The effect of air humidity on decomposition in the dry season

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, 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.

The goal of my work is to characterize litter decomposition resulting from water vapor absorbed during the rainless season and describe the mechanisms involved. We predicted that the high air humidity in this climate zone will allow microbial decomposition of plant litter in the dry season. A litter bag experiment was conducted on the Carmel ridge in Israel which has a Mediterranean climate and is close to the sea. Litter bags containing standard litter (wheat straw) and local litter were placed under evergreen and deciduous shrubs and in open areas to allow different microhabitat conditions. In the field during predawn and midday we measured CO2 fluxes from the litter bags using a respiration chamber connected to an infrared gas analyzer. 

Results/Conclusions

Rates of decomposition of local litter were highly dependent on litter quality. For example, the litter of evergreen shrubs barely decomposed due to high lignin content. The standard litter showed small differences in mass loss between the habitats. This may indicate that when radiation is blocked another decomposition mechanism allows decomposition. During summer measurements the microbial biomass was approximately half of the microbial biomass which was present in the wet season during a rain event. Additionally the CO2 fluxes observed at predawn along 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 content in 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 air humidity appears to be an important driver for decomposition in the dry season.