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Running Head: Soil Respiration, Nitrification, and Denitrification
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Soil respiration, nitrification, and denitrification in a wheat
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farmland soil under different management
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Z.H. HU1,*, H. LING2, S.T. CHEN2, S.H. SHEN1, H. ZHANG2, AND Y.Y. SUN2
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1Yale-NUIST
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Meteorology, Nanjing University of Information Science & Technology, Nanjing, China
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2School
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Technology, Nanjing, China
Center on Atmospheric Environment & Jiangsu Key Laboratory of Agricultural
of Environmental Science and Engineering, Nanjing University of Information Science &
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*
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Z.H. Hu
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E-mail: zhhu@nuist.edu.cn
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Tel: +86-(0)25-58731193
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Fax: +86-(0)25-58731193
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Address: 219 Ningliu Road, Nanjing, China
Corresponding author
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To be submitted to Communications in Soil Science and Plant Analysis
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ABSTRACT
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A field experiment was conducted during the 2010 to 2011 winter wheat-growing
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season to understand the soil respiration (Rs), nitrification, and denitrification rates in
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winter wheat farmland soil under no-tillage (NT) treatment with rice straw
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incorporation. The experimental treatments include NT, NT with rice straw covers on
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the surface (NTS), conventional tillage (CT), and CT with straw incorporation (CTS).
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NT and straw incorporation treatments did not change the seasonal patterns of Rs,
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gross nitrification (Gn), and denitrification (D) rates compared with CT. Compared
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with the CT treatment, the NT, NTS, and CTS treatments significantly reduced Rs
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(p<0.01), and the NT and NTS treatments significantly increased Gn and D (p<0.01).
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CTS also significantly increased Gn (p<0.01) but had no significant effect on D
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(p>0.05). Further analysis showed that the temperature sensitivity of soil respiration
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(Q10) of CT, NT, NTS, and CTS were 4.26, 1.86, 3.25, and 2.36, respectively. Our
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findings suggest that, compared with CT, the NT and straw incorporation treatments
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reduced Rs and Q10 and increased Gn and D.
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Keywords soil respiration, nitrification and denitrification, no-tillage, rice straw
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incorporation, winter wheat
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Introduction
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The carbon content in the soil pool is over 1,750 Gt or approximately thrice the
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vegetation carbon pool and twice the atmospheric pool (Granier et al. 2000).
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Therefore, any small changes in the soil carbon pool can adversely affect global
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carbon cycling. The soil carbon pool in croplands is an important part of the overall
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terrestrial ecosystem carbon pool. Carbon sequestration from croplands plays an
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important role in global change (Smith 2004). Meanwhile, croplands are one of the
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most active ecosystems affected by human activities.
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Soil respiration, including the autotrophic respiration of plant roots and
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heterotrophic respiration of soil microorganisms, plays a key role in global carbon
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cycling (Buchmann 2000; Schlesinger and Andrews 2000). In addition, soil
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respiration and global warming closely influence each other (Sánchez et al. 2003;
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Schlesinger and Andrews 2000). Nitrification and denitrification are important
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processes involving nitrogen turnovers and removals from agricultural ecosystems
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(Currie 1996; Müller, Stevens, and Laughlin 2004; Cookson et al. 2006). Moreover,
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these processes are the main sources of nitrous oxide (N2O) and nitric oxide (NO)
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from soils (Russow, Spott, and Stange 2008; Skiba, Smith, and Fowler 1993).
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Agricultural management affects the soil respiration rate (Rs) (Ding et al. 2007).
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Straw incorporation is an important strategy in organic matter management in
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farmlands (Nie et al. 2007) as it can increase the soil organic carbon content
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(Freibauer et al., 2004) and influence Rs (Duong, Baumann, and Marschner 2009;
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Iqbal et al. 2009). Compared with conventional tillage (CT), the no-tillage (NT)
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approach can protect the structure of soil aggregates (Pikul et al. 2009; Olchin et al.
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2008), strengthen soil mineralization, and increase soil carbon storage capacity
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(Jacobs et al. 2010). Previous studies have suggested that different tillage approaches
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could affect the soil nitrogen mineralization potential and nitrogen transformation
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(Kheyrodin and Antoun 2009; Muruganandam, Israel, and Robarge 2010). These
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approaches could also alter the quantity and microbial activity of soil microorganisms,
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thereby affecting the soil nitrogen cycle (Muruganandam, Israel, and Robarge 2010;
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Mahía et al. 2007). To our knowledge, only Gesch et al. (2007) and Dexter et al.
