Background and aims Rhizodeposition of plants is the most uncertain component of the carbon (C) cycle. By existing approaches the amount of rhizodeposition can only roughly be estimated since its persistence in soil is very short compared to other organic C pools. We suggest an approach to quantify rhizodeposition at the field scale by assuming a constant ratio between rhizodeposited-C to root-C. Methods Maize plants were pulse-labeled with 14 CO 2 under controlled conditions and the soil 14 CO 2 efflux was separated into root and rhizomicrobial respiration. The latter and the 14 C activity remaining in the soil corresponded to total rhizodeposition. By relating rhizodeposited-14 C to root-14 C a rhizodeposition-toroot ratio of 0.56 was calculated. This ratio was applied to the root biomass C measured in the field to estimate rhizodeposition under field conditions. Results Maize allocated 298 kg C ha −1 as root-C and 166 kg C ha −1 as rhizodeposited-C belowground, 50 % of which were recovered in the upper 10 cm. The fate of rhizodeposits was estimated based on the 14 C data, which showed that 62 % of total rhizodeposition was mineralized within 16 days, 7 % and 0.3 % was incorporated into microbial biomass and DOC, respectively, and 31 % was recovered in the soil.Conclusions We conclude that the present approach allows for an improved estimation of total rhizodeposition, since it accounts not only for the fraction of rhizodeposits remaining in soil, but also for that decomposed by microorganisms and released from the soil as CO 2 .

Summary The complexity of soil food webs and the cryptic habitat hamper the analyses of pools, fluxes and turnover rates of carbon (C) in organisms and the insight into their interactions. Stable isotope analysis has been increasingly used to disentangle soil food web structure, yet it has not been applied to quantitatively characterize C dynamics at the level of the entire soil food web. The present study employed 13CO2 pulse labelling to investigate the incorporation of maize root‐derived C into major soil compartments and food web players in an arable field for 25 days. Bulk tissue and compound‐specific (lipids) C isotope ratios were used to quantify pool sizes and 13C incorporation in bacteria and fungi as primary decomposers, nematodes as key drivers of the microfood web and decomposers and predators among the meso‐ and macrofauna. About 20% of the C assimilated by maize was transferred to below‐ground pools. 13C was predominantly incorporated into rhizosphere micro‐organisms rather than in those of the bulk soil. 13C in phospholipid fatty acid biomarkers revealed that root‐derived C was incorporated into the soil food web mainly via saprotrophic fungi rather than via bacteria. Only small amounts of 13C were transferred to higher trophic levels, predominantly into fungal‐feeding nematodes and macrofauna decomposers. Most importantly, C pool size and 13C incorporation did not match closely. Although the fungal C stock was less than half that of bacteria, C transfers from fungi into higher trophic levels of the fungal energy pathway, that is fungal‐feeding nematodes and meso‐ and macrofauna decomposers, by far exceed that of bacterial C. This challenges previous views on the dominance of bacteria in root C dynamics and suggests saprotrophic fungi to function as major agents channelling recent photoassimilates into the soil food web.

To investigate the energy, matter and reactive and non-reactive trace gas exchange between the atmosphere and a spruce forest in the German mountain region, two intensive measuring periods were conducted at the FLUXNET site DE-Bay (<i>Waldstein-Weidenbrunnen</i>) in September/October 2007 and June/July 2008. They were part of the project "ExchanGE processes in mountainous Regions" (EGER). Beyond a brief description of the experiment, the main focus of the paper concerns the coupling between the trunk space, the canopy and the above-canopy atmosphere. Therefore, relevant coherent structures were analyzed for different in- and above canopy layers, coupling between layers was classified according to already published procedures, and gradients and fluxes of meteorological quantities as well as concentrations of non-reactive and reactive trace compounds have been sorted along the coupling classes. Only in the case of a fully coupled system, it could be shown, that fluxes measured above the canopy are related to gradients between the canopy and the above-canopy atmosphere. Temporal changes of concentration differences between top of canopy and the forest floor, particularly those of reactive trace gases (NO, NO₂, O₃, and HONO) could only be interpreted on the basis of the coupling stage. Consequently, only concurrent and vertically resolved measurements of micrometeorological (turbulence) quantities and fluxes (gradients) of trace compounds will lead to a better understanding of the forest-atmosphere interaction

Abstract. Carbon dioxide flux measurements in ecosystem sciences are mostly conducted by eddy covariance technique or the closed chamber method. But there is a lack of detailed comparisons that assess present differences and uncertainties. To determine underlying processes, a 10-day, side-byside measurement of the net ecosystem exchange with both techniques was evaluated with regard to various atmospheric conditions during the diurnal cycle. It was found that, depending on the particular atmospheric condition, the chamber carbon dioxide flux was either (i) equal to the carbon dioxide flux measured by the reference method eddy covariance, by day with well-developed atmospheric turbulence; (ii) higher, in the afternoon in times of oasis effect; (iii) lower, predominantly at night while large coherent structure fluxes or high wind velocities prevailed; or (iv) showed less variation in the flux pattern, at night while stable stratification was present. At night -when respiration forms the net ecosystem exchange -lower chamber carbon dioxide fluxes were found. In the afternoon -when the ecosystem is still a net carbon sink -the carbon dioxide fluxes measured by the chamber prevailed. These two complementary aspects resulted in an overestimation of the ecosystem sink capacity by the chamber of 40 % in this study.

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