The role of physical properties in controlling soil nitrogen cycling across a tundra-forest ecotone of the Colorado Rocky Mountains, U.S.A

There is growing recognition that physical characteristics of landscapes influence nitrogen (N) cycling. The relationships among climate forcing, soil properties, and the fate of N are particularly important in alpine ecosystems vulnerable to climate warming and characterized by shallow, rocky soils. This study evaluated differences in net N mineralization and nitrification rates determined using in-field incubation experiments across patches defined by six plant community types within an alpine catchment of the Colorado Rocky Mountains. We considered not only differences in net N transformation rates across space and time within a growing season, but also whether or not soil properties (i.e., physical and chemical) and conditions (i.e., temperature and moisture) could explain patch-scale variation in rates. Highest net N mineralization and nitrification rates occurred in the dry meadow (3.7 ± 0.5 and 3.4 ± 0.5 μg N cm⁻² d⁻¹, respectively), while the lowest were in the subalpine forest (−0.3 ± 0.4 and 0.0 ± 0.1 μg N cm⁻² d⁻¹), which exhibited net N immobilization. The magnitude of differences in net N transformation rates through time differed among patches and was strongly controlled by soil C:N ratios. Dry and moist meadow communities showed the greatest range in net N transformation rates across the growing season and changes were positively correlated with soil moisture. In contrast, inhibition of nitrification at high soil moisture occurred in wet meadow areas. Our data suggest that as the alpine growing season lengthens in a drier, warmer future, changes in soil moisture will likely be a primary factor driving patterns of net N transformation rates.