Soil pH has contrasting effects on gross and net nitrogen mineralizations in adjacent forest and grassland soils in central Alberta, Canada

Soil pH can be affected by land use change and acid deposition and is one of the primary regulators of nutrient cycling in the soil. In this study, two soils from adjacent forest and grassland sites in central Alberta were subjected to different pH treatments to evaluate the short-term effects of pH on soil gross N transformations using the 15N tracing technique and calculated by the numerical model FLUAZ. For the forest soil, gross N H 4 + immobilization increased faster than gross N mineralization rates with increasing soil pH, leading to a declining pattern in net N mineralization rates; however, none of those rates changed with pH in the grassland soil. In contrast, the increase in pH significantly stimulated gross and net nitrification rates while soil acidification decreased gross and net nitrification rates for both the forest and grassland soils. The ratio of gross nitrification to gross N H 4 + immobilization rates (N/IA) was significantly increased by KOH addition but declined to nearly zero by HCl addition for each soil. The low and high KCl addition treatments partially or completely inhibited gross nitrification rates, respectively, but gross mineralization was less sensitive to salt additions than the nitrification process. We conclude that based on the short-term laboratory incubation experiments both pH and salt (osmotic effect) affected gross N transformations and pH had contrasting effects on gross and net nitrogen mineralization but not on nitrification in the adjacent forest and grassland soils.