Integrative effects of soil tillage and straw management on crop yields and greenhouse gas emissions in a rice–wheat cropping system

Europ. J. Agronomy 63 (2015) 47–54

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Integrative effects of soil tillage and straw management on crop yields and greenhouse gas emissions in a rice–wheat cropping system Li Zhang a , Jianchu Zheng b , Liugen Chen b , Mingxing Shen c , Xin Zhang d , Mingqian Zhang a , Xinmin Bian a , Jun Zhang d , Weijian Zhang a,d,∗ a

Institute of Applied Ecology, Nanjing Agricultural University, Nanjing 210095, China Institute of Agricultural Resources and Environments, Jiangsu Academy of Agricultural Sciences, Nanjing 210014, China c Institute of Agricultural Sciences in Taihu Lake District, Suzhou 215100, China d Institute of Crop Science, Chinese Academy of Agricultural Science/Key Laboratory of Crop Physiology and Ecology, Ministry of Agriculture, Beijing 100081, China b

a r t i c l e

i n f o

Article history: Received 4 August 2014 Received in revised form 18 November 2014 Accepted 18 November 2014 Keywords: Food security Climate change CH4 emission N2 O emission Soil tillage Straw management

a b s t r a c t Significant efforts have been made to assess the impact of tillage regimes on crop yields and/or greenhouse gas (GHG) emissions across single crop growing season. However, few studies have quantified the impact across a whole rotation cycle in multiple cropping systems. Utilizing on a long-term tillage experiment with the rice–wheat rotation system in East China, we examined the GHG emissions under different tillage practices with or without crop straw incorporation. Results showed that compared to the no-straw control, straw incorporation increased wheat yield by 28.3% (P < 0.05), irrespective of tillage practices, but had no significant effect on rice yield. Although straw incorporation did not significantly affect CH4 emissions during the wheat season and N2 O emissions during the whole rice–wheat cycle, it significantly stimulated CH4 emissions by 98.8% (P < 0.01) during the rice season. Also, there were no significant differences in CH4 and N2 O emissions between tillage practices during the wheat season. Compared to plowing, rotary tillage increased CH4 emissions significantly by an average of 38.8% (P < 0.01) but had no significant impacts on N2 O emissions during the rice season. Across the rotation cycle, annual yield-scaled global warming potential of CH4 and N2 O emissions under no-tillage plus rotary tillage was 26.8% (P < 0.01) greater than that of rotary tillage plus plowing with or without straw incorporation. Significant interactions between soil tillage and straw management practices were found on annual GHG emissions, but not on crop yields. Together, these results indicate that plowing in the rice season plus rotary tillage in the wheat season may reduce GHG emissions while increasing crop yield in rice–wheat cropping areas. © 2014 Published by Elsevier B.V.

1. Introduction Soil tillage and straw management can alter carbon (C) and nitrogen (N) dynamics and consequently induce considerable changes in greenhouse gas (GHG) emissions and crop productivity (Paustian et al., 1997; Lal, 2004; Smith et al., 2008; Liu et al., 2014). For example, straw incorporation can stimulate GHG emissions through increasing C availability for methanogenics in paddy soils (Yan et al., 2005) and denitrifiers in dry-land soils (Luo et al., 1999). Also, in comparison to conventional tillage, no-tillage, and reduced tillage can enhance soil C sequestration by decreasing soil

∗ Corresponding author at: Institute of Applied Ecology, Nanjing Agricultural University, Nanjing 210095, China. Tel.: +86 25 84396030; fax: +86 25 84396030. E-mail address: [email protected] (W. Zhang). http://dx.doi.org/10.1016/j.eja.2014.11.005 1161-0301/© 2014 Published by Elsevier B.V.

C decomposition and/or C turnover, thereby mitigating GHG emissions (Six et al., 2000; Al-Kaisi and Yin, 2005; Van Grogenigen et al., 2011; Ruan and Philip Robertson, 2013). Furthermore, reduced tillage and straw incorporation alone or in concert can improve crop yields through improving soil fertility (Malhi and Lemke, 2007; Küstermann et al., 2013). Rice–wheat rotation system is the most popular cropping system in East Asia, covering ca. 13 million hectares each year in China (Frolking et al., 2002). Because this system consists of both irrigated rice and dry-land wheat, it raises unique challenges for quantifying the total GHG emissions. Agricultural practices, such as soil tillage and straw incorporation management, play an important role on crop productivity and/or GHG emissions (Ma et al., 2009; Pandey et al., 2012; Yao et al., 2013; Brennan et al., 2014). Therefore, it is imperative to assess the impact of agricultural practices on crop

