Swiss Farms Cut Nitrogen Pollution for 40 Years, But Their Soils Are Quietly Paying the Price

Nitrogen is the invisible engine of modern agriculture. Since the Haber-Bosch process made it possible to convert inert atmospheric gas into fertilizer more than a century ago, humanity has doubled the amount of reactive nitrogen circulating through the biosphere, and roughly half the global population now depends on crops nourished by synthetic fertilizer. Yet the same element that feeds billions also leaks relentlessly into the environment: ammonia and nitrous oxide drift into the atmosphere, nitrate seeps into groundwater, and rivers carry excess nutrients toward coastal seas, fueling algal blooms and dead zones. Tracking how efficiently a nation uses its nitrogen, and where the losses actually go, has therefore become one of the central challenges of sustainability science. A new study published in Biogeosciences takes on that challenge with unusual ambition, reconstructing four decades of nitrogen flows across the entire agricultural landscape of Switzerland at a resolution of one square kilometer.

The research team, led by Jize Jiang of ETH Zurich together with colleagues from Eawag, Agroscope, and University College Dublin, combined detailed national datasets with the process-based biogeochemical model DayCent to simulate nitrogen cycling in the country’s two dominant agricultural ecosystems: croplands and managed grasslands. DayCent simulates plant growth, soil organic matter decomposition, and the microbial transformations that govern nitrogen fate on a daily timestep, driven by weather data from MeteoSwiss, a new national soil database, land-use statistics, and records of fertilizer application. The simulations covered 14,926 grid cells of cropland and 36,140 grid cells of grassland from 1981 to 2020, capturing fifteen crops that together account for more than 95 percent of Swiss arable land, along with meadows classified by management intensity. To quantify uncertainty, the team ran a Monte Carlo ensemble of 200 iterations across a representative subset of 400 grid cells, varying nine parameters that control leaching, nitrification, and denitrification.

The headline finding is genuinely encouraging. Between the 1980s and the 2010s, nitrogen use efficiency, defined as the fraction of nitrogen inputs that ends up in harvested products, rose from 47 percent to 57 percent in croplands and from 63 percent to 71 percent in grasslands. Over the same period, total nitrogen losses through leaching and gaseous emissions fell by 24 percent in croplands and 4 percent in grasslands, even as yields remained essentially stable. The timing of these improvements is no coincidence. In the 1990s, Switzerland fundamentally restructured its agricultural policy, replacing guaranteed government purchases with direct payments tied to ecological performance. A 1996 constitutional amendment formally recognized agriculture’s environmental stewardship role, and the 1998 Proof of Ecological Performance made subsidies conditional on balanced nutrient budgets. By 2005, 97 percent of Swiss agricultural land was managed under these cross-compliance standards, and average synthetic fertilizer consumption had dropped by roughly a quarter during the decade of reform.

The scale of the policy effect becomes vivid when the results are mapped using the framework of the European Nitrogen Experts Panel, which classifies farmland into six operating regimes based on nitrogen use efficiency, yield, and surplus. In the 1980s, only 26 percent of Swiss croplands and 15 percent of grasslands fell within the so-called characteristic operating space, the zone representing optimal performance. By the 2010s, those figures had climbed to 56 percent and 77 percent respectively. Extremely high fertilizer inputs to grasslands, exceeding 350 kilograms of nitrogen per hectare per year, were largely abolished after 2011, and the optimal zone expanded geographically into the central plateau where intensive crop production dominates. Grasslands showed the most consistent improvement, while croplands displayed a more mixed picture, with some areas shifting from excessive pollution toward regimes of insufficient productivity as inputs were cut back.

But the study’s most striking result lies beneath the surface, literally. Despite the celebrated gains in efficiency, the modeling revealed that Swiss cropland soils have been steadily losing nitrogen. Between 1995 and 2011, cropland soils were depleted at an average rate of 23 kilograms of nitrogen per hectare per year, and the cumulative loss across the full 1981 to 2020 period reached 537 kilograms per hectare. The pattern is corroborated by independent evidence: long-term field monitoring at several arable sites shows soil nitrogen decline, and national soil carbon monitoring recorded a drop in topsoil organic carbon from 62 to 55 tonnes per hectare between the late 1980s and late 2010s, a decline that typically travels hand in hand with nitrogen loss. Grasslands, by contrast, accumulated 728 kilograms of nitrogen per hectare over the same four decades, although the rate of accumulation slowed as fertilizer inputs fell, and by the final five years of simulation both ecosystem types appeared to be approaching equilibrium.

