Effects of Soluble Organic N on Evaluating Soil N-Supplying Capacity

It is important to study the soluble organic N (SON) extracted during water-logged incubation for evaluating soil N-supplying capacity. Soil initial SON and mineral N (Nmin), cumulative soluble organic N and NH4+-N in leachates duringwater-logged incubation, mineralization potentials of both easily decomposable N (ND) and resistant N (NR), and theirrelationships with N uptake by crop in pot experiment were investigated by using 10 kinds of farmland soils with widelydifferent physical and chemical properties on the Loess Plateau, China, and the effects of SON on evaluating soil N-supplying capacity were studied. The results showed that the average content of initial SON (23.9 mg kg-1) of 10 soils was28.8% of initial total soluble N and 2.4% of soil total N. The percentage of cumulative SON in leaching total soluble N(118.1 mg kg-1) was 46.4%, higher than the percentage of initial SON (28.8%), and almost close to the percentage ofcumulative NH4+-N in the leachates. ND had close correlation with total N, and the correlation coefficients were 0.92(P < 0.01, excluding SON in estimating ND) and 0.88 (P < 0.01, including SON in estimating ND), respectively. Nmineralization potential and mineralization rate constant were different with the soil types. ND of Los-Orthic Entisolsand Ust-Sandiic Entisols were lower than that of Eum-Orthrosols. Mineralization rate constant for the fastdecomposable N-fraction (kD) decreased and the mineralization rate constant of resistant materials (kR) increasedwhen SON was taken into account. Cumulative NH4+-N was a better evaluation index of soil N-supplying capacity, andit is not only suitable for the first season crops but also for two successive season crops. Cumulative SON alone wasnot a satisfactory index for the potential of mineralizable N. But it would be more accurate for ND in revealing thepotential mineralizable N when SON was taken into account. Cumulative TSN, to some extent, could also be taken asan index for the potential mineralizable N. Cumulative NH4+-N, total soluble N, and ND were good indexes for estimatingsoil potential mineralizable N, especially for soils of two successive season crops. And cumulative total soluble N andND in evaluating the permanence of soil N-supply is of greater significance when SON was included.