Nutrient fluxes at hill-slope scale in a burned Mediterranean area

Abstract

The International Panel on Climate Change indicated a higher degree of confidence that meteorological conditions associated to climate change will be propitious to increasing extreme events. Impacts on land degradation will also be manifested in bigger and more frequent wildfires. In the Mediterranean area, forest fires are a serious problem since they can change top soils with drastic consequences for important ecosystem services, such as water availability and carbon sequestration. Erosion processes are also enhanced, increasing soil loss, together with its organic matter, iron and nutrients. Whereas it is well documented that wildfires produce significant changes on hydrological and erosion processes, the associated redistribution and fluxes of iron and nutrients have received less attention. This research assesses this gap by studying the transport, redistribution and fate of calcium, potassium, sodium, magnesium, ammonium, nitrate, crystalline and amorphous iron in coupled hillslopes (burned: BU, 39°50\'45.11\"N, 0°22\'20.52\"W and control: CO, 39°51\'08.7\"N, 0°22\'17.6\"W) at the Natural Park of Sierra de Espadán, Azuébar, Spain. The fire occurred the 28/08/2014, with high severity. Soil was sampled (19-9-2014) in the foot’s slope (deposition), middle part (transport) and top (eroding) at two depths (<2 cm, 2-5 cm), and in two environments (under canopy soil: UC; bare soil: BS). Sediments were collected from four sediment fences constructed at the foot’s slope, and together with soils samples, analysed with regard to iron and nutrient contents. The main objective of this work is to increase the understanding on the iron and nutrient fluxes in Mediterranean burned areas experiencing soil erosion and deposition. Based on the data of the closest pluviometre (Sot de Ferrer: 4.5 km), 4 erosive rain events were considered in this study. These were registered in 29/11/2014, 23/3/2015, 15-16/6/2015 and 2/11/2015 with total volumes of 64.2, 103.2, 41.2 and 131.6 L m2, respectively. These events did not produce any sediment in CO hillslope, but in the BU one high yields were measured. Thus, sediment collected after each rainfall were 12.7, 143.6, 12.6 and 62.2 kg corresponding to 10.4, 41.5, 5.0 and 14.6 kg C ha-2, respectively.There are significant differences in the availability of iron and nutrients between BU and CO. The distribution of nutrients also changed due to fire. On the CO hillslope this is influenced by the environment, since more nutrients were found UC than in BS, but depth seems to be more important for the distribution on BU. Regarding the position on the hillslopes, significantly different amounts of iron were observed at the deposition and eroding sites. Erosion by rain events after forest fire caused high nutrient availability in sediments. This means the fertility of the soil on the hillslope decreased due to erosion after fire. Concluding, the nutrient and iron contents are significantly influenced by erosion after forest fires. Although the fertility of the soil can increase strongly at first, erosion will cause a fast decrease in nutrients availability. Fire can cause to a new distribution of nutrients and iron, which is not advantageous for the erosion zone and for the soil on 2-5 cm depth. This means soil fertility and therefore soil quality would be decreased by high severity fires at hillslope scale, despite the fact that significant amounts of nutrients are released by forest fire at short term.