ABSTRACT
Despite experimental limitations, numerous isotopic tracer studies have quantied the amount of C xed in photosynthesis that is subsequently partitioned belowground and then lost via exudation. Although there are numerous caveats in the interpretation of the results (Meharg, 1994; Nguyen, 2003; ¤ornton et al., 2004; Paterson et al., 2005), a meta-analysis of 271 studies undertaken by Jones et al. (2009) has revealed some consensus in C partitioning over a wide range of plant species and experimental conditions. Overall, it appears that when plants are fed with isotopically labeled 13CO2 or 14CO2, approximately 60% ± 10% of
the xed C (range 20%–98%) remains in the shoot, becoming incorporated into new growth and storage pools. ¤e remaining 40% ± 10% is transferred belowground to the root system. Within the root, the newly xed C has a number of fates including the transfer to new growing tips and symbionts (e.g., N2 xers, mycorrhizal fungi), the repair of older tissues, and loss in respiration or loss into the soil as gaseous, soluble, and insoluble exudates. Once in the soil, the released C can then undergo a number of fates including di¦usion away from the root, uptake and respiration by soil microorganisms, uptake and incorporation into new microbial cells, abiotic sorption to soil particles, abiotic mineralization by mineral oxides, leaching through the soil, volatilization from the soil surface, and recapture by the root itself or by neighboring roots (Figure 6.3; Martin, 1970; Jones and Darrah, 1994; Kuzyakov et al., 2003; Paterson et al., 2007; Jones et al., 2009). Separating these numerous ¥uxes, which occur concurrently and which possess many interrelated feedback loops, has proved impossible, and this has limited a holistic understanding of C ¥ow and the importance of individual processes. On balance, it appears that of the C that is transferred belowground, approximately 19% ± 8% remains in the root (range 5%–55%), 5% ± 1% is lost from the root and incorporated into soil organic matter (SOM) (range 2%–18%), and 12 ± 5 is lost as CO2 (Jones et al., 2009). ¤is respiratory loss arises from roots and from the microbial turnover of root exudates (range 2%–30%). Distinguishing between root and microbial respiration has been one of the major constraints to gaining an accurate measure of rhizodeposition in soil-grown plants. Using a range of experimental approaches, it is now believed that roots and microorganisms contribute equally to rhizosphere respiration (Kuzyakov, 2002a, 2006), an assumption that must be considered with caution. Carbon loss by rhizodeposition has therefore been estimated to be around 11% of the net xed C or 27% of C allocated to roots, which corresponds to 0.4-0.6 Mg C ha−1 year−1 in cereals (Kuzyakov and Domanski, 2000). ¤is can be compared to the average organic C content of a typical agricultural topsoil (0-30 cm) of 50 to 150 Mg C ha−1.
