Characteristics Of Soil Organic Carbon Turnover In Paddy Fields Under Long-Term Fertilization

The carbon(C)cycle in terrestrial ecosystems is an important part of global carbon cycle.As the largest carbon pool,soil is the source or sink of greenhouse gases and dominates the global carbon budget in terrestrial ecosystem.The soil C cycle is affected by both the soil properties and external environment.Fertilization and water conditions are two key factors afecting C cycle in paddy soil.Due to long-term flooding,paddy soils are under anaerobic environment.Thus,the carbon sequestration potential in paddy soil differs from that in upland soils.The effect of fertilization on organic C turnover varies in different soil depths.The C turnover and distribution of exogenous C in soil layers under different-fertilization modes were elucidated by using glucose as a typical representative of labile organic carbon.The results provide basic theory for improving soil fertility and increasing soil C sequestration in paddy fields.Therefore,this study selected soil from Ningxiang long-term experimental station and collected soil samples under four different fertilization:straw organic fertilizer(M),organic-mineral combined application(MNPK),mineral fertilizer(NPK)and non-fertilized soil(Control).Four different depths of soil samples were collected for each treatment sampling to 40 cm at 10 cm isometric spacing.The experiment had two groups:no glucose and with glucose addition.The 13C-labeled glucose was used as labile organic C in paddy soils.This study provides basic theory for understanding organic C mineralization and transformation characteristics under fifferent fertilization in different depth soils,and how soil microoragnisims resonse to C-tunover and priming effect in paddy soils.The main results are as follows:(1)Long-term fertilization improved the release of CO2 and the content of C.N and the enzyme activities of BG,NAG and XYL.Soil CO2 cumulative emissions,MBC,MBN,MBP,SOC content in MNPK were higher than other two fertilized soils.(2)The addition of exogenous C promoted CO2 emission,but the amount of CO2 emission decreased with the increase of soil depth.Exogenous C input improve SOC mineralization for different soil depths.This positive PE lasted longer in 20-40 cm with low MBC contnent than in 20-40 cm with low MBC contnent in each soil than other depths.(3)With the soil depth increase and the MBC content decrease,the exogenous C changes from a slow pool to a fast pool.The soil layer of 0-10 cm with high MBC content is mainly composed of fast carbon pools.In the soil layer of 30?40 cm,the microbial activity is low,which lead to the dominance of slow pool.(4)Exogenous C input directly changed the soil physical and chemical environment and microbial biomass activities.The input exogenous C was partly utilized by microoganisms via releasing CO2 for SOM decomposition resulting in rapid increase of DOC and MBC content and decrease of MBP and NH4+ content.The effect of exogenous C input on each fertilization treatment is:MNPK>M>NPK>Control,and the response of cultivated layer soil to exogenous C is higher than that of plow bottom soil.(5)The activity of extracellular enzymes in soil is changed after exogenous C.input The content of nutrient elements in initial stage of incubation are restricted.Under the regulation of microorganisms,C-acquring enzymes activity for SOM mineralization were inhibited,but N-acquring enzymes activity for SOM mineralization was promoted.The response of 10-20 cm soil layer to exogenous C is the stronger than other soil layers.