Mechanisms underlying litter decomposition in dry seasons

Background/Question/Methods

Decomposition of plant litter is a key process in biogeochemical cycles of major elements. In light of the strong link between global warming and the carbon cycle much effort was dedicated in recent years to improve the understanding of plant litter decomposition and to quantify the products of this process such as CO₂. These efforts led to the discovery of new mechanisms for decomposition under dry conditions, such as photodegradation. In addition, the absorbance of air humidity by litter was shown to facilitate decomposition in dry ecosystems, but the mechanisms involved in this process are unknown so far. The goal of this study is to characterize litter decomposition resulting from water vapor absorbed during rainless periods and describe the underlying mechanisms. We predicted that high relative humidity at night will enable microbial decomposition of plant litter in dry seasons. A litter bag experiment was conducted in a shrubland in Israel which is characterized by Mediterranean climate of mild, moist winter and hot, rainless summer.

Results/Conclusions

Rates of decomposition of local litter were highly dependent on litter quality. Leaf litter of evergreen shrubs barely decayed owing to its high lignin content and low water-vapor absorbance capacity. The standard wheat litter showed little differences in mass loss among microsites, but large differences in loss of lignin. This indicates that alternative mechanisms drive decomposition in microsites exposed to solar radiation as compared with those that are protected from radiation. Microbial biomass as estimated by substrate-induced respiration of leaf litter in the dry season was approximately half of the biomass present during a rain event in the wet season. Additionally, in situ CO₂ fluxes from litter measured at predawn throughout the dry season reached a quarter of the fluxes measured in the wet season during a rain event and were strongly correlated with litter moisture across all litter types. Taking into account the rate of mass loss (which reached up to 14% after 3 months), the presence of microbial biomass, and the measured fluxes, microbial activity facilitated by uptake of air humidity by plant litter appears to be an important mechanism of decomposition in dry periods.