Sediment and Nutrient Export After Seasonal Rainfall: Comparing Forests vs. Thinned and Degraded Land

In recent decades, land abandonment due to socioeconomic issues has been a widespread process in different areas of the Mediterranean, altering landscapes and affecting soil properties and erosion processes. The aim of this research was to assess the impact of land use and land cover change on soil properties and sediment composition produced after seasonal rainfall. Mediterranean open forest (OF), pine afforestation (PA), thinned pine (TPA) and barren land (BL) land use/land covers were compared. We analyzed the soil characteristics and sediments that were collected under each form of land use and management across seven seasonal campaigns between July 2016 and September 2017. The relationships between soil particle size, soil organic carbon (SOC) and its fractions, key nutrients (nitrogen, phosphorous, potassium and sulfur) and rainfall characteristics were evaluated. Sediment loads from runoff, collected in trap MATs in monitoring areas under OF and PA, were similar in both quantity and composition. However, the amount of sediment increased after thinning, though it remained significantly lower than in BL. Sediment loads were driven by total rainfall in OF and in TPA, while rainfall erosivity had a clear impact in PA and BL. Afforestation helped to maintain SOC and nutrient levels comparable to those in OF, which were significantly higher than in BL. Nitrogen and phosphorous losses were mainly governed by the total amount of precipitation. However, the effect of rainfall on potassium and sulfur losses was not clearly evident.