Analytical Investigations to Estimate Phosphorus Re-dissolution Rates in Trace Levels of Selected Topsoils and River Sediments

Anthropogenic phosphorus (P) input from fertilised and unfertilised topsoils into surface water and re-dissolution from sediments can be key drivers of eutrophication. This study aimed to (1) analyse the P input processes into streams/rivers particularly via erosion from fertilised and unfertilised fields and (2) study the effectiveness of the riparian strip in reducing P emissions from diffuse sources. For the investigation, Cambisol-Tschernosem and Luvisol samples from Loess were taken from Thuringian test fields (Germany). Three laboratory simulations were designed to analyse P re-dissolution and leaching behaviour from topsoils and sediments and further extrapolated to a realistic scenario based on the P input path into receiving waters via erosion. Organic bonded phosphorus and orthophosphate were leached out at the beginning. Upscaling to a realistic scenario showed that the main source of P in receiving waters was leaching from sediment interstitial sites (57.5%) via percolation while the P re-dissolution via diffusion (13%), due to two heavy rain events (17%), and leaching from soil interstitial sites (12.5%) only played a minor role. The risk of eutrophication exceeded the threshold total P of 0.10 mg L-1 given as an orientation value by the Federal/State water consortium (LAWA). This was observed in percolates from all sandy soils (0.17-0.85 mg L-1), only slightly in the clayey soils (<= 0.11 mg L-1) but not in either streambed sediment (<= 0.08 mg L-1). However, local differences such as steeper slope, different soil compositions such as higher sand and lower clay percentages, and poorer buffering due to lower lime and aluminium content were identified as reasons for a higher risk of eutrophication.

Automated summary

This study investigated the phosphorus input processes from fertilised and unfertilised fields and found that leaching from sediment interstitial sites was the main source of phosphorus pollution. The risk of eutrophication exceeded the recommended threshold, and differences in soil composition and water buffering capacity were identified as factors influencing the risk of eutrophication.