 6-24% of Nt; k = 0.0011
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day-1) (Figure 2 b). The stocks of labile N in 5-25 cm soil depth ranged from 209 to 286 kg
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ha-1 (Figure 2 d). The stocks of the calculated intermediate N pool ranged from 429 to 2943
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kg ha-1 (Table 3; 72-91% of Nt) whereas the passive N pool ranged from 14 to 354 kg ha-1
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(Table 3; 2-13% of Nt) (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 systems are given for each study
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site separately in Table 2. The stocks of the LF fraction were significant higher under NT (5.7
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t ha-1) and MT (4.4 t ha-1) compared to CT (2.9 t ha-1) in 0-5 cm soil depth. For the stocks of
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the HF fraction, significant differences were observed between all tillage treatments in 0-5 cm
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soil depth in the order of CT > NT > MT (Table 4). In 5-25 cm soil depth, the LF and HF
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stocks were not significantly different.
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3.6
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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 4, 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, Nmic, 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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Correlation analyses
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4
Discussion
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4.1
Stocks of Corg and Nt
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The determined stocks of Corg and Nt in the studied soils result from the equilibrium of
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the input and output of C and N. Main input sources in soil agro-ecosystems for C and N are
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fertilization and plant derived OM (i.e., root particles, aboveground crop residues). The output
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mainly derives from OM decomposition, vertical as well as horizontal transport processes,
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and plant uptake (N) followed by harvest. The incorporation and distribution of plant derived
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OM and OM decomposition processes in soils are largely influenced by management options
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such as tillage intensity. Over a time span of 14 to 21 years, different tillage intensities lead in
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the surface soil (0-5 cm) to significantly higher stocks of Corg and Nt under NT compared to
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CT (Table 4). Our results also revealed a positive but not significant effect of reduced tillage
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(MT) on the stocks of Corg and Nt compared to CT. Similar findings were described for silty
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loam soils by Mikha and Rice (2004) and Cosentino et al. (1998). A significant increase in
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Corg and Nt with decreasing tillage intensity was not observed for the subsurface soil (5 - 25
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cm). This indicates that the effect of the tillage intensity on the stocks of Corg and Nt depends
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on the soil depth. A possible explanation for this is the different incorporation depth of C and
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N in form of crop residues. Even if the amount of this input is equal under the three tillage
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systems, the crop residues are incorporated homogeneously into 0-25/30 cm under CT
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whereas such residues are incorporated in 0-10/15 cm under MT and in 0-5 cm depth under
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NT (i.e., seedbed preparation for sugar beets). Consequently, the added C and N amounts are
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distributed in a larger soil volume under CT compared to MT and NT leading to the largest
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accumulation under NT in 0-5 cm depth. Beside this dilution effect, the lower physical impact
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of reduced or no tillage systems compared to CT may lead to an improved physical protection
336
of OM against microbial decomposition due to the occlusion in aggregates and to increased
337
Corg and Nt stocks. Accordingly, Andruschkewitsch et al. (2013) found for the same four long-
338
term field experiments significant higher macro-aggregate contents in soils under NT and MT
339
compared to CT in 0-5 cm depth.
340
Gibt es einen Einfluss der mineralischen N-Düngung auf die Nt Werte???  r = 0.73, p <0.01
341
(stocks kg Nt ha-1, N-Dünger kg ha-1)
342
Gibt es Daten zum Ertrag der Flächen  wäre wichtig um den Eintrag an crop residues
343
abschätzen zu können
344
4.2
Microbial biomass C and N
345
The stocks of Cmic and Nmic were significantly higher under MT and NT compared to
346
CT in 0-5 cm soil depth (Table 4). This is in line with Green et al. (2007) and Balota et al.
