Abstract. The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotopeCorrespondence to: N. Brüggemann (n.brueggemann@fz-juelich.de) studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis. The second major part elaborates on plantinternal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual), including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between Published by Copernicus Publications on behalf of the European Geosciences Union. 3458 N. Brüggemann et al.: Plant-soil-atmosphere C fluxes C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO 2 dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above-and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO 2 fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as well as microbial community effects on C mineralization are reviewed. A further part of the paper is dedicated to physical interactions between soil CO 2 and the soil matrix, such as CO 2 diffusion and dissolution processes within the soil profile. Finally, we highlight state-of-the-art stable isotope methodologies and their latest developments. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or currently impede the interpretation of isotopic signals in CO 2 or organic compounds at the plant and ecosystem level. This review tries to identify present knowledge gaps in correctly interpreting carbon stable isotope signals in the plant-soil-atmosphere system and how future research approaches could contribute to closing these gaps.

The terrestrial carbon (C) cycle has received increasing interest over the past few decades, however, there is still a lack of understanding of the fate of newly assimilated C allocated within plants and to the soil, stored within ecosystems and lost to the atmosphere. Stable carbon isotope studies can give novel insights into these issues. In this review we provide an overview of an emerging picture of plant-soil-atmosphere C fluxes, as based on C isotope studies, and identify processes determining related C isotope signatures. The first part of the review focuses on isotopic fractionation processes within plants during and after photosynthesis. The second major part elaborates on plant-internal and plant-rhizosphere C allocation patterns at different time scales (diel, seasonal, interannual), including the speed of C transfer and time lags in the coupling of assimilation and respiration, as well as the magnitude and controls of plant-soil C allocation and respiratory fluxes. Plant responses to changing environmental conditions, the functional relationship between the physiological and phenological status of plants and C transfer, and interactions between C, water and nutrient dynamics are discussed. The role of the C counterflow from the rhizosphere to the aboveground parts of the plants, e.g. via CO₂ dissolved in the xylem water or as xylem-transported sugars, is highlighted. The third part is centered around belowground C turnover, focusing especially on above- and belowground litter inputs, soil organic matter formation and turnover, production and loss of dissolved organic C, soil respiration and CO₂ fixation by soil microbes. Furthermore, plant controls on microbial communities and activity via exudates and litter production as well as microbial community effects on C mineralization are reviewed. The last part of the paper is dedicated to physical interactions between soil CO₂ and the soil matrix, such as CO₂ diffusion and dissolution processes within the soil profile. From the presented evidence we conclude that there exists a tight coupling of physical, chemical and biological processes involved in C cycling and C isotope fluxes in the plant-soil-atmosphere system. Generally, research using information from C isotopes allows an integrated view of the different processes involved. However, complex interactions among the range of processes complicate or impede the interpretation of isotopic signals in CO₂ or organic compounds at the plant and ecosystem level. This is where new research approaches should be aimed at

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In this review, the current knowledge of nitrous oxide (N 2 O) emissions from soybean (Glycine max (L.) Merr.) ecosystems, particularly on postharvest N 2 O emissions, is summarized and controlling factors of postharvest N 2 O emissions from soybean ecosystems are discussed. A new biological method to mitigate N 2 O emission is also presented. The latest (2006) guidelines of the Intergovernmental Panel on Climate Change (IPCC) concluded that N 2 O emissions derived directly from biological nitrogen (N) fixation were not significant in legume crop ecosystems. The default N 2 O emission factor from biological N fixation was revised to zero, whereas the default N 2 O emission factor for the decomposition of legume crop residues (postharvest N 2 O emissions) was the same as that for nonlegume crop residues (1%). From previously measured field data, the percentage of N in soybean residue emitted as N 2 O after harvest was calculated to determine emission factors. Values ranged from 0.0-10.0% (average 1.3% ± 2.7%, median: 0.2%), indicating relatively low emission factors. The average emission factor calculated for N 2 O emissions from N in soybean residues was slightly higher than the default emission factor for N in crop residue specified in the guidelines. However, the volume of field-measured postharvest N 2 O emissions in soybean ecosystems is low compared with data for N 2 O emissions during the crop growing season. Previous field-measured data demonstrated that N 2 O emissions often peaked sharply immediately after the harvest. However, values for N 2 O fluxes in the currently available field data were determined on a weekly or monthly basis. This large time gap between samplings may have resulted in underestimation of postharvest N 2 O emissions in soybean ecosystems. More detailed field studies on postharvest N 2 O emissions from soybean ecosystems are needed. Nodule decomposition is the major source of postharvest N 2 O emissions in soybean ecosystems. A new microbiological method was recently developed to mitigate postharvest N 2 O emissions from soybean ecosystems utilizing N-fixing bacteria with increased N 2 O-reducing activity (increased expression of nosZ). This approach may potentially be applied to other leguminous crops and non-legume ecosystems. Ongoing research on the relationships between soil N 2 O emissions and nosZ may reveal a new method for N 2 O mitigation in the future.