Supplementary Data for article 'Long-term biochar amendment alters microbial necromass carbon accumulation in topsoil and subsoil of two contrasting croplands: Alfisol and Ultisol' by Liu et al.

Microbial necromass, a vital component of persistent soil organic matter, plays a crucial role in soil carbon (C) accumulation and stability. Biochar is recognized as a promising strategy for enhancing C sequestration and improving soil fertility in agroecosystems. Given its effects on soil nutrient availability and microbial traits, biochar may influence the accumulation and composition of microbial necromass. However, the long-term legacy effects of biochar amendment on microbial necromass in the soil profile remain poorly understood. To address this gap, we conducted an 8-year biochar amendment experiment in two typical cropland soils (Alfisol, C-rich; Ultisol, C-poor) to assess its effects on microbial necromass C (MNC) in both topsoil (0-15 cm) and subsoil (20-40 cm), and to evaluate how MNC dynamics relate to nutrient availability and microbial community features. Compared to the biochar-free control, biochar amendment significantly increased total necromass C (TNC) by 45.9% in the topsoil of Ultisol but decreased it by 9.9% in Alfisol. In the subsoil, biochar amendment decreased TNC by 10.1% in Alfisol and 13.3% in Ultisol. Among MNC components, biochar enhanced the contribution of fungal necromass C to MNC in both soil profiles. Furthermore, this study revealed that the interaction between nutrient availability and microbial traits collectively regulated MNC accumulation, with nitrogen availability following biochar application being the most important controlling factor. These findings deepen our understanding of microbially mediated C sequestration under long-term biochar amendment and provide valuable insights into the dynamics and stabilization of microbial necromass in the soil profile.