Microbial immobilisation of 13C rhizodeposits in rhizosphere and root-free soil under continuous 13C labelling of oats

Abstract

A greenhouse experiment was conducted by growing oats ( Avena sativa L.) in a continuously 13CO 2 labeled atmosphere. The allocation of 13C-labeled photosynthates in plants, microbial biomass in rhizosphere and root-free soil, pools of soil organic C, and CO 2 emissions were examined over the plant's life cycle. To isolate rhizosphere from root-free soil, plant seedlings were placed into bags made of nylon monofilament screen tissue (16 μm mesh) filled with soil. Two peaks of 13C in rhizosphere pools of microbial biomass and dissolved organic carbon (DOC), as well as in CO 2 emissions at the earing and ripeness stages were revealed. These 13C maxima corresponded to: (i) the end of rapid root growth and (ii) beginning of root decomposition, respectively. The δ 13C values of microbial biomass were higher than those of DOC and of soil organic matter (SOM). The microbial biomass C accounted for up to 56 and 39% of 13C recovered in the rhizosphere and root-free soil, respectively. Between 4 and 28% of 13C assimilated was recovered in the root-free soil. Depending on the phenological stage, the contribution of root-derived C to total CO 2 emission from soil varied from 61 to 92% of total CO 2 evolved, including 4–23% attributed to rhizomicrobial respiration. While 81–91% of C substrates used for microbial growth in the root-free soil and rhizosphere came from SOM, the remaining 9–19% of C substrates utilized by the microbial biomass was attributable to rhizodeposition. The use of continuous isotopic labelling and physical separation of root-free and rhizosphere soil, combined with natural 13C abundance were effective in gaining new insight on soil and rhizosphere C-cycling.