Impact of organic amendments on carbon stability and carbon use efficiency in acidic and alkaline soils

Soils represent the largest reservoir of organic carbon in terrestrial ecosystems, yet the mechanisms controlling its stabilization and turnover are still not fully understood, limiting our ability to anticipate their response to climate change. Microbial processes are central to the formation, preservation, and loss of soil organic carbon (SOC), with microbial carbon use efficiency (CUE)—the fraction of assimilated carbon allocated to growth versus respiration—emerging as a key integrative parameter of microbial functioning. While CUE has been proposed as a predictor of SOC persistence, its contribution remains debated. In parallel, CUE is gaining attention in the context of carbon farming policies, as it links microbial functioning with soil carbon sequestration. Among the management practices aimed at enhancing SOC, organic amendments such as compost and biochar stand out for their capacity to influence CUE and improve soil functioning. In this study, we assessed how different organic amendments affect SOC stability and sequestration in two contrasting soils from the Iberian Peninsula: acidic grasslands and alkaline rain-fed soils. The amendments included four biochars, two cattle digestates, a green compost, and a biochar–compost mixture. Over 100 days, soil respiration (CO₂ emissions), microbial biomass, and soil properties were monitored using an automatic respirometer. Microbial CUE and microbial activity largely determined carbon (C) retention in the studied soils. Cow digestate increased microbial activity but reduced microbial CUE in both soils, leading to higher C losses through respiration and lower C retention. In contrast, biochars—particularly those produced from white poplar wood, olive pomace and rice husk—enhanced carbon recalcitrance, extending the residence time of the stable C pool by six to nine times compared with unamended soils. Microbial analyses showed that bacterial loads were 2–3 orders of magnitude higher than fungal loads. Compared with acidic grassland soils, alkaline soils generally showed higher microbial CUE values, reflecting a greater potential for C sequestration. These findings also indicate that microbial CUE exhibited clear soil-specific behavior, being consistently higher in the AS than in the acidic GS. This pattern suggests that differences in microbial community dominance—particularly the relative contribution of bacteria and fungi—may underlie the contrasting CUE responses observed between soils, a topic that warrants further investigation in future studies. In the alkaline soils, digestate amendments resulted in the highest bacterial abundance, whereas rice husk biochar favored fungal growth. Additionally, the high Cu and Zn content of cow manure digestate posed risks in acidic soils. This study also emphasizes that amendment strategies should be tailored to soil type to optimize carbon sequestration. Moreover, a novel thermal–respirometry correlation model was also developed, providing a practical tool for assessing soil carbon dynamics and C stability.