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Table 1 Field experimental treatments and tillage and straw management practices in the rice–wheat cropping system. Treatment

RT–CT NT–RT RT–CT–S NT–RT–S

Tillage regime

Tillage depth (cm)

Straw incorporation

Wheat

Rice

Wheat

Rice

Wheat

Rice

Rotary No Rotary No

Plowing plus rotary Rotary twice Plowing plus rotary Rotary twice

10 0 10 0

15 10 15 10

No No Rice straw Rice straw

No No Wheat straw Wheat straw

yields and GHG emissions and determine the mitigation potential management practices for agronomic innovations. There are an increasing number of studies assessing soil tillage and straw management effects on GHG emissions and/or crop productivity in rice–wheat cropping areas. However, most of them have only considered single cropping season (Zou et al., 2005; Gangwar et al., 2006; Yao et al., 2009) and few have quantified the impact across a whole rotation cycle where multiple cropping is involved. For example, Zou et al. (2005) reported that straw incorporation increased CH4 emission and reduced N2 O emission in the rice season. But Yao et al. (2009) showed that straw incorporation decreased N2 O emission in the non-rice season. Also, reduced tillage perturbations were found to reduce CH4 and N2 O emissions and grain yield (Pandey et al., 2012). To date, there are still many uncertainties about the effects of soil tillage and straw management practices on crop yields and GHG emissions. One of the main reasons might be due to the experimental duration, as most of existing studies were based on very short-term experiments (Ma et al., 2009; Yao et al., 2013). Yet effects of soil tillage and straw management practices on soil C storage and GHG mitigation are complex and may vary with practice duration (Duiker and Lal, 1999; Six et al., 2004; Smith et al., 2008). Also, it is worthy to mention that soil tillage and straw management practices are often intermingled together in field. However, existing field studies have mainly determined the effects of either tillage or straw management practices and the interactions of these two practices are not well documented (Bayer et al., 2014). Therefore, it is essential to further quantify the long-term effects of soil tillage and straw management on crop yields and GHG emissions across a whole rotation cycle in multiple cropping systems. The objective of this study was to assess the effects of soil tillage and straw management on CH4 and N2 O emissions and crop yields in the rice–wheat rotation system. Year-round measurements of CH4 and N2 O emissions were conducted in an existing long-term field experiment to assess the net effect across the whole rotation cycle. Also, field soils were obtained from these long-term plots and incubated to estimate soil C and N mineralization under simulated soil tillage and straw management practices. 2. Materials and methods 2.1. The field experiment 2.1.1. Experimental site description Our field measurements of CH4 and N2 O fluxes were conducted with the long-term tillage experiment located at the Institute of Agricultural Sciences in Taihu Lake District, Wuxi city (31◦ 27 N, 120◦ 25 E), Jiangsu Province, China. The Taihu region represents a northern subtropical monsoon climate with an annual temperature of about 15.7 ◦ C, an average annual precipitation of 1094 mm, and the effective accumulated temperature (above 10 ◦ C) of 4947 ◦ C. The soil has a clayey loam, developed from loessial deposits, with hydromica and semectite as the dominating clay minerals. Before the experiment was initiated in 2005, the surface soil (0–20 cm depth) contained 19.5 g kg−1 organic C, 1.82 g kg−1 total N, and pH (H2 O) of 6.5.

