Soil carbon 4 per 1000: carbon inputs to croplands are unlikely to be the primary constraint

Abstract Carbon (C) inputs to soils are increasingly viewed as the primary constraint on atmospheric carbon sequestration in agricultural systems. Here, we argue instead that the dominant limitation to soil organic carbon (SOC) recovery is not the magnitude of carbon inputs-typically on the order of tonnes per hectare-but rather the rate at which fresh organic matter (OM) is converted into stable soil organic matter (SOM). Evidence from experimental and field studies indicates that this transformation is largely controlled by the availability of the full suite of chemical elements constituting SOM (O, H, N, P, S, K, Ca, Mg, among others). Several of these elements, particularly phosphorus (P) and sulfur (S), are naturally scarce in many cropland soils and may therefore limit microbial metabolism and the stabilization of organic matter. We suggest that nutrient availability, rather than biomass input per se, represents the key bottleneck for increasing SOC stocks in croplands.