Soil N Pools and Transformation Rates Under Different Land Uses in a Subalpine Forest-Grassland Ecotone

Soil nitrogen pools (NP), denitrification (DN), gross nitrification (GN), N2O and CO2 flux rates with their responses to temperature increases were determined under five different land uses and managements in a subalpine forest-grassland ecotone of the eastern Tibetan Plateau. Land uses consisted of 1) sparse woodland, 2). shrub-land, 3) natural pasture, 4) fenced pasture, and 5) tilled pasture mimicking a gradient degenerating ecosystem under grazing impacts. The NO3--N content was higher than the NH4+-N content. Comparing tilled pasture with fenced pasture showed that higher intensive management (tillage) led to a significant decrease of soil organic matter (SOM) (P < 0.05) in the soils, which was in contrast to the significant increases (P < 0.05) of DN, GN, N2O and CO2 flux rates. GN (excluding tilled pasture) and CO2 flux rates increased with a temperature rise, but DN and N2O flux rates normally reached their maximum values at 12-14 degreesC with tilled pasture (the highest management intensity) being very sensitive to temperature increases. There was a difference between net nitrification and GN, with GN being a better indicator of soil nitrification.