2.1.2. Experimental design The long-term tillage experiment was established in 2005, being laid out as a randomized block design with three replicates. There were two tillage practices each for the wheat (rotary tillage, RT, or no-tillage, NT) and rice seasons (plowing plus rotary tillage, CT, or rotary tillage only, RT), forming four tillage regimes for the whole rotation cycle (RT–CT, NT–RT, RT–CT–S, and NT–RT–S, see Table 1). Each plot size was 26 m2 (4.0 m × 6.5 m). Wheat and rice straw were both from the preceding seasons and incorporated freshly in the plots at approximately 3 and 5 Mg ha−1 , respectively. The C:N ratio of wheat and rice straw was 63 and 52, respectively. The cropping sequence was rice (Oryzasativa) followed by winter wheat (Triticum aestivum). Rice was transplanted at the age of about 5 leaves at a density of 32 hills m−2 and 3 plants a hill in mid June and harvested in late October. Wheat was seeded onto the soil surface roughly at a rate of 112.5 kg ha−1 in early November and harvested in late May. Water regimes in the rice season were practiced as early flooding-mid season drainage-intermittent irrigation regimes. The application rates of fertilizers for the rice–wheat rotation were 225 kg N ha−1 and 90 kg K2 O ha−1 for the wheat growing season and 225 kg N ha−1 and 150 kg K2 O ha−1 for the rice growing season, with no P fertilization for either growing season. 2.1.3. CH4 and N2 O measurements We conducted 2-year field measurements of CH4 and N2 O emissions starting from January 2011 to November 2012. CH4 and N2 O fluxes from the rice–wheat rotation system were measured using a static chamber method (Zou et al., 2005). One chamber frame, constructed from polyvinyl chloride (PVC) chamber (50 cm × 50 cm) with a water groove on the top edge, was randomly installed into each treatment plot to a 10 cm soil depth layering in two rows of wheat or rice plants. The chamber frames were kept in the same position over the whole sampling period, except when they were removed due to tillage practices. The top chamber, made of PVC, with a bottom size of 50 cm × 50 cm and a height of 50 cm or 100 cm (in terms of plant height), was equipped properly to collect gases. The water groove was designed to seal the rim of between the chamber and the frame. CH4 and N2 O fluxes were measured on a weekly basis in triplicate plots across the whole rotation cycle, except for about 30-day during the interval of wheat and rice seasons due to farming practices. Gas samples were collected at 0, 5, 10, and 15 min from the inside headspace after chamber closure using a 20-ml plastic syringe and transited into a 20-ml glass vial that were sealed with butyl rubber septa. The air temperature inside the chamber was determined during gas sampling. Collected gas samples were analyzed within 24 h on a modified gas chromatograph (GC) (Agilent 7890A, California, USA) equipped with a flame ionization detector (FID) and electron capture detector (ECD) for quantifying CH4 and N2 O concentrations, respectively. Fluxes were calculated with the linear increase in concentrations of selected sample sets that yield to a linear regression value of r2 > 0.90 (Zou et al., 2005). Average fluxes and standard errors of CH4 and N2 O were determined from triplicate plots. Cumulative CH4 and N2 O emissions were accumulated from the fluxes between every two adjacent sample days of the measurements (Zou et al., 2005).

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2.1.4. Soil and plant sampling To determine soil organic carbon (SOC) and mineral nitrogen (N) contents, soil samples (0–10 cm) were obtained from each plot after wheat harvest in June 01, 2012. We chose the 10 cm depth because it was the soil depth of straw incorporation. A subsample of each field soil was obtained and kept under 4 ◦ C for determining C and N mineralization (see Section 2.2). Fresh soil samples were sieved (2 mm) and extracted with 50 mL 0.5 M K2 SO4 . NH4 + and NO3 − N concentrations in the extraction were determined on an automatic analyzer (AA3, Bran-Luebbe, Norderstedt, Germany). Soil mineral N was calculated by the sum of NH4 + and NO3 − N contents. A 10 g subsample of field soil were air-dried, finely ground and sieved (<0.25 mm) for analysis of SOC on a C/N/H/S-analyzer (Vario Elementar III, Germany). At the plant physiological maturity, all aboveground plant biomass was harvested from each chamber. Wheat and rice plants were separated into leaves, stems, and seeds. Plant materials were oven-dried at 65 ◦ C for 48 h and weighed to determine crop yields. To estimate the long-term integrated impact of soil tillage and straw management practices on CH4 and N2 O emissions, total global warming potential (GWP) was calculated as CO2 equivalent (CO2 eq) on a time horizon of 100-year using the radioactive forcing potential of 25 for CH4 and 298 for N2 O (IPCC, 2007). The equation was as follows: GWP(kgCO2 eq ha−1 yr−1 ) = 25 × CH4 (kg ha−1 yr−1 ) + 298 × N2 O(kg ha−1 yr−1

(1)

Additionally, to associate with GWP and crop production, GWP per unit grain yield, namely yield-scaled GWP (kg CO2 eq Mg grain−1 ), was introduced and calculated following the equation by Pittelkow et al. (2013).

2.2. The incubation experiment Mineralizable soil carbon (C) and soil N2 O N emissions were measured in a laboratory incubation study under controlled aerobic conditions. Field moist soils collected on June 01, 2012 were sieved (<2 mm) to remove large roots and stones. The soil were adjusted to 60% water holding capacity (WHC) and pre-incubated under 25 ◦ C in the dark for 7 days to stabilize the microbial activity and avoid an undesired microbial peak. Emissions of N2 O N and CO2 C were measured in three replicates on days 1–7, 10, 18, 32, 34, 41, 48, 55, 68, 82, 97, 123, and 151 after the start of the incubation. Gas samples in the headspace were collected using gas-tight syringes (Singh et al., 2010) and were analyzed for the concentrations of CO2 C and N2 O N on a on a modified (GC) (Agilent 7890A, California, USA). The CO2 C and N2 O N emissions were calculated following the equation provided by Troy et al. (2013). Cumulative gas fluxes were determined by multiplying each gas flux of the interval between sampling dates. Soil C and N mineralization were then estimated by the cumulative emissions of CO2 C and N2 O N over the 151 days sampling period.

