Assessing the Role of Plant Root Architecture on Soil Carbon Stabilization
Marie-Anne de Graaff
Atmospheric CO2 cycles into terrestrial ecosystems via the process of photosynthesis, and upon plant death this carbon (C) is deposited into the soil. This carbon may either be stored in the soil (stabilization) or respired back into the atmosphere as CO2 by microorganisms. Since atmospheric CO2 is a greenhouse gas and is a factor contributing to climate change, understanding the biological processes by which CO2 is stored in soil rather than respired back into the atmosphere is important for the development of efficacious climate change mitigation strategies. Plant roots regulate both soil carbon input and decomposition processes, and as such are a major component of the global C cycle. It is uncertain, however, whether differences in root architecture among plant species differentially impact soil C stabilization versus soil C respiration. Root systems with a relatively large abundance of small diameter roots are expected to enhance C respiration rather than C stabilization due to their positive impacts on microbial activity. In contrast, plants with a greater abundance of large diameter roots may contribute to soil C stabilization to a greater extent. This study aims to assess the effects of root system architecture on soil carbon stabilization. To assess how root architecture affects soil C sequestration, we measured how a variety of switchgrass cultivars with a range of root structures affect soil C input and stabilization in physically defined soil fractions (i.e. Coarse Particulate Organic Matter (CPOM), Fine Particulate Organic Matter (FPOM), silt, and clay]. The carbon abundance in each soil fraction was determined by collecting soil samples from three switchgrass cultivars with large diameter root architectures, and three switchgrass cultivars with small diameter root architectures. The switchgrass cultivars (C4 plants) were established on soils that supported a stand of C3 grasses for 36 years. Due to different photosynthetic pathways between C3 and C4 plants, there is a natural C13 isotopic difference between the soils and plants. This enables us to use a natural C isotope ratio technique to estimate the contribution of new root-derived C to soil C pools. The soil samples collected were cores (60 cm depth) collected directly over the crown of a switchgrass plant, with four replicate cores taken per cultivar. The cores were divided into depths of 0-10, 10-20, and 20-30 cm, and the soils were separated from the roots with a 2 mm sieve. Each soil depth category was then fractionated. Coarse-POM, and FPOM were fractionated using a wet sieve method, and silt and clay were fractionated by centrifugation. Samples of soil fractions across all depths are analyzed for carbon content δ-C13 signature. Preliminary data suggest that switchgrass cultivars differentially affect soil C sequestration; we are investigating whether any of these differences are associated with differences in root architecture.