Erosion-induced losses of carbon, nitrogen, phosphorus and heavy metals from agricultural soils of contrasting organic matter management.

Water erosion on agricultural land preferentially carries away fine particles. Due to the generally high specific density of sorption sites of these particles, their displacement can be associated with substantial particle-bound transfer also of nutrients and pollutants with low solubility. Organic matter amendments can reduce soil erosion, but to what extent they affect the erosion-induced element losses has not been studied much. Here, we carried out field rainfall experiments on three pairs of soil plots (1.5×0.75m) after incorporating a mixture of wheat straw and grass (2.12gC per kg soil) as organic amendment into the topsoil (0-5cm depth) of one plot in each pair (OI treatment). The other plot was prepared in the same way but without incorporation of the amendment (NI treatment). Artificial rainfall (49.1mmh-1) was simultaneously applied on each pair of plots for approximately 2h, and sediment samples were collected at designated time steps for the analyses of C, N, P, Cu, Zn and stable carbon isotope ratios. The organic amendment substantially reduced element losses, but to a lesser extent than soil loss, as the element concentrations were higher in the exported sediments in the OI than in the NI treatment. With and without organic amendment, the concentrations of the studied elements were consistently higher in the exported sediments than in the bulk soil. They were always maximal at the onset of discharge and then continuously decreased towards the values of the bulk soil. The δ13C values revealed that the eroded C was preferentially derived from the fresh organic residue added to the soil. Pairwise correlations between elements and sediment size indicate that the export of N, Cu and Zn was primarily associated with soil organic matter loss, whereas P export was more associated with mineral fractions.