Physico-chemical and bio-chemical controls on soil C saturation behavior. A renewal for: “Soil C Saturation and Steady-State Level Determine C Sequestration Rate and Capacity”.
Department of Energy (DOE), Biological and Environmental Research, Award #ER63912
Co-Principal Investigator (PI: Johan Six, University of California, Davis)
Soil organic carbon (SOC) stabilization rates and durations vary across ecosystems, management practices, and climate regimes (West and Six 2007), but how the relationship between physico-chemical and bio-chemical SOC characteristics and the C saturation phenomenon influences the rate, limit and permanence of SOC stabilization has yet to be elucidated. In this renewal proposal we build on the concepts developed and results obtained from previous DOE funded projects to further discern how soil C saturation and deficit govern SOC stabilization and permanence, especially how the physico-chemical and bio-chemical characteristics of SOC play a role in SOC stabilization and saturation. The major hypothesis underlying our newly proposed work is: the rate and permanence of soil C stabilization is determined by the soil C saturation phenomenon, which is itself driven by the physico-chemical and bio-chemical characteristics of the soil minerals and SOC. Many soils, especially those that have been significantly depleted in SOC through cultivation show a linear response between annual C input rates and steady-state SOC levels (Paustian et al. 1997a; Kong et al. 2004; West and Six 2007), suggesting that the mechanisms controlling C accumulation are independent of the amount of C already in the soil, that is, there is no inherent C saturation limit in soils. This assumption of ‘first-order’ kinetics, in which the specific decomposition rate (k) is defined as a constant (e.g., dC/dt = I – kC), forms the basis for nearly all of the current soil organic matter and decomposition models (Paustian et al. 1994; Paustian et al. 1997b). However, some mineral soils with relatively high SOC contents show only a weak or no response to increasing rates of C input, suggesting that there is a dependency between SOC levels and the basic kinetics governing SOC stabilization. These soils exhibit the phenomenon of C saturation (Campbell et al. 1991; Paustian et al. 1997a; Six et al. 2002)1. From these and other observations in the literature (e.g., Hassink 1996) we formalized a concept of soil C saturation (Six et al. 2002) and initiated a suite of laboratory and field studies with funding from DOE. Our DOE-funded results support the following basic tenets of the saturation concept: 1) fundamental kinetics of soil C stabilization are dependent on SOC levels, 2) differences in the relative efficiency by which newly added C is stabilized differs across discrete SOC pools and 3) SOC pool saturation with increasing C input limits responses to global change.