Soils potentially vulnerable to phosphorus losses: speciation of inorganic and organic phosphorus and estimation of leaching losses

Eutrophication is an important threat to aquatic ecosystems world-wide, and reliable identification of areas vulnerable to phosphorus (P) losses from diffuse sources is essential for high efficiency of mitigation measures. In this three-step study we investigated (i) relationships between the agronomic (Olsen-P and P-AL) and environmental soil P tests (P-CaCl2) with molecular techniques (P-31 NMR and XANES) followed by (ii) rainfall simulation experiment on topsoil lysimeters and (iii) comparison to long-term field measurements of water quality. Soil samples were collected from seven sites indicated to be vulnerable to nutrient losses due to underlying geology. High P release correlated to standard agronomic P tests (Olsen P, r = 0.67; and P-AL, r = 0.74) and low P sorption capacity (r = - 0.5). High content of iron-bound P compounds indicated more labile P and higher release of dissolved P (r = 0.67). The leaching experiment showed that three out of four soils with high initial soil P status had both higher P leaching concentrations before fertilization (0.83-7.7 mg P l(-1)) compared to soil with low initial soil P status (0.007-0.23 mg P l(-1)), and higher increase in P concentrations after fertilization. Higher soil P sorption capacity reduced P leaching losses. Finally, long-term monitoring data show no significant trends in P losses in a field with low initial P content and moderate P fertilization rates whereas high and over time increasing P losses were recorded in a field with high initial soil P content and repetitively high P fertilization rates.

Automated summary

Eutrophication poses a significant threat to aquatic ecosystems globally, and it is crucial to accurately identify areas susceptible to phosphorus (P) losses from diffuse sources for effective mitigation measures. This study examined the relationship between agronomic and environmental soil P tests and molecular techniques, conducted rainfall simulation experiments, and compared long-term field measurements. The findings indicate that high P release is correlated with standard agronomic P tests and low P sorption capacity. Increasing levels of iron-bound P compounds suggest greater P availability and higher release of dissolved P. The study also highlights the impact of initial soil P status and fertilization rates on P losses over time.