Comprehensive Effects Of Straw Retention In Rice-Wheat Double Cropping System In Taihu Lake Region

Rice-wheat cropping system is one of the main planting systems in Taihu Lake region.Straw retention has become an important agricultural measure for sustainable high yield of local paddy field.However,straw retention also has different effects on farmland ecological environment.Therefore,the identification of the comprehensive effects?economic and ecological benefits?of straw retention is conducive to the selection of suitable straw retention mode for regional farmland ecosystems.This research took the monolith lysimeter experiment started in June 2009 and the long-term fixed position field test started in June 2012 as the research objects,and studied the economic benefits and ecological environmental effects,such as soil physical and chemical property,nutrient runoff loss and greenhouse gas emissions in rice-wheat cropping paddy field under different straw retention modes and nitrogen?N?fertilizer rates.Besides,the effects of straw retention on the bioavailability of heavy metals in soil were studied by pot experiment.The comprehensive effect of straw retention modes was assessed by the comprehensive index method.In this research,W,R,WR represented the mode of wheat straw retention,rice straw retention,and both wheat and rice straw retention,respectively;N0,Nr,N1,and N2 represented no N fertilizer,reduced N fertilizer,recommended N fertilizer,and conventional N fertilizer,respectively.The following are the main results:?1?By the monolith lysimeter located experiment,the amount of ammonia volatilization,total N contents in leachate and soil,the N contents in straw in different growth periods and grains,and the rice yields were investigated.The results showed that wheat straw retention with optimum N rate increased the rice yield by about 2.48%?12.8%compared with the fertilizer treatment,and the highest yield was obtained in WN1 treatment.N use efficiencies were declined with the increasing of N rates in rice reason,which could be enhanced by straw retention.Agronomic N efficiency and N apparent recovery under straw retention with N application treatment increased by 1.4?3.4 kg grain/kg N and 1.8%?4.0%,respectively,relative to the only fertilizer treatment.The amount of N residual,ammonia volatilization and N leaching in paddy field increased with the increasing of N application rate.Compared with no straw retention,the amount of ammonia volatilization and N residual in paddy field with straw incorporation was increased by 18.2%and10.1 kg/hm² respectively at a rate of 240 kg N/hm²,and the N leaching and total N loss were reduced by 30.9%and 6.0%respectively.Thus,wheat straw retention with recommended N application rate?WN1?could increase the N efficiency and rice yield,reduce the total N loss in the rice–wheat cropping system.?2?The results of yield and economic benefit analysis of the straw retention located experiment showed that Straw retention increased rice yield,and rice yield was higher in RNI and WRN1 modes than that in WN1 mode,while,the wheat yield showed a trend of"reduced-stable-increased"with the years of straw returning to the field.Under double-season's straw retentions,the annual yield of rice and wheat treated with recommended N application increased by 2.71%compared with conventional N application.Straw retention increased N use efficiency in rice season,but decreased in wheat season,and annual N use efficiency of wheat and rice showed an increasing trend.Compared with WN1 mode,RN1 and WRN1 mode significantly increased the annual N use efficiency.The annual agronomy efficiency,yield increase rate and marginal yield of straw returned were highest in RN1,WRN1 and RN1 modes,respectively,so RN1 model had a higher effect on annual crop yield than WRN1 model.On the whole,recommend N application and straw retention increased annual net benefit of paddy field,and the rice net benefit accounted for 68.9%?79.4%of the annual net benefit.Compared with N1 mode,the annual net benefit of RN1,WRN1 and WN1 increased by 4825,4149,and 1676 yuan/hm²,respectively.The annual ratios of net benefit of WRN1 and RN1 modes significantly increased by 13.3%and 16.9%compared to WN1 mode,respectively.Therefore,from the perspective of farmers'economic benefits,RN1 or WRN1 mode should be recommended in the rice-wheat cropping farmland ecosystem.?3?After 3 rice-wheat continuous cropping cycles,the physicochemical properties of the topsoil were determined.The results showed that the soil fertility of recommended N treatment was better than that of conventional N treatment.Under the recommended N application,the soil fertility of the topsoil in the paddy field were improved by straw retention.Compared with no straw retention mode,straw retention mode reduced the bulk density and pH of topsoil by 0.09 g/cm³and 0.26 pH units,respectively,and increased the contents of organic matter,total N,alkali-hydrolyzed N,available P,available K and C/N ratio in topsoil by 9.62%,3.19%,13.0%,33.9%,17.8%,and 6.49%on average,respectively.In this experiment,WRN1 and RN1 modes had better effects on improving soil fertility than WN1 mode.?4?By the straw retention located experiment,concentrations of N&P in surface water and runoff water were monitored under different treatments during a continuous cropping cycle.The results showed that straw retention significantly reduced the runoff risk of