View larger version (38K):
[in a new window]
|
Fig. 1. Schematic of photosynthate fate (modified from Smucker, 1984). The C in the various pools (stable SOM [soil organic matter], decomposing tissue, soil organisms, soluble C pools, exudates, etc.) contributes to soil organic C. The rate and amount of translocation depends on age, nutritional status, and genetics of the plant. Labeling studies indicate about 50 to 70% of photosynthetic C remains above ground and 20 to 30% is translocated below ground. Of the translocated C, about 50% is used for root growth, and about 30% moves into the rhizosphere or is released during root respiration (Johnen and Sauerbeck, 1977; Kuzyakov and Cheng, 2001; Kuzyakov 2002a, 2002b). Molina et al. (2001) predicted that 24% of the net C fixed by photosynthesis became rhizodeposition, which is C found in the rhizosphere derived from root exudates, mucigel, and sloughed cells. Rhizodeposition is a critical component for driving the belowground food web. Respired CO2 can be recycled into roots via nonphotosynthetic carboxylation of phosphoenol pyruvate using HCO3– to form oxaloacetate by the ubiquitous plant enzyme phosphoenol pyruvate carboxylase (Latzko and Kelly, 1983). Nonphotosynthetic C fixation may be important for balancing C in roots (Vuorinen et al., 1992). The decomposition and humification of plant and faunal material forms stable SOM. Further decomposition of SOM releases free C compounds and, ultimately, organic C is converted into inorganic CO2.
|
