Wildfire influences on stream water geochemistry in a boreal catchment

Abstract: Forest wildfires are important for the structure and functioning of boreal ecosystems including biogeochemical processes. During the last decades, natural forest fires have been successfully suppressed in Fennoscandia. Therefore, our knowledge on the biogeochemical effects of forest fires is limited. In August 2006, a 1500 ha forest fire occurred near Bodträskfors, northern Sweden. The intensity of the fire varied among vegetation types. Especially in areas dominated by pine forest of the lichen type, vegetation and the humus layer were burnt to a large extent whereas the impact in moist-wet forest patches was less pronounced. Burning and subsequent leaching of plant remainders was expected to affect local surface waters. The main objectives with this thesis were to: 1) investigate the behavior of water quality parameters (pH, conductivity) and dissolved major elements including TOC (Total Organic Carbon) due to leaching of burnt plant remainders and, 2) measure the dissolved concentration and isotopic composition of boron as a potential tracer of forest fires. The project was performed in 2007 – 2009 in a creek draining the burnt area. A creek in a neighboring catchment not affected by the fire was sampled as a reference. Both catchments are characterized by the same type of bedrock and soil cover. The results indicate that leaching of the burnt plant remainders was mainly contributing P, K, Ca and Mg to creek water run-off. This is consistent with the naturally high concentrations of these elements in plant tissue. Na was not leached to any greater extent. Compared to the reference creek, burnt creek concentrations of N were slightly higher and slightly lower for TOC. The pH was ~ 0.5 units lower in the burnt creek during all years of measurement. Conductivity was higher in the burnt creek in 2007 but had decreased to the levels of the reference creek in 2009. Conductivity appeared to be controlled by Ca and Mg concentration as the temporal variations of these parameters/elements were closely correlated. During leaching from burnt plant remainders, P and K concentrations showed similar variation with time. The co-variation of Ca and Mg concentrations marks another behavior but TOC and N each exhibit unique changes of concentration with time. P and K are important nutrients associated to the burnt biomass. This caused extensive leaching of these elements for more than three years after the fire. Ca and Mg are contributed from groundwater to a great extent which was the likely reason for the more short-term post-fire leaching of these elements. As N is also considered an important nutrient it was unexpected that leaching was not as extensive as for P or K. However, the release of N after disturbances in boreal forest catchments has previously been observed to be erratic and depends greatly on the properties of the soil. The hypothesized inhibited release of TOC to the burnt creek was not obvious and the pH difference between both catchments was not considered an effect of the fire. However, since sampling was not conducted for longer than three years after the fire it could not be ruled out that TOC and pH were influenced on a longer term than that. The dissolved B concentrations were substantially higher in the burnt creek in 2007 compared to the reference creek and the isotopic composition (δ11B) was lighter. δ11B was not measured in 2009 but the burnt creek B concentrations were only slightly lower relative to 2007. The measured concentration and isotopic composition of dissolved B in the burnt creek were in line with anomalies of B measured in lake sediments. These anomalies were concluded to be the result of leaching of burnt wood remainders. The lighter B isotopic composition detected in both the present and the previous studies is in line with other findings concluding that forest plants utilize mainly the lighter B isotope (10B) during nutrient uptake from the soil solution. This B is normally released during the winter season inducing a seasonal oscillation of δ11B in the soil solution. The seasonal variation of δ11B was observed in the results from the reference creek. The more extensive leaching of lighter B from the burnt plant remainders most likely caused the overall lighter B isotopic composition and higher B concentrations in the burnt creek relative to the reference creek.