Nitrogen Fertilizer Pollution

Risksv03.00 credits/per_km2

Responsible:Niccolò Calandri

Nitrogen Fertilizer Pollution — Soil & Water Contamination from Agricultural Nitrogen

Overview

Nitrogen Fertilizer Pollution maps the estimated environmental pollution caused by the application of nitrogen-based fertilizers in agriculture. Nitrogen (N) is the most widely applied macronutrient in global farming systems and, while essential for crop growth, a substantial fraction of applied nitrogen is lost to the environment through leaching to groundwater, volatilization as ammonia (NH₃), surface runoff, and microbial denitrification producing nitrous oxide (N₂O).

This indicator is critical for biodiversity monitoring because excess reactive nitrogen is one of the most pervasive drivers of ecosystem degradation worldwide. Nitrogen deposition in aquatic ecosystems triggers eutrophication — the excessive growth of algae and aquatic plants — which depletes dissolved oxygen (hypoxia), collapses food webs, and creates "dead zones" in rivers, lakes, and coastal waters. In terrestrial ecosystems, nitrogen deposition alters soil chemistry, favours fast-growing nitrophilous species at the expense of specialist flora, reduces plant diversity, and degrades habitats for dependent fauna such as pollinators and soil invertebrates. Additionally, N₂O emissions from fertilized soils are a potent greenhouse gas with a Global Warming Potential 298 times that of CO₂, linking fertilizer pollution directly to climate change and its cascading effects on biodiversity.

The data come from the NPKGRIDS dataset (v1.08), a global gridded inventory of nitrogen, phosphorus, and potassium fertilizer application rates at ~10 km resolution (5 arc-minutes), published by the University of Minnesota / Global Landscapes Initiative. The variable Nrate represents the total nitrogen application rate in kg N/ha/year, aggregated across all 175 crop classes present in the dataset. A 50% loss fraction is applied to estimate the portion of nitrogen that becomes environmental pollution, based on established estimates of nitrogen use efficiency in global agriculture.

Calculation Methodology

The indicator is derived through the following processing steps:

NetCDF data access — The NPKGRIDS dataset is read from AWS S3 as NetCDF files via xarray. The variable Nrate (kg N/ha/year) is extracted for each grid cell
Aggregation across crop classes — Nitrogen application rates are summed across all 175 crop classes present in the dataset, producing a single total nitrogen application map
Pollution fraction estimation — A 50% loss fraction is applied to the total nitrogen application rate:
```
N_pollution = Nrate_total × 0.50
```
This fraction accounts for nitrogen lost through: leaching to groundwater (nitrate contamination), volatilization as ammonia (NH₃), atmospheric emissions of nitrous oxide (N₂O), and surface runoff contributing to eutrophication
Bilinear interpolation — The data are upscaled 6× from ~10 km native resolution to approximately 1.7 km output resolution using bilinear interpolation
Colormap and visualization — The gnuplot2 colormap is applied with a PowerNorm normalization (gamma=0.4) to enhance visibility of lower pollution values. The colormap range spans from 0 to 9,720 kg/ha

Buffer: 25,000 m (25 km) around the polygon, to account for the coarse native resolution.

Data Sources

Code	Name	Provider	Resolution	Availability
`WRD_NPKGR_20`	NPKGRIDS v1.08	Nature Scientific Data / University of Minnesota	~10 km (5 arc-min)	2020-01-01 — 2020-12-31

Quality Range and Levels

Indicator	Unit	Range	Inverted
`fertilizer_pollution_n`	kg N/ha/yr	[0, 50, 100, 200, 500, 1000]	Yes

Inverted = Yes: lower values of nitrogen pollution indicate less environmental contamination and conditions more favourable for biodiversity and ecosystem health.

Level	kg N/ha/yr	Interpretation
A (Excellent)	0 – 50	Minimal nitrogen pollution — near-natural conditions, low eutrophication risk
B (Good)	50 – 100	Low nitrogen pollution — moderate agricultural influence, limited ecological impact
C (Moderate)	100 – 200	Moderate nitrogen pollution — noticeable pressure on aquatic and terrestrial ecosystems
D (Poor)	200 – 500	High nitrogen pollution — significant eutrophication risk, groundwater contamination likely
E (Critical)	> 500	Very high nitrogen pollution — intensive agriculture, severe ecosystem degradation

Technical Notes

Nitrogen application rates are summed across all 175 crop classes in the NPKGRIDS dataset
The 50% loss fraction is a global average estimate; actual losses vary by climate, soil type, and management practices (range: 30–70%)
Bilinear interpolation from ~10 km to ~1.7 km improves spatial detail but does not add real information at sub-grid scale
The gnuplot2 colormap with PowerNorm (gamma=0.4) compresses the upper range, improving differentiation at low-to-moderate pollution levels
The 25 km buffer is necessary due to the coarse native resolution of the source dataset
Data represent the year 2020; no temporal series is currently available

References

Lu, C. & Tian, H. (2017). "Global nitrogen and phosphorus fertilizer use for agriculture production in the past half century: shifted hot spots and nutrient imbalance". Earth System Science Data, 9(1), 181–192. DOI: 10.5194/essd-9-181-2017
Lun, F. et al. (2024). "NPKGRIDS — Global gridded dataset of nitrogen, phosphorus, and potassium fertilizer application rates". Nature Scientific Data. https://www.nature.com/articles/s41597-024-04030-4
Galloway, J.N. et al. (2003). "The nitrogen cascade". BioScience, 53(4), 341–356.

Data Sources & References

Sources

WRD_NPKGR_20

Methodology

The Fertilizer Pollution N map shows nitrogen (N) pollution using NPKGRIDS v1.08. Nitrogen application rates are summed over 175 crop classes. A 50% loss fraction is applied (leaching, NH3 volatilization, N2O denitrification, runoff). Data is bilinearly interpolated from ~10 km to ~1.7 km and displayed with gnuplot2 colormap (PowerNorm gamma=0.4).