Nitrogen content and trends in agricultural catchments in Estonia

Several studies have shown that the stream water quality in agricultural catchments could vary according to the changes in land use, production intensities and implemented agri-environmental measures. In present study we investigated the nitrogen content and temporal dynamics in concentration and loss in small streams and catchments in Estonia for the period 1992–2011. The studied catchments represent various agricultural production areas, management practices, soil types, geology and size. A statistical analysis using the Mann Kendall (MK) test was undertaken to discern monotone trends in time series of total nitrogen (TN) and nitrate nitrogen (NO₃–N) concentrations for 8 sampling sites and TN loss for five sites, for which at least a data set of 10 years was available. Additionally the partial Mann–Kendall (PMK) test has been adapted to account for the influence of water discharge to the temporal trends in TN loss in five catchments. The MK trend test revealed one statistically significant (p < 0.05; two-sided test) upward trend in TN concentrations and two upward trends in NO₃–N concentrations. Less significant (p < 0.08) upward trend in TN content was also noted in two streams. Overall, the TN concentrations were rather high and it ranged with 90% probability from 2.3 to 8.9 mg l⁻¹ in the studied streams. The MK test revealed the statistically significant upward trend in TN loss only at the Räpu (p < 0.001) and Jänijõgi (p < 0.05) catchments (one-sided test). After conditioning on trends in water discharge there was only one (Räpu) statistically significant temporal upward trend in TN loss. Elevated nitrogen concentrations in three other catchments (Rägina, Vodja, Porijõgi) explain only part of the upward trend in nitrogen losses and increasing tendencies in losses could mainly be associated with upward tendency in discharge. The direct (not flow-normalised) unit-area loss of nitrogen from agricultural land reached to 63.1 and 82.5 kg N ha⁻¹ yr⁻¹ in the Räpu and Jänijõgi catchments, respectively, which is higher than seen at the Rägina basin (25 kg ha⁻¹ yr⁻¹) and the River Vodja catchment (21.9 kg ha⁻¹ yr⁻¹). Elevated losses are typical for recent years, particularly after 2007.