Recycling soil nutrients by using channel deposits as fertilizers?

Abstract

Topsoil constituents are eroded from agricultural sites and leached towards drainage channels. This transfer can affect aquatic ecosystems and deteriorate the efficiency of drainage systems and fertilisers. As long as erosion cannot be completely avoided, the recycling of sediments and associated nutrients may offer a sustainable solution to these problems. The aim of our case study at the island Sant Erasmo, lagoon of Venice (Italy) was to assess the ecological problems and potentials of sediment recycling. With our assessment we concentrated on (1) the origin of channel sediments, (2) the benefit of sediment application for increasing the nutrient stocks of the soils, and (3) the risk of heavy metal (HM) contamination of arable soils by sediment application. Samples from soils of horticultural sites, sediments, and waters from adjacent drainage channels and lagoon sediments were analyzed for the concentrations of nutrients (P and K) and HM (Cu, Pb, and Zn). Potentially available channel sediment masses and element stocks were calculated for the soil fertility classes of Sant Erasmo based on local measurements of sediment depths and analyses of aerial photographs by a geographic information system. In a column experiment, leaching of both nutrients and Cu from recently dredged sediments was analyzed. Heavy metal concentrations of soils and channel sediments were much higher than of lagoon sediments. The similarity of the chemical properties of the channel sediments and of top soil samples implies that topsoil material is eroded into the channels. The amount of sediments accumulated in the channels corresponded to soil erosion rates between 2 and 23 t ha−1 a−1. Channel sediments contained higher concentrations of nutrients and organic carbon but slightly lower concentrations of HM than the soils of adjacent horticultural sites. Sediment P and K yields would be sufficient to replace fertiliser application at the horticultural sites for up to 51 and 35 years, respectively. The column experiment indicated that Cu mobilization induced by oxidation processes is restricted to the first years after sediments are applied to the soils. Our study emphasizes that for a comprehensive assessment of sediment recycling in agricultural systems the available sediment stocks as well as the contents of nutrients and pollutants of the sediment in relation to soils have to be considered.