Decades of Manure and Straw Reshape the Hidden Bacterial World Beneath Farm Fields

Beneath every farm field lies an invisible workforce that determines how well crops grow, how nutrients move through the soil, and how resilient the land is to drought, disease and disturbance. A new long-term study from a semi-arid research farm in Hisar, India, has now shown that the way farmers feed their soils can profoundly reorganize this hidden workforce. By combining DNA sequencing of soil bacteria with network analysis, researchers found that decades of integrated nutrient management, blending chemical fertilizers with organic amendments such as farmyard manure, wheat straw and green manure, produced richer, more diverse and structurally different bacterial communities than either untreated soil or fertilizer alone.

The research draws on one of the longest running field experiments in the region, established during the Kharif season of 1985 at the research farm of CCS Haryana Agricultural University. The site sits in a hot semi-arid climate classified as BSh under the Köppen system, receiving just 443 millimeters of annual rainfall, most of it between July and September. Summer temperatures can climb to 47 degrees Celsius while winter minima fall to 1 degree. The soil is a sandy loam classified as Typic Ustochrept, and the experimental plots follow an annual pearl millet and wheat rotation. From twelve treatments in a randomized complete block design, the team selected five for metagenomic analysis: an unfertilized control, NPK fertilizer alone, and three integrated treatments combining NPK with farmyard manure, wheat straw or green manure grown from dhaincha, a Sesbania species.

To read the bacterial census of these soils, the researchers extracted DNA and sequenced the V3-V4 region of the 16S rRNA gene, the standard molecular barcode for identifying bacteria. After quality filtering with the QIIME2 pipeline and chimera removal, more than 409,000 high-quality sequences were classified against the SILVA reference database. The team then computed alpha diversity metrics including observed species, Shannon index and Faith’s phylogenetic diversity, and assessed beta diversity using Bray-Curtis and UniFrac distances, visualized through principal coordinate analysis and hierarchical clustering.

The results were striking. Across all five samples, three phyla dominated: Proteobacteria, Firmicutes and Actinobacteria, with smaller contributions from Chloroflexi, Acidobacteria and Bacteroidetes. But the balance shifted dramatically depending on the treatment. In the control, NPK-only and NPK plus green manure soils, Proteobacteria and Firmicutes together accounted for more than 60 percent of all phyla. In the NPK plus farmyard manure and NPK plus wheat straw soils, Proteobacteria and Actinobacteria rose above Firmicutes, signaling a genuine compositional reorganization rather than a minor fluctuation.

Diversity metrics told an even clearer story. The number of observed species ranged from just 260 in the untreated control to 1,250 in the NPK plus farmyard manure soil, nearly a fivefold difference. The control consistently recorded the lowest values for richness, evenness and phylogenetic diversity, while the farmyard manure and wheat straw treatments ranked highest on all three measures. The NPK-only and green manure treatments fell in between. At the genus level, the control and NPK soils were dominated by Pseudomonas and Bacillus, whereas the farmyard manure soil was characterized by Sphingomonas, and the wheat straw soil carried notable abundances of Bacteroides and Nocardioides. The green manure soil hosted elevated levels of Stenotrophomonas, Delftia, Achromobacter and Coxiella.

Statistical tests confirmed that these differences were not noise. Principal coordinate analysis separated the organically amended soils from the control and NPK-only soils along the first two axes, which together explained nearly 74 percent of the variation. A permutational multivariate analysis of variance found that 63 percent of the differences in microbial signatures were directly linked to whether a soil belonged to the unamended or organically amended group, a remarkably high value indicating a coordinated ecological shift rather than random sampling variation. A similarity percentage analysis identified Bacillus, Sphingomonas and Delftia as the largest contributors to the dissimilarity, with just four genera accounting for over 55 percent of the community variance between groups.

Perhaps the most intriguing finding came from differential network analysis, which maps how bacterial genera interact with one another. The unamended and NPK-only soils formed densely connected networks with highly linked hub taxa such as Bacillus, Pseudomonas and Streptomyces. The organically amended soils, by contrast, showed sparser networks with more negative associations. Counterintuitively, the researchers argue this structural loosening is a sign of a healthier ecosystem. Dense, highly interdependent networks are vulnerable, because a disturbance hitting a single hub taxon can cascade through the whole community. Sparser, more compartmentalized networks imply niche partitioning and functional redundancy, where multiple taxa can perform overlapping roles such as nutrient cycling without the system depending on any single vulnerable species.

The specific bacteria enriched under each amendment also make ecological sense. Farmyard manure introduces complex, recalcitrant carbon compounds, favoring metabolically versatile Sphingomonas species equipped with enzymes that degrade aromatic compounds and tough biopolymers. Green manure from Sesbania delivers a rapid pulse of fresh, nitrogen-rich plant tissue, selecting for fast-growing copiotrophs like Stenotrophomonas that excel at breaking down easily decomposable material. Many of the genera detected across the treatments are known plant growth promoters: Bacillus, Pseudomonas and Streptomyces solubilize phosphorus and suppress pathogens, while Bradyrhizobium and related rhizobia fix atmospheric nitrogen. Nitrogen fixers such as Methylobacterium and Acetobacter appeared under NPK treatment, and decomposers like Megasphaera and Succinivibrio were unique to the wheat straw soil.

The study does carry a methodological caveat. Soil from the four field replicates of each treatment was pooled before DNA extraction, which prevents statistical assessment of variation within treatments. The authors acknowledge this limitation but note that the macro-level divergence between the two groups remained statistically robust, supported by the high PERMANOVA value and consistent clustering across multiple analyses. The raw sequence data have been deposited in the NCBI Sequence Read Archive, allowing other researchers to scrutinize and extend the findings.

For a world that must raise agricultural output by an estimated 70 to 100 percent by 2050 to feed a population exceeding nine billion, the message from this semi-arid Indian field is timely. Nearly four decades of combining inorganic fertilizers with farmyard manure, straw or green manure did not merely maintain soil fertility; it cultivated a richer, more resilient bacterial ecosystem beneath the surface. As the authors conclude, aligning agricultural practices with the functional complexity of soil microbial communities may be one of the most powerful and underappreciated levers for building farming systems that can endure the pressures of the coming decades.

Subject of Research: Effects of long-term integrated nutrient management on soil bacterial diversity and community structure

Article Title: Long term integrated nutrient management promotes soil bacterial diversity and beneficial community structure

Article References: Ahlawat, V., Ahalawat, N., Boora, N., Dadarwal, R. S., Dhanda, D., Sangwan, P., Kumar, V., Kumar, R., Kumar, V., & Yadav, P. K. (2026). Long term integrated nutrient management promotes soil bacterial diversity and beneficial community structure. Discover Soil, 3(1), Article 133. https://doi.org/10.1007/s44378-026-00286-x

Image Credits: AI Generated

DOI: 10.1007/s44378-026-00286-x

Keywords: soil microbiome, integrated nutrient management, 16S rRNA sequencing, farmyard manure, green manure, wheat straw, NPK fertilizer, bacterial diversity, Proteobacteria, Actinobacteria, sustainable agriculture, microbial networks