Pathogenic nematodes exploit Achilles' heel of plant symbioses

Cyst nematode parasites disrupt beneficial associations of crops with rhizobia and mycorrhiza. Chen et al. discovered the mechanism and demonstrated that the soybean cyst nematode Heterodera glycines secretes a chitinase that destroys key symbiotic signals from the microbial symbionts. The authors further developed a chitinase inhibitor that alleviates symbiosis inhibition.

Soil carbon-nutrient cycling, energetics, and carbon footprint in calcareous soils with adoption of long-term conservation tillage practices and cropping systems diversification

Calcareous soils, comprising vast areas in northern and eastern parts of India, are characterized by low soil organic carbon (SOC) with high free CaCO3 that results in low nutrient bioavailability with poor soil structure. Improvement of this soil can be achieved with conservation tillage with residue retention coupled with diversification of cropping system including legumes, and oilseeds in the system. Concerning all these, a long-term experiment was carried out in the calcareous soils having low organic carbon and high free CaCO3 (∼33 %) with varied tillage practices, viz. permanent bed with residue (PB), zero tillage with residue (ZT), and conventional tillage without residue (CT); and cropping systems viz. maize-wheat-greengram (MWGg), rice-maize (RM), and maize-mustard-greengram (MMuGg) during 2015-2021. From this study, it was observed that PB and ZT resulted in ∼25-30 % increment in SOC compared to the initial SOC, while CT showed a 4 % decrease in the SOC. Conservation tillage practices also resulted in better soil aggregation and favourable bulk density of the soil. Furthermore, PB and ZT practice exhibited 10-13 %; 15-18 %; 11-15 %; 40-60 %, 20-36 %, and 23-45 % increments in the soil available N, P, K, soil microbial biomass carbon, dehydrogenase activity, and urease activity, respectively over those under CT. Crop diversification with the inclusion of legume and oilseed crops (MMuGg, and MWGg) over cereal-dominated RM systems resulted in better soil health. Maize equivalent yield and energy use efficiency (%) were also found to be the maximum under PB, and ZT, in combination with the MMuGg system. ZT and PB also reduced the carbon footprint by 465 and 822 %, respectively over CT by elevating SOC sequestration. Hence, conservation tillage practices with residue retention coupled with diversification in maize-based cropping systems with mustard and greengram can improve soil health, system productivity, and energetics, and reduce the carbon footprint in calcareous soils.

Native Rhizobia Improve Plant Growth, Fix N2, and Reduce Greenhouse Emissions of Sunnhemp More than Commercial Rhizobia Inoculants in Florida Citrus Orchards

Sunnhemp (Crotalaria juncea L.) is an important legume cover crop used in tree cropping systems, where there is increased interest by growers to identify rhizobia to maximize soil nitrogen (N) inputs. We aimed to isolate and identify native rhizobia and compare their capabilities with non-native rhizobia from commercial inoculants to fix atmospheric dinitrogen (N₂), produce and reduce nitrous oxide (N₂O), and improve plant growth. Phylogenetic analyses of sequences of the 16S rRNA and recA, atpD, and glnII genes showed native rhizobial strains belonged to Rhizobium tropici and the non-native strain to Bradyrhizobium japonicum. Plant nodulation tests, sequencing of nodC and nifH genes, and the acetylene-dependent ethylene production assay confirmed the capacity of all strains to nodulate sunnhemp and fix N₂. Inoculation with native rhizobial strains resulted in significant increases in root and shoot weight and total C and N contents in the shoots, and showed greater N₂-fixation rates and lower emissions of N₂O compared to the non-native rhizobium. Our results suggest that native rhizobia improve plant growth, fix N₂, and reduce greenhouse emissions of sunnhemp more than commercial rhizobia inoculants in Florida citrus orchards.

