Mycorrhizal hyphal turnover as a dominant process for carbon input into soil organic matter

The atmospheric concentration of CO2 is predicted to reach double current levels by 2075. Detritus from aboveground and belowground plant parts constitutes the primary source of C for soil organic matter (SOM), and accumulation of SOM in forests may provide a significant mechanism to mitigate increasing atmospheric CO2 concentrations. In a poplar (three species) plantation exposed to ambient (380 ppm) and elevated (580 ppm) atmospheric CO2 concentrations using a Free Air Carbon Dioxide Enrichment (FACE) system, the relative importance of leaf litter decomposition, fine root and fungal turnover for C incorporation into SOM was investigated. A technique using cores of soil in which a C-4 crop has been grown (delta C-13 -18.1 parts per thousand) inserted into the plantation and detritus from C-3 trees (delta C-13 -27 to -30 parts per thousand) was used to distinguish between old (native soil) and new (tree derived) soil C. In-growth cores using a fine mesh (39 mu m) to prevent in-growth of roots, but allow in-growth of fungal hyphae were used to assess contribution of fine roots and the mycorrhizal external mycelium to soil C during a period of three growing seasons (1999-2001). Across all species and treatments, the mycorrhizal external mycelium was the dominant pathway (62%) through which carbon entered the SOM pool, exceeding the input via leaf litter and fine root turnover. The input via the mycorrhizal external mycelium was not influenced by elevated CO2, but elevated atmospheric CO2 enhanced soil C inputs via fine root turnover. The turnover of the mycorrhizal external mycelium may be a fundamental mechanism for the transfer of root-derived C to SOM.