Modelled global increase in nitrate stored in the vadose zone between 1900 and 2000. Nitrate storage (in Tg N) is modelled under the baseline model run (black) and from sensitivity analyses (red and blue for +/− 50 percent travel time and nitrate leaching, respectively. Graphic: Ascott, et al., 2017 / Nature Communications

By Roger Harrabin
10 November 2017

(BBC News) – Huge quantities of nitrate chemicals from farm fertilisers are polluting the rocks beneath our feet, a study says.

Researchers at the British Geological Survey say it could have severe global-scale consequences for rivers, water supplies, human health and the economy.

They say the nitrate will be released from the rocks into rivers via springs.

That will cause toxic algal blooms and fish deaths, and will cost industry and consumers billions of pounds a year in extra water treatment.

In a paper in Nature Communications, the scientists from BGS and Lancaster University estimate that up to 180 million tonnes of nitrate are stored in rocks worldwide - perhaps twice the amount stored in soils.

They say this is the first global estimate of the amount of nitrate trapped between the soil layer and the water-bearing aquifers below. They warn that over time the nitrate will inevitably slowly seep into the aquifers.

Most nitrate, the team says, is in rocks in North America, China, and Europe where fertiliser has been lavishly applied for decades. […]

Matthew Ascott, hydrogeologist at the BGS and lead author, said: "With big investments being made to reduce water pollution through changes in farming, it is vital that we understand what pollution is already in the environment.

"Water and the pollutant travels through the rocks below our feet very slowly. This and a history of intensive agriculture means that a large store of nitrate pollution has built up over time.

"When this pollution is released, it will continue to impact water quality for decades, in some cases, even where controls on fertiliser use have been put in place."

This is what's known as the “nitrate timebomb”. [more]

Scale of 'nitrate timebomb' revealed


Spatial distribution of nitrate stored in the vadose zone. Global vadose zone N storage (in kg N ha−1) is shown for 1925 (a), 1950 (b), 1975 (c) and 2000 (d). Graphic: Ascott, et al., 2017 / Nature Communications

10 November 2017 (British Geological Survey) – The unsaturated (vadose) zone between the base of soils and the water table can be an important store of nitrate. Water moves slowly downward through the unsaturated zone and so a large store of nitrate can accumulate if this water contains nitrate derived from surface sources such as fertiliser. Release of this store can affect ground- and surface water quality for decades and it can continue for a long time after changes in farming practice that reduce nitrate leaching.

To better understand the extent of the problem, the BGS has made the first global-scale quantification of nitrate stored in the vadose zone between 1900 and 2000.

Increases in nitrate storage in the unsaturated zone though space and time

To estimate nitrate stored in the unsaturated zone, we 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.

Our modelling showed a 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 (Tg) of nitrogen (N). This is equivalent to up to 200 per cent of the inorganic N stored in soils. Spatially, 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 fertiliser use.

Nitrate storage patterns

To better understand trends in nitrate storage, we grouped the nitrate storage responses for river catchments across the globe. The groups show clear differences and trends, with a distinct split between developed (USA, Europe) and developing (Africa, East Asia) countries. In the developed group, current nitrate leaching into the unsaturated zone is decreasing, as a result 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 fertilised agriculture.

Implications for environmental policy

The large store of nitrate in the unsaturated zone means that the use of 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. However, in both cases, it is essential that 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.

Publication

Ascott, M J, Gooddy, D C, Wang, L, Stuart, M E, Lewis, M A, and Binley, A M.  2017.  Global patterns of nitrate storage in the vadose zoneNature Communications. DOI: 10.1038/s41467-017-01321-w

Contact

Contact Matthew Ascott for more information.

Global patterns of nitrate storage in the unsaturated zone


Basin-scale nitrate storage trends. Spatial distribution of the nitrate storage clusters (a), nitrate storage cluster centroids (b), distribution of vadose zone travel times (c) and mean annual nitrate leaching input time series (d) for each cluster. Graphic: Ascott, et al., 2017 / Nature Communications

ABSTRACT: Global-scale nitrogen budgets developed to quantify anthropogenic impacts on the nitrogen cycle do not explicitly consider nitrate stored in the vadose zone. Here we show that the vadose zone is an important store of nitrate that should be considered in future budgets for effective policymaking. Using estimates of groundwater depth and nitrate leaching for 1900–2000, we quantify the peak global storage of nitrate in the vadose zone as 605–1814 Teragrams (Tg). Estimates of nitrate storage are validated using basin-scale and national-scale estimates and observed groundwater nitrate data. Nitrate storage per unit area is greatest in North America, China and Europe where there are thick vadose zones and extensive historical agriculture. In these areas, long travel times in the vadose zone may delay the impact of changes in agricultural practices on groundwater quality. We argue that in these areas use of conventional nitrogen budget approaches is inappropriate.

Global patterns of nitrate storage in the vadose zone

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