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SOIL BIOLOGY & BIOCHEMISTRY

Soil Carbon Saturation: Linking Concept and Measurable Carbon Pools

^a Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523-1499
^b Dep. of Earth and Environmental Science, Univ. of Pennsylvania, Philadelphia, PA 19104-6316
^c Dep. of Soil and Crop Sciences, Colorado State Univ., Fort Collins, CO 80523
^d Dep. of Plant Sciences, Univ. of California, Davis, CA 95616

^* Corresponding author (cstewart{at}colorado.edu).

The soil C saturation concept suggests a limit to whole soil organic carbon (SOC) accumulation determined by inherent physicochemical characteristics of four soil C pools: unprotected, physically protected, chemically protected, and biochemically protected. Previous attempts to quantify soil C sequestration capacity have focused primarily on silt and clay protection and largely ignored the effects of soil structural protection and biochemical protection. We assessed two contrasting models of SOC accumulation, one with no saturation limit (i.e., linear first-order model) and one with an explicit soil C saturation limit (i.e., C saturation model). We isolated soil fractions corresponding to the C pools (i.e., free particulate organic matter [POM], microaggregate-associated C, silt- and clay-associated C, and nonhydrolyzable C) from eight long-term agroecosystem experiments across the United States and Canada. Due to the composite nature of the physically protected C pool, we fractioned it into mineral- vs. POM-associated C. Within each site, the number of fractions fitting the C saturation model was directly related to maximum SOC content, suggesting that a broad range in SOC content is necessary to evaluate fraction C saturation. The two sites with the greatest SOC range showed C saturation behavior in the chemically, biochemically, and some mineral-associated fractions of the physically protected pool. The unprotected pool and the aggregate-protected POM showed linear, nonsaturating behavior. Evidence of C saturation of chemically and biochemically protected SOC pools was observed at sites far from their theoretical C saturation level, while saturation of aggregate-protected fractions occurred in soils closer to their C saturation level.

Abbreviations: Abbreviations: µagg, microaggregate fraction, (53–250 µm) • cPOM, coarse unprotected particulate organic matter (>250 µm) • CT, conventional tillage • iPOM, microaggregate-protected particulate organic matter (heavier than 1.85 g cm^–3, >53 µm in size) • H-dClay, hydrolyzable, easily dispersed, clay-sized fraction (acid soluble, <2 µm) • H-dSilt, hydrolyzable, easily dispersed, silt-sized fraction (acid soluble, 53–2 µm) • H-µClay, hydrolyzable, microaggregate-derived clay-sized fraction (acid soluble, <2 µm) • H-µSilt, hydrolyzable, microaggregate-derived silt-sized fraction (acid soluble, 53–2 µm) • LF, fine, unprotected POM (lighter than 1.85 g cm^–3, 53–250 µm) • NH-dClay, nonhydrolyzable, easily dispersed, clay-sized fraction (acid resistant, <2 µm) • NH-dSilt, nonhydrolyzable, easily dispersed, silt-sized fraction (acid resistant, 53–2 µm) • NH-µClay, nonhydrolyzable, microaggregate-derived clay-sized fraction (acid resistant, < 2 µm) • NH-µSilt, nonhydrolyzable, microaggregate-derived silt-sized fraction (acid resistant, 53–2 µm) • NT, no-till • POM, particulate organic matter • SOC, soil organic carbon