Depth-dependent microbial necromass carbon accumulation responses to long-term biochar amendment in croplands

Soil microbial necromass carbon (MNC), a major contributor to stable soil organic C (SOC), is increasingly recognized as a crucial indicator of long-term C sequestration. However, the depth-dependent responses of MNC to long-term biochar amendment remain poorly understood. Here, we conducted a 12-year field experiment in two contrasting cropland soils—a C-rich Entisol and a C-poor Ultisol—to assess the effects of biochar on MNC accumulation and composition in both topsoil (0–20 cm) and subsoil (20–40 cm). In the topsoil, biochar increased MNC by 23.3% in the Entisol and 39.0% in the Ultisol, with a stronger response in fungal necromass than in bacterial necromass. Conversely, subsoil MNC decreased by 17.9–30.4% across both soils. These contrasting patterns were associated with biochar-induced changes in nutrient availability and microbial traits, including nitrogen availability, enzyme activity, metabolic quotient, and microbial biomass. To complement our findings, a meta-analysis of 85 pairs of observations from 23 peer-reviewed studies confirmed that biochar increased topsoil MNC in 83.5% of cases, with an average increase of 10.2%. The magnitude of the effect was greater in soils with low initial SOC and high sand content, and it intensified over time, peaking 10 years after application. In summary, 12-year biochar amendment enhanced MNC accumulation in the topsoil but reduced it in the subsoil, highlighting the importance of long-term and depth-specific evaluations to fully understand biochar’s role in microbially mediated SOC sequestration.