The fate of soyabean photosynthetic carbon varies in Mollisols differing in organic carbon

The fate of photosynthetically-fixed carbon (C) in the plant-soil-microbe continuum has received much interest because of its relevance to soil C and the global C cycle. However, information on the flow of this plant C below ground and its contribution to soil C sequestration in soils with contrasting organic C (C-org) is limited. In this study, soyabean (Glycine maxL. Merr.) was grown in three Mollisols with low (1.04%), medium (2.90%) and high (5.05%) C-org, respectively. Plants were labelled with (CO2)-C-13 to trace the photosynthetic C dynamics in the plant-soil system for up to 288hours. The total amount of net fixed C-13 by plants ranged from 66 to 78mgpot(-1), and there was no difference between soils. The amount of C-13 in soil organic matter (SOM) increased from 1.9 to 6.1mgpot(-1) over time in the high-C-org soil, while it showed a non-significant change with 2.2mgpot(-1) (on average) in the medium-C-org soil, and decreased from 2.9 to 0.1mgpot(-1) in the low-C-org soil. In the low-C-org soil, the amount of C-13 in soil microbes decreased markedly over time, showing a fast turnover, and had a significant correlation (P0.01) with C-13 in the SOM pool. However, such a relationship was not significant in the soil with high or medium C-org. These results indicate that most of the root-derived C in the low-C-org soil is degraded quickly by microbial activity, while the greater input of the photosynthetic C to SOM in the high- and/or medium-C-org soil can probably be attributed to physical sorption of root-derived C by SOM and minerals, thus protecting it against microbial decomposition.