347
(2004) who also reported higher Cmic contents under NT compared to CT. Similar to the
348
stocks of Corg and Nt no signifcant differences in Cmic and Nmic between the tillage systems
349
were detected in 5-25 cm soil depth. Our data indicate, that the increase in Corg and Nt in 0-5
350
cm with decreasing tillage intensity is accompanied by an increase in easily decomposable
351
OM leading to higher stocks of Cmic and Nmic. This is supported by positive correlations
352
between stocks of Corg and LF (r = 0.62, p <0.01) as well as between stocks of Cmic and Nmic
353
and LF (r = 0.86 and r = 0.7, p <0.01). The LF fraction as separated here represents highly
354
management sensitive free and aggregate occluded organic particles (Six et al., 2002), which
355
were shown to contribute to the easily decomposable OM in topsoils (Strosser, 2010; Kaiser
356
et al., 2010). Similar findings were reported by Alvarez et al. (1995), who detected a higher
357
biological activity of microorganisms (between 0.035 and 0.078 µg CO2-C µg Biomass-C-1 d-
358
1
359
0.24) .. Additional to the larger amount of mineralizable OM, Balota et al. (2004) assumed
360
more favorable conditions for microorganisms in soils without tillage because of improved
361
water supply due to enhanced soil aggregation compared to soils of intensive tillage.
362
Furthermore, especially macro-aggregates in topsoils act as habitats for soil microorganisms
) at higher contents of C in the soil LF (between 1.07 and 1.46 µg C g-1 LF) (r = 0.65, p =
363
(Bailey et al., 2012) and can promote decomposition processes due to the close proximity of
364
decomposers and potential energy sources (i.e., occluded OM) .
365
366
4.3
CO2-C emission and mineralized N
367
The stocks of respired CO2-C and leached Nmin (i.e., NO3-, NH4+) were significantly
368
higher under NT compared to CT in 0-5 cm soil depth (Figure 1a and b) generally confirming
369
results from Balesdent et al. (2000) who investigated the influence of the tillage intensity on
370
the amount of mineralizable N. These results are in line with the above discussed increase in
371
substrate (Corg, Nt, LF) and decomposers (Cmic and Nmic). For samples from 5-25 cm soil
372
depth, the cumulative CO2-C release was significantly higher under CT compared to NT. The
373
higher CO2-C and Nmin stocks under NT compared to CT seems to result from an
374
accumulation of bio-available C and N pools and enhanced microbial OM decomposition in
375
the NT topsoils compared to the CT soils. This is in line with the higher Cmic and Nmic stocks
376
found in NT topsoils compared to CT soils in 0-5 cm depth (Table 4) indicating higher
377
microbial biomass and activity in the NT topsoils. This is supported by the significant
378
correlation of the Cmic and Nmic stocks with the CO2-C (r = 0.72, p <0.01 and r = 0.5, p <0.05,
379
respectively) and Nmin stocks (r = 0.79, p <0.01 and r = 0.6, p <0.01, respectively).
380
In contrast to our results, Alvarez et al. (1995) reported 88-92% greater CO2-C
381
evolution in soils under conventional tillage compared to reduced tillage. The authors
382
concluded an increase in carbon availability and mineralization in conventionally managed
383
soils caused by the incorporation of plant residues and the increased OM decomposition due
384
to reduced spatial disconnection between microorganisms and freshly added OM. The
385
contrary results from Alvarez et al. (1995) may be in part a result from methodological
386
differences. Alvarez and co-workers used 0.01 M NaOH traps in a field experiment and
387
measured the CO2 as emitted from the whole soil profile while in our study the CO2 emission
388
was measured during an incubation experiment separately analysing soil samples from 0-5 cm
389
and 5-25 cm soil depth. Our soil samples from the CT treatments may not contain all
390
incorporated OM because the incorporation depth ranged between 25 and 30 cm under field
391
conditions, which may have caused an underestimation of the CO2 evolution from CT
392
samples.
393
394
395
396
4.4
Labile and intermediate C and N pools
Labile should be influenced by mic, min, LF, intermidte less since it should consist of
OM 10-100 years.