Micromanaging the nitrogen cycle in agroecosystems

While large inputs of synthetic nitrogen fertilizers enable our current rate of crop production and feed a growing global population, these fertilizers come at a heavy environmental cost. Driven by microbial processes, excess applied nitrogen is lost from agroecosystems as nitrate and nitrous oxide (N₂O) contaminating aquatic ecosystems and contributing to climate change. Interest in nitrogen-fixing microorganisms as an alternative to synthetic fertilizers is rapidly accelerating. Microbial inoculants offer the promise of a sustainable and affordable source of nitrogen, but the impact of inoculants on nitrogen dynamics at an ecosystem level is not fully understood. This review synthesizes recent studies on microbial inoculants as tools for nutrient management and considers the ramifications of inoculants for nitrogen transformations beyond fixation.

Profound Change in Soil Microbial Assembly Process and Co-occurrence Pattern in Co-inoculation of Bradyrhizobium japonicum 5038 and Bacillus aryabhattai MB35-5 on Soybean

Rhizosphere microbial communities are vital for plant growth and soil sustainability; however, the composition of rhizobacterial communities, especially the assembly process and co-occurrence pattern among microbiota after the inoculation of some beneficial bacteria, remains considerably unclear. In this study, we investigated the structure of rhizomicrobial communities, their assembly process, and interactions contrasting when Bradyrhizobium japonicum 5038 and Bacillus aryabhattai MB35-5 are co-inoculated or Bradyrhizobium japonicum 5038 mono-inoculated in black and cinnamon soils of soybean fields. The obtained results indicated that the Chao and Shannon indices were all higher in cinnamon soil than that in black soil. In black soil, the co-inoculation increased the Shannon indices of bacteria comparing with that of the mono-inoculation. In cinnamon soil, the co-inoculation decreased the Chao indices of fungi comparing with that of mono-inoculation. Compared with the mono-inoculation, the interactions of microorganisms of co-inoculation in the co-occurrence pattern increased in complexity, and the nodes and edges of co-inoculation increased by 10.94, 40.18 and 4.82, 16.91% for bacteria and fungi, respectively. The co-inoculation of Bradyrhizobium japonicum 5038 and Bacillus aryabhattai MB35-5 increased the contribution of stochastic processes comparing with Bradyrhizobium japonicum 5038 inoculation in the assembly process of soil microorganisms, and owing to the limitation of species diffusion might restrict the direction of pathogenic microorganism movement. These findings support the feasibility of rebuilding the rhizosphere microbial system via specific microbial strain inoculation and provide evidence that the co-inoculation of Bradyrhizobium japonicum 5038 and Bacillus aryabhattai MB35-5 can be adopted as an excellent compound rhizobia agent resource for the sustainable development of agriculture.

Contribution, Utilization, and Improvement of Legumes-Driven Biological Nitrogen Fixation in Agricultural Systems

Legumes improve soil fertility through the symbiotic association with microorganisms, such as rhizobia, which fix the atmospheric nitrogen and make nitrogen available to the host and other crops by a process known as biological nitrogen fixation (BNF). Legumes included in the cropping system improve the fertility of the soil and the yield of crops. The advantages of legumes in the cropping system are explained in terms of direct nitrogen transfer, residual fixed nitrogen, nutrient availability and uptake, effect on soil properties, breaking of pests' cycles, and enhancement of other soil microbial activity. The best benefits from the legumes and BNF system can be utilized by integrating them into cropping systems. The most common practices to integrate legumes and their associated BNF into agricultural systems are crop rotation, simultaneous intercropping, improved fallows, green manuring, and alley cropping. However, the level of utilizing nitrogen fixation requires improvement of the systems, such as selecting appropriate legume genotypes, inoculation with effective rhizobia, and the use of appropriate agronomic practices and cropping systems. Therefore, using legumes at their maximum genetic potential, inoculation of legumes with compatible rhizobia, and using appropriate agronomic practices and cropping systems are very important for increasing food production. Importantly, the utilization of legumes as an integral component of agricultural practice in promoting agricultural productivity has gained more traction in meeting the demand of food production of the world populace. Priority should, thus, be given to value the process of BNF through more sustainable technologies and expansion of knowledge to the system.