Belowground Carbon Cycle of Napier and Guinea Grasses Grown for Biofuel Feedstock Production.

Soil carbon (C) sequestration may partially offset rising atmospheric CO2 concentration. Napier grass (Pennisetum purpureum) and Guinea grass (Panicum maximum), in particular, are perennial C4 grasses with potential to sequester soil C while simultaneously provide aboveground biomass for energy production. For 8 selected grass varieties, the study examined the quantity and quality of C input to belowground and how these factors affect differences in SOC after three cycles of ratooning. Mass balance approach to measure total amount of C plant send belowground was used as quantity variable, whereas decay constant was used as quality of C input into soil. It was hypothesized that (1) grass accessions varies in quantity of pools and fluxes of C by aboveground yield, (2) root biomass from grass accessions varies due to root chemical properties, and (3) higher quantity and lower quality of belowground C input would sequester most SOC. Total belowground C fluxes ranged from 1507 to 1954 g C m^-2 yr^-1 with marginally significant differences among accessions. Soil C stock increased from 2010 to 2011, and highest increase of SOC was observed in Napier grass accession 2 with 7.2 % compared to initial SOC in 2010. As hypothesized, aboveground yield from November 2010 ratoon harvest was positively related to TBCF. Also, root decay constant was strongly and positively linked to initial root lignin concentration. Contrary to expected, soil C stock after three ratoon cycles were significantly and positively linked to root tissue lability. In order to expand beyond bivariate relationships, structural equation modeling was used to reveal causal relationships among multicollinear variables. The results suggested that microbial transformation of labile C in roots rather than chemical recalcitrance was important driver of soil C accumulation in the short-term; however, how this will translate to C sequestration in the long-term is still yet to be discovered.