2.3. Data analysis Differences in CH4 and N2 O emissions, crop yields and other variables among treatments were determined statistically using the PROC MIXED procedure on SAS 9.3 (SAS Institute Inc., Cary, NC, USA, 2011). Data was not transformed before analysis. Comparisons for significance of treatment means were performed using Turkey’s test at the 95% probability level.

Fig. 1. Seasonal variations of CH4 fluxes for the rice (a) and wheat (b) seasons in the rice–wheat rotation system under different soil tillage and straw management practices of 2011 and 2012. RT–CT and RT–CT–S represent rotary tillage in wheat season plus with plowing in rice season without and with straw incorporation, respectively; NT–RT and NT–RT–S represent no-tillage in wheat season and rotary tillage in rice season without and with straw incorporation, respectively. Vertical bars represent standard errors of the three replicates.

3. Results 3.1. CH4 fluxes and seasonal/annual cumulative emissions During both rice seasons, CH4 fluxes were the highest right after transplanting (7 days) across all treatments and remained relatively high for another 3–4 weeks (Fig. 1a). CH4 fluxes decreased quickly after week 5 and remained at a low level since. The highest CH4 fluxes ranged from 391 to 1667 mg CH4 C m−2 d−1 , and from 314 to 1933 mg CH4 C m−2 d−1 in 2011 and 2012, respectively. Averaged cumulative CH4 emissions were 3457 and 6872 kg CO2 eq ha−1 for the non-straw and straw incorporated treatments, respectively (Table 2). Compared to no-straw control, wheat straw incorporation significantly increased cumulative CH4 emissions (P < 0.01) (Table 3). Additionally, seasonal CH4 emissions were 38.8% higher under rotary tillage than plowing (P < 0.01). During both wheat seasons, no clear trend of CH4 fluxes was observed (Fig. 1b). Each treatment plot was either a minor source or sink of CH4 fluxes. Seasonal mean CH4 fluxes were relatively low, ranging from −0.85 to 3.76 mg CH4 C m−2 d−1 and 0.77 to 0.85 mg CH4 C m−2 d−1 in 2011 and 2012, respectively. Cumulative CH4 emissions ranged from −92.0 to 80.3 kg CO2 eq ha−1 , and from 33.7 to 42.1 kg CO2 eq ha−1 , respectively, in 2011 and 2012 (Table 2). Notillage and rice straw incorporation had no significant effect on CH4 emissions in the wheat season (P > 0.05). Across the rotation cycle, CH4 emission in the rice season was the major source that contributes to annual CH4 emissions both in 2011

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Table 2 Impacts of soil tillage and straw management practices on seasonal and annual crop yields and global warming potential (GWP) of CH4 and N2 O emissions for 2011 and 2012. Treatment

2011 RT–CT NT–RT RT–CT–S NT–RT–S

Rice season

Wheat season

Annual

CH4 N2 O kg CO2 eq ha−1

GWP

Yield Mg ha−1

CH4 N2 O kg CO2 eq ha−1

GWP

Yield Mg ha−1

CH4 N2 O GWP kg CO2 eq ha−1 yr−1

Yield Mg ha−1 yr−1

Yield-scaled GWP kg CO2 eq Mg grain−1

3354 4592 6903 7555

4235 5064 7465 8465

9.56 9.13 9.99 9.78

−92 80 −40 14

346 310 385 354

254 391 345 368

4.45 5.20 5.55 6.28

3262 4672 6863 7569

1228 782 947 1264

4490 5455 7810 8833

14.01 14.33 15.54 16.05

325 381 511 548

881 472 562 910 2012

RT–CT NT–RT RT–CT–S NT–RT–S

2133 3748 4911 8119

1113 644 1025 1435

3246 4392 5935 9554

9.85 9.44 10.26 10.40

42 39 39 34

605 660 578 521

647 699 616 554

4.53 4.88 6.27 6.34

2175 3787 4950 7569

1718 1304 1602 1956

3893 5091 6552 10108

14.38 14.32 16.52 16.74

269 356 397 604

2011–2012* RT–CT NT–RT RT–CT–S NT–RT–S

2744a 4170b 5907c 7837d

997a 558a 793a 1172a

3741a 4728b 6700c 9009d

9.70a 9.29a 10.12a 10.09a

−25a 60a −1a 24a

476a 485a 481a 437a

451a 545a 480a 461a

4.49a 5.04a 5.91b 6.31b

2719a 4230b 5906c 7861d

1473a 1043a 1274a 1609b

4191a 5273b 7181c 9471d

14.20a 14.33a 16.03b 16.40b

296a 369b 451c 578d

*

Values are averaged data of the two-year cycles. Different characters in the same column represent significant differences (P < 0.05).

