Another bottleneck for nitrogen mineralization in temperate forest soils: Arginine metabolism in microorganisms

Soil nitrogen (N) mineralization is generally limited by microbial N assimilation when microorganisms are exposed to substrates with high carbon-to-nitrogen (C/N) ratios. We hypothesized that microbial N release is also limited by repression of arginine-degrading activity in forest soils with the high C/N ratios. We analyzed the microbial assimilation and mineralization rates of 14C-labeled amino acid mixture, arginine, ornithine, and urea added to a variety of forest and cropland soils. The proportions of amino acid mixture derived 14C incorporated into microbial biomass (MB14C) in a 24-h incubation (3.7-20.4%) increased with soil C/N ratio and an increase in MB14C retards inorganic N release in the forest soils. Arginine mineralization displays a contrary pattern to amino acid derived MB14C and decreased with increasing soil C/N ratio. The reduced arginine-degrading activity in the forest soils with the high C/N ratios is consistent with general microbial N assimilation associated with growth, but it also correlates with the enhanced fungi-specific N preservation (e.g., recycling of ammonium in urea-ornithine cycle and accumulation of arginine or ornithine in vacuoles). Arginine or ornithine degradation is one of slower amino acid degradation pathways and potentially retards N mineralization in N-limited forest soils.