Decomposition of organic matter is a key process in ecosystem carbon and nutrient cycling, and in the exchange of carbon between the biosphere and the atmosphere. Decomposition is generally considered a direct consequence of microbial activity, as enabled by moisture from precipitation. However, as the climate gets warmer and drier, with a higher frequency of extreme events, such as prolonged droughts, new driving factors, mechanisms and feedbacks may become relevant. We investigated, whether alternative water sources can drive microbial activity in the absence of rain and whether such humidity-enhanced microbial degradation interacts with abiotic mechanisms of litter decay, such as photodegradation and thermal degradation. We experimentally manipulated solar irradiance and nighttime air humidity under field conditions in natural ecosystems with a hot and rainless summer season. Plant litter was successively introduced at the beginning and the end of the summer, and was analyzed for CO2 emissions and changes in mass and functional traits.
Results/Conclusions
Under dry conditions, microbial degradation was enabled by absorption of dew and water vapor by litter at night. We estimated that most of the litter CO2 efflux and decay occurring in the summer was due to nighttime humidity-enhanced microbial degradation, with considerable additional contributions from photochemical and thermal degradation during the day. We further observed mutual enhancement of microbial activity and photodegradation on a daily scale. Moreover, humidity-enhanced microbial degradation and photodegradation jointly modified litter during the dry summer, thus affecting subsequent decay in the wet winter. A large microbial biomass in the summer led to facilitation of mass loss, but also caused strong inhibition of nitrogen loss from litter in the winter. A 2-yr study showed that while decay rate slowed down from the first to the second wet winter season, the rate of decay remained constant during two subsequent dry summer seasons, jointly contributing 30% to the overall mass loss. While these emerging decay mechanisms have recently been described to affect carbon and nitrogen cycling in drylands, they are expected to become relevant in humid zones where they need to be considered to better understand and predict the impact of climate change on biogeochemical cycling.