Effects Of Different Rape Straw Returning Methods On Greenhouse Gas Emissions And Rice Production In Paddyfield

Large amount of crop residues were produced during agricultural activities every year.Currently,the unreasonable use of crop residues（e.g.straw burning）could cause terrible effects on agro-ecosystem,such as the lost of nutrients and atmospheric pollution.Since different residue returning practices certainly would cause diverse impacts on economic benefits and environmental benefits of agro-ecosystem,exploring a residue returning practice which combined economic benefits with environmental benefits was of great importance.All the experiment plots were distributed in a completely randomized block design.Five residue returning treatments were conducted in this experiment:no straw returning（NS）,oilseed rape straw covered on the plot soil（SC）,oilseed rape straw buried into the soil（SB）,oilseed rape straw with straw-decomposing microbial inoculants covered on the plot soil（SDC）and oilseed rape straw with straw-decomposing microbial inoculants buried into the soil（SDB）.The chemical properties of soil,the abundance of soil microbial functional genes related with greenhouse gas emissions（GHGs）,crop yields and GHGs under different residue returning treatments were studied.The primary experimental results were as following:（1）Straw returning methods significantly influenced the soil dissolved organic carbon（DOC）contents in the tillering stage and maturation stage of rice.In the tillering stage,the DOC contents in rice tillering stage were signightly higher by 17.08%-19.80%,21.25%-26.05%,21.11%-24.68%for SB,SDC and SDB respectively compared with the NS treatment.In the maturation stage,the soil DOC contents were signightly higher than NS by 23.45%-24.93%,54.48%-60.96%,25.19%-32.33%and 50.60%-58.81%for SC,SB,SDC and SDB treatments,respectively.Besides,the two-year field experiment exhibited that straw returning significantly increased the TOC contents in paddy soil.（2）Straw retention significantly affected the soil NH₄⁺-N contents in rice seedling stage and heading stage.In rice seedling stage,the soil NH₄⁺-N contents in the SDC and SDB treatments were significantly lower than the NS treatment by 13.06%-20.40%and 15.25%-22.84%,respectively.However,the soil NH₄⁺-N contents at rice heading stage in the SC,SDC and SDB treatments were 1.22-1.87 times,1.16-1.67 times and1.26-1.77 times of that observed in the NS treatment.No significant differences were found the soil NO₃^--N contents among the majority rice growing stages.（3）Straw returning significantly influenced the soil microbial biomass carbon（MBC）contents in rice tillering stage,heading stage and maturation stage.To be specific,the soil MBC contents at rice heading stage in the SDC and SDB treatments were significantly higher than the NS treatment by 44.39%-54.10%and 29.05%-66.38%,respectively.Similarly,the increased ratios were 23.84%-40.73%and 33.37%-43.11%at rice maturation stage.Significantly differences in the soil microbial biomass nitrogen（MBN）contents amoung different residue returning methods were found at rice heading stage and maturation stage.Relative to the NS treatment,the soil MBN contents in the SB,SDC and SDB treatments were significantly improved by 38.63%-46.75%,62.44%-63.22%and 55.77%-75.08%respectively at rice heading stage;the soil MBN contents at rice maturation period in the SDC and SDB treatments were strongly increased by 37.54%-70.54%and 46.14%-82.82%,respectively.（4）The abundances of microbial functional genes related with GHGs emissions were significantly influenced by different straw returning practices.Relative to the NS treatment,the abundance of mcr A were significantly increased by 72.60%-91.62%,137.19%-157.84%,99.26%-107.83%,163.61%-166.45%for SC,SB,SDC and SDB treatments.Relative to the NS treatment,the abundance of nir K in SC,SB,SDC and SDB treatments were increased by 91.28%-97.61%,103.09%-116.22%,170.09%-202.27%and 213.81%-221.53%,and the abundance of nos Z in SC,SB,SDC and SDB treatments were increased by 65.32%-84.34%,73.51%-91.21%,100.81%-110.97%and 119.03%-157.21%.The abundance of nos Z in the SDC and SDB treatments were significantly higher than the NS treatment by 138.54%-141.24%and185.62%-289.32%,respectively.（5）Straw retention increased the methane（CH₄）emissions obviously in paddy field.Relative to the NS treatment,the rice seasonal CH₄ emissions were 20.44%-22.63%,34.47%-41.79%,15.99%-26.41%and 37.64%-42.47%higher in the SC,SB,SDC and SDB treatments.No significant differences were showed in rice seasonal nitrous oxide（N₂O）emissions amoung all the treatments.Different straw returning treatments significantly influenced the rice production.Relative to the NS treatment,the rice yieds of the SDC and SDB treatments were 9.42%-11.03%and 10.31%-11.77%significantly higher.The lowest greenhouse gas intensity were noticed in the SDC treatments among the four straw returning treatments.The SDC treatment had the highest net ecosystem economic benefits（NEEB）among all the treatments in the two-year field experiment.In addition,the NEEB in SDC treatment was significantly improved by 11.17%-12.31%compared with the SB treatment.（6）The rice seasonal CH₄ emissions had a very significant correlation with the DOC contents and the abundance of the mcr A gene（P<0.01）.In conclusion,the SDC treatment increased the crop yields,and had the lowest greenhouse gas intensity and highest net ecosystem economic benefits among different straw returning treatments.So the SDC treatment was recommendable.