Carbon dynamics subsequent to establishment of switchgrass

It is necessary to develop new, clean and renewable biofuels to mitigate the greenhouse effect and reduce dependence on nonrenewable fossil fuels. Switchgrass (Panicum virgatum L.) has been chosen by the US Department of Energy as its model herbaceous energy crop in the Southeast due to high yields, low production costs, economic benefits, and little competition from existing enterprises. However, knowledge of its influence on soil carbon (C) dynamics is limited, as no systematic study of soil C dynamics in switchgrass culture has been conducted. Our objective was to determine C dynamics subsequent to switchgrass establishment and the impact of cultural practices (row spacing and harvest frequency) on C biogeochemical characteristics in a sandy loam soil (Typic Paleudult) and a clay loam soil (Typic Hapludult). Results indicated that soil C characteristics changed over time after switchgrass establishment. Carbon mineralization, microbial biomass C, C turnover, and % microbial biomass C in organic C was generally higher approximately 2 years after switchgrass planting than after its initial establishment in a sandy loam soil. Specifically, C mineralization increased by 112 and 254% at depths of 0–15 cm and 15-30 cm, respectively; microbial biomass C increased by 168% in the top 15 cm of soil; and C turnover increased by 116 and 255% at 0-15 cm and 15–30 cm, respectively. Microbial biomass comprised 0.76±0.09% and 0.75±0.19% of soil organic C in 0–15 cm and 15–30 cm depths, respectively, of sandy loam soil, while it made up 1.37±0.22% and 1.11±0.19% of soil organic C in the same soil depths in clay loam soil. Harvesting once resulted in more C turnover than harvesting twice in the sandy loam soil. Linear regression between soil C characteristics and switchgrass root weight, root C, and root nitrogen (N) returned to soil support the conclusion that soil C accumulation is positively related to root input. It appears that switchgrass establishment may have dual benefits as a source of renewable energy and as a means of improving soil quality. However, longer periods of study will be required to elucidate substantial gains in soil quality owing to switchgrass culture.