Extreme droughts have serious impacts on the pools, fluxes and processes of terrestrial carbon (C) and nitrogen (N) cycles. 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 to droughts (precipitation reduction experiments) in three main natural ecosystems: forests, shrubs and grasslands. Data were collected from 148 publications (1815 sampling data at 134 sites) with the drought experiments from 1 to 13 years.
Drought reduced soil organic C content (-3.3%) mainly because of decreased plant litter input (-8.7%) and reduced litter decomposition (-13%) across all the three ecosystem types. Drought increased mineral N content (+31%) but reduced N mineralization rate (-5.7%) and nitrifcation rate (-14%), 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%) and N content (+33%) 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% increase of NH4+ content 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 nitrifcation. For shrubs and grasslands, however, stabilized or decreased N mineralization and nitrifcation 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 most studies mainly focused on the short-term droughts, and there is a lack of comprehensive understanding of long-term drought effects and 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.
Reference
Deng L, Peng C, Li J, Liu Y, Hai X, Liu Q, Huang C, Shangguan ZP, Kuzyakov Y 2021. Drought effects on soil carbon and nitrogen dynamics in global natural ecosystems. Earth-Science Reviews 214, 103501. https://doi.org/10.1016/j.earscirev.2020.103501
