Synchronous trends in N-NO3 export from N-saturated river catchments in relation to climate

Long term trends (1978-2005) of N-NO3 concentrations in river water were investigated for 10 rivers draining forested catchments in Piedmont, North-Western Italy, and Canton Ticino, Switzerland. All the river catchments come into the category of the medium-high stage of N saturation (levels 2-3 of the Stoddard's classification). The seasonal signal in N-NO3 concentrations and its changes in time over the course of the study period was also evaluated. Single trends were analysed for significance and magnitude; statistical techniques for the detection of common trends were then applied to identify a common pattern in the N-NO3 time series. Both the increasing NO3 levels and the limited seasonal pattern in recent years indicate an aggrading level of N saturation in time. Synchronous trends of N-NO3 export were found for 8 rivers. The main common trend was used to test relationships with: (i) temperature, (ii) precipitation, and (iii) N deposition. Step-changes in the data series were also assessed, and the main points of change are discussed in relation to meteorological factors and response to the N saturation status. Temperature proved to be the main factor affecting the temporal pattern of N-NO3 concentrations: warm periods were usually followed by an N-NO3 increase in river water due to enhanced mineralisation and nitrification in soil.