Disentangling variability of litter decomposition in semiarid ecosystems: A two-phase model linking leaf traits, photodecomposition and rainfall variability

Background/Question/Methods

Recent studies are considering that distinctive factors, such UV radiation, may be playing significant roles in litter decomposition and nutrient cycling in arid environments. In semiarid ecosystems exposure to UV radiation takes place in the dry season and such pre-degradation enhances microbial degradation during the rainy season. Because decomposability may also depend on initial litter chemistry, carry-over effects of UV radiation could vary between coexisting species with contrasting leaf traits. This study proposes a generalized conceptual model of the variation in decomposition processes in semiarid ecosystems illustrating the importance of carry-over effects of photodegradation and litter quality, on the overall rate of decomposition in semiarid ecosystems. This was tested by evaluating the importance of photodecomposition on litter mass loss in two shrub species from semiarid Chile, evergreen Porlieria chilensis and deciduous Proustia pungens. Litter from Proustia and Porlieria was placed under shrubs and on open soils where litter was protected or exposed to natural UV. Secondly, we run a decomposition experiment in the field, with this pre-exposed litter, in two years of contrasting rainfall.

Results/Conclusions

We found that carbon losses after UV-exposure reduced litter biomass by 12% and this effect was only observed on open soils and UV-exposed litter. Such carbon losses were  >20% in Proustia, and only 8% in Porlieria. Further, carry-over effects explained 67% of biomass loss from Proustia in the dry year but only 11% in the wet year. In Porlieria, carry-over effects only explained <6% of litter biomass irrespective of litter position, whereas differences among years explained >81%.Our results show that a that a model that considers carry-over effects of photodecomposition, interannual rainfall variability and litter quality improves our understanding of litter recycling in semiarid ecosystems. We found that during dry spells litter of deciduous Proustiawas predegraded by solar-radiation and changes in litter chemistry made it more degradable by microorganisms. This was particularly relevant in years of low rainfall and when litter was placed on open soils.In contrast, litter decomposition of evergreen Porlieria seems to be controlled mainly by microbial activity triggered by high precipitation. Hence, Porlieria should rely more strongly on microbial decomposers that promote litter breakdown underneath its crown when water is available. These two species with contrasting leaf traits may persist, in this nutrient and water-limited ecosystem, by taking advantage of different and abiotic factors, in other words, may have different “decomposition niches” that promote coexistence in temporally variable environments.