Landscape-scale variability of N mineralization in forest soils

Our understanding of the controls on N-cycling and availability in forest soils following disturbance is limited. A comparative study was conducted to examine the spatial distribution of soil N in forest soils and assess the effects of site disturbance. Sampling grids were established within a 120 × 120 m representative area at a native site, and at recently (i.e. 4-year-old) burned and clear-cut sites. A three-dimensional classification of landscape form was used to stratify each landscape into distinct landform elements. The spatial distribution of inorganic-N was not related to landform element, irrespective of site disturbance, indicating an absence of topographic control at the scale studied. However, a narrowing of the NH4+-to-NO3− ratio at the clear-cut site compared to the native site suggests that N-cycling was influenced by site disturbance. Similarly, an increase in the size of the microbial biomass at the clear-cut site, coupled with a widening of the microbial biomass C-to-N ratio, suggest that disturbance altered both the size and composition of the microbial biomass. Potential N and C mineralization, and net nitrification in the forest floor and surface mineral horizons representing two distinct landform complexes were studied in a controlled aerobic 8-wk incubation experiment. Accumulation of NH4+ and NO3− differed markedly between sites although the effects of topographic position were generally non-significant. Inorganic-N accumulated principally as NH4+ in soils from the native site due to an extended lag in nitrification. In contrast, NH4+ accumulation in soils from the recently disturbed sites remained limited, whereas NO3− accumulation predominated. Thus, although topography did not markedly influence N distribution at the scale studied, site disturbance had a direct effect on N-cycling processes in these forest soils.