Losses of phosphorus, nitrogen, dissolved organic carbon and soil from a small agricultural and forested catchment in east-central Finland

Abstract

Livestock-derived phosphorus (P) loading values are largely based on studies performed in small experimental fields, leading to uncertainties when applying the results at larger scale. This five-year study estimated annual loads of P, nitrogen (N), dissolved organic carbon (DOC) and total suspended solids (TSS) in a small 3.2 km2 agricultural and forested catchment by monitoring water quality and discharge in four ditches reflecting different land uses. The monitored catchment has intensive livestock production, with proportions of grassland, cereal and forest area typical for east-central Finland. Typically, spring runoff started in early April under snow cover and peaked during snowmelt. There was great variation in the amount and pattern of discharge between years. Mean annual total P (TP) load (± SD) from an agricultural sub-catchment was 1.0 ± 0.37 kg ha−1. Mean TP load from a forested (PS) sub-catchment was 0.4 ± 0.17 kg ha−1 when a point source of pollution (PS) was included and 0.06 kg ha-1 when it was excluded. The point source of pollution contributed 12 % of the TP load from the whole 3.2 km2 catchment. The proportion of dissolved reactive P (DRP) to TP load varied between 34 % and 67 % in the study period (largest for the forested (PS) sub-catchment and smallest for the whole catchment). Observed erosion rates were very low when measured as TSS, with the highest (115 ± 103 kg ha−1) and lowest (3.4 ± 2.3 kg ha−1) mean annual erosion rate in the agricultural and forested (PS) sub-catchments, respectively. In the agricultural sub-catchment, the relationship between TP and DOC tended to be stronger than that between TP and TSS. Mean annual total N load from the agricultural sub-catchment was 19.0 ± 7.2 kg ha−1 and from the whole catchment 10 ± 2.7 kg ha−1, while mean annual DOC load was 45 ± 13 kg ha-1 and 92 ± 28 kg ha−1, respectively. These results are important in developing, implementing and targeting mitigation measures to control agricultural nutrient losses to surface waters.