Biophysical controls on soil organic matter protection and respiratory carbon loss in a northern hardwood forest

Soil organic matter (SOM) is a major global carbon (C) pool vulnerable to ongoing warming, as microbial SOM decomposition and CO₂ respiration are sensitive to temperature. We characterized the edaphic characteristics that explain variation in soil C pool size, cycling, and temperature sensitivity (Q₁₀) across two sites of differing elevation, forest community composition, and mineral parent material at Hopkins Memorial Forest, Williamstown Massachusetts USA. We found that higher elevation locations maintained significantly larger surface soil C pools, despite similar aboveground tree biomass, litter production, and belowground C allocation across sites. We found large differences in the fraction of total soil that is protected from microbial decomposition, with enhanced physical protection in macroaggregate-rich, upper elevation soils. These locations maintain a higher relative abundance of plants producing lignin-rich litter, which may fuel aggregate formation and SOM protection. Upper elevation soils in the forest also contain higher aluminum concentrations in surface soils, indicating the potential of cation bridging as a mechanism for increased SOM protection. Experimental addition of glucose, vanillin, and lignin substrates produced broadly conserved respiratory responses across sites, suggesting that microbial communities maintain similar decomposition capacity, although lignin addition induced slightly elevated respiration responses in upper relative to lower elevation plots. Seasonal Q₁₀ of soil respiration was higher at the upper elevation site and increased with soil potassium (K⁺) availability across plots, potentially reflecting K⁺ constraints on autotrophic and heterotrophic metabolic activity. Our findings suggest that variation in the extent of physical protection of soil C, particularly through macroaggregate formation is an important mechanism for long term soil C storage at the site.