Effects Of Biogas Slurry Application On Soil Organic Nitrogen Mineralization In Coastal Wheat-rice Double Cropping System

Biogas slurry is a by-product of anaerobic fermentation of livestock and poultry manure,which contains a lot of N,amino acid,humic acid,active enzyme and other biological active substances.Biogas slurry is a high efficient substitute of chemical N fertilizer,which is of great value in the development strategy of high quality agriculture.A large amount of N from biogas slurry applied to farmland is bound to have significant impact on the turnover and transformation of soil N.However,previous studies on the application of biogas slurry in farmland mainly focused on the amount of chemical N fertilizer replaced by biogas slurry,the way of replacement,environmental effects,etc.,the effects of biogas slurry application on mineralization and fractions of soil organic nitrogen（SON）has not been reported.This paper takes the wheat-rice double cropping system in Dongtai,Jiangsu Province as the object,and collects pig biogas slurry（CSTR process）from the biogas station of COFCO Meat（Jiangsu）Co.,Ltd.,through the field plot experiment,setting up（1）the experiment of biogas slurry dosage:rice field set up four biogas slurry dosage treatments,respectively is 0 m³/ha（CK-R）,250m³/ha（BS1-R）,500 m³/ha（BS2-R）,750 m³/ha（BS3-R）.Wheat fields were set at 0 m³/ha（CK-W）,166 m³/ha（BS1-W）,332 m³/ha（BS2-W）and 498 m³/ha（BS3-W）.（2）the experiment of replacing chemical fertilizer with biogas slurry:on the premise that the total N input of rice field and wheat field is controlled at 225 kg/ha and 150 kg/ha,respectively,four biogas slurry substitution levels are set for rice field and wheat field,respectively,they were 100%urea（CBS0-R,CBS0-W）,33%biogas slurry+67%urea（CBS1-R,CBS1-W）,66%biogas slurry+34%urea（CBS2-R,CBS2-W）and 100%biogas slurry（CBS3-R,CBS3-W）.A total of 8 treatments were set in the field plot experiment to study the characteristics of soil total N（TN）,ammonium N（AN）,amino acid N（AAN）,amino sugar N（ASN）,unknown N（UN）,non-hydrolyzable N（NHN）and SON mineralization,the changes of soil bacterial community,soil p H,EC,soil organic carbon（SOC）and their relationship with soil nitrogen mineralization potential（N₀）were analyzed in order to provide theoretical basis for the determination of the optimum amount of biogas slurry and the management of soil N in the coastal agricultural area of Jiangsu Province.The main results and conclusions of this paper are as follows:（1）For the rice season soil,compared with no biogas slurry（CK-R）,low（BS1-R）,medium（BS2-R）,and high（BS3-R）biogas slurry dosage significantly increased soil N₀,the increase was between 26.7%and 34.95%（P<0.05）,but there was no significant difference between the three dosages.In comparison,under the same amount of N treatment,the order of soil N₀ in the rice season is:CBS3-R=CBS2-R>CBS1-R>CBS0-R.This indicates that the application of biogas slurry can significantly increase the N supply capacity of rice season soil.When the substitution level of biogas slurry is higher than 66%,the N supply capacity of rice season soil is the strongest.For the wheat season,the application of biogas slurry can significantly increase soil N₀.Compared with CK-W,soil N₀ under BS1-W,BS2-W,and BS3-W treatments increased by 103.2%,72.2%,and 129.4%,respectively,and the soil N₀ under BS3-W treatment was significantly higher than BS2-W,with an increase of 33.2%.On the other hand,under the same amount of N treatment,the order of soil N₀ in wheat season is:CBS3-W=CBS0-W>CBS2-W.This shows that under the amount of 332～498 m³/ha of biogas slurry,the N supply capacity of wheat season soil increases with the increase of biogas slurry amount,and the complete substitution of biogas slurry for urea will not cause a decrease in the N supply capacity of the wheat field soil.