How soil microbial properties change in the transitional layer between organic and mineral soil in the Arctic tundra

Low temperatures in Arctic soils slow decomposition and cause thick, nutrient-poor organic (O) horizons to accumulate at the soil surface. Soil microbes in the O horizon predominantly control decomposition, while the underlying transitional layer between organic and mineral horizons mediates process rates in the O horizon through its influence on redox potential and moisture regimes, e.g. due to intermittent thawing and cryoturbation. However, the microbial dynamics and nutrient cycling of the transitional layer, which predominantly contains the properties of O horizon but also possess some properties of mineral soil, are poorly studied. Decomposition and nutrient cycling rates in Arctic soils are expected to increase with warming, and we expect that C and nutrient cycling in transitional soil will control decomposition rate by providing an organic-mineral interface between mineral horizons and the overlying O horizon. This biogeochemically important interface can facilitate nutrient influx to the O horizon from underlying mineral soil. To test this prediction, we conducted an experiment to determine microbial C and nutrient cycling rates in transitional soils under different oxygen availability, moisture, and pH conditions.

We separately incubated surface, transitional, and mineral soils from Arctic tundra spanning different land surface ages, which have contrasting soil pH, and topographies that regulate drainage conditions. Soil extracellular enzyme activities were measured as a proxy of nutrient acquisition by microbes. Salt extractions were used to assess soil N and P mineralization and chloroform fumigation-extraction was used to measure microbial C, N, and P. Greenhouse gas production was measured by gas chromatography and used to calculate fluxes. Differences in C and nutrient cycling rates between transitional, organic and mineral horizons show how the transitional zone mediates nutrient and C availability in the soils above and below.