Particulate Organic Matter Responses to Perennial Grass Production in Midwestern Soil

Abstract:

Terrestrial carbon sequestration is essential to mitigating atmospheric carbon dioxide levels. While annual row crop agriculture contributes to soil carbon loss in the Midwest, the establishment of perennial crops has the potential to increase soil carbon stocks through increased organic inputs and changes soil carbon pools and fluxes. Perennial grasses eliminate the need for tillage and increase belowground biomass, both critical to the conservation of soil organic matter and soil carbon sequestration. The effect of C₄ perennial grasses on particulate organic matter (POM) was evaluated in Illinois, where native switchgrass and a sterile hybrid of the Asian grass Miscanthus were planted at the University of Illinois Energy Farm in 2008. During 6 years after establishment of perennial crops, POM was compared with plots growing a corn-corn-soy rotation typical of the area and a 26-species restored prairie. POM concentrations increased for all crops between 67 and 79% over 6 years, with the greatest increases in prairie and miscanthus soils. POM concentrations were highest at the 0-10 cm depth, however POM accrued faster in the 10-30 cm depth. Isotopic analyses of POM material showed that after 6 years, POM carbon consisted of 22-33% C₄ material under perennial monoculture crops, indicating the incorporation of newly-established plant material to the POM fraction. As POM carbon is primarily plant-derived, increases in POM reflect increases in organic matter inputs as well as the cessation of tillage. While increases in POM under annual row crops reflect the incorporation of aboveground organic matter by tillage, POM increases in untilled perennial crops mirror increases in belowground biomass and the formation large soil aggregates, structures which protect POM carbon from microbial degradation and result in longer residence times for soil carbon. Therefore untilled soils under long-term perennial crop production provide an important environment for the storage and protection of carbon.