Biophysical Controls on Soil Carbon Cycling 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 concentration, 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 the upper site maintained significantly higher surface soil C concentration, despite similar litterfall inputs across sites. We found large differences in the fraction of total soil C that is protected from microbial decomposition, with enhanced physical protection in macroaggregate-rich, upper site soils. Upper site plots maintain a higher relative abundance of plants producing lignin-rich litter, which may fuel aggregate formation and 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 site plots. Seasonal Q₁₀ of soil respiration was higher at the upper 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. Despite enhanced SOM physical protection at the upper site, the higher temperature sensitivity of soil respiration may reduce soil C in the context of future warming.