LANDSCAPE-SCALE VARIABILITY OF N MINERALIZATION IN FOREST SOILS

Our understanding of the controls on N-cycling and availability in for est soils following disturbance is limited. A comparative study was co nducted to examine the spatial distribution of soil N in forest soils and assess the effects of site disturbance. Sampling grids were establ ished within a 120 x 120 m representative area at a native site, and a t recently (i.e. 4-year-old) burned and clear-cut sites. A three-dimen sional classification of landscape form was used to stratify each land scape into distinct landform elements. The spatial distribution of ino rganic-N was not related to landform element, irrespective of site dis turbance, indicating an absence of topographic control at the scale st udied. However, a narrowing of the NH4+-to-NO3- ratio at the clear-cut site compared to the native site suggests that N-cycling was influenc ed by site disturbance. Similarly, an increase in the size of the micr obial biomass at the clear-cut site, coupled with a widening of the mi crobial biomass C-to-N ratio, suggest that disturbance altered both th e size and composition of the microbial biomass. Potential N and C min eralization, and net nitrification in the forest floor and surface min eral horizons representing two distinct landform complexes were studie d 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 accum ulated principally as NH4- in soils From the native site due to an ext ended lag in nitrification. In contrast, NH4+ accumulation in soils fr om the recently disturbed sites remained limited, whereas NO3- accumul ation predominated. Thus, although topography did not markedly influen ce N distribution at the scale studied, site disturbance had a direct effect on N-cycling processes in these forest soils.