Soil Nitrogen Leaching And Microbial Characteristics Following Biogas Slurry Application To Oilseed Rape

In the anaerobic fermentation of livestock and poultry manure in biogas projects,the by-product biogas slurry is often returned to the field for consumption.The karst mountainous area of southwest China has barren soil and heavy erosion,and the biogas slurry has high NH₄⁺-N content and high fertility,but its nitrification product NO₃^--N is theoretically easy to be leached out.The risk of N leaching from the methane return to the field in this region needs to be assessed.Therefore,this paper investigates the dynamic changes in soil nitrogen availability and leaching over time through rape pot and plot trials,and determines the environmental consumption of methane;explores the response characteristics of microbial processes related to N leaching to methane application,and identifies the microbial mechanisms of N leaching from methane irrigation.The pot experiment was conducted with no fertilizer（CK）,chemical fertilizer(CF,480 kg N·hm^-2),low(R120,120 kg N·hm^-2),medium(R240,240 kg N·hm^-2)and high(R480,480 kg N·hm^-2)application rates of methane,and no crop application of methane(NP480,480 kg N·hm^-2).Six treatments were applied.In the small-scale trial,no fertiliser was applied to oilseed rape（CK）,and four treatments were applied at low(R120,120 kg N·hm^-2),medium(R240,240kg N·hm^-2)and high(R480,480 kg N·hm^-2)rates.The results of the study showed that.The ratio of NO₃^--N to TN leaching amount was 39.2%～60.7%and 29.8%～81.0%in pot and plot experiments respectively;the NO₃^--N leaching amount of pot R480treatment was CK,CF,NP480,R120,R240 treatment 2 times,35%,67%,1.8 times and 1.4 times;the amount of NO₃^--N leaching in the cell R480 treatment was 6.9,3.1,2.1 times that of the CK,R120,R240 treatments,respectively.Therefore,there is a risk of nitrogen leaching in biogas slurry irrigation,which is dominated by NO₃^--N load,and the risk of NO₃^--N leaching increases with the increase of nitrogen application.There is a very significant positive correlation between NO₃^--N leaching amount,and the correlation coefficients r are 0.989（p<0.01）and 0.954（p<0.01）respectively,but the risk is generally lower than that of chemical fertilizers,which can be reduced by nitrogen absorption in rapeseed.The soil NO₃^--N content of pot R480 treatment was 5.8 times,43%,1.2 times,4.5 times,and 3.4 times that of CK,CF,NP480,R120,and R240 treatments,respectively;R120,R240 treatments 2.8 times,3.1 times,1.7 times,biogas slurry irrigation increased soil NO₃^--N content accumulation;while potted and plot soil NO₃^--N content and NO₃^--N leaching amount were extremely significant The correlation coefficients r were 0.958（p<0.01）and 0.808（p<0.01）,respectively,so the accumulation of soil NO₃^--N content may be the driving factor for the risk of NO₃^--N leaching.There was a significant positive correlation between the amount of nitrogen applied in biogas slurry,hydroxylamine dehydrogenation activity and relative abundance of nitrifying bacteria Nitrosospira,indicating that the application of biogas slurry could promote soil nitrification.In this paper,the promoting effect of biogas slurry on nitrification led to the accumulation of soil NO₃^--N,because the soil NO₃^--N content was closely related to the relative abundance of nitrifying bacteria Nitrosospira,Nitrospira,enzyme hydroxylamine dehydrogenase activity,nitrification function gene pmo C-amo C,The relative abundances of pmo A-amo A,pmo B-amo B and hao all had obvious positive correlations.There was a positive correlation between nitrogen application rate and the relative abundances of denitrifying bacteria Acidovorax,Acidovorax,Bradyrhizobiu,Mesorhizobium,functional genes nap A,nap B,nir K,nor B,nar G,and nar H,but negatively correlated with soil nitrate reductase and nitrite reductase activities.Correlation,indicating that biogas slurry generally promotes soil denitrification.This promotion can theoretically reduce soil NO₃^--N content,but the potted soil NO₃^--N content is positively correlated with nitrate reductase activity,denitrification function gene nap B,denitrifying bacteria Acidovorax,Acidovorax relative abundance,it remains to be further clarified whether the nitrification-promoting effect of biogas slurry reduces soil NO₃^--N and affects nitrogen leaching.The above shows that the application of biogas slurry on the one hand promotes the nitrification of rapeseed soil,which leads to the accumulation of soil NO₃^--N,which in turn drives the leaching of nitrogen,and on the other hand,promotes the denitrification of rapeseed soil,but whether this promotion reduces soil NO₃^--N and its effect on leaching require further study.Although there is a risk of nitrogen leaching in the application of biogas slurry,the risk is lower than that of chemical fertilizers with the same amount of nitrogen.Under the premise of controlled application,biogas slurry has the potential to replace nitrogen in chemical fertilizers.In this paper,considering nitrogen leaching,soil fertility,agronomic traits and yield of rapeseed,it is recommended that for one season of rapeseed with yellow soil,the nitrogen application rate of biogas slurry should not exceed 240 kg N·hm^-2,corresponding to the consumption of biogas slurry of 85.7m³·hm^-2.The breeding carrying capacity is 3.2 cows/ha.In addition,biogas slurry increased the content of available phosphorus in the 25～50 cm soil of the plot test by25%～40%,and the content of available potassium by 20%～48%.Phosphorus leaching risk of biogas slurry irrigated rapeseed with yellow soil needs attention.