The effect of different fire temperatures on the water repellency parameters of forest sandy soil under different types of vegetation

Soil under specific tree forest species (e.g. pines) can be naturally water repellent. Forest fire can strengthen or destroy soil water repellency (SWR). Fire induced SWR have many direct or indirect effects, including increased preferential flow rate and risk of ground water contamination, increased surface runoff and soil erosion, increased amount of carbon stored in soil, reduced levels of seed germination and plant growth. Understanding the post-fire hydrologic response of forest soil is paramount for effective risk management and mitigation of post-fire hydrologic hazards. Three experimental sites were located in the Borská nížina lowland (southwestern Slovakia). Eolian (wind-blown) sand dunes form the central part of the Borská nížina lowland, which make it a specific region within Central Europe. Pines have been planted here for sand dune stabilization since the 18th century and today cover a huge part of the lowland. The first site IL1 represent 100-years-old stand of Scots pine ( Pinus sylvestris ), the second site IL2 is a 30-years-old stand of Scots pine ( Pinus sylvestris ) and the third site LL is a deciduous stand with a predominance of alder ( Alnus glutinosa ). The disturbed mineral soil samples were taken from 2.5–5.0 cm depth of soil horizon. The organic horizon (0–2.5 cm) was sampled separately before mineral soil. In the laboratory, the samples from each site in 5 replicates were placed into a muffle furnace and exposed to a temperature from 50 to 900°C. The persistence of SWR in soil samples was measured using the water drop penetration time (WDPT) test. Our goal was to quantify the changes of SWR of the naturally water repellent soil induced by different fire temperatures under age and species different forest stands. Forest stands were selected to include different vegetation age and type of litter (surface organic horizon) under the relatively same site conditions (climate, soil and relief conditions). The measured values of natural background water repellency decreased in order IL1>IL2>LL. The highest value of induced SWR (WDPT max ) was measured at IL1 and further declined in the order LL˃IL2; however increase of SWR after heating, estimated as a difference between maximal induced and natural SWR had different trend (LL>IL2˃IL1). Mean value of parameter WDPT max -WDPT n at IL1 was statistically different from values estimated at sites IL2 resp. LL. The changes in natural and induced SWR that we have found may be attributed partially to the quantity and to the origin of organic material (litter of the plant communities with different age and composition of the species).