Decoding the rhizodeposit-derived carbon’s journey into soil organic matter

Net rhizodeposition corresponds to the portion of living root carbon (C) that remains in the soil after microbial processing and partial decomposition. Although it is assumed that this C input exerts an important role in the formation of soil organic matter (SOM), its contribution to distinct SOM pools is still not fully understood. In this study, we aimed to (i) quantify the retention of net rhizodeposition C in the different SOM fractions and in reactive Al and Fe mineral phases and (ii) investigate how rhizodeposition drives the spatial distribution of microbial communities in the rhizosphere. To track the transfer of net rhizodeposition into the soil, we used artificially labeled eucalypt (Eucalyptus spp.) seedlings under a 13C-CO2 atmosphere (multiple-pulse labeling). Combining physical SOM fractionation and the chemical extraction of aluminum (Al) and iron (Fe) reactive phases, we studied the distribution of net rhizodeposition into different soil fractions. We also assessed the 13C incorporation into microbial phospholipid fatty acids (PLFAs) at different distances from the roots. Our results show that 76 % of the net rhizodeposition 13C was retained within the mineral-associated organic matter (MAOM) fraction. About 28 % of net rhizodeposition 13C within the MAOM fraction was retained within the Al and Fe reactive phases, indicating that this is a sizeable mechanism for the retention of net rhizodeposition in soil. Rhizodeposition increased the abundance of microbial PLFAs exclusively in the soil close to the roots (0–4 mm), with prominent incorporation of net rhizodeposition 13C into fungal biomarkers. Overall, our findings underscore the importance of mineral associations for the retention of net rhizodeposition in the soil. We also highlight the role of fungi in transferring the root-derived C beyond the root vicinity and promoting the formation of occluded SOM.