Drought effects on soil carbon and nitrogen dynamics in global natural ecosystems

Extreme droughts have serious impacts on the pools, fluxes and processes of terrestrial carbon (C) and nitrogen (N) cycles. A deep understanding is necessary to explore the impacts of this extreme climate change events. To investigate how soil C and N pools and fluxes respond to drought and explore their mechanisms we conducted a meta-analysis synthesizing the responses of soil C and N cycles to droughts (precipitation reduction experiments) in three main natural ecosystems: forests, shrubs and grasslands. Data were collected from 148 recent publications (1815 sampling data at 134 sites) with the drought experiments from 1 to 13 years across the globe. Drought reduced soil organic C content (-3.3%) mainly because of decreased plant litter input (-8.7%) and reduced litter decomposition (-13.0%) across all the three ecosystem types in the world. Drought increased mineral N content (+31%) but reduced N mineralization rate (-5.7%) and nitrification rate (-13.8%), and thus left total N unchanged. Compared with the local precipitation, drought increased the accumulation of dissolved organic C and N contents by +59% and +33%, respectively, due to retarded mineralization and higher stability of dissolved organic matter. Among the three ecosystem types, forest soils strongly increased litter C (+64%, n=8) and N content (+33%, n=6) as well as microbial CO2 (+16%, n=55), whereas total CO2 emission remains unaffected. Drought decreased soil CO2 emission (-15%, n=53) in shrubs due to reduction of microbial respiration and decreased root biomass. The 98% (n=39) increase of NH4+ concentration in forest soils corresponds to 11% (n=37) decrease of NO3- and so, it reflected the increase of N mineralization rate, but the decrease of nitrification. For shrubs and grasslands, however, stabilized or decreased N mineralization and nitrification mean less N uptake by plants under drought. Overall, the effects of drought on soil C and N cycles were regulated by the ecosystem type, drought duration and intensity. The drought intensity and duration intensify all effects, especially on the decreasing total (CO2) emission. However, the most studies mainly focused on the short-term droughts, and there is a lack of comprehensive understanding of how drought effects in a long-term consequences. So, future studies should strengthen drought frequency impacts on ecosystem C and N dynamics in the long-term sequence (> 10 years) in order to face the impacts of global change.