State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, Nanjing, 210008, China; University of Chinese Academy of Sciences, Beijing, 100049, China.
Institute of Red Soil and Germplasm Resources, Jinxian, 331717, China.
Environ Res. 2024 Jul 1;252(Pt 3):118923. doi: 10.1016/j.envres.2024.118923. Epub 2024 Apr 16.
Symbiotic nitrogen fixation of Chinese milk vetch (Astragalus sinicus L.) can fix nitrogen from the atmosphere and serve as an organic nitrogen source in agricultural ecosystems. Exogenous organic material application is a common practice of affecting symbiotic nitrogen fixation; however, the results of the regulation activities remain under discussion. Studies on the impact of organic amendments on symbiotic nitrogen fixation have focused on dissolved organic carbon content changes, whereas the impact on dissolved organic carbon composition and the underlying mechanism remain unclear. In situ pot experiments were carried out using soils from a 40-year-old field experiment platform to investigate symbiotic nitrogen fixation rate trends, dissolved organic carbon concentration and component, and diazotroph community structure in roots and in rhizosphere soils following long-term application of different exogenous organic substrates, i.e., green manure, green manure and pig manure, and green manure and rice straw. Remarkable increases in rate were observed in and when compared with that in green manure treatment, with the greatest enhancement observed in the treatment. Moreover, organic amendments, particularly pig manure application, altered diazotroph community composition in rhizosphere soils, therefore increasing the abundance of the host-specific genus Mesorhizobium. Furthermore, organic amendments influence the diazotroph communities through two primary mechanisms. Firstly, the components of dissolved organic carbon promote an increase in available iron, facilitated by the presence of humus substrates. Secondly, the elevated content of dissolved organic carbon and available iron expands the niche breadth of Mesorhizobium within the rhizosphere. Consequently, these alterations result in a modified diazotroph community within the rhizosphere, which in turn influences Mesorhizobium nodulation in the root and symbiotic nitrogen fixation rate. The results of the present study enhance our understanding of the impact of organic amendments on symbiotic nitrogen fixation and the underlying mechanism, highlighting the key role of dissolved organic carbon composition on diazotroph community composition in the rhizosphere.
紫云英（Astragalus sinicus L.）的共生固氮可以从大气中固定氮，并作为农业生态系统中的有机氮源。外源有机物质的应用是影响共生固氮的常见做法；然而，调节活动的结果仍在讨论中。关于有机肥料对共生固氮影响的研究集中在溶解有机碳含量的变化上，而对溶解有机碳组成及其潜在机制的影响仍不清楚。本研究采用 40 年农田长期定位试验平台上的土壤，通过原位盆栽试验，研究了长期施加不同外源有机底物（绿肥、绿肥+猪粪和绿肥+稻草）后，共生固氮速率趋势、根和根际土壤中溶解有机碳浓度和组成以及根瘤菌群落结构的变化。与绿肥处理相比，和处理中的固氮速率显著增加，其中处理中的增幅最大。此外，有机肥料，特别是猪粪的施用，改变了根际土壤中根瘤菌群落的组成，从而增加了宿主特异性属 Mesorhizobium 的丰度。此外，有机肥料通过两种主要机制影响根瘤菌群落。首先，腐殖质底物的存在促进了溶解有机碳的组分增加，从而提高了有效铁的含量。其次，溶解有机碳和有效铁含量的增加扩展了 Mesorhizobium 在根际的生态位宽度。因此，这些变化导致根际中根瘤菌群落发生改变，进而影响根中的 Mesorhizobium 结瘤和共生固氮速率。本研究结果提高了我们对有机肥料对共生固氮的影响及其潜在机制的认识，强调了溶解有机碳组成对根际中根瘤菌群落组成的关键作用。
