Within this project I sampled three soils at three agricultural drainage monitoring sites in Latvia where subsurface drainage was installed 30 – 50 years ago. The soils were sampled at 5 distances from a drain, reflecting a water flux gradient - quicker closer to the drain and slower further from the drain. The soil properties further from the drain were assumed to be more similar to natural soil conditions, while soil properties closer to the drain were assumed to be more affected by subsurface drainage. The soil samples were analyzed for various parameters – pH, texture, carbonate content, several element concentrations, soil organic carbon, nitrogen and phosphate sorption potential. Additionally, I used water flux data from 30 years of agricultural drainage monitoring to extrapolate water quality and suspended solid results, which I collected during 2020.
Soil sampled at one of the sites exhibited a strong drainage induced gradient for carbonate concentration that was reflected also in the concentrations of Ca and Mg, and soil pH. Carbonates act as buffers against soil acidification, help in the stabilization of soil organic matter, and are beneficial for the growing of many crops with many soils that lack carbonates having to undergo liming (addition of carbonates). The soil furthest from the drain contained up to 10 times more carbonates in 4 of the sampled 5 soil horizons compared to the soil closer to the drain, which contained carbonates only in the deepest soil horizons sampled. This was also reflected in a soil pH difference of up to 0.3 units with soil pH values in all profiles varying from 7.1 to 8.3. A hypothesized gradient in clay and Fe stocks was not observed. Nor was a gradient found for soil organic carbon concentrations, nitrogen mineralization and phosphorus sorption potential. This strengthens the assumptions that underly nutrient, carbon and pollutant modelling in soils, e.g. in Earth System Models. The conditions at the other two sites were not favorable for observing drainage induced soil changes due to very heterogeneous parent material and, for one site, low lying landscape position.
At all sites, the sediment composition was dominated by iron (hydr)oxides, and they were mostly clay-sized with most of them being <1 µm in diameter. At the site where the effect of drainage was observed, data on the oxidation-reduction potential of the drainage water suggested that it did not experience reducing (limited oxygen) conditions. Considering water flux data from the last 30 years, it is possible that the subsurface drainage system in the soil is quite efficient, and the soil horizons sampled often do not experience reducing conditions. Without reducing conditions iron cannot be mobilized. Clay-sized particles rich in Fe are clearly washed out from the soil but the main pathway for this process might be macropore flow.
Result exploitation and dissemination:
• Data on soil properties were made available for further use in calibrating a hydrological model used as part of the agricultural drainage monitoring program that includes the sampled sites.
• Intermediary results were presented at the Nordic Clay Meeting, 2021.
• Materials from the project were used to create a digital booth (“How soil feeds water”) at the Science is Wonderful! Exhibition in 2020.
The attached image shows the effect of subsurface drainage on soil water fluxes and carbonate distribution with distance (in meters) from the drain. The blue simple line indicates a heightened groundwater level (e.g. after snowmelt), the lines with arrows indicate water flow. The more arrows stacked together, the faster the flow of water. The gradient coloring in the bars intensifies with higher carbonate concentration.
