Evaluation of soft N-supplying capacity on the Loess Plateau under long-term alternate leaching aerobic incubation conditions

The long-term alternate leaching aerobic incubation is the most important method for evaluating soil N supply- ing capacity.Initial mineral N(N_(min)),extractable total soluble N and soluble organic nitrogen(SON),mineral N(MNL= NO_3~--N+NH_4~+-N),total soluble N(TNL=SON+NL)and cumulative SON leached during aerobic incubation,and their correlations with crop N-uptake of 10 types of farmland soils with different physical and chemical properties on the Loess Plateau were investigated.Soil N mineralizable capability and effect of mineralized N with or without SON on soil N-supplying capacity was studied.The results showed that mean content of initial SON(N 23.9 mg/kg)across 10 soils was 28.8% of initial total soluble nitrogen and 2.4% of total soil N.After 217d's aerobic leaching incubation,the mean cumulative SON content was N 28.8 mg/kg,19.8% of cumulative TNL.Correlation analysis results showed that cumulative SON had significant correlation with N-uptake of two successive season crops while no significant correlation with the first season crop.With or without SON,TNL and MNL were significantly correlated with crop N-uptake,and the correlation with N-uptake of two successive season crops was stronger than that with N-uptake of the first season crop. Furthermore,a significant correlation(P0.01)was observed between TNL,MNL and N-uptake of two successive sea- son crops.In summary,MNL and TNL could be used as indices for potential mineralizable N,which are not only suitable for the first season crop but also suitable in evaluating soil N-supplying capacity for two successive season crops;while SON,total soil N and soil microbe N were not good indices for soil potential mineralizable N in short-term.