Effects Of Straw Return On Soil Carbon And Nitrogen Components And Yield In Rice-Wheat Rotation System Of Easten China

Straw return is an important farmland management measure.Different methods and years of straw return have important impacts on the soil organic carbon and nitrogen turnover and crop production.The Middle-lower Yangtze Plain is one of the major grain producing areas in China.Intensive wheat-rice cropping in this region guarantees the high and stable yield of crops,but also cause to non-point source pollution and soil degradation.Therefore,exploring soil carbon and nitrogen pool and grain yields affected by different straw returning methods and years have important theoretical and practical significance for promoting the sustainable development of rice-wheat rotation systems.This study consisted of two experiments.Experiment I was set up in 2013,include four treatments:minimum tillage with straw retention(MT),rotary tillage with straw retention(RT),conventional tillage with straw retention(CT),and conventional tillage without straw retention(CT0)4 treatments.Experiment ? was set up in 2010,include no straw return(NR)and 1-7 years of straw return(SR1-SR7,2017)or 1-8 years of straw return(SR1-SR8,2018).Through the analysis of effect of different methods and years of straw return on the soil organic carbon pool and nitrogen pool and their impacts on grain production,we hope to propose reasonable carbon and nitrogen management strategies and straw management measures in rice-wheat rotation areas.The main results of this study were as follows:1.The depth of straw distribution was different under different straw return methods,and the soil organic carbon(SOC)and total nitrogen(TN)content of the corresponding soil layer were increased respectively after 4 years of appling different straw return methods.Compared with CTO,MT significantly increased the SOC and TN at 0-5 cm soil layer,and SOC and TN stratification rate of 0-5 cm soil layer to the other layers.CT increased the SOC and TN content(9.08%-12.10%)at 0?20 cm soil layer,significantly increasing the SOC content of 10-20 cm soil layer compared to CTO.The greater the tillage intensity,the lower the C:N ratio.2.Different straw return methods adjusted the content of SOC and TN compontents in each soil layer within 0-20 cm after 4 or 5 years,but had different effects on different soil layers.MT significantly increased the content of LFOC,HFOC,POC,MOC and PTN at 0-5 cm soil,but had the lower HFOC,POC,MOC and LFTN,HFTN,PTN and MTN contents than RT and CT treatment at 10-20 cm soil.CT increased LFOC,POC,LFTN and PTN contents at each soil layer compared to CTO.The content of LFOC,HFOC,POC and MOC(LFTN,HFTN,PTN and MTN)were extremly significantly correlated with the content of SOC(TN),all of which could be used as indicators to indicate the change of SOC(TN).The change rates of LFOC and LFTN were highly correlated with that of SOC and TN,and show the highest sensitivity to farming methods,indicated that LFOC and LFTN were the best indicators of the SOC and TN change affected by straw return methods,respectively.3.The organic carbon and total nitrogen were mainly distributed in the>0.25 mm water-stable aggregates at each treatments after 4 or 5 years of appling different straw return methods.The SOC content in the macroaggregates is 30.53%-44.96%higher than that in the microaggregates.The content and the contribution rate to soil total organic carbon and nitrogen of>2 mm water-stable aggregates were the aggregate size that was mostly affected by straw return methods,and both decrease with the increase of tillage intensity.MT significantly increased the ratio of>2 mm aggregates and>0.25 mm macroaggregates.Compared with CTO,CT increased the SOC contents in each class of aggregates,but had no significant effect on the TN content in aggregates.Straw returning methods significantly affected the contribution rate of SOC(TN)in aggregates>2 mm to SOC(TN).Compared with CTO treatment,MT significantly increased the contribution rate of SOC in>2 mm aggregates to SOC.There was a significant positive correlation between the SOC content at 0-20 cm soil and the content of soil macroaggregates.The contribution rate of SOC in macroaggregates under CT increased 9.4%compared to CTO,and the increase rate was 13.24%.The contribution rate of TN in>2 mm aggregates of MT was significantly higher than that of CT and CTO.The contribution rate of TN in 0.25?0.053 mm and<0.053 mm aggregates of CTO were significantly higher than of other treatments.4.Straw return methods significantly affected the SOC and TN stock.MT increased the SOC stock in 0-20 cm soi profile and TN stock in 0-10 cm soil profile.The SOC stock at each soil layer in CT was higher than that in CTO,and the difference was significant at 0-10 cm and 0-20 cm soil layer.The TN stock at 0-20 cm soil layer in CT was also higher.5.The content of SOC and TN at each soil layer