and 2012. Averaged annual CH4 emissions ranged from 2719 kg CO2 eq ha−1 for RT–CT to 7861 kg CO2 eq ha−1 for NT–RT–S. Annual CH4 emissions under straw incorporation were 117.2% and 85.8% higher (for NT–RT–S and RT–CT–S) than that of no-straw control (for NT–RT and RT–CT), respectively. Annual CH4 emissions were 55.6% and 33.1% higher under NT–RT and NT–RT–S than that of RT–CT and RT–CT–S, respectively. 3.2. N2 O fluxes and seasonal/annual cumulative emissions During both rice seasons, similar patterns of N2 O fluxes were observed across all treatments with varying magnitude of N2 O fluxes (Fig. 2a). A large peak of N2 O fluxes was observed both in September 2011 and 2012. Averaged N2 O fluxes were 0.90–1.23 mg N2 O N m−2 d−1 and 1.09–2.77 mg N2 O N m−2 d−1 in 2011 and 2012, respectively. Cumulative N2 O emissions were lowest in NT–RT (mean: 558 kg CO2 eq ha−1 ), followed by RT–CT–S, RT–CT, and NT–RT–S. There was no clear trend of N2 O fluxes during both wheat seasons (Fig. 2b). N2 O fluxes remained low at background levels averaging 0.51 mg N2 O N m−2 d−1 in the first season. Distinct peaks of N2 O fluxes were observed during the midseason in the second season, from February to April 2012. Mean N2 O fluxes were 1.50 mg N2 O N m−2 d−1 , 0.99 mg N2 O N m−2 d−1 , 1.28 mg N2 O N m−2 d−1 , and 1.61 mg N2 O N m−2 d−1 , respectively, for RT–CT, NT–RT, RT–CT–S, and NT–RT–S. However, there was no significant difference in cumulative N2 O emissions (P > 0.05) (Table 2). Across the rotation cycle, there were no significant effects of soil tillage and straw management practices on N2 O emissions (P > 0.05) (Table 2). Annual N2 O emission was significantly increased by the combination of reduced tillage and straw incorporation (mean: 1609 kg CO2 eq ha−1 for NT–RT–S) (P < 0.05).

3.3. Crop yields Crop yields of wheat and rice showed variable responses to soil tillage and straw management practices (Table 2). Soil tillage had no significant effect on seasonal and annual crop yields. Straw incorporation significantly increased wheat yield by 28.3% (P < 0.05) but did not significantly affect rice yields. Annual crop yields of wheat and rice were 13.7% higher with straw incorporation than those of non-straw incorporation (P < 0.01).

3.4. GWP and yield-scaled GWP There were significant differences of the total global warming potential (GWP) of emitted CH4 and N2 O across all treatments, ranging from 4191 to 947 kg CO2 eq ha−1 (Tables 2 and 3). Straw incorporation had significant effect on annual GWP, with an averaged increase of 71.3% and 79.6% for RT–CT–S and NT–RT–S compared to RT–CT and NT–RT, respectively (P < 0.01). Similarly, soil tillage had significant effect on annual GWP with an averaged increase of 25.8% and 31.9% for NR–RT and NT–RT–S compared to RT–CT and RT–CT–S, respectively (P < 0.01). Soil tillage and straw incorporation interactively increased annual GWP of CH4 and N2 O emissions (P < 0.05). Annual GWP was contributed mainly by CH4 and N2 O emissions in rice season, averaging approximately 92.6% across all treatments. The contribution of CH4 emission to total GWP was greater than N2 O emission, both for the rice season and rotation cycle, with an average of 85.4% for the former and 79.3% for the latter. Both straw incorporation and soil tillage had significant effects on yield-scaled GWP (P < 0.01 vs. P < 0.01). Yieldscaled GWP were increased by 26.8% under no-tillage plus rotary tillage, irrespective of straw incorporation (mean: 369 and 578 kg