The mechanism behind cropland depletion is a convergence of pressures. Nitrate leaching, which accounts for roughly 80 percent of total nitrogen losses from croplands and more than 40 percent of all nitrogen inputs, is amplified by annual cultivation, which disturbs soil structure and leaves fields bare and vulnerable during winter. Wheat, Switzerland’s most widely grown crop, is a particularly aggressive extractor of soil nitrogen, so harvest itself becomes a major export pathway. Meanwhile, the policy-driven reduction in fertilizer inputs, while cutting pollution, also narrowed the margin between what farmers put into their fields and what crops, microbes, and water remove. Scenario simulations underscored this tension: cutting fertilizer rates by 20 percent reduced nitrogen losses by about 20 percent and harvests by only 3 to 6 percent, but it deepened soil nitrogen depletion in croplands and slowed accumulation in grasslands. Reducing losses alone, the authors conclude, is not enough to protect soil fertility if inputs fall in step.

Climate adds another complicating layer. Sensitivity tests showed that a uniform 2-degree Celsius increase in air temperature raised simulated nitrogen losses by about 6 percent in croplands and 16 percent in grasslands, because warming accelerates soil nitrogen turnover and gaseous emissions. This suggests that climate change has partially offset the pollution benefits of reduced fertilization, particularly in grasslands, where relative losses barely declined despite substantially lower inputs. The study also found that the amount of nitrogen applied matters far more than its source: swapping organic manure for synthetic fertilizer, or the reverse, changed total losses by only a few percent, whereas cutting the total input produced large reductions. Notably, biological nitrogen fixation by legumes and grass-clover mixtures supplied 4 to 15 percent of cropland inputs and 22 to 33 percent of grassland inputs, and systems relying more on fixation tended to achieve higher efficiency.

What makes this work resonate far beyond the Alps is its diagnostic framework. Most national nitrogen assessments track inputs, outputs, and surplus, but neglect the soil nitrogen stock itself, on the assumption that depletion only occurs where inputs are insufficient. The Swiss case demolishes that assumption: soil nitrogen mining can occur even under high-input, intensively managed agriculture, hidden behind impressive efficiency statistics. The authors propose a three-pillar evaluation, jointly considering nitrogen use efficiency, nitrogen losses, and soil nitrogen stock changes, as a transferable tool for any country pursuing pollution targets while safeguarding long-term productivity. Similar policy-driven turnarounds have been documented in China, where targeted nitrogen management between 2007 and 2017 delivered simultaneous gains in yields and environmental outcomes, suggesting that system-level change is achievable within a single generation when science, practice, and policy align.

The caveats are acknowledged with unusual candor. DayCent’s simplified water-flow module may overestimate drainage, its treatment of ammonia volatilization is coarse and likely underestimates those emissions while inflating nitrate leaching, and cover crops, a policy measure introduced in the late 1990s that could reduce leaching by around 10 percent, were not represented due to insufficient data. Yet the Monte Carlo analysis showed that parameter uncertainty was smaller than the observed long-term trends, lending confidence to the central conclusions. For Switzerland, the message is clear: the 1990s reforms genuinely worked, delivering stable production with less pollution, but the nation must now widen its lens to include the soil bank account it has been quietly drawing down. For the many high-income countries facing the same productivity-versus-pollution trade-off, and for low- and middle-income nations at risk of mining their soils, the Swiss four-decade record offers both a warning and a roadmap: efficiency and emission targets are necessary, but only integrated nitrogen management that watches the soil itself can secure agriculture’s foundations for the long run.

Subject of Research: National-scale assessment of nitrogen use efficiency, nitrogen losses, and soil nitrogen depletion in Swiss agriculture over 40 years

Article Title: Balancing nitrogen use efficiency, losses and soil nitrogen depletion to evaluate national scale agri-environmental performance over 40 years

Article References: Jiang, J., Winkel, L. H. E., Wüst-Galley, C., Bretscher, D., Necpalova, M., Stenke, A., & Six, J. (2026). Balancing nitrogen use efficiency, losses and soil nitrogen depletion to evaluate national scale agri-environmental performance over 40 years. Biogeosciences, 23(18), 6741-6761. https://doi.org/10.5194/bg-23-6741-2026

Image Credits: AI Generated

DOI: 10.5194/bg-23-6741-2026

Keywords: nitrogen use efficiency, nitrogen losses, soil nitrogen depletion, Swiss agriculture, DayCent model, biogeochemical modeling, agri-environmental policy, nitrate leaching, grasslands, croplands, biological nitrogen fixation, sustainable agriculture