Quantifying Soil Carbon Protection by Anaerobic Microsites

Abstract:

Soils are the largest carbon (C) stock with decadal or faster turnover times. Anaerobic microsites may serve as a soil C protection mechanism as anaerobic respiration is inherently slower than aerobic. Consequently, aeration of anaerobic microsites may increase microbial respiration rates and soil C loss as carbon dioxide. We leverage this framework to study the role of anaerobic microsites in soil C protection across soils of varied moistures and textures. We examine anaerobic microsites based on their response in greenhouse gas evolution upon increased oxygenation. We present both a field method that simulates tillage by disaggregating soils and a laboratory method that employs an enhanced O₂ atmosphere (30-35% v/v O₂) to more closely examine the role of O₂ supply in anaerobic microsite formation. Iron and manganese reduction in the soils confirm the prevalence of anaerobic metabolisms in both methods. Consistent with convention, we find that wetter soils tend to have greater anaerobic volume. We also demonstrate that the increase in CO₂ evolution observed after physical soil disturbances is due not only to access to occluded C but also the aeration of anaerobic microsites. Enhanced atmospheric O₂ partial pressure does not necessarily increase soil CO₂ production, indicating O₂ demand is also a key regulator of anaerobic microsite formation. The ratio of mineral associated organic C to particulate (or dissolved) organic C thus appears to play a key role in anaerobic microsite formation and the susceptibility of soil C to respiration upon aeration. Our results elucidate the mechanisms that decrease soil C stocks post-tillage and the role of anaerobic microsites in soil C storage.