Soil Biology & Biochemistry
Soils that exhibit soil organic carbon (SOC) saturation provide an opportunity to examine mechanisms of C storage in soils with increasingly limited C-stabilization potential. A manure rate experiment in Lethbridge, Alberta, in which SOC responded asymptotically to long-term manure C additions, allowed us to assess changes in SOC biochemical composition in response to soil C saturation. By quantifying the cupric oxide oxidation products of lignin, cutin, and suberin in fractionated SOC pools that are characterized by chemical (i.e., mineral-associated), physical (i.e., microaggregate-associated), or no protection (i.e., free particulate organic matter), we evaluated the interaction between C saturation and the biochemical characteristics of SOC. We tested the specific responses of soil fraction lignin, cutin, and suberin to C saturation level by using the bulk soil to approximate C-input composition across manure input treatments. Carbon-normalized lignin (lignin-VSC/OC) in the chemically protected fractions did not differ, while in the non-protected and physically protected soil fractions, it decreased with C saturation level. Neither the stabilization of cutin and suberin, nor the lignin:cutin + suberin ratio, differed in any of the measured soil fractions in response to C saturation level. These results indicate that with C saturation and decreased C stabilization potential, lignin, cutin, or suberin were not preferentially stabilized or depleted in mineral protected soil C pools. The lack of evidence for biochemical preference in mineral associations with C saturation supports the existence of an outer kinetic zone of organomineral associations, in which partitioning of organic compounds, rather than sorption, controls mineral SOC accumulation at high SOC loadings. Furthermore, despite theories of inherent lignin recalcitrance, depleted lignin concentrations with C saturation in the non-protected and aggregate protected fractions indicate that lignin was, in this study, preferentially decomposed when not protected by association with mineral phases in the soil. In conclusion, C-input quantity, and not quality, combined with physical and chemical protection mechanisms that govern long-term C storage, appeared to control C saturation and stabilization at this site.
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