Increased Soil Nitrogen Availability Suppresses Annual Soil Respiration in Mixed Temperate Forests Regardless of Acidification

Soil respiration (Rsoil) is the second largest terrestrial carbon (C) flux, and therefore, it is imperative to understand and quantify its responses to global environmental change. Rsoil consists of two component CO₂ fluxes: autotrophic respiration from the metabolic activity of roots (Ra_-root) and heterotrophic respiration (Rh) derived from the metabolic activity of mycorrhizal fungi and microbial decomposition of detritus, soil organic matter, and rhizodeposits. Increased nitrogen (N) availability often reduces Rsoil in forest ecosystems, but it remains unclear which contributing fluxes govern Rsoil responses and if suppression of Rsoil results from increased N availability itself or from the tendency of added N to acidify soil. Here, we address these uncertainties in a long-term, large-scale factorial N × pH experiment in six temperate forest stands in central New York, USA. We anticipated that increasing soil N availability would decrease plant belowground C allocation and related root-associated respiration and that soil acidification would suppress microbial decomposition, thereby reducing Rh. We found that both acidifying and deacidifying N additions suppressed annual Rsoil by 19% and 13%, respectively (-1.8 Mg C ha^-1 year^-1 overall), but acidification (from pH 4.67 to 4.22) alone did not detectably affect this flux. Annual Rsoil decreased steeply (R² = 0.66, p < 0.001) as soil N availability increased. Nitrogen additions generally suppressed Rh, especially in the forest floor (-34%), whereas the effects of acidification alone varied by soil depth, with substantial suppression in the forest floor (-33%) partially offset by stimulation at depth. A novel partitioning of Rsoil component responses suggests that N additions suppressed root-associated respiration by ~1.1 Mg C ha^-1 year^-1 (62% of the Rsoil suppression), while acidification alone had no effect. Our findings demonstrate that soil N availability, not soil pH, is the predominant biogeochemical control over Rsoil in these temperate forests, with larger responses of plant-driven C fluxes than microbial-driven C fluxes.