S-CLIMA · Drought impacts on plant-soil interactions and ecosystem stability

Drought impacts on plant-soil interactions and ecosystem stability

The overall aim of the S-Clima project was to characterise the mechanisms by which drought and affects plant-soil interactions and ecosystem stability in Mediterranean habitats. The project began with a review of the literature on root exudation under drought, followed by two large-scale drought and nutrient (nitrogen and phosphorus) manipulation experiments including the measurement of a wide range of plant and soil traits and responses. The plant species used in the project are two tree species found in the Mediterranean region. Holm oak (Quercus ilex) is very characteristic throughout the whole area and is known to be relatively drought-tolerant. Scots pine (Pinus sylvestris) is an economically important species and is less tolerant to water stress, and its distribution within in the Mediterranean comprises habitats at high altitudes and at the northern limits of the biome. The traits measured encompassed a large range of scales, to enable a good mechanistic understanding of the relevant plant-soil interactions. The capacity for plant and soil traits to recover after a period of optimum conditions was also assessed. Above-ground plant performance under drought was assessed through measures of photosynthesis and chlorophyll fluorescence. Rhizodeposition (and in particular root exudation) was measured using a recent in situ measuring technique to estimate how the amount of carbon entering the soil from the roots changes under different water and nutrient regimes. Changes in the composition of exudates were assessed using modern metabolomics methods. Metabolomics techniques were also used to monitor impacts of the experimental treatments on leaves and soils. In relation to soil-based measurements, three methods were used to assess changes in microbial populations. These were (1) DNA sequencing of bacteria and fungi, (2) phospholipid fatty acid (PLFA) analysis of microorganisms, and (3) use of Biolog Eco-plates to assess community level changes in the soil microbiota. The final results of the project will have impacts on both the scientific community and also in wider society. The first result of the project was the publication of a literature review of the effects of drought on rhizodeposition under drought and the consequences for soil communities and ecosystem resilience. Summarizing the previous knowledge in this area showed that moderate drought tends to lead to an increase in rhizodeposition (when standardised by plant biomass) but when drought is more extreme it leads to more variable responses. Regarding the experimental work, data from the first experiment indicated that the soil microbial community is more resistant to water stress than the plant community and had a higher capacity for recovery (shown by above-ground plant measurements). The community level analysis of the microbial community indicated that drought led to changes in soil community diversity and abundance. Increasing drought also led to differences in the relative abundance of fungi and bacteria in the soil, which has implications for soil function. Soil microbial DNA analyses will provide further information on the relationship between species diversity and functional diversity of microbial communities, and if function of soil microbes can be maintained under drought. Information about how soil communities change under drought is needed to inform landowners about risks to maintaining soil performance. This project also has strong potential for helping to predict impacts of drought on Mediterranean forests and their soils, in particular being able to quantify at what level of drought the effects are irreversible. The project was successful in measuring the amount of root exudation in two species that have been little-studied in this regard before, and are economically and socially important. Exudate composition was also successfully measured and changes related to water stress have been characterised for the first time in these species. These results will inform the future scientific research on the relationships between trees and soils, and in particular will offer new avenues of investigation for forestry scientists to pursue whilst trying to increase resilience of forest systems. The second experiment, which incorporated nutrients, will enable us to document potential interactions between soil fertility and drought, which is an area that has been identified as a research priority. Increased use of fertilisers and high levels of pollution mean that nutrient imbalances are becoming more striking in many ecosystems throughout the world. Therefore, our results will be useful for informing predictions of how multiple drivers of environmental change interact with each other. Overall, our project has a strong focus on characterising and understanding soil resilience, which is the ability for soils to recover from disturbances such as water stress. Soils are increasingly being seen as a vital ecosystem service to protect, therefore this work potentially has wide socio-economic influence, particularly in agricultural and forestry sectors.

Data: CORDIS, © European Union