Mechanisms Underlying The Effects Of Potassium Management Measures On Soil Potassium Fertility In Rice-oilseed Rape Rotation System

Potassium（K）is a macronutrient for plant,and plays an important role in plant growth,development and physiological metabolism of crops such as rice and oilseed rape.Rice-oilseed rape rotation is one of the most important crop rotation systems in the Yangtze River Basin,in China.The stability of this rotation is of great significance for maintaining food security and ensuring oil supply.This rotation is characterized by high degree of intensification and unbalanced application of chemical fertilizers,resulting in a lack of K in the farmland in this region.Under the intensive system,crop straws are huge in quantity and rich in K resources,and straw return is an effective way to utilize straw resources and alleviate soil K deficiency.However,the potential mechanism of soil K fertility improving after straw return is still unclear,and the appropriate K fertilizer rate after straw return needs to be further clarified.Therefore,this research is carried out to study the effects of straw return and K application on the relationship of soil K quantity-intensity,the changes of soil aggregate assicaited K and the K release from insoluble minerals.On this basis,with the help of field experiments of K management measures,the suitable soil K management strategies for this rotation were summarized based on the changes of soil K fertility and the crop yield.The main results are as follows:（1）Straw return and K fertilization increased the soil K activity,the equilibrium water-soluble K,the equilibrium exchangebale K,the K content held at non-special sites and the minmum exchangebale K in the 0-20 cm layer.At the same time,the soil K activity in the20-40 cm layer also improved,thereby improving soil K supply capacity.Compared with no K application,straw return and K application increased soil K activity,the equilibrium water-soluble K,the equilibrium exchangebale K,the K content held at non-special sites and the minmum exchangebale K by 107%-577%,55%-137%,167%-233%,14%-28%,and 17%-33%,respectively,the K activity in 20-40 cm soil also increased by 56%-131%.（2）Straw incorporation improved the adsorption of K by increasing the soil humic acid in macroaggregates and enhanced soil K fertility.Straw retrun and K application synergistically increased soil organic carbon and humic acid content,and increased the proportion of aggregates>2 mm.Compared with no straw return,soil organic carbon and humic carbon content increased by 16.6%and 15.8%,respectively,and>2 mm fraction increased by 23.0%after straw return.Straw return and K fertilization increased soil exchangeable and non-exchangeable K pools in>2 mm aggregate fractions,thereby inmproved soil aggregate associated K pools.Compared with no straw return and no K application,the soil exchangeable K pools and non-exchangeable K pools in>2 mm fraction increased by 45.4%and 13.4%,respectively after straw return and K application.Structural equation modeling shows that soil organic carbon and aggregate fractions significantly affected the adsorption of K,among them,organic carbon affected soil non-exchangeable K,and aggregate fraction affected exchangeable K and non-exchangeable K pools.（3）Under K deficient conditions,the soil hydromica transformed into 1.4 nm transition minerals and kaolinite,thereby improving soil K supplying capacity.Compared with K fertilization and straw return,the soil treated with no K application showed higher proportion of kaolinite and lower proportion of hydromica,and it had lower cumulative K release.In order to clarify the thermodynamic mechanism of the K release from K-bearing minerals in K deficient conditions,the simulation studies were used to explore the K release from K bearing minerals through oxalic and citric acid dissolving biotite,the results showed that organic acids can promote K release.Further analysis found that the binding energy of carboxyl and hydroxyl functional groups were 7.0-10.3 k J mol^-1 and 4.2-8.5 k J mol^-1,respectively,the carboxyl and hydroxyl functional groups are adsorbed on the surface of soil K-bearing minerals by hydrogen bonds and van der Waals forces,respectively,thus promoted the release of the insoluble mineral K.（4）Straw return and K fertilization can maintain the high and stable yield of crops and soil K fertility,and improve K use efficiency in the rotation system.Compared with no K application,the yield of rice and rape increased by 12.7%-27.8%and 19.2%-29.8%,respectively,under different K management measures,and the K uptake of the crop rotation system increased by 58.6%-147.6%,the average K deficiency each year in soil alliveated-2-229 kg K hm^-2,and the soil exchangeable K and non-exchangeable K contents increased by 30.4%-186.4%and-1.8%-29.3%,respectively.Considering the soil K fertility of the soil-crop system and crop yield,straw return and input the same amount of K that taken away by grain is the best K management strategy.In conclusion,straw return and K application can improve the quantity and intensity of soil K supplying,increase soil exchangeable and non-exchangeable K in soil macroaggregates,and maintain the relative composition of K-bearing minerals.Therefore,it can maintain the high and stable yield of crops and soil K fertility in the rice-oilseed rape rotation.