Can nitrogen budgets explain differences in soil nitrogen mineralization rates of forest stands along an elevation gradient?

Abstract

Nitrogen (N) mineralization rates in forest systems typically decrease with decreasing temperature. Temperature decreases with increasing elevation. Thus, N mineralization rates are expected to decrease with increasing elevation. However, soil N mineralization rates at the Coweeta Hydrologic Laboratory, NC, USA are higher at the highest elevation. Causes of higher mineralization rates at the higher elevation have yet to be explained. Alternative hypotheses to explain patterns of mineralization rates along this elevation gradient include (1) a mineralization promoter in decomposing herbs, leaf litter, or soil of the high elevation, (2) low pH in the low elevation soils which inhibit mineralization, (3) greater total N pools at the high elevation, (4) low moisture availability in low elevation soils, (5) differences in soil texture, and (6) differences in biological communities. Previous studies did not support our N mineralization promoter hypothesis, nor does soil pH explain mineralization rates. In this study we constructed N budgets for a high elevation northern hardwood and a low elevation oak-pine site to determine if differences in N pools and fluxes are sufficient to explain differences in N mineralization rates. Evaluating N in the upper 0–10 cm of mineral soil, forest floor, overstory biomass, annual canopy litter fall, understory herb turnover, rainfall, canopy throughfall, and in soil solution, we found that the high elevation stand has more stored N and greater N fluxes than does the low elevation stand. The high elevation stand has many characteristics of a stand in an early stage of N saturation, while the low elevation stand characterizes an N-limited forest. Causes of greater N storages and fluxes at the high elevation northern hardwood site are not obvious.