Application of <i>δ</i><sup>13</sup>C and <i>δ</i><sup>15</sup>N isotopic signatures of organic matter fractions sequentially separated from adjacent arable and forest soils to identify carbon stabilization mechanisms

Identifying the chemical mechanisms behind soil carbon bound in organo-mineral complexes is necessary to determine the degree to which soil organic carbon is stabilized belowground. We used the δ¹³C and δ¹⁵N isotopic signatures from two organic matter (OM) fractions from soil to identify the likely binding mechanisms involved. We used OM fractions hypothesized to contain carbon stabilized through organo-mineral complexes: (1) OM separated chemically with sodium pyrophosphate (OM(PY)) and (2) OM stabilized in microstructures found in the chemical extraction residue (OM(ER)). Furthermore, because the OM fractions were separated from five different soils with paired forest and arable land use histories, we could address the impact of land use change on carbon binding and processing mechanisms within these soils. We used partial least squares regression to analyze patterns in the isotopic signature of OM with established proxies of different binding mechanisms. Parsing soil OM into different fractions is a systematic method of dissection, however, we are primarily interested in how OM is bound in soil as a whole, requiring a means of re-assembly. Thus, we implemented the recent zonal framework described by Kleber et al. (2007) to relate our findings to undisturbed soil. The δ¹⁵N signature of OM fractions served as a reliable indicator for microbial processed carbon in both arable and forest land use types. The δ¹³C signature of OM fractions in arable sites did not correlate well with proxies of soil mineral properties while a consistent pattern of enrichment was seen in the δ¹³C of OM fractions in the forest sites. We found a significant difference in δ¹³C of pooled OM fractions between the forest and arable land use type although it was relatively small (<1‰). We found different binding mechanisms predominate in each land use type. The isotopic signatures of OM fractions from arable soils were highly related to the clay and silt size particles amount while organic matter not directly bound to mineral surfaces in the contact zone was involved in cation bonding with Ca. In forest soils, we found a relationship between isotopic signatures of OM(PY) and the ratio of soil organic carbon content to soil surface area (SOC/SSA). For arable soils, the formation of OM(PY)-Ca-mineral associations seems to be a relevant OM stabilization mechanism while the OM(PY) of forest soils seems to be separated from layers of slower exchange not directly attached to mineral surfaces. This means there is a potential to build multiple OM layers on mineral particles in the arable soil and thus the potential for carbon accumulation