Nitrate trends in United Kingdom watersheds since 1868: can we reverse the trend?

The intensity with which land is used for food production has increased dramatically over the last century, due to improved farming methods, mechanisation and production of fertilisers on an industrial scale. But this has come at a price: agriculture is now the main source of nitrate entering the terrestrial biosphere. Nitrate concentrations and fluxes in the world’s rivers have been rising in response to these changes in agriculture and have become a cause for concern in recent decades. Here we use records of nitrate concentration and flux from a number of UK rivers, including the world’s longest record (River Thames, London, 1868 to 2008), to show that dramatic and sustained increases in nitrate concentrations and fluxes occurred immediately following agricultural intensification in the middle part of the 20th Century, and that more recent introduction of source control measures (i.e. fertiliser controls) to reverse these increases have been ineffective, in the short to medium term at least. This evidence fundamentally questions our ability to mitigate the effects of large-scale diffuse pollution, and suggests the widely-accepted strategy of fertiliser control management as means of reducing nitrate concentrations in rivers to be flawed. Because these records show no significant system recovery over the last forty years, conclusions drawn from short-term monitoring, certainly less than 15 years, could be erroneous unless viewed in an appropriate historical context. We recommend that, in any region, a few, very long benchmark monitoring sites must be maintained to provide this context.