Drought, heat, and management interact to affect soil carbon and nitrogen losses in a temperate, humid climate

Some areas with historically mesic climates are predicted to experience more climate extremes, including longer droughts combined with hotter days and more intense precipitation. Drought and rewetting are known to alter carbon (C) and nitrogen (N) cycling. However, little information is available on how the effects of drought on C and N cycling differ with temperature and land use in soils from humid regions. We evaluated several metrics of C and N cycling under drought with or without heat stress in a forest site and conventionally and organically managed arable sites. We sampled undisturbed soil cores from 0 to 10 cm and incubated them under either reference conditions (REF), drought (DRT), or drought combined with heat stress (D + H). Metrics of C and N cycling, including actual and potential mineralization, enzyme activities, microbial biomass, and dissolved organic C and N, and microbial community structure were assessed at the end of the stress period and 14 and 28 d after rewetting. We found that the effects of D + H differed in magnitude and direction from those of DRT: cumulative C and N mineralization followed the order DRT < REF ≤ D + H. Land management affected stress response: mineralization was always greater in the forest and organic sites than in the conventional site. Post-wet pulses of C and potential net N mineralization were 1.7 and 3.6 times higher, respectively, in the D + H soils than DRT soils, and were greatest at the forest site. Only the organic site was sensitive to DRT alone. Across sites, microbial biomass N was reduced more by stress than C, and only N-cycle parameters failed to reach reference levels after the recovery period. In agreement with previous studies, the N cycle was more affected than the C cycle. Our results suggest that climate change-induced heatwaves during drought have implications for ecosystem C and N balance in mesic climates.