Abstract
Wildfires affect vast areas of Mediterranean forests, thereby triggering changes in hydrological and geomorphological processes that can negatively affect both terrestrial and aquatic ecosystems. Although several studies have evaluated the post-fire hydrological and erosive response in burnt forest areas, an important knowledge gap remains with respect to nutrient mobilization by overland flow. To address this gap, a recently burnt area was selected near the Ermida village (north-central Portugal). The study area was instrumented shortly after a wildfire that occurred in July 2010, to evaluate the export of dissolved (nitrate – NO3-N and orthophosphate – PO4-P) and total nitrogen (TN) and phosphorus (TP) forms by overland flow. This was done in two burnt eucalyptus plantations due to their contrasting slope aspects (south-east vs. north-west). Bounded micro-plots were installed in August 2010 and monitored over two years. During this period, overland flow samples were collected on a 1- to 2-weekly basis, depending on the occurrence of rain. Results showed that the north west-facing slope (BE-N) presented higher nutrient losses than the south east-facing slope (BE-S), contradicting the findings of previous studies in the Mediterranean region. A logging operation that had taken place at the BE-N site shortly before the fire might account for these findings, by causing soil compaction and/or reducing the protective vegetation and litter cover. TN and TP exports were particularly pronounced during the first four months following the wildfire. After this initial period, further peaks in TN and TP exports occurred sporadically, mainly associated to intense rainfall events. The observed mid-term post-fire nutrient losses not only suggested a threat to the soil nutrient balance of Mediterranean eucalypt forests but also a potential risk of eutrophication of downstream water bodies.
Highlights
The warmer and drier conditions forecasted for the Mediterranean region during the 21st century are expected to change the region's wildfire regimes (Turco et al, 2014; Viedma et al, 2015)
No significant differences in total nitrogen (TN) (t = −1.933; p = 0.085; Fig. 5) and Pav (t = −0.212; p = 0.837; Fig. 5) contents were observed between the ashes collected at the BE-S and BE-N sites
This was in line with the findings of Ferreira et al (2016a, 2016b) for two other eucalypt sites burnt at moderate severity within the same study area
Summary
The warmer and drier conditions forecasted for the Mediterranean region during the 21st century are expected to change the region's wildfire regimes (Turco et al, 2014; Viedma et al, 2015). Fire-induced changes in vegetation and soil physical (e.g. decrease in soil porosity and decrease in water holding capacity and infiltration), chemical (e.g. decrease in soil organic matter) and biological (e.g. reduction of soil microbial biomass and activity) properties can greatly affect the nutrient cycles, both directly and indirectly, through enhanced post-fire exports by runoff (Certini, 2005; Costa et al, 2014; Ferreira et al, 2016a, 2016b; Hosseini et al, 2017; Knoepp et al, 2005; Machado et al, 2015; Moody et al, 2013) This is a major concern for the sustainable management of forest soils of Mediterranean hillslopes and mountains as these soils are often shallow and poor in nutrients (Caon et al, 2014; Ferreira et al, 2005; Mayor et al, 2016; Shakesby, 2011; Shakesby and Doerr, 2006). Aside from decreasing the on-site nutrient pools (Caon et al, 2014; Certini, 2005; Pausas et al, 2008; Shakesby, 2011), runoff-induced nutrient losses enhance the risk of eutrophication to downstream water bodies and the associated deterioration of chemical and biological water quality (Abraham et al, 2017; Bladon et al, 2014; Emelko et al, 2016; Silins et al, 2014; Smith et al, 2011; Verkaik et al, 2013)
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