Estimating the attainable soil organic carbon deficit in the soil fine fraction to inform feasible storage targets and de-risk carbon farming decisions

Abstract

Context

Defining soil organic carbon (SOC) ‘potential’ storage, underpins the economic feasibility of carbon sequestration; however, ‘potential’ storage is not quantifiable using historical and current empirical data. We propose a framework to define ‘attainable’ SOC storage that varies with soil properties, environmental conditions and management practices.

Aims

Within the soil fine fraction, we quantified additional storage capacity of the fine fraction SOC attainable deficit (FF_SOC__{Attainable_Def}) by the difference between attainable (FF_{SOC_Attainable}) and actual fine fraction SOC.

Methods

Using three analyses, we developed a framework to: (1) estimate the FF_{SOC_Attainable_Def} of the fine fraction of Australian agricultural soils within broad mean annual precipitation ranges and soil depth classes; (2) establish rapid prediction capability for the FF_SOC__{Attainable_Def} using infrared/partial least square regression modelling; and (3) generate spatial FF_{SOC_Attainable_Def} estimates for agricultural regions with ensemble Random Forest modelling.

Key results

Global analyses of FF_SOC__{Attainable_Def} do not consider key environmental drivers of carbon inflows and outflows nor soil depth. Separate analyses of soils derived from different combinations of precipitation and soil depth need to include variations in environmental conditions and soil properties to accurately define FF_{SOC_Attainable} and FF_{SOC_Attainable_Def} within the fine fraction. Spatially estimated FF_SOC__{Attainable_Def} stocks revealed an opportunity to increase current fine fraction SOC stock by 3.47 GT (0–0.10 m depth) and 3.24 GT (0.10–0.30 m depth).

Conclusions

Our findings suggests that FF_SOC__{Attainable_Def} is dynamic, not static. Caution is needed when interpreting the results from this analysis.

Implications

Deriving estimates of FF_SOC__{Attainable_Def} will reduce risks in decision making on carbon farming in national policies.