Study On The Component Change And Release Characteristics Of Soil Phosphorus Under Rice-crayfish Co-culture Mode

As a new rice farming model in recent years,Rice-crayfish co-culture（RC）model has been rapidly popularized in the middle and lower reaches of the Yangtze River.In this mode of combination of culture and planting,the soil properties will change,which also has both positive and negative effects on the environment.Phosphorus is a necessary nutrient element for crop growth.In this study,taking the southwest of Jianghan Plain as the study area,through the analysis and determination of rice monoculture（RM）and RC model after field sampling,indoor simulation and model prediction,the long-term characteristics of soil phosphorus accumulation and bioavailability in RC model were studied,and the risk of soil phosphorus release in RC model was evaluated.It provides the basis and support for the input and management of soil phosphorus and the environmental risk of phosphorus release,and promotes the green and sustainable development of RC model.The main results are as follows:（1）The accumulation of total phosphorus（TP）and available phosphorus（Olsen-P）in soil under RC model for a long time showed an increasing trend,among which the increasing trend of planting and co-culture years≥11 years was the most significant.With the extension of planting and co-culture years,the content of inorganic phosphorus（Inorg-P）and the proportion of inorganic phosphorus to TP in RC model soil increased significantly,while the content of organic phosphorus（Org-P）had no obvious change trend,and the ratio of organic phosphorus to TP showed a downward trend.Therefore,RC model is beneficial to the increase of Inorg-P content,and Inorg-P plays a major contribution to the increase of TP content.（2）The content and proportion of closed occluded phosphate（O-P）in RM soil and calcium phosphate（Ca-P）in RC model soil were the highest and its proportion to Inorg-P was the highest.With the increase of planting and co-culture years,the content and proportion of aluminium phosphate（Al-P）,dicalcium phosphate（Ca₂-P）and octadecalcium phosphate（Ca₈-P）to Inorg-P increased,among which the co-culture years≥11 years was the most significant.With the increase of co-culture years,the content of ferric phosphate（Fe-P）and O-P did not change significantly,compared with RM,the ratio of Fe-P and O-P to Inorg-P decreased significantly,but there was no significant difference among different co-culture years,while compared with RM,the content of hydroxylapatite(Ca₁₀-P)and its ratio to Inorg-P increased significantly,but there was no significant difference among different years.RC model is not conducive to the accumulation of soil Fe-P and O-P,which tend to transform to Al-P.The increase of Inorg-P content in RC model is due to the increase of Al-P and various forms of Ca-P.（3）Correlation analysis showed that the first source of available phosphorus in RC model soil was Ca₂-P,and the determination coefficient was 0.94,followed by Al-P,Fe-P and Ca₈-P,and the determination coefficients were 0.88,0.66 and 0.43,respectively.Stepwise regression analysis showed that the contribution of Ca₂-P to available phosphorus was the highest in RC model soil,followed by Al-P.（4）The soil phosphorus adsorption isotherm experiment of RC model in 5 sites in this study area showed that soil maximum phosphorus uptake(X_max)and phosphorus sorption index（PSI）were significantly negatively correlated with p H and positively correlated with soil organic matter（SOM）content.The phosphorus adsorption and fixation capacity of soil decreased with the increase of p H,and increased with the increase of SOM content.（5）73.33%of the soil PSI in the RC model in the study area was above the"30"cut-off point.The results of laboratory simulation experiments showed that Olsen-P was76.34mg/kg and degree of phosphorus sorption saturation（DPSS）was 11.24%was11.24%.At present,the average Olsen-P content in soil was 24.34mg/kg,and the average TP content in field water and surrounding ditch water at the end of crayfish culture period was 3.44%.The TP content in the surface water environmental quality standard（GB 3838-2002）is lower than the II water standard.Therefore,at present,soil phosphorus fixation capacity of RC model in this study area is strong,soil phosphorus accumulation has not reached the mutation point of loss,and phosphorus content of surface water in the late stage of crayfish cultivation is low,resulting in a low degree of water pollution during drainage.The risk of soil phosphorus release in RC model is low and the environmental risk is small in the study area.