Nitrogen Loss By Microbial-driven Processes And Their Environmental Implications In Farmland Soil

Farmland is an important part of agricultural ecosystem,which provides guarantee for human production and life.With the rapid development of China’s economy and agriculture,the amounts of nitrogen（N）fertilizers are applied to farmland soils.However,the excessive use of N fertilizers has not only decreased N-utilization efficiency by crops,but also caused nitrogen overload in soil,leading to pollution in farmland ecosystem.It is necessary to pay attention to the mechanism of nitrogen transformation and cycling in farmland,and to further reveal the mechanism and contribution of microbial-driven nitrogen loss processes for evaluating and predicting the changes of farmland ecosystem services and management decisions.Microbial-driven nitrogen loss processes can play a key role in nitrogen cycle,which can transform bioavailable nitrogen into gaseous nitrogen.Denitrification and anammox are two familiar nitrogen loss processes in farmland ecosystem,and Feammox is a newly discovered nitrogen loss process in recent years.Feammox is driven by microorganism,using Fe（III）as electron acceptor to oxidize ammonium（NH₄⁺）to nitrogen gas（N₂）,nitrite（NO₂^-）and nitrate（NO₃^-）as the end products.The discovery of Feammox has broken the understanding of microbial nitrogen loss pathway,which will also affect the whole nitrogen cycle of farmland ecosystem.Based on this,taking the farmland in Wanshandang area of Wuxi City as the research object,¹⁵N isotope tracing and molecular techniques were used to investigate the seasonal and vertical distributions of nitrogen loss processes,the effects of agricultural activities（fertilization and rice planting）on nitrogen loss processes,and the ecological and environmental implications of nitrogen loss processes in farmland ecosystem.This study clarified the status and contribution of denitrification,anammox and Feammox in farmland ecosystem,hoping to provide scientific basis for farmland salinization control and agricultural environmental management.The main results are as follows:（1）Based on the study of farmland soil,the seasonal and vertical distribution characteristics of denitrification,anammox and Feammox processes were explored,the contributions of denitrification,anammox and Feammox to N loss were compared,and the key environmental driving factors were analyzed.The results showed that the rates of denitrification,anammox and Feammox had significantly seasonal and vertical distribution differences in farmland ecosystem.In the seasonal distribution,the total N loss rates showed the highest in summer,followed by spring and fall,and the lowest in winter.In the vertical distribution,denitrification rate was the highest at 0-10 cm and gradually decreased with the increase of soil depth;anammox rate was the highest at10-20 cm and basically exhibited the surface soil layer higher than the deep soil layer;and Feammox rate was the highest at 20-30 cm and basically exhibited the deep soil layer higher than the surface soil layer.In terms of the contribution to N loss,denitrification was the main contributor,especially in the topsoil,which could contribute more than 70.0%of N₂ production;anammox was also an important contributor,which could contribute more than 50%of N₂ production with the increase of soil depth;Feammox had a small contribution,but the biggest contribution of Feammox to nitrogen loss was 25.5%in the 30-40 cm soil layer.The statistical analysis indicated that temperature,soil total organic carbon（TOC）,NO₃^--N,nir K and nir S genes were the key factors affecting the potential denitrification rate;temperature,soil NO₃^--N,NO₂^--N,and 16S r RNA gene of anammox bacteria were the key factors affecting the potential anammox rate;and temperature and soil Fe（III）reduction rate was the key factor affecting the potential Feammox rate.（2）The effects of agricultural activities（fertilization and rice planting）on denitrification,anammox and Feammox rates were investigated.The results demonstrated that fertilization significantly increased soil nutrient content,decreased soil p H,stimulated the growth of related microorganisms,and then promoted the activities of denitrification,anammox and Feammox.Compared with no fertilization,the rates of denitrification,anammox and Feammox increased by 0.3-4.2,0.3-1.4 and0.1-0.4 times respectively,indicating that a high potential N loss rates may existe in fertilized soil.Rice planting affected the physical and chemical properties in the root zone soil,and stimulated the growth of denitrifying bacteria and iron reducing bacteria in the root zone,which were the main reason for the difference of denitrification,anammox and Feammox activities.In the root zone soil,the rates of denitrification,anammox and Feammox were 35.8%,51.9%and 65.5%higher than those in the non-root zone soil respectively,indicating that a high potential N loss rates may existe in the rice root zone.（3）A simple model of N transformation and N balance was constructed to clarify the role of soil microbial-driven nitrogen loss processes in farmland nitrogen cycle,investigate the removal path of NH₄⁺-N and NO₂^--N in farmland soil,and explore the ecological and environmental implications of farmland N loss process.The results indicated that the N loss fluxes of denitrification,anammox and Feammox were 414.3kg N ha^-1 yr^-1,16.3 kg N ha^-1 yr^-1 and 13.9 kg N ha^-1 yr^-1 in farmland surface soil,respectively.In addition,through analysis,it can be seen that NO₂^--N removal was mainly in 0-10 cm soil layer,and NH₄⁺-N removal was mainly below 10 cm soil layer.This may indicate that different measures can be adopted in the soil layer polluted by different forms of inorganic N,which is of great significance for maintaining the environmental health of farmland ecosystem.（4）Based on the measured data of this study,the measures were put forward,which can provide basic theoretical data for ensuring the use of N by crops and alleviating the N overload in farmland soil,and ultimately serve to maintain the ecological and environmental health in farmland ecosytem.For example,reasonable fertilization（fertilization in 10-30 cm soil layer,long-term cultivation in farmland,appropriate increase of fertilization in summer）,reducing N loss（reducing denitrification activity,reducing ammonia volatilization,avoiding excessive leaching loss）.