The impact of changing freeze-thaw dynamics under recent climatic changes on nutrient leaching in a Swedish agricultural field

Agriculture can be a significant contributor of nutrients, such as phosphorus (P) and nitrogen (N) to surface water, increasing the risk of eutrophication. Soil frost and freeze-thaw (FT) cycles impact both the transport of nutrients through changes in the hydrologic regime of the field and the mobility/availability of nutrients through changes in the biogeochemistry of the field.  With a changing climate, changes in the frequency and duration of FT cycles are expected in regions of higher latitudes and altitudes. However, there is a knowledge gap related to the response of nutrient leaching with changing FT patterns in a changing climate.

The aim of this study was to investigate the impact of soil freezing and thawing on nutrient leaching (N, P) from an agricultural field in northern Sweden for the period 1989-2021. The FT dynamics were modelled in terms of a soil temperature profile using an explicit soil moisture and energy-based process model – the COUP, at an hourly time step. Long term environmental monitoring data of surface and drainage runoff, combined with soil temperature and soil moisture data were used for model calibration and validation. Finally, the modelled FT dynamics and measured nutrient concentrations and runoff were statistically related to each other.

Our preliminary findings confirm the importance of soil frost occurrence for the separation of surface runoff and drainage. However, no clear relationship between soil FT dynamics and nutrient loads (or concentrations) in surface or drainage water could be observed. This suggests that changes in the hydrological regime through freezing and thawing are most important for the amount and export pathways of nitrogen and phosphorous as compared to alternative mechanisms of nutrient mobilisation.