Soil salinization and alkalinization are major constraints to agricultural productivity globally, particularly in arid and semi-arid regions. These conditions severely limit the activity of soil microbes, which play a crucial role in nutrient cycling. Sustainable agricultural practices are needed to enhance crop yields in these challenging environments. Organic amendments, such as bio-fertilizers and rotten straw, offer a promising approach to improve soil health and crop productivity compared to inorganic amendments. Bio-fertilizers introduce beneficial soil microorganisms that enhance nutrient uptake by plants. Rotten straw, derived from plant residues, improves soil organic matter and nutrient availability. The combined application of bio-fertilizers and rotten straw has shown potential for remediating saline-alkaline soils, but further research is needed to understand their effects on rhizosphere microbial communities and crop yields under different plant genotypes. This study hypothesizes that the addition of bio-fertilizer and rotten straw to saline-alkaline soil will alter rhizosphere soil physicochemical properties, microbial communities, and improve the productivity of oats with different saline-alkaline tolerance levels. Two oat cultivars, one tolerant (Baiyan2) and one sensitive (Caoyou1) to saline-alkaline conditions, were used to investigate the combined effects of these amendments in a field experiment in northern China.

Previous studies have demonstrated the beneficial effects of organic amendments on soil health and crop production in saline-alkaline environments. Bio-fertilizers, composed of plant growth-promoting microorganisms, have been shown to improve soil fertility and productivity by enhancing nutrient availability to plants. Rotten straw, a readily available byproduct of agriculture, accelerates nutrient release compared to untreated straw, positively influencing soil physicochemical properties and enzyme activities. Combined application of bio-fertilizers and rotten straw is increasingly recognized as an effective strategy for saline-alkaline soil remediation. However, the impact of these amendments on rhizosphere bacterial communities and their interactions with different plant genotypes requires further investigation. Research has shown varying responses of different plant species and cultivars to soil amendments due to differences in rhizosphere microbial communities. While some studies have shown genotypic differences in alfalfa rhizosphere communities, others have found no such differences in soybean, canola, wheat, rice or maize. This study addresses the knowledge gap by focusing on how organic amendments affect the rhizosphere bacterial community structure of oat cultivars with varying levels of saline-alkaline tolerance.

A field experiment was conducted from 2016 to 2017 in a saline-alkaline region of Inner Mongolia, China. Two oat cultivars (Caoyou1 – sensitive, Baiyan2 – tolerant) were grown under four treatments: control (no amendment), bio-fertilizer only, rotten straw only, and combined bio-fertilizer and rotten straw. Soil physicochemical properties (pH, electrical conductivity, available N, P, K) and enzyme activities (catalase, alkaline phosphatase, urease, sucrase) were measured. Rhizosphere soil samples were collected for high-throughput sequencing of the V4 region of the 16S rRNA gene to analyze bacterial community composition and diversity. Data were analyzed using general linear models, permutational multivariate analysis of variance (PERMANOVA), principal component analysis (PCA), and redundancy analysis (RDA) to assess the effects of treatments and cultivars on soil properties, enzyme activities, oat yield, and bacterial community structure. Spearman's correlation analysis examined relationships between bacterial communities, soil properties, and oat yield. The experiment was a 4x2 factorial design in a randomized complete block design with three replications per year. Rotten straw was prepared by fermenting corn straw. Oat seeds were sown at a rate of 150 kg ha⁻¹ year⁻¹, and diammonium phosphate was applied as basal fertilizer. Amendments were incorporated into the soil before sowing. Soil samples were collected at pre-seeding and heading stages. Rhizosphere soil was carefully collected from around the roots. DNA was extracted from fresh soil samples using a Soil DNA Isolation Kit. 16S rRNA gene amplicon sequencing was performed using Illumina HiSeq2500. Bioinformatics analysis involved demultiplexing, quality filtering, chimera removal, OTU clustering, and taxonomic classification using the SILVA database. Statistical analysis was conducted using SAS 9.0 and R.

