The six rights of how and when to test for soil C saturation

Abstract. The concept of soil organic carbon (SOC) saturation emerged a bit more than 2 decades ago as our mechanistic understanding of SOC stabilization increased. Recently, the further testing of the concept across a wide range of soil types and environments has led some people to challenge the fundamentals of soil C saturation. Here, we argue that to test the concept, one should pay attention to 6 fundamental principles or rights (R’s): the right measures, the right units, the right fractionation method, the right soil type, the right mineralogy, the right saturation level. Once we take care of those 6 rights across studies, we find a maximum of C stabilized by minerals and estimate based on current data available that this maximum stabilization is around 82 ± 4 g C kg^-1 silt+clay for 2:1 clay dominated soils while most likely only around 46 ± 4 g C kg^-1 silt+clay for 1:1 clay dominated soils. These estimates can be further improved using more data, especially for different mineralogies across varying environmental conditions. However, the bigger challenge is on how and which C sequestration strategies to implement in order to effectively reach this 82/46 g C kg^-1 silt+clay in soils across the globe.

In recent years, several studies (e.g., Begill et al. 2023; Salonen et al. 2023) have questioned the concept of soil carbon (C) saturation, i.e., organic C stabilized by soil minerals (Hassink, 1997; Six et al. 2002). Here, we want to draw attention to six fundamentals that we think one should be cognizant about when “testing” and “questioning” soil C saturation.