Effect Of Substitution Of Chemical Fertilizer With Biogas Slurry On Paddy Soil Denitrification And Dissimilatory Nitrate Reduction To Ammonium Characteristics Under The Measures Of Straw Return

With the rapid development of the livestock breeding industry in the coastal area,a large amount of biogas slurry was produced.Biogas slurry is rich in nutrients required by crops such as nitrogen(N),and its farmland consumption played an important role in the country's combination of planting and breeding,and fertilizer substitution and reduction strategies.Straw return is an important farmland management measure in coastal areas.It not only affects the soil carbon and N balance and changes the soil properties,but also has a significant impact on the turnover of soil N.Denitrification(Den)and dissimilatory nitrate(NO₃^--N)reduction to ammonium(DNRA)are important processes of soil NO₃^--N dissimilation and reduction.They are closely related to soil greenhouse gas N₂O formation.In the past,studies on the effects of substitution of chemical fertilizer with biogas slurry and combined use of chemical fertilizers and straws on soil Den had been sufficient,but simultaneous research on the process of soil Den and DNRA,especially related research under the background of straw return has not been reported.In this paper,taking the paddy in the coastal saline soil farming area of Dongtai,Jiangsu Province as an object,with the help of ¹⁵N isotope tracing technology,the following research was carried out:(1)Indoor cultivation experiment with biogas slurry replacing chemical fertilizer:set up four treatments of blank(C0,0 kg N/hm²),low(C1,75 kg N/hm²),medium(C2,150 kg N/hm²)and high(C3,225 kg N/hm²)fertilizer treatments,and the same amount N of biogas slurry replaced chemical fertilizer(B0,0 kg N/hm²;B1,75 kg N/hm²;B2,150 kg N/hm²;B3,225 kg N/hm²)treatments to study the effect of replacing chemical fertilizer with biogas slurry on the rate of soil Den and DNRA.(2)A field experiment with biogas slurry replacing chemical fertilizer and straw return:set up blank(CK,0 kg N/hm²),only adding biogas slurry(B,225 kg N/hm²),adding fertilizer only(C,225 kg N/hm²),adding biogas slurry and straw(BS,225 kg N/hm²,straw return at a rate of 7.5 t/hm²),adding fertilizer and straw(CS,225 kg N/hm²,straw return at a rate of 7.5 t/hm²)treatments to study the change characteristics of soil Den and DNRA rate in different growth periods(tillering,heading,and maturity)of rice.(3)Indoor cultivation experiment with biochar addition:set up CK,B,C,BS and CS treatments to analyze the effect of biochar(biochar and soil weight ratio:0 and 1%)on soil Den and DNRA rates.Simultaneously analyzed the relationship between soil Den and DNRA rates and soil properties under different treatments.Clarified the impact of substitution of chemical fertilizer with biogas slurry on the characteristics of paddy soil Den and DNRA under the measures of straw return.It was expected to provide a theoretical basis for the evaluation and regulation of soil N₂O emissions under the measures of replacing chemical fertilizers with biogas slurry in coastal saline soil farming areas.The main research results and conclusions of this paper are as follows:(1)Substituting biogas slurry for chemical fertilizer treatments(B1,B2 and B3)for 1 h,the soil Den rate was 4.6-7.3?g N₂O-¹⁵N/kg/h,which was of 1.9-2.9?g N₂O-¹⁵N/kg/h under the chemical fertilizer treatments(C1,C2 and C3);At 24 h,the soil Den rate of replacing chemical fertilizer with biogas slurry and chemical fertilizer treatment had no significant differences(P<0.05).This indicated that substitution of chemical fertilizers with biogas slurry treatments significantly accelerated the soil Den rate in a short period of time(1-24 h).After 72 h,there was no significant difference in the soil Den rate of the chemical fertilizer treatment,but the soil Den rate of the biogas slurry treatment showed B2>B3>B1.This showed that the substitution of biogas slurry for chemical fertilizers reduced the loss of soil N in the form of Den,and contributed to the reduction of soil N₂O release.In addition,soil DNRA rate showed a negative value after adding biogas slurry or chemical fertilizer for 48 h,which indicated that the NH₄⁺-N produced by the DNRA process may be lost through ammonia volatilization or anaerobic ammonia oxidation.(2)At the maturity stage of rice(after rice harvest),the soil Den rate of substitution of biogas slurry for chemical fertilizers treatment(compared between B and C)decreased by 31.8%(P<0.05).In comparison,after straw return,the substitution of biogas slurry for chemical fertilizers(compared between BS and CS)in all observed three growth periods increased the soil Den rate by 16.8%to 53.6%.This showed that under the measures of straw return,the substitution of biogas slurry for chemical fertilizers could increase the soil Den rate.For different growth periods,the highest soil Den rate of B,C,BS and CS treatments all appeared in the mature period.The order of soil N₂O emissions during the entire growth period was:BS(34.6mg/kg)>C(20.9 mg/kg)>CS(20.4 mg/kg)>B(15.3 mg/kg)>CK(2.9 mg/kg),this showed that the substitution of biogas slurry for chemical fertilizers was conducive to the reduction of soil N₂O emissions,but the addition of straw would promote its emissions.In addition,regardless of whether the straw was returned or not,the soil DNRA rate of each treatment in the rice tillering stage was the highest.Further analysis found that regardless of whether the biogas slurry replaces chemical fertilizers after straw return soil Den was positively correlated with pH,and negatively correlated with C/N.After straw was added,the content of NO₃^--N and NH₄⁺-N dominated the change of soil Den rate.(3)After adding biochar,the soil Den rate of treatment B had no significant change,while that of treatment C increased by 26.9%.In comparison,the addition of biochar decreased the soil Den rate treated by BS and CS by 11.8%and 32.3%,and the rate of DNRA by 14.7%and 52.6%,respectively.This showed that under the measures of straw return,the application of biochar could simultaneously reduce the soil Den and DNRA rate.It was worth noting that the soil DNRA rate by BS before adding biochar was 18.1%lower than that of CS,but after adding biochar,BS was 52.0%higher than that of CS.It showed that after straw return,adding biochar to replace chemical fertilizer with biogas slurry was more conducive to the soil DNRA process,which was helpful to the preservation of soil N and reduced the loss of soil N₂O.