Carbon Turnover in the Rhizosphere

Abstract Considerable progress has been made during the last decade towards understanding and quantifying the input and turnover of plant carbon in the rhizosphere. This was made possible by the development (partially by the authors) and combination of appropriate new methods, such as: –homogeneous labelling of whole plants with 14C –distinction between root and microbial respiration –separation of soil zones of known distances from the roots –determination of microbial soil biomass. These methods were applied to study the following aspects: –release of organic plant carbon into the soil by growing roots –utilization of this plant carbon by the microbial biomass in the rhizosphere –related influence on the turnover of soil organic matter, and –spatial range of such root influence in the soil. About 19% of the total photosynthetic production of the investigated plants was released into the rhizosphere as organic material. Most of this (15%) was transformed by the rhizosphere microorganisms into CO2, while only a small fraction (4%) remained in the soil, mainly as microbial cells (2.5%). As a result, microbial rhizosphere biomass increased considerably. Relative to the organic C‐input, however, the incorporation of root derived carbon by the microbial biomass was remarkably low (13%). Along with the increase in microbial rhizosphere biomass, the presence of plant roots also enhanced the decomposition of soil organic matter and affected soil aggregate stability. Root carbon and root influences were even detected up to 20 mm away from the roots. This may be partially attributed to the contribution of root derived volatiles. Accordingly, both the actual volume of the rhizosphere and its metabolic significance is greater than what has so far been assumed. Possible interactions involving root, soil and microbial carbon are discussed.