Table 3 Statistical analysis for the effects of soil tillage and straw management practices on crop yields (yield) of wheat and rice, global warming potential (GWP), yield-scaled GWP, CH4 , and N2 O emissions, and seasonal GWP (wheat and rice seasons) for the two years. Values are Pr > F. Effect

Yield

GWP

Yield-scaled GWP

CH4

N2 O

Wheat season

Rice season

Straw Tillage Year Straw × tillage Straw × year Tillage × year Straw × tillage × year

0.0009 0.6245 0.3082 0.8116 0.5072 0.7377 0.9684

<0.0001 <0.0001 0.8257 0.0267 0.5318 0.1562 0.2844

< 0.0001 <0.0001 0.5926 0.0628 0.8407 0.1024 0.3349

<0.0001 <0.0001 0.2174 0.2165 0.7071 0.1986 0.3197

0.3408 0.8047 0.0063 0.0585 0.6615 0.9286 0.9945

0.7121 0.6087 0.0009 0.4406 0.4091 0.5600 0.9970

<0.0001 < 0.0001 0.5753 0.0188 0.4269 0.1234 0.2777

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18.3 g kg−1 for RT–CT–S vs. NT–RT–S) than no-straw control (18.7 g kg−1 vs. 17.1 g kg−1 for RT–CT vs. NT–RT). No-tillage tended to decrease SOC compared to rotary tillage in the wheat season. Compared to rotary tillage, soil mineral N was increased by 22.1% under no-tillage (P < 0.05). The highest mineral N was observed in NT–RT–S (93.5 mg N kg−1 ), which was greater than that of RT–CT–S (72.9 mg N kg−1 ), RT–NT (20.0 mg N kg−1 ), and NT–RT (41.0 mg N kg−1 ) (P < 0.05). 3.6. Soil C mineralization and N2 O N emission

Fig. 2. Seasonal variations of N2 O fluxes for the rice (a) and wheat (b) seasons of the rice–wheat rotation system under different soil tillage and straw management practices in 2011 and 2012. RT–CT and RT–CT–S represent rotary tillage in wheat season plus with plowing in rice season without and with straw incorporation, respectively; NT–RT and NT–RT–S represent no-tillage in wheat season and rotary tillage in rice season without and with straw incorporation, respectively. Vertical bars indicate standard errors of three replicates.

CO2 eq Mg grain−1 for NT–RT and NT–RT–S) compared to RT–CT and RT–CT–S (mean: 296 and 451 kg CO2 eq Mg grain−1 ). 3.5. Soil organic C and mineral N Straw incorporation in the wheat season had significant effect on soil organic carbon (SOC) and mineral nitrogen (N) (P < 0.05) (Fig. 3). SOC was higher under straw incorporation (19.4 vs.

CO2 C emissions were higher under straw incorporation than no-straw control, except day 4, day 7, day 10, and day 34 (P < 0.05) (Fig. 4a). Across the whole incubation period, cumulative CO2 C emissions were significantly increased by 28.1% under straw incorporation (1175 vs. 1052 mg CO2 C kg−1 for RT–CT–S vs. NT–RT–S) compared to no-straw control (860 vs. 882 mg CO2 C kg−1 for (RT–CT vs. NT–RT) (P < 0.05). However, soil tillage had no significant effect on cumulative CO2 C emissions (P > 0.05). The patterns of cumulative N2 O N emissions were similar for all treatments (Fig. 4b). High N2 O N emissions occurred in the first 32 days of the experiment (113, 112, and 124 ␮g N2 O N kg−1 h−1 for RT–CT, NT–RT, and NT–RT–S, respectively). N2 O N emissions from NT–CT–S were relatively low over the whole incubation period. But there were no significant differences in N2 O N emissions between the treatments (P > 0.05). This was largely due to the high variability between columns from the same treatment. Over the 151 days sampling period, cumulative N2 O N emissions were significantly higher under RT–CT (748 ␮g N2 O N kg−1 ), NT–RT (617 ␮g N2 O N kg−1 ) and NT–RT–S (654 ␮g N2 O N kg−1 ) compared with RT–CT–S (404 ␮g N2 O N kg−1 ) (P < 0.05). However, the addition of straw to soil had no significant effect on N2 O N emissions (P > 0.05). 4. Discussions 4.1. Effects of soil tillage and straw incorporation on CH4 emission Our results showed that straw incorporation can have significantly different effects on CH4 emissions between rice and wheat growing seasons (Fig. 1; Table 2). These differences likely stem from the resulting impacts of different management regimes on soil C and O2 for methanogens. Two pathways of CH4 production contribute to CH4 emissions in rice paddies. Acetotrophy process (C6 H12 O6 3CO2 + 3CH4 ) is considered to be the dominant process, contributing about 2/3 of the CH4 produced (Segers, 1998; Le Mer and Roger, 2001). The other process is CO2 + H2 -dependent

Fig. 3. Soil organic C (SOC) (a) and mineral N (b) contents response to soil tillage and straw management practices. RT–CT and RT–CT–S represent rotary tillage in wheat season plus with plowing in rice season without and with straw incorporation, respectively; NT–RT and NT–RT–S represent no-tillage in wheat season and rotary tillage in rice season without and with straw incorporation, respectively. Vertical bars indicate standard errors of three replicates.

