Straw, Lupine Green Manure and PGPR Cut Soil Nitrous Oxide Emissions in Crop Rotation

Nitrous oxide is one of agriculture’s most stubborn climate problems. It is a greenhouse gas roughly 300 times more potent than carbon dioxide over a century, it is the dominant ozone-depleting substance emitted in the twenty-first century, and about 52 percent of total global emissions come from agricultural soils. Now a long-term field experiment in Ukraine has shown that a deceptively simple recipe — wheat straw, a lupine green manure crop, and a handful of beneficial bacteria — can cut nitrous oxide losses from fertilized soil by 16 to 29 percent while actually raising crop yields. The findings, published in Frontiers in Agronomy, offer a rare example of a climate mitigation strategy that does not ask farmers to sacrifice productivity.

The research team, led by Vitaliy Volkogon of the Institute of Agricultural Microbiology and Agro-Industrial Production of the National Academy of Agrarian Sciences of Ukraine, measured nitrous oxide fluxes from a leached light-loam Chernozem soil near Chernihiv over three growing seasons, from 2019 to 2021. The measurements were taken within a four-crop rotation of potato, spring barley, pea, and winter wheat, part of a field experiment established in 2009. By the time emissions monitoring began, the plots had undergone more than a decade of systematic fertilization, meaning the soil biological processes in each treatment had acquired distinct characteristics shaped by the type and combination of fertilizers applied.

The experimental design was ambitious. The team compared an unfertilized control with organic systems — 5 tonnes per hectare of winter wheat straw, post-harvest lupine green manure, or both — and with mineral fertilizers at low, medium, and high rates, from N40P40K40 up to N120P120K120 for potatoes and N30P30K30 to N90P90K90 for the grain crops. Crucially, they also tested mineral fertilizers applied against the background of the organic amendments, tracking both the direct effect in the year of incorporation and the residual effects over the following three years. Each treatment was duplicated in parallel blocks, one without microbial inoculation and one with pre-sowing treatment using plant growth-promoting rhizobacteria, or PGPR: Azospirillum brasilense for potato tubers and barley seeds, Rhizobium leguminosarum for peas, and Paenibacillus polymyxa for winter wheat.

Measuring nitrous oxide from field soil is technically demanding because the gas is produced by two microbial processes — aerobic nitrification, in which ammonia is oxidized, and anaerobic denitrification, in which nitrate serves as the terminal electron acceptor under low-oxygen conditions. The team used closed static chambers inserted 5 centimeters into the soil, sealed with a water lock, and exposed for three hours. To capture the full extent of nitrogen losses, they employed the acetylene inhibition technique: cups of water containing calcium carbide released acetylene inside the chambers, which blocks the reduction of nitrous oxide to benign dinitrogen. This forces any nitrous oxide that would otherwise have been further reduced to accumulate, allowing the researchers to quantify total emissions from both nitrification and denitrification. Gas samples were then analyzed by gas chromatography with an electron capture detector, and fluxes were extrapolated to a per-hectare, per-day basis.

The results confirmed what many studies have shown before: mineral fertilizers alone increased nitrous oxide emissions, and the losses rose proportionally with nitrogen rates. Under potato cultivation, applying 40 kilograms of nitrogen per hectare increased N-N2O losses by 72 to 98 percent relative to the control, depending on the growth stage; at 80 kilograms the increase reached 102 to 127 percent, and at the highest rate of 120 kilograms it climbed to 125 to 163 percent. But when the same fertilizer rates were applied over the background of straw and lupine green manure, the picture changed dramatically. Emission intensity dropped by 7 to 12 percent under potatoes, with similar reductions at the medium and high fertilizer rates.

The residual effects of the organic amendments proved equally important. In the first year after incorporation, when spring barley followed the potato crop, nitrous oxide losses under mineral fertilization combined with the residual organic background fell by 6 to 13 percent compared to mineral fertilizers alone. In the second year of residual action, under peas, the reduction widened to 10 to 18 percent across developmental stages. By the third year, under winter wheat, however, the mitigating capacity of the straw and green manure had largely faded — differences compared to mineral-only treatments were either statistically non-significant or marginal. The organic carbon, in other words, buys roughly two years of emission relief before its effect dissipates.

The PGPR inoculation added a persistent layer of mitigation on top of the organic carbon effect. Across all four crops and all fertilization backgrounds, pre-sowing inoculation lowered nitrous oxide emissions, with the largest reductions occurring when mineral fertilizers, organic matter, and bacteria were combined. Under peas, the integrated system of the lowest fertilizer rate plus residual straw, green manure, and Rhizobium inoculation cut emissions by 23 to 29 percent across growth stages compared to the same fertilizer rate applied alone. Under barley, the highest fertilizer rate combined with organic residues and Azospirillum reduced losses by up to 28 percent at germination compared to mineral fertilization alone.

