Soil organic matter dynamics after afforestation of mountain grasslands in both a Mediterranean and a temperate climate |
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Authors: | Carlos Ortiz Eduardo Vázquez Agustín Rubio Marta Benito Andreas Schindlbacher Robert Jandl Klaus Butterbach-Bahl Eugenio Díaz-Pinés |
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Institution: | 1.Department of Agricultural Production,Technical University of Madrid,Madrid,Spain;2.Department of Natural Systems and Resources,Technical University of Madrid,Madrid,Spain;3.Department of Forest Ecology and Soils, Federal Research and Training Centre for Forests,Natural Hazards and Landscape - BFW,Vienna,Austria;4.Institute of Meteorology and Climate Research,Karlsruhe Institute of Technology,Garmisch-Partenkirchen,Germany |
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Abstract: | We studied the effect of mountain grassland afforestation with conifer trees (Pinus sylvestris, Picea abies and Pinus cembra) on soil organic matter (SOM) cycling and carbon (C) isotopic composition in two contrasting climate areas using a regional approach. Seventeen paired sites (each containing at least 40 years prior afforested and grassland plots) were investigated in the mountains of Central Spain and Western Austria. Topsoil CO2 effluxes were monitored under standardized conditions for six months as a proxy for soil organic carbon (SOC) mineralisation. The bulk C and nitrogen (N) concentrations and their isotopic composition in the soil and in the plants were assessed. The soil C:N ratio was consistently greater after afforestation in both regions, which in Spain was caused by a significant decrease in N concentration. No consistent effect was found on mineralisation rates due to vegetation change. Afforestation produced a more consistent soil 13C enrichment in the Spanish than in the Austrian sites. Our work strongly suggests that increasing altitude in Mediterranean mountain grasslands alleviates water limitation, favouring both plant growth and SOM decomposition, and ultimately accelerating C cycling. In contrast, temperate grassland areas at high altitudes were associated with severe temperature limitations, which constrained SOM transformation processes. In spite of the impact of afforestation on soil biogeochemical processes, C concentrations were marginally affected. We therefore conclude that grassland conversion to coniferous forests does not enhanced C sequestration in the mineral soil, for at least 40 years after land-use change. |
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