Effects of combined nitrogen fertilization with biochemical inhibitors on leaching characteristics of soil potassium in yellow clay soils

Soil nutrient leaching in yellow clay soils, especially nitrogen (N) and potassium (K) leaching, is extremely serious in South China. This has resulted in not only waste of resources and potential environmental threat, but also in serious restriction on sustainable production of crops. In this study, the effects of urease inhibitorN-(n-butyl) thiophosphoric triamide, NBPT, nitrification inhibitor2-chloro-6-(trichloromethyl) pyridine, CP and their combined application on leaching characteristics of K in yellow clay soils were investigated in an indoor soil column simulation with urea (U) and urea ammonium nitrate (UAN) application. The study aimed to improve the capability of soil K through rational application of biochemical inhibitors. In the leaching solution of different N fertilizers, higher average K⁺ concentration was obtained under UAN treatments (average in 103.0 mg·kg^-1) than under U treatments (average in 93.9 mg·kg^-1), with obvious differences among inhibitor treatments. At the end of incubation (72 days after incubation), UAN treatments increased K⁺ leaching average by 6.7% more than U treatments. K⁺ accumulation in leaching solution under U treatments was in the order of U > U+NBPT > U+NBPT+CP > U+CP > CK. Compared with U treatment, U+NBPT, U+CP and U+NBPT+CP treatments reduced K⁺ accumulation by 8.7%, 20.2% and 14.9%, respectively. K⁺ accumulation under UAN treatments was in the order of UAN > UAN+NBPT > UAN+NBPT+CP > UAN+CP > CK. Compared with UAN treatment, K⁺ accumulation in UAN+NBPT, UAN+CP and UAN+NBPT+CP treatments were reduced respectively by 6.0%, 13.8% and 9.2%. Additionally, leaching rate of K⁺ across different treatments was in the order of UAN > UAN+NBPT > U > UAN+NBPT+CP > UAN+CP > U+NBPT > U+NBPT+CP > U+CP. In the middle of incubation (36 days after incubation), soil available K content of fertilizer microsites under U and UAN treatments decreased significantly. The addition of CP effectively maintained high availability of K content in the topsoil. Compared with the addition of NBPT alone, combined application of NBPT and CP reduced NO₃^- leaching, increased K⁺ fixation on soil lattice and mitigated leaching risk of K⁺ for more than 72 days in yellow clay soils. Equation models were used to describe the relationship between K⁺ accumulation (y) and NO₃^- accumulation (x) in leaching solution. The linear equation (y=ax+b) and Elovich equation (y=alnx+b) fitted well, with a and b values for inhibitor treatments obviously different. In conclusion, the application of CP alone or in combination with NBPT in yellow clay soils effectively increased the adsorption of K⁺, minimized soil K⁺ leaching loss, mitigated the risk of nutrient leaching and improved fertilizer utilization rate.