Disentangling controls on mineral-stabilized soil organic matter using a slurry incubation

Abstract:

Mineral-stabilized organic matter (OM) is the largest and oldest pool of soil carbon and nitrogen. Mineral stabilization limits OM availability to soil microbes, preventing its decomposition and prolonging its turnover. Thus, understanding controls on the decomposition of mineral-stabilized OM is key to understanding soil carbon and nitrogen dynamics. The very slow turnover of mineral-stabilized OM makes it challenging to study in a typical incubation, and as a result, many potential controls (temperature, OM chemistry, and mineralogy) on its turnover remain unclear. We aimed to better understand controls on decomposition of mineral-stabilized OM by employing a slurry incubation technique, which speeds up microbial processing of OM by maximizing OM accessibility to microbes. In a slurry incubation, we expect that any OM that is not stabilized on mineral surfaces will be available for decomposition and will be converted to CO₂. Using this technique, we studied the interactive effects of incubation temperature, plant material type (aboveground vs. belowground), and soil fraction (silt vs. clay) on CO₂ efflux and OM stabilization. We separated silt-sized and clay-sized fractions from an agricultural soil, added aboveground or belowground plant material to each, and incubated them at 15°C, 25°C and 35°C. The added plant material was isotopically labeled (¹³C and ¹⁵N), which allowed us to trace it through the system and distinguish between the responses of the new (derived from the plant material) and old (derived from what was already present in the silt and clay) OM to warming. We measured CO₂ efflux and ¹³CO₂ efflux throughout the incubation. We performed one short-term harvest at day 6 and one final harvest at day 60. Initial results show higher cumulative CO₂ efflux at warmer temperatures regardless of plant material type or soil fraction. A larger fraction of that CO₂ came from OM that was initially present in the silt and clay, rather than from the plant material that we added, which suggests faster turnover of that “old” OM at warmer temperatures. We will present CO₂ efflux data in addition to total [C] and [N] and the isotopic ratios of ¹³C and ¹⁵N in the silt and clay at each harvest to explain how the interactions between warming, plant material type and soil fraction affect turnover of mineral stabilized OM.