The assessment of nitrogen concentrations in agricultural ditches during the vegetation period in Latvia

Agricultural ditches are essential components of the hydrological network and agricultural landscape in Latvia, yet they serve as primary pathways for nutrient transport from agricultural fields to downstream water bodies. The water quality monitoring during the vegetation period is needed to understand the levels and patterns of nitrogen losses and identify measures to reduce negative impact of agricultural activities on the environment.

Within this study 112 agricultural ditches representing all four geomorphological regions of minimum runoff in Latvia were selected for water sampling. The water samples were collected in 107 agricultural diches using a manual grab sampling method during the time period from September 20 to October 30, 2025. In 5 ditches the water flow was not observed. In the accredited laboratory the water samples collected were tested for concentrations of total nitrogen (TN), nitrate nitrogen (NO₃-N) and ammonium nitrogen (NH₄-N) according to the national standard methods. The Corine Land Cover 2018 datasets have been utilized to study the effects of land use distribution on nitrogen concentrations relevant for the catchment areas of the selected ditches.

The monitoring results showed a wide range of concentrations across the study sites. TN concentrations varied from 0.49 to 13.95 mg l^-1, while NO₃-N between 0.001 and 13.62 mg l^-1. The wide range of TN and NO₃-N concentrations indicate for a heterogeneous intensity of agricultural activities carried out within the catchment areas of interest, where the intensity agricultural activities differs from very low to very high. The water samples collected in 37 agricultural ditches indicated for the increased ratio (over 75%) between NO₃-N and TN concentrations. NO₃-N, as a highly soluble and mobile form of nitrogen, is lost from agricultural fields mainly via subsurface drainage systems when the nitrogen surplus is present and excess water percolates through the soil profile until reaches the depth of subsurface drainage systems. The increased ratio between NO₃-N and TN concentrations showcase the negative impact of tile drained arable land on nitrogen losses. Contrary, 17 monitoring sites showed a low NO₃-N and TN ratio (below 25%) thus indicating for the minor impact of arable land on water quality. NH₄-N concentrations in general terms were low with the mean concentration of 0.066 mg l^-1, minimum of 0.002 mg l^-1 and maximum of 1.609 mg l^-1 showing that nitrogen losses from sources of organic origin are not present in the catchments, except of one case.

TN concentrations have a visually inexpressive and statistically insignificant relationship with the share of agricultural lands in the catchment areas of interest. A closer relationship between the two parameters involved in this analysis is limited by the specificity of the data sets applied, when in the cases of increased share of agricultural lands both high and low TN concentrations have been observed.

This study was funded by the Ministry of Agriculture of the Republic of Latvia within the scope of the research project “The assessment of hydrological conditions and water quality during the vegetation period in agricultural ditches in Latvia”, the decision No. 10.9.1-11/25/1545-e.