What makes these results particularly compelling is the mechanism the researchers propose. The straw, with its wide carbon-to-nitrogen ratio of 77:1, and the lupine biomass, with a narrower ratio of about 16:1, together supplied roughly 3,600 kilograms of carbon per hectare. This carbon drives microbial immobilization of the mineral nitrogen that crops fail to assimilate — a fraction that can be substantial, given that nitrogen use efficiency from fertilizers typically ranges between 35 and 65 percent. Rather than leaving that surplus nitrogen available to nitrifiers and denitrifiers, the added carbon channels it into microbial biomass and organic pools, where it is temporarily locked away and can later mineralize as a slow-release nitrogen source for subsequent crops.

The bacterial strains appear to work through two complementary routes. Azospirillum brasilense and Rhizobium leguminosarum possess functional denitrification capabilities, meaning they can enzymatically reduce nitrous oxide to benign dinitrogen. But even Paenibacillus polymyxa, which lacks denitrifying capacity, still significantly reduced emissions under winter wheat. This strongly supports the hypothesis that inoculation-enhanced plant nitrogen uptake depletes the soil mineral nitrogen pool accessible to native nitrifiers and denitrifiers. Phytohormone production by the bacteria stimulates root system expansion, accelerating nutrient flux from the bulk soil — and crucially, this root growth stimulation occurs independently of soil nitrogen levels, unlike root nodulation in legumes.

Perhaps the most surprising finding was the yield data. Contrary to the conventional agronomic dogma that straw incorporation requires supplemental mineral nitrogen — typically at least 10 kilograms per tonne of straw — to avoid transient nitrogen immobilization that impairs crop nutrition, the experiment revealed consistent yield increases when straw and green manure were combined with mineral fertilizers. Under potatoes, the medium fertilizer rate plus organic residues produced 38.46 tonnes per hectare compared to 36.35 under mineral fertilization alone, a statistically significant gain. Under peas, all fertilizer doses applied over the second-year residual organic background produced significant yield increases. Pre-sowing PGPR inoculation added further productivity gains across the rotation, with the highest potato yield of 45.25 tonnes per hectare achieved under the fully integrated system of high fertilizer rates, organic residues, and inoculation — a 152 percent increment over the unfertilized control.

The authors argue that this challenges a long-standing assumption. Under modern, high-input fertilizer regimes, a fraction of applied mineral nitrogen routinely remains unassimilated by the crop. Rather than compensating with supplemental nitrogen when straw is incorporated, they suggest, that residual pool should be biologically immobilized into organic forms. Straw and related crop residues should serve as an exogenously supplied carbon source to drive microbial assimilation of surplus fertilizer nitrogen, simultaneously advancing soil carbon sequestration. The combined organic inputs also balanced each other’s mineralization rates — the straw releasing carbon slowly, the lupine biomass more rapidly — optimizing the timing of microbial carbon availability.

The findings arrive at a moment when alternative mitigation technologies face mounting scrutiny. Enhanced-efficiency fertilizers, nitrification inhibitors, and urease inhibitors can improve nitrogen use efficiency, but chemical inhibitors raise production costs and pose non-target environmental risks. Nitrapyrin, a widely used nitrification inhibitor, has been detected in stream water, indicating off-site transport, while residues of dicyandiamide have been found in cow milk in New Zealand, prompting regulatory bans on its use in pasture systems. Biological approaches based on PGPR strains avoid these pitfalls, and recent work with Stutzerimonas stutzeri has shown similar potential for mitigating soil nitrous oxide emissions through microbial community shifts.

The study is not without limitations. The acetylene inhibition technique, while cost-effective and feasible for large-scale field evaluations, is less sensitive than nitrogen-15 isotope tracing, and the researchers acknowledge specific methodological constraints. The literature on crop residue effects also remains conflicting: some studies report straw mulching reducing emissions by 23 to 34 percent, while others document increased fluxes when straw is co-applied with mineral fertilizers, attributed to enhanced carbon and nitrogen substrate availability promoting microbial activity. A meta-analysis of cover crops found increased nitrous oxide emissions in 60 percent of evaluated studies, regardless of species. The Ukrainian team hypothesizes that these discrepancies derive from differences in residue carbon-to-nitrogen ratios, cropping system architecture, and agronomic management practices.

Nevertheless, the overall message is clear and operationally actionable. Integrating mineral nitrogen fertilizers with straw, intercropped green manures, and pre-sowing microbial inoculation provides an effective management strategy to suppress gaseous nitrogen losses and improve yields within crop rotations. These practices align directly with the principles of sustainable agricultural intensification: achieving higher yields from existing arable land while minimizing environmental externalities and enhancing ecological services. For a world grappling with both food security and climate change, a strategy that cuts a potent greenhouse gas by up to 29 percent while boosting potato yields by half again over the control is the kind of trade-off agriculture has been searching for.

Subject of Research: Soil N2O emission intensity under different crop fertilization systems and the use of PGPR

Article Title: Soil N2O emission intensity under different crop fertilization systems and the use of PGPR

Article References: Volkogon, V., Volkogon, K., & Sydorenko, V. (2026). Soil N2O emission intensity under different crop fertilization systems and the use of PGPR. Frontiers in Agronomy, 8, Article 1959485. https://doi.org/10.3389/fagro.2026.1959485

Image Credits: AI Generated

DOI: 10.3389/fagro.2026.1959485

Keywords: Soil, emission, intensity, under, different, crop, fertilization, systems, PGPR, scientific research