397
We proposed to separate the bulk SOM into pools of C and N with different turnover
398
dynamics and to reveal effects of the tillage intensity on these pools. The stocks of the
399
modeled labile C and N and calculated intermediate C and N pools were significantly larger
400
under NT compared to CT (Figure 2a and b) in 0 - 5 cm soil depth. The intermediate C and N
401
pools are up to 6 times larger than the labile C and pools. In the soil samples from 5-25 cm
402
depth we found slightly but not significantly higher labile C pools under CT compared to MT
403
and NT. The higher intermediate C pools under MT, followed by NT and CT. Hermle et al.
404
(2008) also investigated a possible effect of tillage on carbon pools in a sandy loam soil under
405
a wheat-maize-wheat-canola rotation. They found a significantly higher carbon storage under
406
no tillage and reduced tillage compared to conventional tillage only in the labile C pool. The
407
intermediate C pools was unaffected by tillage in their study. They have adjudged only a
408
minor influence of tillage on soil C dynamics and C sequestration. Our results also indicate a
409
tillage effect on the intermediate OM pool and refute the statement of Hermle et al. (2008).
410
Correlation analyses showed the labile C and N pool to be heavily influenced by the light
411
fraction (LF) (Table 5). An association of the labile C pool to C in the LF (r=0.76) was also
412
found by Alvarez et al. (1995). But apparently, the LF partially also contributes to the
413
intermediate C pool as described by a coefficient of r=0.56 (Table 5). The LF fraction
414
comprises free and aggregate occluded organic particles. It is known, that aggregate occluded
415
organic particles are stronger protected against microbial decomposition than free organic
416
particles. Therefore, we assume that the amount of aggregate occluded particles contributes
417
stronger to the intermediate C and N pool and that the amount of free organic particles
418
contribute stronger to the labile C and N Pool. However, because we did not separated free
419
and aggregate occluded organic particles selectively are more precise differentiation is not
420
possible. Probably, after a separation of LF into free and occluded LF, the free LF relates to
421
the labile OM pool, while the occluded LF (physical protected OM by inclusion in
422
aggregates) contributes to the intermediate soil OM pool. For this protected OM, Six et al.
423
(2002) described a tillage effect with a loss of soil OM by breaking up the aggregates at
424
increased cultivation.
425
The suggestion, that microbial biomass decompose mainly labile OM was not fully
426
supported by our correlation analyses because next to the positive correlation of Cmic and Nmic
427
with the modeled labile C and N pools, Cmic and Nmic correlated also with the intermediate C
428
and N pools (Table 5).
429
4.5
430
The experimentally determined passive C and N pool was not influenced by tillage system,
431
which was in line with Helfrich et al. (2007) and Hermle et al. (2008). Because the passive
432
OM pool has a MRT of more than 100 years, the fourteen to twenty-one years of continuous
433
tillage intensity in our long-term field experiment should not affect the passive C and N pool,
434
as passive OM is independent by any management (i.e. fertilization, tillage, cropping).
Passive C and N pools
435
Passive (or stable) OM pools are related to soil mineral characteristics (Alox, clay,
436
HF), which were unaffected by tillage treatment. The highly positive correlation of passive C
437
and N pools with Alox and clay indicated a strong binding affinity of passive OM to soil
438
mineral components. Also the aggregate formation through binding of OM to clay particles
439
results in a spatial inaccessibility of OM for microbial decomposition. This OM occluded
440
within aggregates and clay structures < 20 µm is long-term stabilized (von Lützow et al.,
441
2006), as tillage do not destruct this small aggregates.
442
443
5
Conclusion
444
The fractionation of soil organic matter with chemical and biological methods and the
445
estimation of the size of pools with different stabilities by modeling lead to a comprehension
446
of soil organic matter dynamics in soils dependent on tillage intensity.
