Multiple constraints cause positive and negative feedbacks limiting grassland soil CO2 efflux under CO2 enrichment

Authors: Fay, Philip; HUI, DAFENG - Tennessee State University; JACKSON, ROBERT - Stanford University; Collins, Harold; REICHMANN, LARA - University Of Texas At Austin; ASPINWALL, MICHAEL - University Of Texas At Austin; Jin, Virginia; KHASANOVA, ALBINA - University Of Texas At Austin; HECKMAN, ROBERT - University Of Texas At Austin; Polley, Herbert

Submitted to: Proceedings of the National Academy of Sciences (PNAS)
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/6/2020
Publication Date: 1/12/2021
Citation: Fay, P.A., Hui, D., Jackson, R.B., Collins, H.P., Reichmann, L.G., Aspinwall, M.J., Jin, V.L., Khasanova, A.R., Heckman, R.W., Polley, H.W. 2021. Multiple constraints cause positive and negative feedbacks limiting grassland soil CO2 efflux under CO2 enrichment. Proceedings of the National Academy of Sciences(PNAS). 118(2). Article e2008284117. https://doi.org/10.1073/pnas.2008284117.
DOI: https://doi.org/10.1073/pnas.2008284117

Interpretive Summary: The effects of atmospheric carbon dioxide (CO2) enrichment on grasslands depend strongly on how much CO2 is subsequently lost back to the atmosphere through the process known as ‘soil respiration’. This loss can represent a sizeable amount of the carbon gained by ecosystems through photosynthesis, so understanding the controls on this loss is critical to understanding how much carbon is stored by ecosystems, where it cannot contribute to climate warming. In a long-term study of the carbon gains and losses in grasslands in Central Texas, we found that the increase in grassland productivity with CO2 enrichment was the most consistent predictor of the amount of loss, but that at elevated CO2 levels productivity continued to increase at a faster rate than did losses to soil respiration. This implies increased storage of carbon in this grassland at future elevated CO2 concentrations, although there was also evidence for considerable variation among coarse and fine textured soils. The causes of buildups or losses of carbon in grasslands is critical information for land managers and policy makers, enabling management decisions and policies that will improve the sustainability of grasslands managed for agricultural production and other ecosystem goods and services.

Technical Abstract: Terrestrial ecosystems are increasingly enriched with resources such as atmospheric CO2 that limit ecosystem processes. The consequences for ecosystem carbon cycling depend on the availability of other limiting resources and feedbacks from the plant community. These feedbacks remain poorly understood for soil CO2 efflux, JCO2, a primary carbon flux from the biosphere to the atmosphere. We applied a unique CO2 enrichment gradient (250 to 500 µL L-1) for eight years to grassland plant communities on soils varying in texture. We identified the trajectory of JCO2 responses and feedbacks from the plant community and other resources. JCO2 is expected to increase with CO2 enrichment. We found a linear increase in JCO2 on coarse sandy loam and fine clay soils, and a saturating increase on an intermediate silty clay soil. The varying CO2 responses of JCO2 arose from soil-specific feedbacks from the plant community and soil water availability. On the clay soil, CO2 was the dominant limitation, and on the sandy loam the JCO2 response was reinforced by positive feedbacks from aboveground net primary productivity and effective species richness (exp(H)). On the silty clay, the saturating JCO2 response to CO2 arose from a negative feedback between soil water potential and exp(H). These findings apply conceptual models of multiple resource limitation to reveal how the primary flux of carbon to the atmosphere depends on whether there are positive or negative feedbacks between limiting resources and plant community structure. Incorporating feedbacks from biodiversity will improve models of terrestrial carbon sequestration and ecosystem services.