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conditions and water management. Because of the dominant effect of O2 , effects of soil tillage and straw incorporation on CH4 emissions in upland soil will be less important or even undetectable under low soil moisture and aerobic conditions (Hütsch, 2001; Xu and Hosen, 2010). 4.2. Effects of soil tillage and straw incorporation on N2 O emission

Fig. 4. Soil C and N mineralization as affected by soil tillage and straw management practices. (a) Cumulative CO2 C emissions; (b) cumulative N2 O N emissions. RT–CT and RT–CT–S represent rotary tillage in wheat season plus with plowing in rice season without and with straw incorporation, respectively; NT–RT and NT–RT–S represent no-tillage in wheat season and rotary tillage in rice season without and with straw incorporation, respectively.

methanogenesis under anaerobic conditions (Conrad and Klose, 1999). In irrigated systems such as rice paddies where anaerobic conditions dominate, CH4 emissions are primarily controlled by the availability of C substrates in soil (Yan et al., 2005). Straw incorporation buries residues into soil and increases available C (Fig. 4a), particularly labile C for methanogens. Different tillage methods can have significantly different effects on CH4 emissions, as evidenced by 38.8% higher CH4 emission under rotary tillage than plowing (Table 2). Again, this is likely due to different C availability for methanogens. Rotary tillage mixes residue into the surface 5–10 cm depth soil. In contrast, plowing buries residues into deeper layer of soil (10–15 cm depth), reducing access of these residues by microbes through protection of soil matrix (Hütsch, 1998). Also, rotary tillage may increase residue fragmentation, facilitating residue decomposition to intermediate products that serves as substrates for methanogens (Segers, 1998; Le Mer and Roger, 2001). Higher total organic C under plowing than rotary tillage (Fig. 3a) also indicated higher decomposition in the rotary tillage system. Significantly higher CO2 evolution in the soil from plowing than rotary tillage plots in our incubation experiment (Fig. 4a) provided further evidence showing that there was more labile C in the plowing soil. In upland systems such as wheat fields, O2 functions as the major driver controlling both CH4 production and oxidation (Le Mer and Roger, 2001). Therefore, CH4 emissions from soils amended with crop straw during the non-rice season is strongly influenced by soil moisture (Xu and Hosen, 2010), which depends on climatic

In rice paddies, soil N2 O emissions mainly stem from denitrification and straw amendment may affect N2 O emissions through increasing available C for microbes and microbial O2 consumption, leading to conditions favorable for denitrification (Miller et al., 2008). In our field experiment, N2 O emissions were not significantly affected by straw incorporation in both rice seasons (Table 2). These results were in contrast to other previous reports of inhibitive effect of straw incorporation on N2 O release (Zou et al., 2005; Yao et al., 2009; Yao et al., 2013). The effect of straw incorporation effects on N2 O emissions can be two-folds depending on residue C:N of crop residues incorporated and soil N content because denitrification process dominates N2 O production. Denitrifiers are usually heterotrophic microbes that need organic C. Therefore, when residues have relative low C:N ratios or the soil has high N content, residue incorporation can enhance denitrification and therefore, N2 O production (Huang et al., 2004; Miller et al., 2008; Shan and Yan, 2013). In contrast, when soil N is low, incorporation of high C:N residues may facilitate microbial growth and microbial N immobilization into microbial biomass. In our experiment, straw incorporation increased organic C substrates in the soil (Fig. 3a) for microbial growth and thus promotes microbial N assimilation, reducing ammonium (NH4 + ) for nitrifiers. Although residue input increases soil mineral N (Fig. 3b) through microbial N mineralization and nitrification, In our aerobic incubation experiment, N2 O production may largely stem from nitrification and microbial NH4 + utilization may contribute to observed N2 O reduction (Fig. 4b). Different soil tillage practices had no significant effect on N2 O emissions (Table 2). Similarly, this is likely due to the differential effects of tillage practices on the C and N availability. SOC was greater under plowing than rotary tillage (Fig. 3a) while soil mineral N was greater under rotary tillage than plowing (Fig. 3b). Because either C or N availability might limit N2 O production, these differential effects of soil tillage may lead to similar net effects on N2 O emission. Also, soil N2 O emission in dry-land soils is mainly controlled by soil temperature and moisture (Zou et al., 2004; Chen et al., 2013). The effects of no-tillage and straw incorporation on N2 O emission may depend on soil aeration and climatic conditions (Six et al., 2004; Smith and Conen, 2004; Shan and Yan, 2013; Yao et al., 2009; Van Kessel et al., 2013; Sheehy et al., 2013). Under notillage system, crop residues are evenly left to soil surface and may enhance soil moisture (Gregorich et al., 2006) so that it may be potentially conducive to N2 O emissions. However, surface placement of residues reduces microbial access to organic C, reducing energy sources for microbial denitrification. 4.3. Effects of soil tillage and straw incorporation on crop yields Our results showed that straw incorporation had significantly different effect on the yields of rice and wheat (Table 2). Wheat yield was greater under straw incorporation than no-straw control. But straw incorporation had no significant effect on rice yield. Increased soil organic C and soil fertility in croplands caused by straw incorporation has the potential to improve crop productivity (Pan et al., 2009). Also, straw incorporation may enhance soil moisture and organic C, benefiting wheat growth, but having little effect on rice. Gangwar et al. (2006) also showed that

