Carbon loading regulates soil organic matter dynamics after digestate applications

Increasing soil organic carbon (SOC) in croplands is crucial for enhancing soil health and improving nutrient cycling. Anaerobic digestate solids (ADS) are increasingly applied as an organic amendment, yet the mechanisms controlling how ADS-derived carbon (C) contributes to persistent SOC pools remain uncertain. It is unclear how soil C loading—the ratio of mineral-associated organic carbon (MAOC) to silt + clay—regulates SOC formation following ADS additions. Here, we distinguish between two complementary metrics: marginal formation, the increase in an SOC pool per unit of ADS-C applied, and formation efficiency, the proportion of C input incorporated into that pool. We hypothesized that increasing C loading reduces marginal MAOC formation by decreasing reactive mineral surfaces but has no effect on particulate organic carbon (POC). We conducted a 6-month incubation using two agricultural soils of similar texture but contrasting C loading levels (low, Soil L; high, Soil H) to quantify ADS and native SOC decomposition, enzyme responses, and MAOC and POC formation across four ADS-C input rates. Consistent with prior field studies, increasing C loading decreased the SOC formation efficiency of ADS. However, the underlying mechanisms differed from our expectations: marginal MAOC formation increased (from 0.13 to 0.21), whereas marginal POC formation decreased (from 0.18 to 0.10), with higher C loading. These contrasting responses were more consistent with microbial controls associated with resource availability than with predictions based on C saturation alone. Soil H—characterized by higher concentrations of labile C and nutrients—likely supported greater microbial growth and higher carbon use efficiency, conditions favorable for microbial biomass production and residue accumulation. In contrast, Soil L had lower C and nutrient availability, which constrained microbial metabolism and favored ADS-derived C accumulation in the POC pool. Overall, our study highlights that MAOC and POC formation are jointly shaped by resource availability and microbial physiology, rather than mineral saturation alone. These findings underscore the importance of integrating multiple control factors when evaluating SOC dynamics under ADS management.