Assessing microbial necromass carbon across contrasting tropical land uses

Nearly 40% of globe’s land surface is used for crop and livestock production, generating one quarter of total greenhouse gas emissions and making agricultural landscapes key targets for climate mitigation. Integrated production systems are proposed as a nature-based solution to directly enhance soil organic carbon (SOC) sequestration by increasing aboveground diversity and stimulating belowground microbial processes. However, indirect microbial contributions, particularly through their cellular residues, must also be considered, as microbial necromass carbon (MNC) has emerged as a major long-term carbon pool in soils. We quantified SOC and MNC across five land-use systems (Eucalyptus forest, cropping, pasture, crop-livestock (CLi), and crop-livestock-forest integrations (CLFi)). Soil samples (0-10 cm) were analyzed for total organic carbon by dry combustion and amino sugar biomarkers by HPLC to estimate bacterial and fungal necromass. SOC ranged from 26 to 34 g kg^-1, with highest values in pasture and CLFi, intermediate in CLi, and lowest in cropping and Eucalyptus forest systems. MNC showed a similar pattern, peaking in pasture (20 g kg^-1) and CLFi (19 g kg^-1) and remaining lowest in cropping systems (14 g kg^-1). Fungal necromass consistently exceeded bacterial necromass across all land uses. Although less diverse, pasture alone displayed SOC and MNC levels comparable to CLFi, demonstrating that substrate quality and continuous root-derived inputs can regulate carbon accumulation even under low aboveground diversity. Overall, microbial necromass represents a central link between land-use management, substrate quality, and long-term carbon stabilization in tropical soils, with grass-based systems, whether isolated or integrated, favoring soil carbon accumulation.