N in rice and wheat seasons relative to no straw retention,and the concentration of NH₄⁺-N and NO₃^--N in surface water in rice season decreased by 31.5%and 47.1%respectively,and that in wheat season decreased by 19.6%and 11.7%respectively.However,straw retention increased the runoff risk of P in paddy field,increasing by 17.8%in rice season and 30.3%in wheat season respectively.Among the three straw retention modes investigated herein,the total dissolved N?DTN?and total soluble P?DTP?concentration under RN1 mode were lower than those under WRN1 and WN1modes,and the DTN concentration was significantly higher than that of N1 treatment,while there was no difference in DTP concentration between RN1 and N1 treatment.Compared with WRN2treatment,crop yield and DTP concentration in surface water in WRN1 treatment did not change significantly,but DTN concentration decreased by 12.4%.Therefore,on the premise of guaranteeing the annual yield of wheat and rice,RN1 mode could effectively prevent and control the annual N&P runoff loss of paddy field in rice-wheat cropping area.?5?By the straw retention located experiment,the greenhouse gases emission of paddy field in rice season under different treatments.The results showed that CH₄ emissions were mainly concentrated in the tillering fertilizer stage and accounted for 54.2%?87.5%of the total emissions during the rice season;N₂O emissions were primarily concentrated in the panicle fertilizer stage and accounted for 46.7%?51.4%total emissions during the same period.The warming potential of CH₄ alone was responsible for 87.5%?98.5%of the total CH₄ and N₂O global warming potential?GWP?during the rice season.Thus,CH₄ was the main greenhouse gas contributor and reduction target in the paddy field.The N₂O emissions in the paddy field decreased with straw retention,but CH₄ emissions significantly increased,which resulted in a significant increase in the total GWP.The total GWP of WN1,WRN1,and RN1 was 3.45,3.73,and 1.62 times higher than that of the N1 treatment,respectively;and the greenhouse gas intensity?GHGI?was 3.00,2.96,and 1.52 times that of the N1 treatment,respectively.Thus,the rice straw retention mode had the smallest GWP and GHGI and thereby made the smallest contribution to the greenhouse effect among the three straw retention modes.Under straw retention,the recommended N fertilization rate not only maintained rice yield but also significantly reduced the GWP and GHGI by 16.5%and 30.1%,respectively,relative to the conventional N fertilizer treatment.Results from this study suggest that adopting RN1 mode in the rice–wheat cropping system prevalent in Taihu Lake region will aid in mitigating the contribution of straw retention to the greenhouse effect.?6?A rice pot experiment was carried out to investigate the effects of straw retention on the bioavailability of heavy metal in soil and the heavy metal concentrations in rice grain.The results showed that straw retention increased the rice yield in the heavy metal contaminated soil.Compared with CK treatment with no straw retention,the yield of S1 treatment with half of harvested straw retention significantly increased by 21.0%,which was the highest in different treatments.Soil organic matter in the topsoil under straw retention increased by 10.0%,pH slightly decreased,and Eh significantly decreased,relative to CK treatment.The ability of contaminated soil to fix the heavy metals was strengthened by straw retention,so that the concentrations of Cr,Cu,Ni,and Pb in soil solution were decreased,which bioavailability was reduced.Compared with CK treatment,the concentrations of Cr,Cu,Ni,and Pb in rice grains in the straw retention treatments reduced by 7.14%,9.77%,30.1%,and 8.65%,respectively.On the whole,straw retention increased the rice yield,reduced the bioavailability of heavy metals in the soil,and reduced the risk of heavy metal pollution in rice grains.In this experiment,the remediation effects of half of harvested straw retention?S1?on heavy metal contaminated soil was better than that of all of harvested straw retention.?7?Based on the monitoring data of the straw retention located experiment,comprehensive index method was used to assess the comprehensive effect of straw retention modes by taking yield benefit of rice and wheat,soil nutrients,DTN and DTP concentrations of surface water in rice season,and emissions of CH₄ and N₂O in rice season as evaluation indexes.The results showed that the weight of the ecological benefit index in the evaluation index system was 0.741,which was higher than the weight of the yield benefit index.Under the recommended N application rate,all the straw retention modes increased the paddy field's comprehensive benefits,and the comprehensive index value of rice straw retention mode was highest?0.808?,followed by the double season's straw retentions mode?0.716?,and the wheat straw retention mode was lowest?0.511?.Under double season's straw retentions,excessive N application?N2?reduced the paddy field's comprehensive benefit,and the comprehensive index value of WRN2 treatment decreased by 0.195 compared with the WRN1 treatment.Therefore,in the rice-wheat cropping farmland ecosystem in the Taihu Lake region,it is suggested that the local government promote the“rice straw retention+recommended N fertilizer”mode?RN1?,which has the highest comprehensive benefits and can consider both crop yield benefits and eco-environmental benefits.