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Thus, soil organic C storage is linked to N availability and can result in N immobilization in SOM with feedbacks to plant productivity. In-N-Out: A hierarchical framework to understand and predict soil carbon storage and nitrogen recycling Conversely, when N is available in excess of plant and microbial demand, the C and N cycles decouple, and N is lost from the system. We further hypothesize that C input-limited systems have N in excess of demand, resulting in decoupling of the N and C cycles, and the opening of N cycle to losses from the soil. [Extracted from the article]

A meeting of plant physiologists, ecologists and soil scientists examined the hypothesis that the changed chemistry of dead leaves, grown in an environment high in CO2, will mean that there will be a decline in the decomposition rate. It was concluded that there was insufficient evidence to support such a hypothesis. However other potential feedbacks on nitrogen availability are still important and there is still a need to distinguish the processes controlling decomposition in Co2-enriched ecosystems.

Elevated CO2 increases intrinsic water use efficiency ( WUE i) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood., We combined tree ring analysis with isotope measurements at three Free Air CO2 Enrichment ( FACE, POP- EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used δ13C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO2 effects on stomatal conductance were explored using δ18O as a proxy., Across all the sites, elevated CO2 increased 13C-derived water-use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms., Our findings provide a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modelling elevated CO2 and climate impacts on forest productivity, carbon and water balances. [ABSTRACT FROM AUTHOR]

Abstract: The palatability to isopods and microbes of a broad range of hardwood leaf litter, derived from three field CO2-enrichment experiments in the USA, was investigated, using δ 13C, to trace the C flow from litter to isopods and to CO2 respired by microbial decomposition. Leaf litter grown under elevated CO2 had δ 13C values ranging from −39 to −45‰, which were significantly different from ambient litter δ 13C values of around −30‰. Litter palatability to isopods of the Porcellio sp. was tested by incubating ambient- and elevated-CO2 litter, and a mixture of the two, in the presence of isopods for 14 days, under environmentally controlled conditions; δ 13C was measured on litter and isopods'' body before and after incubation. In an additional experiment, litter was incubated in the absence of fauna for 30 days, and on five occasions the δ 13C of the CO2 respired from litter was measured. The 13C label was clearly carried from the litter source to the isopods'' bodies, and their faeces. For microbial-respired CO2, δ 13C was significantly higher than that of the litter source, suggesting preferential degradation of substrates enriched in 13C as compared to those in the overall litter. With the exception of Quercus myrtifolia leaf litter, elevated CO2 did not affect the palatability to isopods nor the microbial degradation of any of the litters, possibly as a result of unaltered litter N concentration. However, significant differences in litter palatability and decay rates were observed among the different species. With this study, the use of isotopically labelled litter material was confirmed as a key methodology that can significantly contribute to the advancement of the understanding of litter decomposition and of the quantification of C fluxes in the process. [Copyright &y& Elsevier]

Abstract: Owing to the continuously increasing concentration of atmospheric CO2, it has become a priority to understand if soil organic matter (SOM) will behave as a sink or a source of CO2 under future environmental changes. Although many studies have addressed this question, a clear understanding is still missing, particularly with respect to long-term responses. In this study, we quantified soil C stores and dynamics in relationship to soil aggregation and pool composition in a Californian chaparral ecosystem exposed for 6 years to a gradient of atmospheric CO2 concentrations, ranging from pre-industrial levels 250 to 750μll−1 CO2. Fossil fuel-derived CO2 depleted in 13C was used for the fumigation, thus providing a tracer of C input from the vegetation to the soil. Long-term CO2 exposure invariably affected soil aggregation, with a significant decrease in the macroaggregate fraction at highest CO2 levels relative to the other two size fractions (i.e. microaggregates and silt and clay). This soil structural change most likely reduced the stability and protection of SOM, and C content generally decreased in most fractions over the CO2 treatments, and induced faster turnover of recently fixed C at high CO2 levels. The strongest response was found in the C content of the microaggregates, which decreased significantly () with rising levels of CO2. We conclude that increasing atmospheric CO2 concentrations will decrease soil C in chaparral ecosystems, and that the microaggregate fraction is the most responsive to increasing concentrations of atmospheric CO2. [Copyright &y& Elsevier]

Soil fauna have been found to accelerate litter decomposition rates across many ecosystems, but little is known about their impact on soil organic matter formation during decomposition and their influence on ecosystem carbon and nitrogen cycling during this process. In a three-year litterbag-free decomposition study, we suppressed microarthropod abundance by 38% and tracked the fate of 13C- and 15N-labeled litter into different soil organic matter fractions and the microbial community. Microarthropod suppression slowed litter mass loss and decreased litter carbon input into the soil and soil microbes during the first 18 months of decomposition. The microarthropod suppression did not alter the total amount of carbon and nitrogen incorporated in the soil after complete surface litter mass loss. However, lower early-stage microbial carbon uptake due to lower early-stage litter inputs to the soil, as well as a significant decrease in the C:N ratio of litter-derived organic matter inputs to the mineral soil fractions, made less nitrogen available for plant uptake in the microarthropod suppression treatment. Thus, the acceleration of early-stage, more labile litter inputs to the soil altered the timing and availability of carbon and nitrogen inputs to the soil. A simulation of these effects on the tallgrass prairie ecosystem using the DayCent model predicts lower net primary productivity and lower total soil C and N mineralization when soil microarthropods are less abundant. Our results highlight the importance of soil microarthropods for ecosystem functioning through their role in transforming decomposing litter organic matter into soil organic matter and the feedback of this process to ecosystem productivity and soil C sequestration. [ABSTRACT FROM AUTHOR]

A new principle for measuring soil CO2 efflux at constant ambient concentration is introduced. The measuring principle relies on the continuous absorption of CO2 within the system to achieve a constant CO2 concentration inside the soil chamber at ambient level, thus balancing the amount of CO2 entering the soil chamber by diffusion from the soil. We report results that show reliable soil CO2 efflux measurements with the new system. The novel measuring principle does not disturb the natural gradient of CO2 within the soil, while allowing for continuous capture of the CO2 released from the soil. It therefore holds great potential for application in simultaneous measurements of soil CO2 efflux and its δ13C, since both variables show sensitivity to a distortion of the soil CO2 profile commonly found in conventional chamber techniques. [Copyright &y& Elsevier]

Elevated CO₂ increases intrinsic water use efficiency (WUE(i) ) of forests, but the magnitude of this effect and its interaction with climate is still poorly understood. We combined tree ring analysis with isotope measurements at three Free Air CO₂ Enrichment (FACE, POP-EUROFACE, in Italy; Duke FACE in North Carolina and ORNL in Tennessee, USA) sites, to cover the entire life of the trees. We used δ¹³C to assess carbon isotope discrimination and changes in water-use efficiency, while direct CO₂ effects on stomatal conductance were explored using δ¹⁸O as a proxy. Across all the sites, elevated CO₂ increased ¹³C-derived water-use efficiency on average by 73% for Liquidambar styraciflua, 77% for Pinus taeda and 75% for Populus sp., but through different ecophysiological mechanisms. Our findings provide a robust means of predicting water-use efficiency responses from a variety of tree species exposed to variable environmental conditions over time, and species-specific relationships that can help modelling elevated CO₂ and climate impacts on forest productivity, carbon and water balances., (© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.)