Biophysical Controls On Ecosystem Nitrogen Cycling along a Soil Texture Gradient.

Most ecosystems retain a majority of reactive N inputs, transforming reactive mineral N into relatively nonreactive stable organic N. Mechanistic explanations for these observations focus on C-dependent processes, and in particular wide C:N ratios. However, in some ecosystems C-dependent mechanisms leave unexplained variation in reactive N retention. The potential for soil texture to explain variation in temperate forest N retention remains largely unexplored. Soil texture may help to explain N retention through two mechanisms: Negatively charged clay particles can promote the adsorption and physical protection of NH4; in contrast, coarse soil texture can promote rapid soil water flow, allowing reactive N to bypass plant, microbial and SOC sinks. Using a small forested catchment containing large gradients in soil texture and SOC, we provide evidence for interaction between these mechanisms. Tracer 15NH4 and 15NO3, applications show a positive correlation between silt content and 15NH4 transfer to the insoluble soil organic pool, but no correlations between SOC and 15NH4 or 15NO3 transfer to the insoluble soil organic pool. Consistent with these data, soil solution NO3 and hydraulic conductivity are negatively correlated with silt content. Coupled lysimeter and soil moisture data show that precipitation-induced spikes in soil moisture are coincident with large reductions in soil solution NO3 in sandy soils, but not silty soils. These data suggest that downward transport of NO3 in silty soils is inhibited by greater transfer of NH4 to the insoluble organic pool and lower hydraulic conductivity.