Predicting mobilization of suspended sediments and phosphorus from soil properties: a case study from the north west Po valley, Piemonte, Italy

Pedotransfer functions are useful for helping environmental agencies to identify soils vulnerable to both total suspended sediment (TSS) and phosphorus (P) mobilization and transport in storm runoff in order to establish strategies to control eutrophication. The DESPRAL test was applied to 90 different and representative agricultural soils of the Piemonte region in the west Po valley, Italy, to investigate the relationships between soil properties and potential TSS and P mobilization in land runoff. Sixty six percent of the variance in TSS is explained by clay, silt and Olsen P content using a multiple regression. Particulate P (PP) was the dominant fraction [89% of total P (TP)] dispersed by the test. The P enrichment of the dispersed particles is strongly related to the amount of TSS and soil clay content according to an exponential function, with a threshold of 20% clay below which sediment P content and PER (P enrichment ratio) increased. The ratio of PP to TSS is also significantly related to the P concentration of soil clay content (r² = 0.69). The soluble reactive P (RP) fraction could be estimated by soil Olsen P (r² = 0.74). Whilst the study suggests some key relationships between soil properties and potential mobilization of sediment and soluble P from cultivated soils of the Piemonte region, soil properties could not be used to estimate the amount of TP or particulate P dispersed by the test. Identification of the risk of TSS, PP and TP transfer within individual fields cannot simply rely on pedotransfer functions but must take account of local site conditions, crop cover and variable management.