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VULCAN Report Summary

Project ID: EVK2-CT-2000-00094
Funded under: FP5-EESD
Country: United Kingdom

Soils responses to climate change

A range of soil measurements have been made at all sites to both quantify the services shrubland soils provide in terms of nutrient supply, carbon sink and water regulation, and the potential effects of future climate change. The results have provided a unique dataset of this understudied ecosystem type, which can be used for future parameterisation of models.

In addition, the results from the climate change experimental treatments indicate that both warming and repeated summer drought will affect these services. Specifically, we have demonstrated the greater sensitivity of soils in northern regions to warming. Sensitivity of soil carbon loss (soil respiration) was greater by a factor of 6 at the Welsh site compared to sites in Spain and Italy. This was confirmed in the experimental treatments where a 1-degree temperature rise resulted in accelerated carbon loss in Wales and Denmark whereas a decline was observed in all other sites. Soil moisture thresholds were identified for all sites and were found to vary with water limitation of respiration at 5.5% to 25% g/g wet soil whilst excess water supressed soil respiration above 55% g/g wetsoil. Repeated summer drought generally decreased soil carbon loss except in Wales where stimulation in soil carbon loss was observed. Unexpectedly, there was also evidence of persistent reduced soil moisture in drought plots in both Wales and the Netherlands throughout the year suggesting either increased plant water use or a structural change in soil, which reduces soil moisture holding capacity. Thus, in some northern soils this creates a longterm positive feedback with enhanced soil CO2 fluxes throughout the whole year beyond the period of the summer drought.

Net change in soil carbon storage also depends on the quantity and and fate of photosynthetically fixed carbon from plants. The effect of climate change on the quantity of litter is described in the plant work package. In the soils work package litter bag studies were used to identify the quantity of recently fixed carbon passed into long term stable soil organic matter pool. Warming increased the initial rate of litter decomposition, but did not affect the amount incorporated into the stable soil organic matter pool (known as the critical limit). We found plant species (i.e. litter quality) was the primary factor determining this critical limit thus shifts in overall plant productivity and/or a shift in species composition will be the key factors determining the input of crabon into the stable organic pool either offsetting or enhancing changes in carbon losses through soil respiration. Evidence from the Danish site suggests for northern sites there is a net reduction in the soil carbon sink as there has been a 15% drop in soil organic matter after 6 years of climate treatments.

In most systems, soils are important sources of plant nutrients. We quantified the controls on the release of nitrogen from the soil store and also identified the impact of climate change treatments. Measurable positive net nitrogen production (i.e. mineralisation) was only found when soil moisture was within above 20% g / g wet soil or below 60% g /g wet soil. Warming will have greatest effect between these two threshold. Increased summer drought is therefore likely to have greatest impact on nitrogen availability for plants in our wettest site in Wales where soil moisture content is usually above the 60% water excess threshold. We found no evidence of changes in phosphorus demand by plants in response to climate treatments.

The implications of these results are that northern sites are at greatest risk from both summer drought and warming with respect to all three services studied; carbon store, nutrient supply and water regulation emphasising the need for reductions in greenhouse gas emissions if these systems are to be protected. Further work is needed to combine the data from the plant work package to assess the overall ecosystem carbon balance for the different climate treatments. Treatments are planned to continue at all sites using national or other sources of funding. These results have been disseminated through attendance at a range of meetings, the VULCAN newsletters and a synthesis paper in the journal Ecosystems. A further paper is planned for submission in July 2005. A journal paper is also planned for the next VULCAN special issue.

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