Effects Of Different Straw Return Years On Greenhouse Gas Emissions In Paddy Fields And Rice Yield

The Northeast rice region is an important rice producing area in China and produces a large amount of crop straw every year.The unreasonable use of straw（e.g.straw burning,etc.）can have many negative effects on the agroecosystem,such as the loss of soil nutrients from farmland and air pollution.Therefore,the rational use of rice straw is of great importance to safeguard agricultural production.Straw return is an important conservation tillage measure.Years of straw return has an important effect on soil nutrients,crop growth and development,and yield formation.It is of great theoretical and practical significance to explore the effects of straw return years on greenhouse gas emissions and rice yield in paddy fields to promote sustainable agricultural development in the Northeast rice region.For this reason,this trial was conducted in 2021 and 2022 at the National Crop Variety Validation Characterization Station of Jilin Agricultural University in Changchun,Jilin Province.The local main rice variety Jinongda667 was used as the test variety.The soil chemical properties of rice fields,photosynthetic characteristics of rice sword leaves,rice population quality,greenhouse gas emission characteristics of rice fields and rice yield performance under no straw return（S0）,one year of straw return（S1）,two consecutive years of straw return（S2）,three consecutive years of straw return（S3）and four consecutive years of straw return（S4）were investigated by means of field location experiments,with a view to providing a theoretical basis for the construction of a reasonable straw return system in the Northeast China rice region.The main findings of the experiment are as follows:1.Straw return increased soil NH₄⁺-N content,NO₃^--N content,available phosphorus content and available phosphorus content in rice fields from jointing stage（JS）to maturity stage（MS）.Straw return increased soil organic carbon content and total nitrogen content in MS rice fields.Soil NH₄⁺-N content,NO₃^--N content,available phosphorus content,available phosphorus content,organic carbon content and total nitrogen content gradually increased with the increase of return years.It indicates that continuous straw return is beneficial to improve soil fertility level.2.Straw return increased net photosynthetic rate（Pn）,stomatal conductance（Gs）,apparent leaf mesophyll conductance（AMC）,water use efficiency（WUE）,and SPAD of the rapier leaves from JS to MS in rice,with an increasing trend with the increase in the number of years of return to the field.This is due to the fact that the nitrogen released from straw decomposition provided more sufficient nitrogen for the growth of rice from jointing to maturity as the number of years of field return increased,which delayed the senescence of the plant and thus improved the photosynthetic production capacity of the rapier leaves and seed filling capacity in the middle and late stages of rice growth.3.Straw return inhibited the occurrence of tillering and dry matter accumulation in rice,which improved the rate of rice spike production.The maximum number of tillers,dry matter accumulation and spike rate of rice increased with the increase of return years.Compared with S0,S1 and S2 treatments reduced the leaf area index of rice,and S3 and S4 treatments increased the leaf area index of rice at the heading stage（HS）and filling stage（FS）.This indicates that continuous straw return facilitates the accumulation of rice biomass.4.Straw return increased CO₂and CH₄emissions from rice fields,with no significant effect on N₂O emissions.Compared with S0,the global warming potential increased by 22.63%,30.94%,26.97%and 21.34%under S1,S2,S3 and S4 treatments,respectively,and the greenhouse gas intensity increased by 43.57%,38.24%,27.88%and 16.04%,respectively.It shows that the greenhouse gas intensity tends to decrease with the increase of straw return years under the straw return conditions.There was a highly significant negative correlation between CO₂and CH₄emissions and maximum tiller number and effective spike number,indicating that increasing maximum tiller number and effective panicle number could help reduce greenhouse gas emissions.5.The rice yield showed S4>S0>S3>S2>S1.S2 and S3 treatments had no significant effect on yield,indicating that the positive effect of increased panicle length,panicle weight and spikelets per panicle offset the negative effect of lower effective spike number.And the yield of S4 increased by 4.64%compared to S0 rice,and the increase was mainly due to the increase in panicle length,panicle weight and spikelets per panicle.And straw return increased the harvest index of rice,which increased with the number of years of return.Yield was significantly and positively correlated with total nitrogen,ammonium nitrogen,nitrate nitrogen,maximum tiller number,spike rate,spike length and effective spike number,and was highly significantly and positively correlated with net photosynthetic rate,leaf area index,dry matter accumulation,spike weight and grain number per spike,and was highly and negatively correlated with greenhouse gas emission intensity.In summary,continuous straw return helps improve soil nutrients in paddy fields,enhance photosynthetic characteristics of rice from nodulation to maturity,increase spike rate,spike length and spike weight,increase the number of grains per spike,and promote yield formation.Although straw return treatment increases CO₂and CH₄emissions from rice fields,the greenhouse gas intensity tends to decrease as the number of years of straw return increases.The results of the study can provide data basis and theoretical support for the rational use of straw,protection of black land,improvement of crop yield and sustainable agricultural development in northeast China.