Land Use History Mediates Soil Biogeochemical Responses to Drought in Temperate Forest Ecosystems

Terrestrial ecosystems are experiencing increasing frequency and intensity of droughts as a result of climate change. Despite a wealth of previous studies investigating soil responses to drought, the importance of historical land use in mediating drought effects remains poorly understood. To identify interactions between drought and historical land use, we sampled soils from two adjacent forested watersheds at the USFS Coweeta Hydrologic Laboratory: a ‘reference’ forest that has remained undisturbed for approximately a century and a ‘disturbed’ forest with a history of conversion to pasture followed by abandonment to forest succession. We incubated intact soil cores in the laboratory under one of two moisture treatments: a constant moisture control and a drought-rewet treatment, which involved a six-week drought followed by rewetting and six additional weeks at constant moisture. We measured respiration and characterized soil C and N pools and microbial communities multiple times throughout the experiment. Soils exposed to drought-rewetting had higher cumulative respiration than control soils, which was driven by particularly strong respiration responses of disturbed soils. In addition, drought-rewetting reduced microbial biomass and increased total extractable N and NH4+, with greater responses in reference watershed soils. Microbial communities also exhibited responses, with increased biomass C:N, increased fungal:bacterial ratios, and decreased nitrifier abundance in response to drought-rewetting. Overall, our results show that drought responses of forests will vary among land use histories and among soil parameters, which should be considered when seeking to predict biogeochemical responses of temperate ecosystems to climate stressors such as intensifying droughts.