Soil moisture regulates the response of soil respiration to long-term warming in a southern boreal forest

Abstract The effects of long-term climate warming on soil respiration and its drivers remain unclear in forests, which store approximately 40% of global soil carbon (C). This uncertainty may hinder realistic predictions of the global C cycling feedbacks under future climate warming. To address this knowledge gap, we conducted a long-term (13 years) climate change experiment, B4WarmED, at two southern boreal forest sites in northern Minnesota, USA. Treatments included simultaneous above- and belowground warming (ambient, + 1.7°C, and + 3.3°C) under different rainfall scenarios (100% and 60% of summer rainfall) and contrasting canopy conditions (open and closed). Across all treatments and years, soil respiration increased by 7% and 17% under + 1.7°C and + 3.3°C, respectively, compared to the ambient temperature plots. In all contexts (all warming levels, rainfall amounts, and canopy conditions), warming increased soil respiration more when soils were moist but less (or even decreased soil respiration) when soils were dry. Furthermore, these responses occurred regardless of whether soil moisture deficits were driven by low ambient rainfall, experimental rainfall reduction, canopy removal, or experimental warming. Our results suggest that factors affecting soil moisture should be accounted for when predicting or modeling the response of soil respiration to warming. Otherwise, warming effects on soil respiration, and thus the positive climate feedback, may be overestimated in forests that periodically experience low soil moisture, and especially those that will do so more frequently under future climate.