gradually increased with the increase of the straw return years,but the increase rate gradually decreased.The SOC.content of the 5-10 cm soil layer of each treatment was higher than that of other soil layers.Compared with NR,the significant difference inflection point of SOC content at each soil layer was 1 to 3 years of straw return,which of TN content was 1 to 2 years of straw return.The soil C:N ratio at the 0-5 cm soil layer increased significantly with the increase of the straw return years in short period(?3 a),and there was no obvious change trend at other layers and the years of straw return.With the increase of straw return years,the stratification rate of organic carbon of 0-5 cm soil layer to the other layers increased first and then decreased,while the trend of the total nitrogen stratification rate was opposite.When the years of straw return were less than or equal to 3,the soil C:N ratio at 0-5 cm soil layer increased significantly with the increase of straw return years,6.The content of SOC components and TN components increased with the increase of straw return years,but the growth rate of LFOC,POC,LFTN and PTN were higher than that of LFOC,MOC,HFTN and MTN,among which the increase rate of HFOC and MTN decreased after 6 years of straw return.With the increase of soil depth,the content of POC in each treatment increased first and then decreased,but the characteristics of MOC content depended on the straw return years.When the straw return years were less than 3,the MOC content of each treatment increased first and then decreased with the increase of soil depth.When the straw return years were more than 3,the content of MOC of each treatment gradually decreased with the increase of soil depth.There was an extremly significant positive correlation between LFOC,HFOC,POC,MOC(LFTN,HFTN,PTN,MTN)and SOC(TN),which can be used as indicators to indicate SOC changes.Among these SOC or TN components,LFOC or LFTN had higher correlations with SOC or TN,and showed the highest sensitivity to the straw return years,which was the best indicator of SOC or TN change affected by straw return years.7.With the increase of straw return years,the change trend of the proportion of different aggregates classes was different.The proportion of>2 mm aggregates gradually increased,which of 2?0.25 mm and 0.25?0.053 mm increased first and then decreased,while that of<0.053 mm gradually decreased.The SOC and TN content in each soil aggregates classes increased with the increase of straw return years(except for the 0.25-0.053 mm particle size).The contribution rate of SOC and TN of>2 mm aggregates increased with the increase of straw return years,while the other classes were decreased.>0.25 mm macroaggregates have the largest contribution rate to SOC and TN.8.The SOC and TN stock in each soil layer of 0-20 cm soil profile increased with the increase of the straw return years,but the increase rate decreased after 6 years of straw returned.9.Under different straw return methods treatments,the rice yields followed by CT>RT>MT>CT0.Soil LFOC and PTN content at 5-10 cm soil layer,SOC,HFOC,POC,MOC,LFTN,and PTN at 10-20 cm showed significant positive correlation with rice yields,and the correlation gradually increased with the soil depth deepened.The MOC content at 10-20 cm soil layer had the greatest impact on rice yields among all the SOC components.The LFTN had the greatest impact on rice yields among all the total nitrogen components,followed by PTN.The SOC content in each aggregate classes and TN in>2 mm and 2-0.25 mm aggregates were positively correlated with rice yield,and the SOC and TN content in>2 mm aggregates had the highest correlation,while the TN content in 0.25-0.053 mm and<0.053 mm aggregates was negatively correlated with rice yield.This showed that the increase of SOC and TN content in macroaggregates could increase rice yield,while the increase of those in microaggregates was not conducive to increasing rice yield.The SOC and TN stock in 0-20 cm soil profiles were positively correlated with rice yield,and the impact increased with the increase of soil depth.10.Under different straw return year treatments,the wheat yield decreased firstly and then increased with the increase of the straw return years.Except HFOC,MOC at 5-10 cm soil layer and MOC at 10-20 cm soil layer,contents of the other SOC and TN components in each soil layer of 0-20 cm were significantly positively correlated with wheat yields.The increase in the LFOC or LFTN content contributed the most to the increase in wheat yield among all SOC or TN components.Contents of SOC and TN in>2 mm aggregates had the highest correlation with wheat yield among all soil aggregates classes.Wheat yields were significantly correlated with SOC and TN stock.For every 1Mg·hm-2 increase in soil organic carbon and total nitrogen storage in 0-20 cm soil profile,wheat yield increased by 47.4 kg·hm-2 and 621.3kg·hm-2,respectively.