（2）Compared with CK-R,the AAN content of the rice season soil was significantly reduced under BS1-R treatment（P<0.05）,with a decrease of 20.7%,and there was no significant change in AN content under BS1-R and BS2-R treatments,but it increased by 14.8%under BS3-R treatment.This indicates that the application of low and medium amount of biogas slurry did not significantly contribute to the increase of AN and AAN content in rice season soil.In comparison,under the same amount of N treatment,compared with 100%application of urea（CBS0-R）,the substitution of biogas slurry for urea significantly increases the content of all SON components,the content of AN,AAN and NHN increased by 15.1%～17.6%,25.6%～39.2%,and 38.9%～73.9%,respectively.This shows that the substitution of biogas slurry for urea can significantly improve the easily mineralized N pool and the hard-to-mineralize N pool in the rice season soil.For wheat season soil,compared with no biogas slurry（CK-W）,NHN content decreased by 57.9%under BS2-W treatment,and increased by 218.3%under BS3-W treatment.Under the same amount of N treatment,CBS1-W and CBS2-W significantly increased the NHN content,compared with CBS0-W,NHN content increased by 47.6%and 35.4%,respectively.This shows that only when the amount of biogas slurry is higher than 332 m³/ha or the level of substituted urea is between 33%and 66%,the hard-to-mineralize N pool of wheat season soil will increase significantly.（3）The second-generation high-throughput sequencing results of 16s rDNA showed that:for rice season soil,based on the relative abundance≥1%,compared with CK-R,the ratios of the Sum of Relative Abundances of Disappeared and newly Emerged genera（SRADE）of BS1-R,BS2-R,BS3-R treatment to the total relative abundance of the main bacterial genera of CK-R treatment was 12.7%,16.7%,and 20.7%,respectively.In comparison,under the same amount of N treatment,compared with CBS0-R,the ratios of SRADE under BS1-R,BS2-R,BS3-R to the total relative abundance of the main bacterial genera under CBS0-R treatment was 11.1%,18.4%,and 23.5%,respectively.This indicates that different biogas slurry dosages and alternative urea levels have similar effects on the composition and structure of the main bacterial genera in rice season soil,with changes ranging from 11.1%to 23.5%.For wheat field soil,the ratios of SRADE under BS1-W,BS2-W,BS3-W to the sum of relative abundance of main bacterial genera under CK-W treatment was 6.3%,22.6%,and 25.3%,respectively.In comparison,under the same amount of N treatment,the ratios of SRADE under CBS1-W,CBS2-W,CBS3-W to the sum of relative abundance of main bacterial genera under CBS0-W treatment was 7.9%,11.1%,and 13.2%,respectively.This indicates that the higher the quantity of biogas slurry or the level of substitution,the greater the influence on the composition and structure of the main bacteria in the soil.On the other hand,BS3-R treatment significantly reduced the Chao1 index,Shannon index and Simpson index of the soil bacterial community in the rice season（P<0.05）.This indicates that too high biogas slurry dosage may reduce the diversity of soil bacterial communities in rice field.（4）For rice season soil,Pedosphaeraceae＿unclassified,KD4-96＿unclassified,and SOC under different biogas slurry dosages had significant effects on soil N₀,and their contribution to N₀ was 34.9%,32.3%,and 19.1%,respectively.This indicates that biological factors are the key factors leading to SON mineralization in the rice season under different biogas slurry dosages.In comparison,under the same amount of N treatment,soil TN,Thermosulfovibrionia＿unclassified,Latescibacteria＿unclassified,Geobacter,and S085＿unclassified had extremely significant effects on soil N₀（P<0.01）,and their contribution to soil N₀ was 56.4%,19.5%,10.0%,8.2%,5.9%,respectively.This shows that when biogas slurry replaces urea,both non-biological factors and biological factors have an important impact on the N₀ of rice season soil.For wheat season soil,Saprospiraceae＿unclassified,soil AAN,Pedosp-haeraceae＿unclassified,and Subgroup＿10 under different biogas slurry dosages had significant effects on soil N₀,and their contribution to soil N₀was 51.6%,39.3%,5.1%,4.0%,respectively.This indicates that both biological and non-biological factors have important effects on the mineralization of SON in wheat season.In comparison,under the same amount of N treatment,only soil AN and NHN had a very significant correlation with soil N₀,with contributions of 51.0%and 36.5%,respectively.This shows that in the mode of biogas slurry instead of urea,non-biological factors are the key for leading to the SON mineralization of wheat season.