Plant and microbial controls on ecosystem respiration

Ecosystem respiration (Reco ) is a major determinant of the net CO 2 exchange between ecosystems and the atmosphere (NEE) and thus plays an important role in the global carbon (C) cycle. While CO 2 flux networks provide increasing information on the bulk ecosystem respiration for different types of vegetation, there is still a limited functional understanding of Reco in relation to its components, and of how they are influenced by plant C assimilation, nutrient supply and by interactions between plants and microorganisms in the field. The proposed project intends to explore these questions for mountain grasslands differing in land use, including a meadow, a pasture and an abandoned grassland. The overall C balance of these selected sites has been studied within the EU- FP5-project Carbomont, which will permit an integration of complementary data sets. The study will include - an assessment of the seasonal variation of NEE, Reco and its components (soil, leaf, stem, root, and microbial respiration in litter, rhizosphere and bulk soil), and of litter decomposition, - in situ experimental testing of effects of shading and clipping on Reco and its components, and on the transfer of C from above to below-ground compartments ( 13C pulse labelling), - in situ experimental testing of effects of species composition (at the level of functional groups) and C/N ratios on litter decomposition and related microbial community structure and activity, - an assessment of microbial diversity and community structure at the grassland sites and their link to C fluxes, - the development of a process-based model linking C assimilation, non-structural carbohydrates and plant and microbial respiration. By focussing on the processes underlying ecosystem C fluxes in differently managed grasslands the proposed project will answer questions about the influence of land use and land management on short-term controls on C- sequestration of grasslands and contribute to an understanding of feedbacks between land-use changes and the mechanisms of above and below ground C-transfer processes.

Ecosystem respiration (Reco ) is a major determinant of the net CO 2 exchange between ecosystems and the atmosphere (NEE) and thus plays an important role in the global carbon (C) cycle. While CO 2 flux networks provide increasing information on the bulk ecosystem respiration for different types of vegetation, there is still a limited functional understanding of Reco in relation to its components, and of how they are influenced by plant C assimilation, nutrient supply and by interactions between plants and microorganisms in the field. The proposed project explored these questions for mountain grasslands differing in land use, including a meadow, a pasture and an abandoned grassland. A comparative study of NEE and its components across these mountain grasslands showed that land use and management affected seasonal NEE, canopy photosynthesis (GPP) and Reco , which all decreased from managed to unmanaged grasslands. While Reco and GPP were generally closely coupled during most of the growing season, Reco was less affected by land management (mowing, grazing) and season. A major effort was made to understand regional and local patterns of the by far largest component of Reco , soil respiration (Rsoil), and how it is driven by abiotic and biotic controls. Whilst climate has a major effect on soil CO 2 emissions, the importance of photosynthetic C input on respiratory activity in the soil has only started to be unravelled. This aspect was analysed in detail across a range of scales, indicating a very close coupling between photosynthesis and soil respiration at timescales from annual to diurnal. The latter was demonstrated with the first published study documenting the respiratory use of freshly assimilated C in soil processes for a grassland at unprecedented time resolution, combining isotopic pulse labelling and tunable diode laser spectroscopy. To account for slower processes involved in the C turnover in mountain grasslands differing in land use a range of litter experiments were conducted. Besides photosynthates, litter is the major input of C from plant into the soil, and thereby may decisively fuel microbial activity (and thus Reco ) as well the C sequestration by the soil. Experiments incubating a range of different litter mixtures from the three different grasslands with different substrates showed that the functional composition of litter affects that of associated microbial communities as well as non-additive effects on microbial substrate utilisation, and that litter quality (as reflected e.g. by C/N ratios), as well as plant functional group identity are of overriding importance in determining litter mass loss in grasslands differing in land use. In conclusion the project has shown how intimately plant and microbial processes are linked in the soil environment, and that photosynthetic activity is an important and immediate driver of ecosystem and soil respiration across a range of scales.