Estimation of rhizodeposition at field scale: upscaling of a C-14 labeling study

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. Maize plants were pulse-labeled with (CO2)-C-14 under controlled conditions and the soil (CO2)-C-14 efflux was separated into root and rhizomicrobial respiration. The latter and the C-14 activity remaining in the soil corresponded to total rhizodeposition. By relating rhizodeposited-C-14 to root-C-14 a rhizodeposition-to-root 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. 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 C-14 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. 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 CO2.