Fingerprinting changes of source apportionments from mixed land uses in stream sediments before and after an exceptional rainstorm event

Abstract

The loss of fertile topsoil is one of the principal soil degradation problems in mountain agroecosystems worldwide. Soil erosion rates reach their maximum during exceptional storm events that remove soil particles, especially from unprotected topsoil. In Mediterranean mountainous environments, several centuries of non-irrigated agriculture and the subsequent removal of natural vegetation for developing agriculture has increased the surface area prone to erosion. In addition, the irregularity in exceptional precipitation events results in a great loss of fertile soil, the subsequent siltation of reservoirs and a decrease in water quality. To analyse the soil response to exceptional events, 161 source samples were collected in a 23 km2 catchment that was mostly cultivated at the beginning of the last century. Source samples were distributed over the five main land use/land covers such as agricultural land, pine afforestation, open forest, bare soil and channel bank areas. Furthermore, 20 channel bed sediment samples were collected along the main streams before and after the exceptional storm event to document changes in the sediments. In addition, floodplain sediments were collected to provide a close replication of sediments deposited during regular storm events. Source apportionments were calculated using the FingerPro unmixing model in the pre-event, regular events and post-event scenarios.The unmixing outputs displayed a large variation of source apportionments from the upper part to the lower part of the catchment and from pre- to post-event sediments. After the event, a decrease of >70% of the clay fraction and its associated elements such as Fe, Al, K, Ba, Sr, Rb, Pb, Zn, V, 137Cs, 40K, 232Th and SOC along with a rise in contents of elements associated with the coarse fraction (Si, Nb, Zr, Ti, P and 226Ra) was recorded in the channel bed sediments. At the catchment outlet, the pre-event sediment showed substantial contributions from bare soil (29%) and from agriculture and channel banks, which both reached 35%, while the channel bank was the main source along the catchment ranging between 44 and 71%. The low contribution from soil under natural covers with a mean value <4% underlines the benefits of vegetation to prevent soil loss. In the post-event sediment, the channel bank contribution increased up to 63% at the catchment outlet. Our findings highlight the hazards of exceptional storm events on modifying sediment source contribution and exporting fine sediment.