447
As expected, no-till increases the labile and intermediate organic matter pool
448
compared to conventional tillage but also reduced tillage leads to a build up of more than less
449
easy decomposable organic matter stocks. This larger stocks of labile and intermediate C and
450
N were unaffected by site characteristics. By modeling the labile C and N pools and
451
calculation of intermediate C and N pools, we could show, that tillage intensity affected both
452
of these pools. The increase of intermediate C and N stocks under reduced tillage and no-till
453
compared to conventional tillage is of main interest for global warming, as Corg and Nt storage
454
in soils leads to decreased C and N losses through smaller CO2 emissions and mineralized N
455
losses. Due to the high mean residence time of stable organic matter in soil, the passive C and
456
N pools are not affected by the comparatively short times of tillage practices, as expected, but
457
we observed a study site effect on passive C and N stocks with larger stocks at high clay
458
contents.
459
A shift from intensive to reduced tillage systems gains to a short-term and medium-
460
term storage of soil C and N with turnover times of less than 10 years and 10 to 100 years,
461
respectively for short and medium-term storage. But one has to keep in mind, that a
462
conversion of long-term reduced or no tillage systems to more intensive tillage systems will
463
lead to an escalate rise of CO2 and mineralized N release by mineralization of the previous
464
stored organic matter.
465
466
References: prüfen!!! Dort fehlen noch die REF die nachträglich in den Text gefügt
467
wurden
468
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469
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563
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. The soil data shown are mean values of three pseudo replicates and the standard errors are
given in parenthesis.
Site
564
565
566
567
a
Site characteristics
Alti- Tempe- PrecipiYear estabtude
rature tation
lished
/m
/ °Ca
/ mma
Soil properties
Sand
Clay
(g kg-1)
Soil typeb
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
according to the World Reference Base for Soil Resources (2006)
b
Silt
Table 2: Site, tillage system, and soil depth and the respective stocks of organic C (Corg), total N (Nt), clay, the oxalate soluble Fe and Al (Feox,
Alox), and the light and heavy fractions (LF, HF). The data shown are mean values of three pseudo replicates and the standard errors are given in
a
parenthesis.
CT: conventional tillage; bMT: reduced tillage; cNT: no-till
Site
Tillage
system
Friemar
CTa
MTb
NTc
Grombach
CT
MT
NT
Lüttewitz
CT
MT
NT
CT
Zschortau
568
569
570
MT
Soil
depth
Corg
Nt
Clay
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)
0-5
5-25
NT
8.9 (0.1)
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)
571
572
573
574
575
Table 3: Site, tillage system, and soil depth and the respective stocks of the labile, intermediate, and passive C and N pools, the microbial biomass
C and N (Cmic, Nmic), and the mineralized C (CO2-C) and N (Nmin). The data shown for passive C and 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, CO 2-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
576
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
577
578
579
580
581
Table 4: Tillage system and soil depth and the respective stocks of the soil organic C (Corg), total
N (Nt), the light (LF) and heavy fractions (HF) (dry mass), the microbial biomass C and N (Cmic,
Nmic). 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
582
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
2668 (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)
583
584
585
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 oxalate 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
586
587
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)
588
589
590
591
592
593
594
595
Figure 1: Cumulated stocks of emitted CO2-C and mineralized net N (Nmin) of soils in 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.
596
597
598
599
600
601
Figure 2: Stocks of the labile, intermediate, and passive C (t ha-1) and N pools (kg ha-1) of soils in 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.
602
603
604
Figure 3: Stocks of the labile C pool versus stocks of (a) the light fraction (LF), (b) microbial biomass C (Cmic), and (c) the microbial biomass N (Nmic)
and stocks the passive C pool versus stocks of the (d) oxalate soluble Al (Alox),(e) the clay fraction, and (f) the heavy fraction (HF).
605
606
607
608
Figure 4: Stocks of the labile N pool versus stocks of the (a) light fraction (LF), (b) microbial biomass C (Cmic), and (d) microbial biomass N (Nmic)
and stocks of the passive N pool versus stocks of the (d) oxalate soluble Al (Alox) and (e) clay-size fraction.