Fate of biuret 15N and its effect on net mineralisation of native soil N in forest soils
Biuret (C2H5N3O2) priming effect on mineralisation of native soil N has not been precisely quantified in previous studies, although it is a potential microbial activity regulator and slow-release N fertiliser. Following application of biuret at concentrations of 0 (B0) and 100 (B100) mg/kg (oven-dried) soil, we measured the dynamics of biuret-derived 15N in soil N pools, soil C mineralisation, and microbial biomass C in a sandy loam and a silt loam during a 112-day-long incubation to investigate the fate of biuret 15N and its effect on net mineralisation of native soil N. Biuret was decomposed faster in the sandy loam soil than the silt loam soil. In the sandy loam soil, the stabilised N pool was a strong sink for the biuret-derived 15N and accumulated about half of the applied 15N at the end of incubation. In the silt loam soil, 68% of the 15N applied was recovered in the NO3−-N pool and the stabilised N pool accumulated only about 25% of the applied 15N at the end of incubation. Biuret addition increased the turnover rate constant of soil organic matter and caused a real priming effect on net mineralisation of native soil N in both soils. The additional mineralisation of native soil N was 20.1 mg/kg (equivalent to 27.3 kg N/ha) in the sandy loam soil and 20.5 mg/kg (equivalent to 57.3 kg N/ha) in the silt loam soil. Biuret priming effect was related to the acceleration of soil organic matter decomposition by increased microbial activity at an early stage and the death/decay of microbes at a later stage of incubation. The native soil N released through the priming effect was partially from soil non-biomass organic matter and partially from soil microbial biomass.