Mineralization responses at near-zero temperatures in three alpine soils

Cold-season processes are known to contribute substantially to annual carbon (C) and nitrogen (N) budgets in continental high elevation and high-latitude soils, but their role in more temperate alpine ecosystems has seldom been characterized. We used a 4-month lab incubation to describe temperature (−2, 0, 5°C) and moisture [50, 90% water-holding capacity (WHC)] effects on soil C and N dynamics in two wet and one dry meadow soil from the Sierra Nevada, California. The soils varied in their capacity to process N at and below 0°C. Only the dry meadow soil mineralized N at −2°C, but the wet meadow soils switched from net N consumption at −2°C to net N mineralization at temperatures ≥0°C. When the latter soils were incubated at −2°C at either moisture level (50 or 90% WHC), net NO₃ ⁻ production decreased even as NH₄ ⁺ continued to accumulate. The same pattern occurred in saturated (90% WHC) soils at warmer temperatures (≥0°C), suggesting that dissimilatory processes could control N cycling in these soils when they are frozen.