The role of nitrogen deposition in the recent nitrate decline in lakes and rivers in Northern Italy

Deposition of inorganic nitrogen (N) in north-western Italy is around 20–25 kg N ha− 1 y− 1, and has remained constant during the last 30 years. This flux of N caused saturation of terrestrial catchments and increasing levels of nitrate (NO3) in surface waters. Recently, monitoring data for both rivers and lakes have shown a reversal in NO3 trends. This change was widespread, affecting high-altitude lakes in the Alps and subalpine lakes and rivers, and occurred at almost the same time at all sites. The seasonal pattern of NO3 concentrations in running waters has shown a change in the last few years, with a tendency towards slightly lower leaching of NO3 during the growing season. Atmospheric input of N has also shown a recent decrease, mainly due to decreasing emissions and partly to the lower amount of precipitation occurring between 2003 and 2009. Surface waters are probably responding to these changing N inputs, but a further decrease of N deposition, especially reduced N, will be required to achieve full recovery from N saturation.

► We analyzed long-term trends of N in deposition and surface waters in N-Italy.
► An overall decrease of nitrate was observed in rivers and lakes in recent time.
► The main cause was a decrease of N deposition in the period 2003–2010.
► NH4 deposition is increasingly important compared to NO3 in terms of N input.
► National legislation will not enable a satisfactory level of N deposition to be reached.