How is litter decomposition affected by climate change in Mediterranean forests? Soil fauna has the key

Lead author Pablo Homet discusses recently published article: Soil fauna modulates the effect of experimental drought on litter decomposition in forests invaded by an exotic pathogen. Find out more about how litter mesofauna plays an important role in litter decomposition in a Mediterranean forest.

Decomposition processes of dead organic matter are fundamental for life cycling. In terrestrial ecosystems, especially in forests, leaf litter decomposition regulates the release of nutrients necessary for many living organisms, therefore, playing a major role in maintaining the functioning of forest systems. Moreover, litter decomposition also liberates important amounts of carbon to the atmosphere affecting climate regulation through feedback processes, crucial in a climate change context.

Multiple factors such as litter characteristics, decomposer organisms and environmental conditions modulate litter decomposition rates; all of them entangled in a bank of complex interactions. This interacting network is susceptible climate change and other global change drivers such as invasive species. Yet, the extent to which several global change drivers modulate or are related to the different litter decomposition compartments, and the way in which these changes occur, is still poorly understood.

Rainfall exclusion plot from inside with a basket to collect freshly fallen leaves (left). Rainfall exclusion plot viewed from outside (Right). (Photographs: Pablo Homet Gutierrez)

To study how litter decomposition is affected by drier conditions expected as a result of climate change and the presence of invasive species, we took the challenge to build a rainfall exclusion structure in a mixed Mediterranean forest of South Spain (Los Alcornocales Natural Park) reducing the 30% of natural precipitation (following realistic climate predictions for the area). This is currently the southernmost European rainfall exclusion experiment. Moreover, the soils of this forest are currently invaded by the exotic soil-borne oomycete Phytophthora cinnamomi, a pathogen causing tree decline.

After a year running the rainfall exclusion to assure that the infrastructure built was producing desired reductions in water interception and soil moisture, we performed a litter decomposition experiment using litter bags. To do so, we first collected freshly fallen leaves individually for each tree inside our plots, then we characterized their properties (C, and N content, SLA, and thickness) and finally filled the self-made litter bags with the collected material (everyone who has ever performed a litter decomposition experiment will know how hard is this process, in particular to sew 800 litterbags and to collect and process enough freshly fallen leaves to fill them). This procedure allows us to match collected litter to the closest tree, and therefore, we could follow the microclimatic and soil conditions underneath each tree.

Examples of the two types of litter bags (see the difference on mesh size) (left). Litterbags placed under a tree within experimental plots (center). Some of the most characteristic mesofauna present in our samples, oribatid mites, a collembola from the order Symphypleona and a spider (from left to right in the picture of the right) (Photographs: Pablo Homet Gutierrez)

The combination of the rainfall exclusion, the use of the litter bags, and the fine characterization of processes occurring at local scales, allowed us to put great detail on assessing the role of soil fauna on litter decomposition. For this, we built two types of decomposition bags with a different mesh size to allow or prevent the entrance of different decomposer guilds. After 3, 9, and 18 months in the field, bags were harvested, mesofauna was extracted and identified, and litter was characterized in terms of weight, C and N loss to determine litter decomposition dynamics.

Graphical abstract explaining experimental design and the most important results, made in BioRender.

After all this effort, we were very excited to see that mesofauna was key to understanding the effect of global change on litter decomposition dynamics for two main reasons. First and counter to expectations, drier conditions obtained by the rainfall exclusion modified soil fauna diversity and composition in such a way that accelerated (and not reduced) C loss and decreased the temporal variation in litter N dynamics. Second, soil fauna was correlated with the abundance of the invasive pathogen. This latter surprising result deserves further exploration to unveil whether this relationship is due to a trophic relationship or whether both respond to similar environmental conditions. Overall, our work reveals that exploring drivers of litter decomposition under natural conditions is fundamental to understand the role of soil biodiversity on modulating global change effects on nutrient cycling processes.

Pablo Homet Instituto de Recursos Naturales y Agrobiología de Sevilla, Consejo Superior de Investigaciones Científicas (IRNAS-CSIC), Spain

You can read the full article online: Soil fauna modulates the effect of experimental drought on litter decomposition in forests invaded by an exotic pathogen

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