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(2000) have documented the effects of tillage and plant residue management on the
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soil respiration of agricultural soil. Thus, the possible impacts of tillage and straw
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incorporation on soil respiration, nitrification, and denitrification in typical croplands
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need further investigation.
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In this study, we hypothesized that NT and straw incorporation approaches might
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reduce the Rs and increase the gross nitrification (Gn) and denitrification (D) rates in
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farmland soil. To test this hypothesis, we performed a field experiment on a winter
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wheat farmland. The objective was to investigate the effects of NT and straw
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incorporation treatments on the Rs, Gn, and D in cropland soil.
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Materials and Methods
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Experimental site
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The field experiment was conducted in a farmland at the Ecological and Agricultural
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Meteorology Experimental Station (32.16° N, 118.86° E), Nanjing University of
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Information Science and Technology, Southeast China. Annual rotations, such as
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paddy rice (Oryza sativa)–winter wheat (Triticum aestivum) and soybean (Glycine
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max)–winter wheat, are the main agricultural cropping systems in this area. The
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annual average temperature and rainfall in this site are 15.6 °C and 1,100 mm,
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respectively. The soil (0 cm to 20 cm) is classified as hydromorphic, containing 26.1%
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clay with an initial pH (H2O) level of 6.20, total organic carbon of 14.98 g·kg-1, and
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total nitrogen of 0.77 g·kg-1.
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Field experiments
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The field experiments were conducted during the 2010–2011 winter wheat-growing
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season. A local prevailing winter wheat cultivar, Yangmai 12, was sown on November
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3, 2010 and harvested on June 1, 2011. Table 1 shows the different growth stages and
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fertilization of wheat. The experiment followed a completely randomized plot design,
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with three replicate plots (1 m × 2 m area each) for every treatment. The treatments
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include NT, NT with rice straw covers on the surface (NTS), conventional tillage (CT,
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with depth of approximately 15 cm), and CT with rice straw incorporation (CTS,
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where the straws were buried after plowing). The rice straws were cut into short
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sections (2 cm to 3 cm in length), and the dry weight of the rice straws employed was
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225 g·m-2.
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Determination of soil respiration
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Rs was measured using the LI-8100 automated soil CO2 flux system (LI-8100,
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LI-COR, Lincoln, NE, USA). An integrated pump circulates the headspace air from
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the chamber to the nondispersive infrared gas analyzer during the closed state of the
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chamber, and the CO2 concentration data and calculated flux rates in the system are
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recorded. Before the measurement, the PVC soil collars (outer and inner diameters of
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21.3 and 20.3 cm, respectively) were inserted 1 cm to 2 cm deep into the soil, with the
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top edges kept at approximately 7 cm to 8 cm above the soil surface. The collars were
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left on the field throughout the experiment. However, all plants inside the collars were
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removed during the study period. Each treatment had three soil collars as replicates,
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and 12 soil collars were randomly placed in the experimental field. Rs was measured
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once a week during the no-rain period. The Rs measurements started at 8:00 in the
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morning to minimize errors due to temperature.
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The soil temperature and moisture (5 cm depth) were simultaneously measured
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using probes equipped with LI-8100.
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Determination of Gn and D
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The Gn and D rates during three crucial growth stages of wheat (turning
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green-elongation, booting-flowering, and grain filling-ripening stages) were
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determined using the BaPS technique. The BaPS system was equipped with a
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container holding a maximum of seven soil cores (Ingwersen et al. 1999). Circular
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stainless covers (7 cm in diameter) were used to collect soil samples. Twelve intact
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soil samples were taken from each treatment, and four cores were simultaneously
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measured in the BaPS container. In other words, three parallel replicates were set for
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each treatment.
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BaPS is based on the determination of the CO2, O2, and total gas balances of the
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soil samples. Nitrification and denitrification are the main biological processes
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responsible for gas pressure changes inside such an isothermal and gas-tight system
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(Breuer, Kiese, and Butterbach-Bahl 2002). Gn and D can be calculated based on gas
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and inverse balance approaches. The unit of Gn and D is μg N·kg-1·h-1, which
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represents the amount of N production per dry weight of soil (kg) per hour. A detailed
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description on BaPS and relevant measuring processes can be found in the papers of
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Ingwerson et al. (1999) and Breuer et al. (2002).
