Effect of Soluble Organic Nitrogen on Evaluating Soil N-Supplying Capacity

【Objective】It is important to study the soluble organic nitrogen (SON) extracting in water-logged incubation for evaluating soil N-supplying capacity. 【Method】Soil initial SON and mineral nitrogen (Nmin), cumulative soluble organic nitrogen and NH4+-N in leaching solution during water-logged incubation, mineralization potentials of both decomposable nitrogen (ND) and resistant nitrogen (NR), and their correlations with crop N-uptake of 10 kinds of farmland soils with different physical and chemical properties on the Loess Plateau were investigated. Effect of SON on evaluating soil N-supplying capacity was studied. 【Result】The results showed that mean content of initial SON (23.9 mg·kg-1) of 10 soils was 28.8% of initial total soluble nitrogen (TSN) and 2.4% of total nitrogen (TN). The percent of cumulative SON in leaching TSN during incubation was higher than 28.8% and as high as the percent of cumulative NH4+-N. After 217 days water-logged incubation, the mean cumulative SON content was 118.1 mg·kg-1, 46.4% of cumulative TSN. ND had close correlation with TN, the correlation coefficients were 0.92 (P0.01, without SON) and 0.88 (P 0.01, with SON), respectively. N mineralization potential(ND) and mineralization rate constant(KD) were different with the soil types..ND of Los-Orthic Entisols and Ust-Sandiic Entisols were lower than that of Eum-Orthrosols. KD decreased and the mineralization rate constant of resistant materials (KR) increased when considering SON. 【Conclusion】Cumulative NH4+-N is a good evaluating index of soil N-supplying not only for the first season crop, but also for two successive season crops. ND is more correct in revealing the potential mineralizable nitrogen when SON was regarded as mineralized N, although cumulative SON alone is not a good index for the potential of mineralizable nitrogen. And cumulative TSN could also be taken as an index for the potential mineralizable nitrogen to some extent. Cumulative NH4+-N, TSN and ND are satisfactory indexes for the potential mineralizable nitrogen, especially in estimating the potential mineralizable nitrogen of the soil for two successive season crops. And cumulative TSN and ND in evaluating the permanence of soil N-supply has better effects when SON was included.