Evaluation of soil 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 supplying capacity. Initial mineral N(Nmin), extractable total soluble N and soluble organic nitrogen(SON), mineral N(MNL=NO3--N+NH4+-N), total soluble N(TNL=SON+MNL) 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 season 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.