The radiocarbon age of soil carbon is a function of vertical transport rates

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Abstract

The radiocarbon content of soil organic carbon (C) is largely assumed to be a reflection of the carbon’s reactivity and a metric of its potential response to either warming or CO2 enhanced soil C inputs. Here we show that advective vertical transport of soil C is an important process affecting apparent soil C ages. This then leads to the conclusion that the increase in soil radiocarbon ages with depth significantly reflects the time of vertical transport, and has no immediate bearing on the carbon’s reactivity without the insight provided through a reaction-transport framework. Thus, the increasing radiocarbon ages of C with depth do not directly imply an increase in soil C resistance to warming or other perturbations. Based on theory, and an independent assessment of soil C decomposition rates, the radiocarbon profiles (and content for a given depth) were calculated, based on the shape of the C depth profile, for over 3000 soils in the USA, and were compared to observational results based on measured soil radiocarbon. The first-order coherence between the two entirely differing approaches suggests the fundamental importance of transport, and the implication that soil C is largely equally responsive to environmental change at all depths. This perspective provides insights that can serve to reduce biases in Earth System Model projections of depth-resolved soil C and its radiocarbon content.

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