Temporal Trends in Nitrogen Concentrations and Losses from Agricultural Monitoring Sites in Latvia

Water quality in small agricultural catchments is affected by anthropogenic (crops, field management operations, fertilizer application) and natural (soil texture, topography, meteorological and hydrological conditions) factors. It is important to estimate long-term (1995–2018) trends of nitrogen losses from small agricultural catchments and subsurface drainage fields to surface water bodies in order to evaluate the impacts of anthropogenic and natural factors and to cover management needs such as implementation of national legislation and the EU directives. The content of the Common Agricultural Policy is currently under development, therefore, there is a need to consider existing monitoring results to address water quality goals within the next planning period. This study examines temporal trends in water discharge, total nitrogen (TN) and nitrate–nitrogen (NO₃-N) concentrations and losses from three agricultural runoff monitoring sites in Latvia including Berze, Mellupite, and Vienziemite. The yearly data series of TN and NO₃-N concentrations and loss were tested for statistical trends using the Mann-Kendall test. The study results show high variations in TN and NO₃-N concentrations and losses among monitoring sites both in subsurface drainage field and small catchment scale. For TN and NO₃-N losses five out of six monitoring sites showed an upward trend, while for TN concentrations three out of six monitoring sites and NO₃-N four out of six monitoring sites showed an upward trend. The results of this study indicate that the upward trends in TN losses are affected by hydrological conditions at the study sites.