HCO₃^- Conversion And Reorientation Of Biogas Slurry To Be Utilized In Paddy Fields In Coastal Agroforestry Area Of Jiangsu Province

Jiangsu coastal agroforestry zone is an important reserve base for national food security and the most important ecologically sensitive zone in coastal zone.Ecological shelterbelt is nested with farmland,and large-scale intensive standard breeding farms are orderly distributed in it.Biogas slurry produced during anaerobic fermentation is the main by-product of aquaculture.Biogas slurry is rich in nitrogen,which makes it favored in field fertilizer replacement research.In the past,important progress had been made in the research on yield increase,disease resistance,soil improvement effect and environmental quality assessment of biogas slurry returning to farmland,which has laid a solid foundation for the rapid development of planting and breeding industry.However,in addition to nutrients,there is a large amount of HCO₃^-in biogas slurry.The study showed that there would be a short-term peak of CO₂ emission after the biogas slurry was returned to the field.It was previously thought that biogas slurry accelerated the mineralization of soil organic carbon.However,if the presence of HCO₃^-is fully considered,previous studies may"overestimate"the mineralization of soil organic carbon by biogas slurry.In addition,the transformation of HCO₃^-in soil and the CO₂ may be reused.However,the effects of HCO₃^-conversion of biogas slurry on soil organic and inorganic carbon pools,CO₂ emissions and rice growth are still insufficient.In this paper,taking the rice field of Jiangsu Dongtai Binhai Agroforestry Area as the model area,it was designed to apply biogas slurry（BS,biogas slurry contains N,P,K,HCO₃^-and other components）,apply fertilizer with the same total amount of biogas slurry（CF,N,P,K and the same amount of biogas slurry）,apply fertilizer with the same total amount of biogas slurry NPK and HCO₃^-（CF-H,N,P,K,and HCO₃^-the same amount as in biogas slurry）.For each treatment,the equivalent N amount was 450 kg·ha^-1,P amount was 135 kg·ha^-1（calculated as P₂O₅）,and K amount was 330 kg·ha^-1（calculated as K₂O）（with reference to local field application）.Through rice pot experiment（whole growth period,150 days）,¹³C stable isotope labeling method was used to study:（1）the effect of HCO₃^-in biogas slurry on the release rate and amount of CO₂ in paddy soil,and the changes of soil p H,EC,HCO₃^-,TN,NH₄⁺-N and NO₃^--N contents were measured synchronously.（2）The relationship between HCO₃^-in biogas slurry and the contents of soil dissolved organic carbon（DOC）,total organic carbon（SOC）and inorganic carbon（SIC）;（3）The effects of HCO₃^-in biogas slurry on photosynthesis and biomass and yield in different tissues（roots,stems,leaves and grains）of rice,and the distribution characteristics of HCO₃^-source C in paddy field CO₂ emissions,soil SOC and SIC pools,rice tissues and grains,in order to provide theoretical and practical basis for the efficient,safe and sustainable utilization of biogas slurry and its effect evaluation.The main research results and conclusions of this paper are as follows:（1）During the entire growth period of rice,the application of biogas slurry（instead of chemical N fertilizer）did not significantly increased the rate of CO₂ release and total emissions from the paddy soil.The presence of HCO₃^-significantly reduced soil CO₂ emissions,and other components（such as small molecule organic matter）in the biogas slurry may increase CO₂emissions except for N,P,K,and HCO₃^-.HCO₃^-and other components contributed to the release of CO₂ in a"trade-off"manner.During the seedling,tillering,and maturity stages of rice,88.54%,91.03%,and 93.11%of C in HCO₃^-treated with BS were converted into CO₂,respectively.However,the cumulative release of this CO₂ only accounted for 0.25%of the total soil release.This indicated that the CO₂ directly converted from HCO₃^-in the biogas slurry directly could be negligible for the CO₂ released from the soil.（2）HCO₃^-contributed to an increase in soil DOC content,with little contribution to SOC and SIC,but other components in biogas slurry might significantly contributed to the increase in SOC.During the seedling,tillering,and mature stages of rice,0.59%,0.69%,and 0.55%of HCO₃^-were converted into SOC under BS treatment,accounting for 0.02‰,0.02‰,and 0.03‰of soil SOC content,respectively;Meanwhile,10.87%,8.28%,and 6.34%of HCO₃^-were converted into SIC,accounting for 0.05‰,0.05‰,and 0.07‰of soil SIC content,respectively.Further analysis indicated that soil p H and NH₄⁺-N content are key factors affecting the conversion of HCO₃^-into SOC and SIC in biogas slurry.（3）Compared with the application of chemical fertilizers,the application of biogas slurry could significantly improve the net photosynthetic rate,intercellular CO₂ concentration,and leaf transpiration rate of rice seedlings.Under BS treatment,the accumulation of ¹³C in the roots,stems,leaves,and grains of rice during maturity was 176.32,respectivelyμg.555.27μg and159.92μg.The utilization rates of HCO₃^-were 1.56‰,4.91‰,and 1.41‰,respectively,with the utilization rate of the aboveground part being significantly higher than that of the underground part.（4）After the application of biogas slurry in rice fields,88.54%～93.11%of C in HCO₃^-was released in the form of CO₂,with SIC accounting for 6.34%～10.87%and SOC accounting for0.55%～0.69%.The proportion of aboveground and underground tissues in rice varied between2.93‰～6.32‰and 0.34‰～1.56‰.