To estimate nitrate stored in the unsaturated zone on a global scale between 1900 and 2000, the British Geological Survey (BGS) linked mathematical models of nitrate leaching from the base of the soil zone with estimates of groundwater recharge, unsaturated-zone porosity, and the depth to groundwater.
British Geological Survey quantifies nitrate storage in unsaturated zone
Its modeling showed substantial continuous increase in nitrate stored in the unsaturated zone over the last century. The peak nitrate storage in 2000 was estimated to be up to 1814 teragrams of nitrogen. This is equivalent to up to 200% of the inorganic nitrogen stored in soils. The unsaturated-zone nitrate storage is greatest in North America, China, and central and eastern Europe, where the depth to water table is large and there is an extensive history of agricultural fertilizer use.
To better understand trends in nitrate storage, the BGS grouped the nitrate storage responses for river catchments across the globe. The groups show clear differences and trends, with a distinct split between developed (United States, Europe) and developing (Africa, East Asia) countries.
In the developed countries, current nitrate leaching into the unsaturated zone is decreasing because of improved regulation and farming practice. In contrast, in the developing countries nitrate leaching shows continuous increases associated with rapid, early development and growing intensification of fertilized agriculture.
The BGS concluded the large store of nitrate in the unsaturated zone means using soil-nitrate leaching estimates alone as an indicator of nitrate pollution is likely to be inappropriate. The distribution of unsaturated-zone nitrate storage can give policy makers and decision makers a first global indication of where this store may be significant and where delays in improvements in groundwater and surface water quality can be expected. This is important for assessing the effectiveness of nitrate management measures and timescales for achieving environmental objectives.
The different nitrate storage characteristics observed highlight the need for different management strategies to tackle nitrate pollution across developing and developed countries. In both cases, it is essential catchment retention processes—such as unsaturated-zone storage—are considered. Other temporary stores of nitrate such as storage in soil organic matter and in riparian zones also need to be quantified if fully integrated pollution-management strategies are to be developed.