The combined bio-fertilizer and rotten straw amendment treatment resulted in the highest oat yields for both cultivars, significantly increasing grain yield and dry biomass production. The tolerant cultivar (Baiyan2) consistently exhibited higher yields than the sensitive cultivar (Caoyou1). The combined amendment treatment significantly reduced soil pH and increased soil water content but also resulted in higher soil salt content compared to the control. Rotten straw significantly increased available potassium, phosphorus, and nitrogen in the rhizosphere soil, whereas bio-fertilizer had less impact on these nutrients. Catalase and alkaline phosphatase activities were significantly increased by rotten straw and bio-fertilizer respectively, while the combined treatment decreased urease activity. All amendment treatments reduced bacterial richness in Baiyan2 but increased richness in Caoyou1. Baiyan2 displayed higher bacterial alpha diversity (Shannon index and Chao1) than Caoyou1. Rotten straw treatments decreased bacterial diversity, particularly in Caoyou1. PERMANOVA and PCA revealed significant effects of cultivar, amendment, and their interaction on bacterial community composition. The dominant phyla were Proteobacteria, Actinobacteria, and Firmicutes. Baiyan2 had higher Proteobacteria and lower Firmicutes abundance compared to Caoyou1. Rotten straw increased Firmicutes and decreased Proteobacteria abundance. RDA showed that communities in rotten straw treatments correlated positively with soil salt, available K, P, and grass yield. Spearman's correlation analysis revealed that soil salt, available K, P, and N were positively correlated with Firmicutes (Bacillus, Pseudarthrobacter, Planomicrobium) and negatively correlated with Proteobacteria (Sphingomonas, Massilia, Nocardioides, Pseudomonas).

The combined application of bio-fertilizer and rotten straw significantly improved oat productivity, particularly grain yield, in a saline-alkaline environment. This improvement can be attributed to several factors. The reduction in soil pH likely enhanced the availability of essential nutrients like nitrogen, phosphorus, and potassium. The increased availability of these nutrients, especially through the application of rotten straw, appears to have directly stimulated plant growth. The shift in the rhizosphere bacterial community, notably the increase in Firmicutes, which are often associated with nutrient cycling and stress tolerance, likely contributed to the improved plant growth. However, the increase in soil salinity associated with the combined treatment is a concern and requires further attention. While the tolerant cultivar (Baiyan2) inherently performed better, the combined amendment treatment significantly improved the yield of the sensitive cultivar (Caoyou1), bringing it closer to the yield of Baiyan2. The findings suggest that the combined use of organic amendments is a promising strategy for improving oat productivity in saline-alkaline soils, especially when combined with tolerant cultivars. However, monitoring and managing soil salinity are crucial when implementing this approach.

This study demonstrates the synergistic effects of bio-fertilizer and rotten straw amendments in improving oat productivity in saline-alkaline soil. The combined treatment significantly enhanced grain yield and biomass, primarily by reducing soil pH, increasing nutrient availability, and altering the rhizosphere bacterial community. The tolerant cultivar (Baiyan2) responded more favorably, but the combined treatment also significantly benefited the sensitive cultivar (Caoyou1). Further long-term studies are needed to investigate the long-term effects of these amendments on soil salinity and overall soil health. Considering the trade-off between improved nutrient availability and elevated soil salinity, employing tolerant cultivars remains a crucial aspect of successful remediation strategies.

This study was conducted over a two-year period. Long-term studies are needed to evaluate the sustained impact of these amendments on soil health and crop yields. The study focused on two specific oat cultivars; further research is warranted to assess the effects across a wider range of cultivars and other crops. The field experiment may not fully represent the variability encountered in different saline-alkaline environments. Finally, the mechanisms underlying the observed changes in bacterial communities and their relationship to plant productivity remain to be further elucidated.