| "The application of biogas slurry in field" has attracted more and more attention in agriculture because of its advantages of low cost,simple technology and resource utilization of waste.However,the high nitrogen contents in biogas slurry（BS）often form nitrogen accumulation in soil and lead to nitrogen leaching driven by rainfalls,resulting in the hidden danger of nitrogen pollution in groundwater and surrounding surface water.This has become the bottleneck that limits the popularization of the application of BS.This research explored the response of paddy soil to the application of BS during the whole growth cycle of rice.And the laboratory simulation system was constructed to explore the effect of rainfall on nitrogen leaching of soil that applied BS before.Subsequently,researches on the impacts of different initial rainfall intervals as well as the control effects of straw mixing with different materials on the nitrogen leaching after the application of BS in soil were performed,in order to provide technical support for alleviating the risk of nitrogen leaching after the application of BS in field.The main results of this research are as follows:（1）Through the research on the response of the paddy soil to the application of BS during rice growth,it was found that the application of BS was generally helpful to improve the nutrient levels in the paddy.Compared with CF（Chemical Fertilizer Treatment）,the concentrations of total nitrogen,total phosphorus and organic matter in paddy water of BCF（Biogas Slurry plus Partial Chemical Fertilizer Treatment）averagely increased by 48.02%,118.75% and 55.23%,and they all showed an trend of rising and then falling during rice growth.The total phosphorus and soil organic carbon contents in the topsoil of BCF increased by 1.92%-13.54% and 4.70%-21.31%during different growth stages of rice than those in CF,respectively.Conversely,the total nitrogen contents in the topsoil of BCF were generally lower that those in CF,especially during the grain filling stage（21.17% lower）.Additionally,it was found that BS stimulated the increase of the relative abundances of rhizospheric bacteria and fungi related to N-,P-and C-cycling,promoting nutrient cycles in soil.Phosphate-solubiliziing-and cellulolytic-related bacteria（eg,Bacillus）and fungi（eg,Mortierella）as well as nitrogen cycle-related bacteria（eg,Nitrosospira）were more abundant in rice rhizosphere of BCF than those in CF,especially during the reproductive period of rice（booting-,filling-stage）.In addition,the relative abundance of DNRA-（nitrate dissimilatory reduction to ammonium）related bacteria gradually increased while the denitrifying-related bacteria decreased during rice growth in BCF.Furthermore,correlation network showed that nutrient-cycles-related microbes were more closely interconnected in BCF than in CF,which played an important role in nutrient cycling of the soil.（2）The research on the effects of different rainfall intensities on nitrogen leaching of soil applied BS showed that the inorganic nitrogen in the soil leachate was mainly in the form of nitrate in the beginning and was mainly in the form of ammonium in the later period of the simulated rainfall.The total leaching amount of nitrate was obviously positively correlated with the background accumulation in the original soil,while the leaching rate had no obvious correlation with the rainfall intensity.Differently,the accumulative leaching amount of ammonium showed a good correlation with the leaching time and rainfall intensity.This difference was mainly due to the adsorption of ammonium generated by the cation exchange capacity of soil.Additionally,the changes of ammonium contents in soil under heavy rainfall（3.04-14.74 mg/kg）were significantly higher than those under weak rainfall（0.27-4.80 mg/kg）,which further showed that rainfall intensity significantly affected the leaching level of soil ammonium.Therefore,it was suggested that BS could be applied in field in the season with low rainfall intensity to reduce the risk of soil nitrogen leaching.（3）Research on the effects of initial rainfall at different time intervals（1 d,5 d）on soil nitrogen leaching after the application of BS showed that when the initial rainfall interval was 5 d,the accumulative amount of total nitrogen（TN）in soil leachate under the BS application intensities of BS1（63 kg N/ha）,BS2（130 kg N/ha）and BS3（270 kg N/ha）was 14.81%,20.47% and 21.37%lower than that in the interval was 1 d,respectively.This was mainly because the extension of the initial rainfall interval made the topsoil stay aerobic for a longer time,promoting the nitrification conversion of ammonium.And partial nitrogen was removed in the aerobic-anoxic-anaerobic environment formed in the soil column when the initial rainfall interval was 5 d,resulting the reduction of the leaching amount of TN.The results showed that the extension of the initial rainfall interval was helpful to reduce the risk of nitrogen leaching of soil applied BS.（4）The research on the control effects of straw mixing with different materials on nitrogen leaching of BS applied soil found that the accumulative amount of TN in the soil leachate of the S（straw）and the S+I（straw mixed with iron base）addition group was 23.23% and 36.32% lower than that in the blank group,respectively,while the accumulative amount of TN in the soil leachate of B（biochar）and S+B（straw combined with biochar）addition group was 21.39% and8.59% higher than that in the blank group,respectively.The results of soil nitrogen contents analysis,high-throughput sequencing and PICRUSt function prediction showed that the contents of ammonium and nitrate and the relative abundances of genus Anaeromyxobacter and Nrf AH genes related to the DNRA process in the topsoil of the S group were relatively higher while the S+I group had the lowest inorganic nitrogen content and higher relative abundances of denitrification-related microbes such as Bacteroidota,norank＿f＿Anaerolineaceae and functional genes Nir K,Nor BC and Nos Z in the topsoil.These indicated that the S group promoted the microbial activity related to the DNRA process and improved the nitrogen retention capacity in the topsoil,while the S+I might strengthen the denitrification process of the topsoil to reduce the migration of nitrogen to deep soil.Due to the relatively oxygen-rich conditions caused by the pore structure of biochar,the nitrification process might be stronger in B group.In the S+B,this process was weakened thereby nitrogen leaching was reduced.And compared with B,the relative abundances of microbes and functional genes related to the nitrification process in S+B group also decreased.Therefore,this research indicated using straw and straw compound iron-based materials were both useful to control the nitrogen leaching in the soil applied BS. |
