Think Deep: Understanding the Importance of Rooting Depth in the Carbon Sequestration Potential of Bioenergy Crops.

Sustainable management of agricultural systems requires maintenance of soil organic matter (SOM) levels, or increases in SOM where soils have been degraded. In agricultural systems, roots are typically the primary organic input to soil following harvest of crops. This is particularly true for cellulosic biofuel production, where the removal of aboveground residues can lead to significant reduction in SOM levels. Select bioenergy feedstocks may have the capacity to mitigate this impact through deep and extensive root production. Roots are primary contributors to SOM via fine root turnover and exudation, particularly in the deep soil, where over 50% of the total soil carbon (C) is stored. C sequestration in the deep soil can be an effective strategy to mitigate climate change and sustain SOM levels for agricultural production. Choosing bioenergy feedstocks, such as Sorghum bicolor, that can contribute to climate change mitigation will simultaneously increase their viability as bioenergy feedstocks. Several varieties of sorghum produce roots over 1-2m soil depth. However, we lack a basic understanding of how decomposition of root tissues leads to new SOM formation along the soil profile, particularly in the deep soil.

We present an innovative method to accurately quantify the contribution of roots to new SOM formation and/or loss of older SOM up to 90cm soil depth in situ. The isotopic enrichment of the roots allows us to follow the movement of decomposition products to physically and chemically stabilized SOM fractions. Soils will be fractionated by aggregate size class and then further fractionated by size and density, into primary organo-mineral fractions. This approach will provide accurate quantification of root and soil C and N dynamics at depth in agricultural systems. We will present data of these C and N flow dynamics in the deep soil from the first harvest of a two-year study.