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Statistical analysis
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The average Rs, Gn, D, and standard errors (SEs) were calculated based on triplicate
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measurements. One-way analysis of variance (ANOVA) was used to test the data
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variability among different treatments. To examine the temperature sensitivity of soil
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respiration, we performed regression analysis using y = aebx, where y is the Rs, x is the
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soil temperature, coefficient a is the intercept of respiration at 0 °C, and coefficient b
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represents the temperature sensitivity of soil respiration. The exponential regression
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was used to analyze the relationship between the data (Rs, Gn, and D) and the
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environmental factors (soil temperature and moisture). In this study, significance
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implies p = 0.05 unless stated otherwise. All statistical analyses were conducted using
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SPSS 13.0 (SPSS Inc., Chicago, IL, USA).
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Results and Discussion
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Effects of NT and straw incorporation on soil respiration
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The Rs values of the different treatments are shown in Figure 1. Compared with CT,
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the NT and straw incorporation treatments did not change the seasonal pattern of Rs.
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In the turning green-elongation stage, the Rs values of the four treatments were
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relatively lower. During the booting-flowering and grain filling-ripening stages, the Rs
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values increased. Figure 2 shows the average Rs of the four treatments in different
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growth stages. The average Rs values of NT and CT were significantly different
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(p<0.05). Compared with CT, NT significantly decreased the average Rs. The NT and
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NTS results during the turning green-elongation and booting-flowering stages were
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significantly different. The CT and CTS results during the turning green-elongation
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and grain filling-ripening stages were also different, indicating that straw
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incorporation reduced Rs. Meanwhile, straw incorporation in CT significantly
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increased Rs compared with that in NTS. In all growth stages, the average Rs of CT
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was the highest.
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No-tillage soils had higher soil organic carbon than CT soils (Jacobs et al. 2010),
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which improved the utilization efficiency of the microbial carbon source (Cookson,
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Murphy, and Roper 2008; Wang et al. 2008). This improvement leads to more reduced
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soil CO2 emissions under NT and NTS, compared with CT and CTS. Further analysis
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showed that the average Rs of CTS was higher than that of NT. This difference may be
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attributed to the fact that the disruption of soil aggregates has stronger influence on
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the efficiency of C stabilization than the application of straw into the soil profile
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(Olchin et al. 2008).
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Effects of NT and straw incorporation on the temperature sensitivity of soil
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respiration
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Regression analysis indicates that Rs increased exponentially with increasing soil
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temperature (Figure 3), thereby showing that soil respiration is positively correlated
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with soil temperature. This result is consistent with the findings of previous studies
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that soil temperature is the main factor affecting soil respiration, along with soil
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organic matter (Ussiri and Lal 2009; Chen et al. 2010).
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The re-estimated b values were used to calculate the soil respiration quotient (Q10 =
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e10b). The Q10 values of different treatments were 4.26 (CT), 3.25 (NTS), 2.36 (CTS),
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and 1.86 (NT). In contrast to CT, the NT and straw incorporation (NTS and CTS)
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treatments reduced Q10, indicating that the NT and straw incorporation treatments
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reduced the temperature sensitivity of soil respiration. Our results suggest that the Q10
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value of soil respiration was significantly affected by NT and straw incorporation
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(p<0.01). Further studies on plant chemistry and the decomposition processes in soil
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are needed to understand the mechanisms of the effects of NT and straw incorporation
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on the temperature sensitivity of soil respiration. Further studies on the different
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responses of autotrophic and heterotrophic respiration to NT and straw incorporation
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should also be conducted.
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Effects of NT and straw incorporation on Gn and D
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The Gn rates of the different treatments are shown in Figure 4. In the turning
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green-elongation stage, compared with CT, the NT and NTS treatments significantly
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increased Gn by 124% (p<0.01) and 252% (p<0.01), respectively. Meanwhile, CTS
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increased Gn by 58% (p>0.05). In the booting-flowering stage, the Gn rates in NT,
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NTS, and CTS soils were 6.62, 8.66, and 3.47 times higher than that in CT soil
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(p<0.01), respectively. In the grain filling-ripening stage, the Gn rates in NT, NTS,
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and CTS soils were 3.07, 4.97, and 1.62 times higher than that in CT soil (p<0.01),
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respectively.
