277-4 What can we learn from carbon isotope profiles of soil organic matter?

Session: Critical Zone Science: Intersection of Processes Linked to Geomorphology, Ecology, Fire and Climate

Presenting Author:

Authors:

Abstract:

In hillslope and other well-drained soils, the d¹³C values of soil organic carbon almost always increase whereas radiocarbon activities tend to decrease with depth. These trends relate to the stabilization of organic carbon, but what, in particular, can we learn from them? We begin with an investigation of radiocarbon and distinguish between two basic conceptual models that explain generalized profiles: 1) the downward advection of soil organic carbon, which we call the ‘conveyor belt’ model, and 2) the vertical decrease in microbial carbon consumption constants, which leads to slower turnover with depth and which we term the ‘convenience store’ model. We suggest that the convenience store model may represent the priming of microbial communities by root exudates and test this idea with Bayesian inversion of profile data uisng two versions of a numerical soil carbon model, one that does and one that does not include scaling of microbial consumption with root density. We also suggest that neither endmember model is sufficient on its own and offer that we do not know how their relative importance varies with the factors of soil formation. We then present an approach involving joint inversion that should both elucidate controls on stable carbon isotope profiles and provide tight constraints on soil carbon cycling parameters. Application of these joint inversions across rooting depth and permeability gradients, and perhaps to soils undergoing woody encroachment, might be especially helpful in understanding the relative importance of the conveyor belt and convenience store.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-11083

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