Supplementary Data for article '<b>Long-term biochar amendment alters microbial necromass carbon accumulation in topsoil and subsoil of two contrasting croplands: Alfisol and Ultisol</b>' 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.微生物残体（Microbial necromass）作为持久性土壤有机质的核心组成部分，在土壤碳（C）固存与稳定性维持中扮演着关键角色。生物炭（biochar）被视为提升农田生态系统碳封存效率、改善土壤肥力的极具潜力的技术手段。鉴于其对土壤养分有效性与微生物性状的调控效应，生物炭或可对微生物残体的积累与组成产生显著影响。然而，长期施用生物炭对土壤剖面中微生物残体的遗留效应迄今仍鲜有深入研究。为填补这一研究空白，我们在两种典型农田土壤（淋溶土（Alfisol）为富碳土壤，老成土（Ultisol）为贫碳土壤）中开展了为期8年的生物炭定位试验，以评估其对表层土壤（0-15 cm）与亚表层土壤（20-40 cm）中微生物残体碳（microbial necromass C）的影响，并揭示微生物残体碳动态与养分有效性、微生物群落特征之间的关联机制。与无生物炭的对照组相比，生物炭施用使老成土表层土壤的总残体碳（total necromass C）显著提升45.9%，却使淋溶土表层土壤的总残体碳降低9.9%。在亚表层土壤中，生物炭施用使淋溶土的总残体碳降低10.1%，使老成土降低13.3%。在微生物残体碳的各组分中，生物炭提升了两个土壤剖面中真菌残体碳（fungal necromass C）占微生物残体碳的比例。此外，本研究发现，养分有效性与微生物性状的协同作用共同调控了微生物残体碳的积累，其中施用生物炭后土壤的氮有效性是最为关键的调控因子。上述研究结果深化了学界对长期施用生物炭条件下微生物介导的碳封存过程的理解，同时为阐明土壤剖面中微生物残体的动态变化与稳定机制提供了重要的科学参考。