Root dynamics of bioenergy crops: Scaling from the field to landscapes

Background/Question/Methods

Belowground processes controlling biogeochemical cycles in bioenergy feedstock production systems will drive impacts to ecosystem services derived from soil carbon (C) capture.   Soil C inputs occur through root production, turnover, and decomposition; an understanding of the controls on these processes is vital for parameterizing ecosystem- and landscape-scale models needed for predicting regional carbon storage. These approaches require a greater understanding of the interactions between edaphic factors and the dynamics of root systems across landscapes.   Additionally, estimates of potential long-term soil C storage rates for perennial systems such as switchgrass cannot be accurately determined at present as the processes of root production and turnover have not been previously reported.  The objective of this study was to estimate annual root production and turnover of two contrasting bioenergy cropping systems (continuous corn and switchgrass) in the top 30 cm of soil on five landscape positions located across a topographic gradient.  Annual root productivity was determined from root ingrowth cores, and root turnover was estimated in switchgrass to determine annual inputs to soil organic C pools.  Additionally, fine root decomposition after one month were assessed using intact soil cores.

Results/Conclusions

Results show significant differences in root production among the three cropping systems, with switchgrass having the highest level of belowground production.  Landscape position affected root productivity differently depending on cropping system; continuous corn showed significantly higher rates of root growth in the backslope position, while landscape position did not significantly affect switchgrass root productivity. Averaged across all landscape positions, switchgrass root production was found to be nearly double that of corn.  Turnover of switchgrass roots was not affected by landscape position, but differed between the top 20 cm compared to the deeper 20-30 cm soil depth.  Turnover rate was twice as fast in the in the 20-30 cm depth compared the upper 20 cm of soil.  Switchgrass root decomposition averaged 19% of total root biomass after one month, while corn root decomposition rates were similar but represented a much larger proportion (46%) of total root biomass.  Our results provide critical data on variation on root dynamics of bioenergy feedstocks and will be useful for the parameterization of biogeochemical and predictive C storage models important for producing regional estimates of soil C sequestration and informing global climate forcing models.