Turnover and stabilization of root derived carbon and nitrogen in soils along climate and land-use gradients in the Biodiversity Exploratories.

Emily Solly1, Schrumpf Marion1, Schöning Ingo1, Trumbore Susan1
1 Biogeochemical Processes, Max Planck Institute for Biogeochemistry

O 2.4 in Environmental controls on fluxes and processes in ecosystems

17.07.2014, 10:00-10:20, H18

Root inputs have increasingly been shown to have a key role in the formation of soil organic matter, the largest active terrestrial reservoir in the global carbon cycle storing about 1,500 Pg of carbon in the upper meter of the mineral soil (1 Pg=1015 g). The amount of organic matter stored in soils is not permanent but dependent on a number of environmental factors which regulate the balance between the inputs, mainly through root litter, and the outputs through decomposition. But how much root litter is stabilized in the soil? How long does it stay there? And how is its residence time influenced by climate and land-use change? Answering these questions is important given the need to improve our knowledge of belowground nutrient fluxes and develop predictive models to understand how ecosystems will respond to environmental change. 

We performed field experiments in 150 grassland and 150 forests sites managed with different land-use intensity and located in three German regions of the interdisciplinary project Biodiversity Exploratories. Each region differs in climate and soil properties. To quantify the contribution of decomposing roots to the formation of stable mineral associated matter, and how much is transferred to dissolved organic matter we placed 1 g of 13C,14C and 15N homogenously labelled root litter in sieved-soil filled lysimeters. We used density fractionations in combination with isotopic analyses to determine the residual particulate root litter and its contribution to the mineral associated organic matter. We determined the decomposition rates of fine root litter and how they are affected by environmental site conditions and litter chemistry, by burying 5112 litterbags between 5 and 35 cm soil depth.

With our experiments we were able to quantify the amount of root derived C and N incorporated in the stable soil organic matter. We observed that these quantities vary according to the local soil texture. Our results indicate that at the regional scale root decomposition is influenced by soil moisture, soil temperature and soil nutrient content which together explain 24% of the variance of the decomposition rates in forests and 34% in grasslands. Additional variation is explained by root litter quality (11% in forests and 15% in grasslands). We found that land-use, in particular N addition through fertilization, decreases the rates of fine root decomposition in grasslands mediated by changes in the quality of the litter.

We conclude that the fraction of root litter which is stabilized strongly depends on the physical texture of the soils; while the amount of labile root litter that is decomposed is mainly controlled by the local climate and root chemical composition. Moreover, land use affects root turnover both directly through its influence on litter quality, but also indirectly by changes in soil properties and plant species composition. These findings improve our ability to predict and model belowground nutrient fluxes.

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Letzte Änderung 19.06.2014