Deeply Rooted: Evaluating Plant Rooting Depth as a Means for Enhanced Soil Carbon Sequestration

Project Overview

Soils store three times as much carbon (C) as the atmosphere, but are not at capacity, and enhanced soil C storage is considered an essential strategy to mitigate rising atmospheric carbon dioxide (CO2) levels. Agricultural soils have experienced substantial C loss in the past century because of poor agricultural practices and erosion. A shift toward deep-rooting crops and low-impact soil management could potentially increase long-term sequestration of C fixed by plants and stored in their root tissues, particularly for crops that have naturally deep root systems (greater than 1 meter). A substantial amount of the CO2 taken up by plants is allocated to their root systems, and because C deposited in deep soil layers has a longer residence time (up to millennia, in contrast to C deposited in topsoils), C increases at depth may have better long-term C sequestration potential than topsoils. However, the accrual, turnover, and stabilization of C in subsoils is a critical knowledge gap.

We investigated a deeply rooted plant, switchgrass (Panicum virgatum), as a means of increasing carbon stocks in marginal and agricultural soils. We hypothesized that deep (greater than 30 cm) soil organic carbon (SOC) stocks would be greater under bioenergy crops relative to stocks under shallow-rooted conventional crop cover. To test this hypothesis, we compared soil depth profiles beneath deeply rooted switchgrass (cultivated for 4 to 30 years) and paired shallow-rooted annual controls. We studied 12 field sites, 3 that were collected in 2018 before the start of the project as part a Department of Energy (DOE) Sustainable Biofuels study, and 9 that we collected in 2019 on a national field sampling campaign across the eastern U.S. In our publication from the 2018 study, which was written in collaboration with the Lawrence Livermore National Laboratory Soil Microbiome scientific focus area (SFA), we found that C stocks increased under switchgrass, but that the increases were dependent on soil texture. In the 2019 study, we found that carbon accrual tended to occur most consistently in low C soil in the southern U.S, which could indicate that perennial grasses may be a viable strategy to increase SOC in marginals soils in this region. We have published two studies from the 2019 sampling campaign thus far, we measured microbial growth parameters that will aid in modeling subsoil carbon cycling, and we found that switchgrass appears to move water upward in the soil profile and could promote drought tolerance, a phenomenon commonly performed by trees known as ‘hydraulic redistribution'. Our study is the first to show this can occur in deep-rooted grasses. Finally, we published a modeling paper in collaboration with the Livermore Soil Microbiome SFA, where we found that poorly crystalline minerals are abundant and strongly correlated with organic C in geographically limited zones with enhanced weathering rates. Our results will inform technological development in the agricultural carbon sequestration sector as well as future negative emissions policies.

Mission Impact

Our research directly supports Livermore's mission research challenge in energy and resource security, as well as the soil carbon pillar of the Director's Engineering the Carbon Economy Initiative for negative emissions—focused on quantifying and engineering soil systems that store C in agriculture and natural ecosystems. The C-cycle studies complement the earth and atmospheric science core competency by enabling improved resiliency of energy and environmental systems to climate change. The work leveraged unique resources supported by the nuclear, chemical, and isotopic science and technology core competency, via our use of high-throughput carbon-14 measurements at the Center for Accelerator Mass Spectrometry (CAMS), and Nuclear and Chemical Sciences Division's (NACS') unique hydrogen-3 isotopic tracer and Stable Isotope Probing capabilities. It also supports the bioscience and bioengineering core competency via our focus on bioenergy agriculture and understanding how root microbiomes shape environmental health. Our LDRD also established a fruitful collaboration with Dr. Asmeret Asefaw Berhe's lab at the University of California, Merced, which will continue as part of Nuccio's DOE Early Career award.

Our LDRD research addresses DOE's energy and environmental security missions. Its focus on bioenergy agriculture and understanding how root microbiomes shape soil and environmental health are directly relevant to the DOE Office of Science mission to promote the development of sustainable biofuels and to determine the mechanisms that control large-scale C cycling processes in terrestrial ecosystems. Our research also supports the NNSA goal to advance the science, technology, and engineering competencies that are the foundation of the NNSA mission, and our results will inform efforts to develop science and technology tools and capabilities to meet future national security challenges.

Publications, Presentations, and Patents

Min K, Slessarev E, Kan M, Pett-Ridge J, McFarlane K, Oerter E, Nuccio E, Berhe A. 2020. "Microbial growth kinetics under deeply- vs. shallow-rooted plants with soil depth profiles." AGU Fall Meeting (poster).

Min K., Slessarev E., Kan M., McFarlane K.J., Oerter E., Pett-Ridge J., Nuccio E.E., and A. Asefaw Berhe. 2021. "Active microbial biomass decreases, but microbial growth potential remains similar across soil depth profiles under deeply- vs. shallow-rooted plants." Soil Biology and Biochemistry. 162: 108401.

Oerter E., Slessarev E., Visser A., Min K., Kan M., McFarlane K.J., Saha M.C., Asefaw Berhe A., Pett-Ridge J., and E.E. Nuccio. 2021. "Hydraulic Redistribution by Deeply Rooted Grasses and its Ecohydrologic Implications in the Southern Great Plains of North America." Hydrological Processes. 35(9): 1-13.

Slessarev, E., Nuccio, E.E., McFarlane, K. J., Ramon, C., Saha, M., Firestone, M. K. and J. Pett-Ridge. 2020. "Quantifying the effects of switchgrass (Panicum virgatum) on deep organic C stocks using natural abundance 14C in three marginal soils." GCB Bioenergy. 12(10): 834-847.

Slessarev E.W., Chadwick O.A., Sokol N.W., Nuccio E.E., and J. Pett-Ridge. 2021. "Rock weathering controls the potential for soil carbon storage at a continental scale." Biogeochemistry Letters. In press.