Dataset S1.xlsx

Paddy fields, as the largest anthropogenic wetlands on Earth, face a high risk of micronutrient loss through surface runoff and leaching due to their frequent irrigation-drainage cycles, as well as removal with crop harvest. While micronutrient’s losses largely impede biological nitrogen fixation (BNF) in soils, agricultural practices that retain micronutrients and thus increase BNF in paddy soils remain underexplored. Using a long-term (40 years) field experiment, we showed that manure and straw inputs substantially increased the content and bioavailability of micronutrients, including molybdenum (Mo) and vanadium (V), compared to the soil without fertilization or with mineral fertilization only. The total content and bioavailability of Mo and V had a strong correlation with microbial necromass and short-range ordered minerals. Metagenomic sequencing analysis further indicated that the content and bioavailability of micronutrients were strongly correlated with the abundance of the key N-fixing genera (i.e., Azospirillum and Bradyrhizobium). Unexpectedly, structural equation modeling (SEM) identified that microbial necromass exerted the strongest control on N-fixing genera, highlighting an underappreciated role of microbial necromass as a reservoir of micronutrients. Based on micronutrient’s bioavailability and metagenomic sequencing, we conclude that micronutrients are the key factor for BNF in paddy soils, offering significant implications for managing BNF in paddy soils.

稻田作为全球规模最大的人工湿地，因其频繁的灌排循环以及随作物收获被移除，面临着通过地表径流和淋溶作用大量流失微量营养元素的高风险。尽管微量营养元素的流失会在很大程度上抑制土壤中的生物固氮（biological nitrogen fixation, BNF）过程，但能够保留微量营养元素并由此提升稻田土壤生物固氮效率的农艺措施仍有待深入探索。本研究依托一项为期40年的长期田间定位试验，结果表明：与不施肥或仅施用化肥的土壤相比，施用有机肥与秸秆大幅提升了包括钼（molybdenum, Mo）、钒（vanadium, V）在内的微量营养元素的总量及其生物有效性。钼与钒的总含量及其生物有效性与微生物残体（microbial necromass）以及短程有序矿物（short-range ordered minerals）存在显著相关性。宏基因组测序（metagenomic sequencing）分析进一步显示，微量营养元素的含量与生物有效性与关键固氮菌属（即固氮螺菌属Azospirillum和慢生根瘤菌属Bradyrhizobium）的丰度显著相关。值得注意的是，结构方程模型（structural equation modeling, SEM）分析结果表明，微生物残体对固氮菌属的丰度具有最强的调控作用，这凸显了微生物残体作为微量营养元素储存库的未被充分认知的重要功能。基于微量营养元素的生物有效性与宏基因组测序结果，本研究得出结论：微量营养元素是调控稻田土壤生物固氮过程的关键因子，这为稻田土壤生物固氮的管理提供了重要的理论依据与实践启示。