Distribution of recently fixed photosynthate in a switchgrass plant-soil system

The use of switchgrass (Panicum virgatum L.) as an energy crop has gained great importance in past two decades due to its high biomass yields on marginal lands with low agricultural inputs and low maintenance requirements. Information on the allocation of photosynthetically fixed C in the switchgrass-soil system is important to understand the C flow and to quantify the sequestration of C in soils. The allocation of 13C labeled photosynthates in shoot, root, soil, and in microbial biomass carbon (MBC) of rhizosphere and bulk soil of 45 days old, greenhouse grown-switchgrass was examined during 20 days 13C-CO2 pulse labeling period. The total 13C recovered in the plant-soil system varied from 79% after 1 day to 42% after 20 days of labeling. After labeling, 54%, 40%, and 6% excess 13C resided in shoot, root and soil, respectively on day 1; 27%, 61% and 11%, respectively on day 5 and 20%, 63% and 17%, respectively day 20 after labeling. The maximum incorporation of 13C from roots into the MB of rhizosphere soil occurred within the first 24 h of labeling. The excess 13C values of rhizosphere soil and rhizosphere MBC were significantly higher than excess 13C values of bulk soil and the bulk soil MBC, respectively. The proportion of excess 13C in soil as MBC declined from 92 to 15% in rhizosphere soil and from 79 to 18% in bulk soil, for 1 day and 20 days after labeling, respectively. The present study showed the effectiveness of 13C labeling to examine the fate of recently photosynthesized C in soil-plant (switchgrass) system and dynamics of MBC.