Fungal and bacterial necromass: Opposite drivers of mineral-associated organic carbon gains and losses

Microbial necromass is a major contributor to mineral-associated organic carbon (MAOC), widely recognized as the primary and stable carbon (C) pool in soils. Nevertheless, long-term land use changes modify MAOC content and composition, including its fungal and bacterial contributions, which depend on soil types, particularly its hydraulic properties. Here, the impacts of over 30 years of land use, encompassing dryland pasture (DryPast), irrigated-pasture (IrrPast) and cropland (IrrCrop), on MAOC were evaluated across three representative soils with varying drainage characteristics: well-drained Lismore soil (LIS), moderately drained Templeton soil (TEM), and poorly drained Waterton/Temuka soil (WAT). Soil organic carbon (SOC) content and MAOC content decreased in order of IrrPast > DryPast > IrrCrop. MAOC determined the total SOC gains and losses after DryPast conversion to IrrPast and IrrCrop, respectively. Land use change had varying impacts on MAOC, which were influenced by the specific soil types. The well-drained LIS soil showed the highest MAOC increase, rising by 26 % following the conversion from DryPast to IrrPast. In contrast, the poorly drained WAT soil experienced the most significant MAOC reduction, decreasing by 22 % after the conversion to IrrCrop. Fungal necromass dominated MAOC gains, while bacterial necromass drove MAOC losses. Furthermore, the response of MAOC to land uses was primarily affected by labile C as the major source of microbial activity and binding agents. The stabilized fungal necromass is primarily protected within micropores of 0.2–3 µm. In contrast, bacterial necromass is largely constrained by the availability of labile nitrogen. Overall, the increase in fungal necromass and the decrease in bacterial necromass, driven by the interplay between specific management (e.g., irrigation or cultivation) and soil type (e.g., hydraulic properties), are important to interpret MAOC responses to changes in land use.