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 during water-logged incubation, mineralization potentials of both easily decomposable N (ND) and resistant N (NR), and their relationships with N uptake by crop in pot experiment were investigated by using 10 kinds of farmland soils with widely different 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 was 28.8% 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 of cumulative 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. N mineralization potential and mineralization rate constant were different with the soil types. ND of Los-Orthic Entisols and Ust-Sandiic Entisols were lower than that of Eum-Orthrosols. Mineralization rate constant for the fast decomposable N-fraction (kD) decreased and the mineralization rate constant of resistant materials (kR) increased when SON was taken into account. Cumulative NH4+-N was a better evaluation index of soil N-supplying capacity, and it is not only suitable for the first season crops but also for two successive season crops. Cumulative SON alone was not a satisfactory index for the potential of mineralizable N. But it would be more accurate for ND in revealing the potential mineralizable N when SON was taken into account. Cumulative TSN, to some extent, could also be taken as an index for the potential mineralizable N. Cumulative NH4+-N, total soluble N, and ND were good indexes for estimating soil potential mineralizable N, especially for soils of two successive season crops. And cumulative total soluble N and ND in evaluating the permanence of soil N-supply is of greater significance when SON was included.