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Figure 5 shows the D rates of the different treatments. In the turning
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green-elongation stage, the D rates in NT and NTS soils were 4.30 and 6.21 times
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higher than that in CT soil (p<0.01). In the booting-flowering stage, compared with
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CT, the NT and NTS treatments significantly increased D by 393% and 476%
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(p<0.01), respectively, whereas the CTS treatment did not increase D (p>0.05). In the
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grain filling-ripening stage, the NT and NTS treatments increased D by 66% and
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189% (p<0.01), respectively, whereas the CTS treatment decreased D by 62%
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(p<0.01).
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The Gn rates of the different treatments were NTS > NT > CTS > CT. The D rates
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were NTS > NT > CT  CTS. Two processes of nitrogen cycle exhibited similar
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characteristics. This similarity may be attributed to the close association among the
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different transformation processes involved in the nitrogen cycle (Nannipieri and Paul
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2009). The NTS and NT treatments had higher Gn and D rates, whereas CT treatment
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had lower Gn and D rates. This discrepancy may be due to the varying nitrogen
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content in the different treated soils. Groffman (1985) also found that nitrification and
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denitrification activities were higher at the upper 5 cm of NT soils than in CT soils. In
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contrast to CT, NT and straw incorporation treatments had higher N transformation
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and mineralization rates (Kheyrodin and Antoun 2009; Muruganandam, Israel, and
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Robarge 2010). Moreover, NT and straw incorporation could increase the quantity and
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microbial activities of soil microorganisms and thus accelerate the soil nitrogen cycle
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(Muruganandam, Israel, and Robarge 2010; Mahía et al. 2007; Phillips 2008). In
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general, the amount of available inorganic N is one of the factors regulating D rates in
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soils (Phillips 2008). Clark et al. (2009) also suggested that the nitrification process is
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consequently limited by NO 3 and NH 4 availability in soil.
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Furthermore, the Gn and D rates in different treatments had similar seasonal
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patterns, showing higher values in the early growth period which then decreased
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gradually. This phenomenon may be attributed to the freezing and thawing in the early
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spring season, which are beneficial to the mineralization, nitrification, and
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denitrification of nitrogen in soils and straw (Su, Kleineidam, and Schloter 2010).
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Moreover, the seasonal variation of Gn rates in soils can be readily predicted by soil
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moisture and temperature (Stange and
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Butterbach-Bahl 2008). In the current study, all Gn and D rates, except the D rates of
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CT and CTS, are negatively correlated with soil temperature and positively correlated
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with soil moisture (Table 2 and Table 3). These results are consistent with previous
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studies (Stange and Neue 2009; Kiese, Hewett, and Butterbach-Bahl 2008; Chen et al.
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2011). Rice and Smith (1983) also found that nitrification rates are higher in NT soils
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than in CT soils because the former type has more favorable soil moisture conditions.
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Conclusions
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Compared with CT, NT and straw incorporation treatments did not change the
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seasonal pattern of soil respiration. However, they reduced the Rs values and Q10
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values. The Gn and D rates of the different treatments had the same seasonal pattern,
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suggesting higher values in the turning green-elongation stage which then decreased
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gradually in the booting-flowering and grain filling-ripening stages. NT and straw
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incorporation significantly increased the Gn and D rates. Therefore, Gn and D rates
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Neue 2009; Kiese, Hewett, and
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are negatively correlated with soil temperature and positively correlated with soil
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moisture.
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Acknowledgements
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This work was supported by the National Natural Science Foundation of China
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(41175136, 41005088), the Ministry of Education of China (grant PCSIRT), and the
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Meteorological Industry Research Fund (GYHY201306046).
