Grassland duration affects soil organic carbon pools only in the topsoil and has limited effects on microbial metabolism

The sustainability of agricultural production relies on the regeneration of soil organic carbon (SOC), both in particulate organic carbon (POC) and mineral-associated (MAOC) forms. Incorporating grass into cropping systems has often been claimed beneficial for increasing SOC. However, the impact of grass duration -ranging from months to years or decades-, on SOC pools and microbial metabolism throughout the soil profile remains unclear. In this study, the effects of three grass-based forage systems on SOC stocks and their distribution across POC and MAOC fractions, after 29 years in a long-term field experiment in NW Italy was investigated. The grass-based forage systems included: permanent grassland; four years of temporary (ley) grassland in rotation to three years of maize monoculture; and an annual double cropping with eight months of grass in rotation to four months of maize. We also evaluated the relationship between these forage systems and soil microbial traits. Our findings showed that SOC, free POC, occluded POC, and MAOC stocks were significantly lower in temporary grassland and double cropping forage systems compared to permanent grassland, but these effects were confined to the topsoil. SOC stocks and their distribution across fractions were similar in temporary grassland and double cropping grass-based forage systems. No differences related to the grass duration in the forage systems were detectable in the subsoil. Microbial carbon use efficiency was poorly affected by the grass duration, but showed a significant depth gradient, decreasing in the topsoil and increasing in the subsoil, primarily driven by changes in pH and soil C:N ratios. Similarly, C-degrading enzymatic activities were influenced more by soil depth than by the grass-based forage system. Overall, our results highlighted the critical role of permanent grassland in promoting SOC accumulation. However, TG as well as DC also played an important role in mitigating SOC depletion.