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incorporation of crop residues increased yield of wheat and SOC. However, the effect of straw incorporation on rice yield may depend on straw application methods (Eagle et al., 2000; Ma et al., 2009; Xu et al., 2010). Soil tillage had no significant effect on crop yield in both rice and wheat seasons (Table 2). Soil tillage may alter crop yields through changing soil properties, particularly soil organic ˜ ˜ niga matter and N availability (Malhi and Lemke, 2007; Patino-Zú et al., 2009). However, the response of crop yield is also influenced by cultivations and climatic conditions. Therefore, the effect of soil tillage on crop yield would be more complex and variable (Malhi et al., 2006; Thomsen and Sørensen, 2006; Van Grogenigen et al., 2011; Yao et al., 2013; Brennan et al., 2014).

4.4. Effects of soil tillage and straw incorporation on the GWP of CH4 and N2 O emission We examined yield-scaled GWP (total CH4 and N2 O emissions per unit of crop yield) because this ratio may be more relevant while considering the net effect on agronomic and environmental parameters. Across the whole rotation cycle, yield-scaled GWP under rotary tillage plus no-tillage was greater than plowing plus rotary, irrespective of straw incorporation (Table 2). Yield-scaled GWP was also significantly increased by straw incorporation, regardless of tillage practices. These results indicate that tillage regimes of rotary tillage in the wheat season plus plowing in the rice season, with or without straw incorporation, may generate less greenhouse gas emissions while maintaining high crop yield. Although straw incorporation may induce labor and energy cost (Glithero et al., 2012; Khakbazan and Hamilton, 2012), it can avoid the emissions of GHG, notably CH4 , generated from straw burning (Smith et al., 2008). Besides, crop yield should be considered along with the mitigation strategies (Lal, 2004; Ma et al., 2009; Pittelkow et al., 2013) as straw incorporation enhances soil fertility (Singh et al., 2004; Gangwar et al., 2006; Malhi and Lemke, 2007). Taken together, rotary tillage in the wheat season plus plowing in the rice season without (RT–CT) or with (RT–CT–S) straw incorporation may be an effective way for rice–wheat cropping system from climate change or food security perspectives.

5. Conclusions Our assessment of GHG emissions and crop yields across the whole rotation cycle showed different a picture from results either from the rice or wheat season alone. Also, trade-off occurred between crop yield and CH4 emissions as influenced by soil tillage and straw incorporation management practices. Based on annual GWP and crop yield analyses, tillage regimes of rotary tillage in the wheat season followed by plowing in the rice season, irrespective of straw management was an optimum practice with high yield and low GHG emissions. These results suggest that the adoption of rotary tillage in the wheat season, combined with plowing during the rice season, may provide a promising management practice to attain the dual objectives of sustaining grain yield and mitigating GHG emissions in East China.

Acknowledgements This work was supported by the National Key Technology Support Program of China (2011BAD16B14), the Program for New Century Excellent Talents in University (NCET-05-0492), the GEF Project of Climate Smart Staple Crop Production in China (P144531), the Innovation Program of CAAS and the grant of China Scholarship Council (CSC).

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