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Table 1. Main growth stages and fertilization schedules of winter wheat
Date
Growth stages
Date
3 November 2010
Sow
28 December 2010
Tillering
16 January 2011
4 February 2011
Turning green
21 February 2011
17 March 2011
Elongation
9 April 2011
Booting
17 April 2011
Heading
23 April 2011
Flowering
28 April 2011
Grain filling
1 June 2011
Harvest
3 November 2010
374
375
376
377
378
379
380
381
382
383
384
18
Fertilization
100 kg N ha-1 (urea),
78 kg P ha-1, 100 kg
K ha-1
50 kg N ha-1, 21 kg P
ha-1, 27 kg K ha-1
50 kg N ha-1
385
Table 2. Effects of no-tillage and straw incorporation on the relationships between
386
gross nitrification rate (Gn) and soil temperature and moisture
Soil temperature (t)
Soil moisture (θv)
Treatments
Gn
R2
p value
NT
868.2e-0.128t
0.919
<0.01
NTS
1174.3e-0.124t
0.940
<0.01
CT
272.5e-0.138t
0.655
<0.05
CTS
700.7e-0.148t
0.937
<0.01
NT
37.7e0.160θv
0.892
<0.01
NTS
82.0e0.060θv
0.931
<0.01
CT
9.0e0.374θv
0.832
<0.01
CTS
11.9e0.273θv
0.912
<0.01
387
388
389
390
391
392
393
394
395
396
397
19
398
Table 3. Effects of no-tillage and straw incorporation on the relationship between
399
denitrification rate (D) and soil temperature and moisture
Soil temperature (t)
Soil moisture (θv)
Treatments
D
R2
p value
NT
215.3e-0.065t
0.758
<0.01
NTS
233.2e-0.048t
0.950
<0.01
CT
16.8e0.022t
0.183
>0.05
CTS
30.4e-0.041t
0.295
>0.05
NT
45.2e0.077θv
0.670
<0.01
NTS
85.2e0.022θv
0.815
<0.01
CT
23.2e0.002θv
0.000
>0.05
CTS
8.8e0.096θv
0.438
>0.05
400
401
402
403
404
405
406
407
408
409
410
20
411
Figure Legends
412
Figure 1. Temporal variation of soil respiration rate (Rs) in different treatments soils.
413
Values are expressed as means (± SE), with the error bars showing SE.
414
Figure 2. Average soil respiration rate (Rs) values in different growth stages. Different
415
letters represent significant differences between treatments. Values are expressed as
416
means (± SE), with the error bars showing SE.
417
Figure 3. Soil respiration rate (Rs) in relation to soil temperature.
418
Figure 4. Gross nitrification rates (Gn) of different treatments soils during different
419
growth stages. Values are expressed as means (± SE), with the error bars showing SE.
420
Figure 5. Denitrification rates (D) of different treatments soils during different growth
421
stages. Values are expressed as means (± SE), with the error bars showing SE.
422
423
424
425
426
427
428
429
430
431
432
21
-2 -1
Soil respiration rate (μmol m s )
10
NT
NTS
CT
CTS
8
6
4
2
433
05-27
05-19
05-13
05-07
05-02
04-25
04-18
04-14
04-04
03-29
03-24
03-15
0
Date(mm-dd)
434
Figure 1. Temporal variation of soil respiration rate (Rs) in different treatments soils.
435
Values are expressed as means (± SE), with the error bars showing SE.
436
437
438
439
440
441
442
443
444
445
446
447
22
8
b
NT
NT S
CT
CT S
c
6
c
c
-2
-1
(μmol·m ·s )
Average soil respiration rate
10
a
4
2
c
b
a
a
a
a
b
0
Turning green-elongation Booting-flowering stage
stage
448
Grain filling-ripening
stage
Growth stages
449
Figure 2. Average soil respiration rate (Rs) values in different growth stages. Different
450
letters represent significant differences between treatments. Values are expressed as
451
means (± SE), with the error bars showing SE.
452
453
454
455
456
457
458
459
460
461
462
23
463
464
Figure 3. Soil respiration rate (Rs) in relation to soil temperature.
465
466
467
468
469
470
471
472
473
474
475
476
24
b
NT
NT S
CT
CT S
360
-1
-1
Gross nitrification rate (μg N kg h )
450
477
a
270
a
ac
180
a
c
c
90
b
a
b
c
d
0
Turning green-elongation Booting-flowering stage
stage
Groth stages
Grain filling-ripening
stage
478
Figure 4. Gross nitrification rates (Gn) of different treatments soils during different
479
growth stages. Values are expressed as means (± SE), with the error bars showing SE.
480
481
482
483
484
485
486
487
488
489
490
491
25
-1
-1
Denitrification rate (μg N kg h )
200
NT
NT S
CT
CT S
b
150
b
a
a
100
b
a
50
c
c
c
c
c
d
0
Turning green-elongation Booting-flowering stage
stage
Growth stages
Grain filling-ripening
stage
492
493
Figure 5. Denitrification rates (D) of different treatments soils during different growth
494
stages. Values are expressed as means (± SE), with the error bars showing SE.
495
26