In the Appalachian region of the eastern US, climate records from the past century show a trend towards more frequent and severe summer droughts followed by increasingly wet autumn periods. This trend is concerning from an ecosystem functioning standpoint because soil re-wetting after drought stimulates microbial-mediated carbon (C) and nitrogen (N) cycle processes. Further, historically disturbed ecosystems (i.e. forest clear-cut) in the Appalachian region are predisposed to cycling N at elevated rates, suggesting that intensifying droughts will threaten forest ecosystem services, particularly in previously disturbed environments. To elucidate potential interactions between disturbance and drought in Appalachian forests, we conducted a drought-rewetting experiment with in-tact 10cm depth mineral soil cores collected from both recently disturbed and undisturbed reference watersheds at the Coweeta Hydrologic Lab in western North Carolina. Throughout the six-week experimental dry and wet periods, we monitored soil respiration as well as several soil C and N pools. We predicted that microbial communities from disturbed soils would exhibit larger drought responses than reference soil communities, resulting in larger changes in C and N pools and process rates following soil drying and rewetting.
Results/Conclusions
Our results show that all soils exposed to drought from both reference and disturbed forests exhibited declines in respiration during the drought period, followed by large spikes in respiration following soil re-wetting. However, soils from disturbed forests displayed a larger respiration response to re-wetting, highlighting the potential for increased soil C loss from recently disturbed ecosystems in response to drought. This magnified respiration response may be attributed to higher relative abundance of copiotrophic (i.e., r-selected) bacterial taxa previously reported in disturbed forest soils from this region. Soil NH4+ pools increased in both disturbed and reference soils following re-wetting, though the effect was larger in reference soils, likely due to higher microbial biomass present in these sites. In contrast, NO3- responses to drought-rewetting were only observed in previously disturbed soils, likely reflecting higher nitrification potentials due to larger resident nitrifier populations reported from disturbed forest sites in the region. Further, this nitrification response was related to higher cumulative NO3--N leached from disturbed soil cores exposed to drought-rewetting relative to all other treatments. Overall, this study demonstrates that responses to drought may vary across temperate forest ecosystems and that disturbance history must be considered when assessing potential threats to ecosystem services due to climate change in Appalachian ecosystems of the eastern US.