Fate and dynamics of recently fixed C in pasture plant-soil system under field conditions

The flow of photosynthetically fixed C from plants to selected soil C pools was studied after (CO2)-C-13 pulse labeling of pasture plants under field conditions, dynamics of root-derived C in soil was assessed and turnover times of the soil C pools were estimated. The transport of the fixed C from shoots to the roots and into the soil was very fast. During 27 h, net C belowground allocation reached more than 10% of the fixed C and most of the C was already found in soil. Soil microbial biomass (C-MIC) was the major sink of the fixed C within soil C pools (ca 40-70% of soil C-13 depending on sampling time). Significant amounts of C-13 were also found in other labile soil C pools connected with microbial activity, in soluble organic C and C associated with microbial biomass (hot-water extract from the soil residue after chloroform fumigation-extraction) and the C-13 dynamics of all these pools followed that of the shoots. When the labelling (2 h) finished, the fixed C-13 was exponentially-lost from the plant-soil system. The loss had two phases; the first rapid phase corresponded to the immediate respiration of C-13 during the first 24 h and the second slower loss was attributable to the turnover of C-13 assimilated in C-MIC. The corresponding turnover times for C-MIC were 1.1 days and 3.4 days respectively. Such short turnover times are comparable to those measured by growth kinetics after the substrate amendment in other studies, which indicates that microbial growth in the rhizosphere is probably not limited by substrate availability. Our results further confirmed the main role of the soil microbial community in the transformation of recently fixed C, short turnover time of the easily degradable C in the rhizosphere, and its negligible contribution to more stable soil C storage.