Constraints on mineral-associated and particulate organic carbon response to regenerative management: carbon inputs and saturation deficit

Regenerative management has potential to increase soil organic carbon (SOC), which will be crucial for mitigating climate change and improving soil health. Distinct fractions of SOC, particulate organic carbon (POC) and mineral-associated organic carbon (MAOC), have been posited as having contrasting responses to regenerative management. The POC response has been proposed as a leading indicator of total SOC response, whereas MAOC response has been seen as eventually limiting SOC response due to its saturation behavior. We explored these and linked expectations of SOC, POC and MAOC response by leveraging multiple datasets of regenerative management with cover crops or perennial crops as contrasted with conventional management (n = 42 sites). Across sites, POC as a percentage of SOC was on average 19%. Regenerative management increased both POC and MAOC but increases in POC were smaller and narrower (0.78 +/- 0.26 g C kg soil(-1)) than increases in MAOC (1.41 +/- 0.80 g C kg soil(-1)). Changes in POC were only weakly correlated with changes in SOC (p < 0.001 but R-2 = 0.11), revealing that absolute changes in POC at any timepoint should not be taken as indicative of total SOC responses. The MAOC response made up the majority of SOC response at 58% of sites. Changes in POC and MAOC with regenerative management were also not related (p = 0.72), indicating that comprehensive assessment of POC and MAOC is needed to understand SOC accumulation under regenerative management. Increases in POC were explained by annual average of increase in root C inputs with regenerative management, suggesting a limited ability for the current portfolio of regenerative management practices to increase POC, even when implemented over decadal timescales. In contrast, increases in MAOC were partially explained by cumulative-for-the-trial increases in root C inputs and were not clearly constrained by silt + clay-estimated saturation deficit. Our results indicate that the increase of MAOC storage in agricultural soils is not limited by saturation but rather by the extent to which root C inputs can be augmented, and that increasing POC storage will require novel practices to overcome current limitations on POC accumulation.