Soil carbon responses to past and future CO2 in three Texas prairie soils

Authors: PROCTER, ANDREW - Duke University; GILL, RICHARD - Brigham Young University; Fay, Philip; Polley, Herbert; JACKSON, ROBERT - Duke University

Submitted to: Soil Biology and Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/16/2014
Publication Date: 1/29/2015
Publication URL: https://handle.nal.usda.gov/10113/60438
Citation: Procter, A.C., Gill, R.A., Fay, P.A., Polley, H.W., Jackson, R.B. 2015. Soil carbon responses to past and future CO2 in three Texas prairie soils. Soil Biology and Biochemistry. 83:66-75.

Interpretive Summary: The concentration of carbon dioxide (CO2) gas in air has increased by about 40% since industrialization and is expected to reach twice the pre-industrial concentration by mid-century. Soils play a pivotal role in regulating the rate at which CO2 concentration rises because soils contain huge quantities of plant-derived (organic) carbon that can be consumed by soil microorganisms (microbes) to release CO2. On the other hand, CO2 enrichment may increase organic carbon pools in soil by stimulating plant growth and increasing the subsequent addition of dead roots and aboveground plant tissues to soil. Increased soil carbon, in turn, will reduce the rate at which atmospheric CO2 concentration rises. We grew grassland vegetation on each three soil types (clay, sandy loam, silty clay) at pre-industrial to anticipated CO2 concentrations in order to determine the role of soil type in mediating CO2 effects on soil carbon. CO2 enrichment for four years did not affect total organic carbon pools in any soil type, but increased pools of fast-cycling soil organic carbon (SOC) in the clay and silty clay soils. This most recently-added and readily-decomposed fraction of SOC increased by four-fold across the CO2 gradient in the clay soil with 55% clay content, but increased by only 50% across the gradient in the sandy loam soil with 15% clay content. By contrast, the oldest and least-readily decomposed fraction of SOC declined by 23% across the CO2 gradient in the silty clay soil with 45% clay content. Our data indicate that soil physical characteristics, notably the clay fraction, strongly regulate CO2 effects on soil carbon dynamics and imply a role of geographic variation in soil type in regulating future responses of soil carbon to CO2 and of feedbacks of carbon storage on atmospheric CO2 concentration.

Technical Abstract: Changes in soil carbon storage could affect and be affected by rising atmospheric CO2. However, it is unlikely that soils will respond uniformly, as some soils are more sensitive to changes in the amount and chemistry of plant tissue inputs while others are less sensitive because of mineralogical, textural, or microbial processes. We studied soil carbon and microbial responses to a preindustrial-to-future CO2 gradient (250-500 ppm) in a grassland ecosystem in the field. The ecosystem contains three soil types: a sandy loam Alfisol, a silty clay Mollisol, and a black clay Vertisol with clay fractions of 15% to 55%. Soil and microbial responses to CO2 are plant mediated, and aboveground plant productivity in this ecosystem increased linearly with CO2 in the sandy loam and silty clay. Although total soil organic carbon (SOC) did not change with CO2 treatment after four growing seasons, fast-cycling SOC pools increased with CO2 in the two clay soils. Microbial biomass increased 18% and microbial activity increased 30% across the CO2 gradient in the black clay (55% clay), but neither factor changed with CO2 in the sandy loam (15% clay). Similarly, size fractionation of SOC showed that coarse POM-C, the youngest and most labile fraction, increased four-fold across the CO2 gradient in the black clay, but increased by only 50% across the gradient in the sandy loam. Interestingly, mineral associated C, the oldest and most recalcitrant fraction, declined 23% across the gradient in the third soil type, a silty clay (45% clay). Our results provide evidence for priming in this soil type, as labile C availability and decomposition rate also increased across the CO2 gradient in this soil. In summary, CO2 enrichment in this grassland increased the fast-cycling soil organic carbon pool as in other CO2 studies, but only in the two high-clay soils. Priming in the silty clay could lead to decreased total SOC after prolonged CO2 exposure. Because soil texture varies geographically, including data on soil types could enhance predictions of soil carbon and microbial responses to future CO2 levels.