| China is a great agricultural country and one of the major wastes in agricultural production is crop straw.It is very important to the sustainable development of agriculture that how to solve these straw reasonable and effective.As one of the main ways to utilize crop straw,straw returning not only can improve soil fertility,increase crop productivity,but also can alleviate the environmental pollution,which caused by straw burning.Rice-wheat is one of the main cropping systems in our country,and the planting area is about 1.05×107 hm-2.Crop straw often returned to fields by rotary tillage,plowing tillage and so on in this area.However,in recent years,with the increase of grain yields,the amount of crop straw is also increasing.The large amount of straw not only makes the mechanized farming difficult,reduces the emergence rate of crops,but also causes a lot of greenhouse gases(GHG)emissions.Straw ditch-buried returning as a noval straw returning mode has been reported.It can not only realize the full amount of straw returning to the field,but also beneficial for the soil carbon sink and increase the crop yield.However,compared with the conventional straw returning method,it is not known whether it has a better comprehensive effect on the crop production,soil fertility and environment.In this study,a field experiment from June,2013 to June,2015 was carried out in the rice-wheat rotation system in the middle and lower reaches of Yangtze River in China.Four wheat straw returning modes,no straw returning(CK),wheat straw retuning with rotary tillage(WR),wheat straw retuning with plowing(WP),and ditch-buried wheat straw returning(WD)were set.The effects of different straw returning modes on soil carbon sequestration,GHG emissions and crop yields,and the carbon footprint analysis were studied.At the same time,the simulation experiment was carried out to verify and analyze the results of the field experiment.So as to comprehensively evaluate the effect of different wheat straw returning modes on farmland soil,crop yield and environmental effect,and provide the theoretical basis for exploring the efficient,safe and ecological way of straw returning mode for this rice-wheat rotation system.Main results are as follows:1.Soil organic carbon fractions and carbon sequestrationIn rice season,wheat straw returning could increase soil total organic carbon content.Compared to CK,soil labile organic carbon and dissolved organic carbon contents were significantly higher under WR,and soil microbial biomass carbon content was significant higher in WP.Soil carbon pool management index(CPMI)was significantly higher under WR,which increased 41.01%than CK.In wheat season,compared with CK,wheat straw returning also could improve soil total organic carbon content,which was significantly higher under WD.WR had the highest soil labile organic carbon,dissolved organic carbon and microbial biomass carbon contents,which were also significantly higher than CK.Compared to CK,WR significantly increased 108.62%soil CPMI.After two years rice-wheat rotations,wheat straw retuning treatments could increase soil organic carbon sequestration,compared with CK,and especially,WD had significantly higher soil organic carbon sequestration than CK and WP.Among three straw returning modes,the soil organic carbon sequestration rate was highest under WD.Therefore,WD treatment could improve soil quality,increase soil organic carbon under short-term,but with the increasing of straw returning,WD may be a great potential mode for carbon sequestration.2.Greenhouse gases emissionsDuring the two rice-wheat rotations,soil CH4 and N2O emissions were higher in higher temperature and more rainfall conditions under the same management practices.Compared with CK,wheat straw returning not only could increase CH4 emission in rice season,but also increase CH4 emission in following wheat season.Whether in rice or wheat season,seasonal average CH4 emission under WD was significantly lower than WR.Annual CH4 emission was significantly lower under WD than WR,which was also lower than WP.Compared to CK,wheat straw returning treatments significantly increased N2O emissions in the first rice season and the second wheat season.WD had significantly lower annual N2O emission than WP,but significantly higher than WR and CK.Therefore,compared with the conventional straw returning with rotary and plowing tillage modes,wheat straw ditch buried may reduce GHGs emissions in the rice-wheat rotation system.The results of simulation experiment showed that soil CO2 emission rate under simulative wheat straw buried returning was lower than that simulative wheat straw returning with rotary tillage and plowing tillage,indicating that the rate of straw decomposition lower.However,compared with simulative WR and WP treatments,soil dissolved organic carbon content under simulative wheat straw buried returning was significant higher,showing that the rate of anaerobic decomposition of CH4 was slower,thus leading less CH4 emission.In flooding and non-flooding period,simulative wheat straw returning modes could alter N2O emission by improved soil NH4+-N content.Therefore,wheat straw returning modes could affect the CH4 and N2O emission by affecting the availability organic carbon and nitrogen contents in the soil.3.Greenhouse gas emission intensity and net global warming potentialDuring the two rice-wheat rotations,compared to CK,wheat straw returning could increase rice and wheat yields,but didn't reach the significant level.Annual grain yields under WP and WD were significantly higher than CK,with the increase of 6.5%and 7.1%,respectively.Compared to CK,wheat straw returning significantly increased the greenhouse gas emission intensity and net global warming potential.The reason was that straw returning significantly increased CH4 emission in rice season,and the GWP was mainly dominated by CH4 emission in rice season.Among three wheat straw returning modes,greenhouse gas emission intensity was lower in WD than WR and WP.Compared with WR and WP,WD could reduce net global warming potential by 13.41%and 17.07%.4.Carbon footprintIn this rice-wheat rotation,the proportion of soil CH4 emission was the largest in the total amount of carbon footprint,followed by fertilizer input in the rice season.Among the three straw returning modes,carbon footprint was lowest in WD,which decreased 5.94%and 3.37%respectively,compared to WR and WP.In wheat season,fertilizer input dominated the largest proportion of carbon footprint,followed by soil N2O emission.Wheat straw returning had significantly higher yield-scaled carbon footprint than CK.Among three straw returning modes,compared with WR and WP,WD could reduce yield-scaled carbon footprint by 9.43%and 8.33%.Therefore,reducing CH4 emission and improving nitrogen fertilizer utilization efficiency is the main way to mitigate carbon emission in rice season,while decreasing fertilizer input is the important approach to decrease carbon emission in this rice-wheat rotation.Based on these results,wheat straw returnng treatments could increase soil organic carbon and labile organic carbon fractions content,improve rice and wheat grain yields,but also caused a lot of CH4 and N2O emissions.In the rice-wheat rotation system,reducing CH4 emission in rice season and improving nitrogen fertilizer utilization efficiency is the important way to realize energy saving and emission mitigation.Among three different straw returning modes,wheat straw ditch buried in rice seasons might be a great potential mode to reduce GHG emissions while sustaining increasing crop yields and soil carbon sequestration in the rice-wheat rotation system compared with straw returning via rotary tillage or plowing. |
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