Recycling of phosphorus from dredged lake sediment: Importance of iron-bound phosphates for plant growth

Phosphorus (P) is critical for food production. However, it has been managed unsustainably for decades and geopolitical challenges complicate its availability. While accessible P-rock deposits are linearly exploited, excessive fertilization practices lead to P loss from land to water, and thus, eutrophication. The release of legacy P from sediments to the water column, i.e. internal P loading, sustains global eutrophication issues. Sediment removal and its subsequent reuse as soil amendment can simultaneously lower internal P loadings and create a new P resource. However, the plant bioavailability of sedimentary P, especially Fe-P, is rather controversial. In this study, the direct P fertilizer effect of fresh lake sediment, lake sediment after Fe-P removal, amorphous Fe-P, and the reduced Fe-P mineral vivianite on barley was investigated and compared to the conventional mineral P fertilizer triple superphosphate (TSP). Fresh sediment, amorphous Fe-P, and vivianite fertilization significantly increased biomass and P uptake compared to the 0-control, while the Fe-P removal from the sediment reduced both effects. The P use efficiency was generally lower than for TSP and decreased in the order amorphous Fe-P > fresh sediment > vivianite > sediment after Fe-P removal. In a parallel soil incubation without barley growth P diffusion from the tested alternative substrates was not observed. We conclude that fresh lake sediment has P fertilizer potential with amorphous Fe-P as a significant contributor. Further, fertilization with fresh sediment and Fe-P can increase soil adsorptive capacities potentially reducing leaching but also creating dependency of plant P bioavailability on plant-soil interactive mechanisms.