Effects Of Long-term Carbonization Straw Return On Soil Denitrification In Paddy Fields

Paddy field is an important part of the Chinese agricultural ecosystem,which guarantees the food security production of China.Chemical fertilizer application has been substantially expanded in order to sustain the high production of paddy fields.However,nitrogen utilization efficiency is low in China,which leads to a large amount of nitrogen loss.The long-term flooding situation of rice cultivation creates optimal circumstances for anaerobic nitrogen conversion.The anaerobic nitrogen conversion process includes denitrification process and anaerobic ammonia oxidation process,both of which transform nitrate into N₂via microbial action in anaerobic environment and are collectively referred to as the net loss of nitrogen.Microbial-mediated denitrification process is the main cause of nitrogen loss in paddy fields.Because of its soil improvement,carbon sequestration,and emission reduction properties,straw biochar is frequently employed as a soil improver.Biochar application will change the living environment of microorganisms involved in nitrogen transformation and their activities,thus affecting the soil nitrogen removal process in paddy fields.So based on the long-term returning straw carbonized rice fields of taihu lake watershed as the research object,through the indoor helium gas airtight incubation test and in-situ monitoring field,using the 15 n isotope MIMS quantitative markers and molecular biology technology to study the long term returning straw carbonization(0BC:0 t hm^-2per season;1BC:2.25 t hm^-2per quarter;5BC:11.3 t hm^-2per quarter;10BC:22.5t hm^-2per season)and straw returned(RST:2.25 t hm^-2per season)different depth（0～20,20～40,40～60,60～80 and 80-100 cm）denitrification intensity and main influence factors.Meanwhile,the occurrence regularity and influencing factors of net nitrogen removal rate of soil-water system in situ surface layer（0～10cm）were monitored.The main conclusions are as follows:1.The denitrification rate in different soil layers under different treatments is between（0.150±0.049）～（4.424±0.373）nmol N g^-1h^-1,and the anammox potential range is 0～（1.255±0.101）nmol N g^-1h^-1.The contribution rate of nitrogen removal in total denitrification process is 39.65-100%.At the spatial scale,the denitrification potential decreased with the increase of soil depth,while the anammox showed vertical differentiation.The loss of nitrogen removal in paddy field is mainly caused by surface denitrification.The denitrification and anammox rates were decreased by the application of high biochar,but the differences are not significant in the60～100cm deep soil.2.The application of biochar changed the physical and chemical properties of soil profile and the abundance of microbial functional genes.The copy number of hzs B was much lower than that of nir K and nir S,and the copys of nir K and nir S decreased gradually with the increase of soil depth.Decreased the abundance of hzs B functional genes.The nitrogen removal process was significantly correlated with soil environmental factors and the abundance of functional genes nir S and hzs B,but the abundance of functional genes was not the key limiting factor affecting the nitrogen removal process.3.There is no accumulation of soluble N₂in surface water,and the peak of soluble N₂O concentration is relate to field fertilization.The application of biochar and straw increased the concentration of dissolved N₂O in surface water,the more biochar,the higher the concentration of soluble N₂O.DOC content is the key factor affecting soluble N₂O content.High biochar content reduced the net nitrogen removal rate of surface soil-water system through soil and water NO₃^--N content.4.In the soil-water system of paddy field,the net nitrogen removal rate decreased first and then increased on time scale.The net nitrogen removal rate was affected by both surface water and soil substrate concentration.The accumulated nitrogen removal loss during paddy field flooding ranged from31.2 to 44.6 kg N ha^-1,accounting for 13.03 to 18.56%of the nitrogen application level in paddy field.The1BC,5BC,10BC and RST treatments reduced nitrogen loss by 9.62%,12.22%,14.75%and 29.8%on average.The results showed that the denitrification rate in paddy soil profile was higher than that of anammox,and the denitrification rate in surface soil was higher than that in deep soil.Therefore,surface soil denitrification was still the main process in the denitrification process.The N₂O produced by denitrification will accumulate in field water as dissolved gaseous nitrogen.The application of high biochar reduced the nitrogen loss in rice field.