Black carbon contributes to organic matter in young soils in the Morteratsch proglacial area (Switzerland)

Most glacier forefields of the European Alps are being progressively exposed since the glaciers reached their maximum expansion in the 1850s. Global warming and climate changes additionally promote the exposure of sediments in previously glaciated areas. In these proglacial areas, initial soils have started to develop so that they may offer a continuous chronosequence from 0 to 150-yr-old soils. The build-up of organic matter is an important factor of soil formation, and not only autochthonous but also distant sources might contribute to its accumulation in young soils and surfaces of glacier forefields. Only little is known about black carbon in soils that develop in glacier forefields, although charred organic matter could be an important component of organic carbon in Alpine soils. The aim of our study was to examine whether black carbon (BC) is present in the initial soils of a proglacial area, and to estimate its relative contribution to soil organic matter. We investigated soil samples from 35 sites distributed over the whole proglacial area of Morteratsch (Upper Engadine, Switzerland), covering a chronosequence from 0 to 150 yr. BC concentrations were determined in fine earth using the benzene polycarboxylic acid (BPCA) marker method. We found that charred organic matter occurred in the whole area, and that it was a main compound of soil organic matter in the youngest soils, where total Corg concentrations were very low. The absolute concentrations of BC in fine earth were generally low but increased in soils that had been exposed for more than 40 yr. Specific initial microbial communities may profit from this additional C source during the first years of soil evolution and potentially promote soil development in its early stage

Only small changes in soil organic carbon and charcoal concentrations found one year after experimental slash-and-burn in a temperate deciduous forest

International audienceAnthropogenic fires affected the temperate deciduous forests of Central Europe over millennia. Biomass burning releases carbon to the atmosphere and produces charcoal, which potentially contributes to the stable soil carbon pools and is an important archive of environmental history. The fate of charcoal in soils of temperate deciduous forests, i.e. the processes of charcoal incorporation and transportation, and the effects on soil organic matter are still not clear. In a long-term experimental burning site, we investigated the effects of slash-and-burn and determined soil organic carbon, charcoal carbon and nitrogen concentrations and the soil lightness of colour (L*) in the topmost soil material (0?1, 1?2.5 and 2.5?5 cm depths) before, immediately after the fire and one year after burning. The main results are that (i) only few charcoal particles from the forest floor were incorporated into the soil matrix by soil mixing animals. In 0?1 cm and during one year, the charcoal C concentrations increased only by 0.4 g kg?1 and the proportion of charcoal C to SOC concentrations increased from 2.8 to 3.4%; (ii) the SOC concentrations did not show any significant differences; (iii) soil lightness significantly decreased in the topmost soil layer and correlated with the concentrations of charcoal C (r=-0.87**) and SOC (r=?0.94**) in samples 0?5 cm. We concluded that the soil colour depends on the proportion of aromatic charcoal carbon in total organic matter and that Holocene burning could have influenced soil charcoal concentrations and soil colour

Preservation of fire-derived carbon compounds and sorptive stabilisation promote the accumulation of organic matter in black soils of the Southern Alps

Cryptopodzols are black soils that occur under forests dominated by chestnut trees (Castanea sativa) in Southern Switzerland. Their soil organic carbon (SOC) stocks reach an average of 150 t C ha-1 and are thus among the highest of European forest soils. We investigated the processes leading to the accumulation and stabilisation of SOC in these soils by analysing three Cryptopodzols and one Cambisol for charred organic matter content (macrocharcoal and BPCA), the amounts of Fe and Al, and the colour and SOC content in bulk soil and density fractions. The results showed that charred organic matter produced by frequent ␣res in the area for more than 10,000 years is highly abundant in Cryptopodzols: the stocks of macrocharcoal and BPCA-C amount to up to 31 t ha-1 and 17 t ha-1, respectively. These high amounts of charred organic matter are responsible for the dark soil colour and high SOC concentrations that are, however, also closely related to Fep and Alp concentrations. We concluded that the occurrence of charcoal across the whole pro␣les of Cryptopodzols seems to be the dominating factor, although both the formation of organo-metallic or organo-mineral complexes in the subsoil and the high abundance and stability of charred organic matter are responsible for the high SOC stocks in Cryptopodzols

Pedogenesis of Chernozems in Central Europe — a review

Since Dokuchaev's investigations of Russian Chernozems, Central European Chernozems were established as steppe soils, with their pedogenesis dominated by humus accumulation as a result of dry continental climate and steppe vegetation, with carbonaceous parent material and bioturbation as other prerequisites. The WRB-FAO classification defined Chernozems by their morphological characteristics, but was biased by the climo-genetic formation model. However, the assumption that modern Central European Chernozems are relics of steppe soils conflicts with palaeobotanical evidence from an early reforestation that started in the Late Glacial, and also with pedological studies that dated Chernozem formation to the Early Holocene. In this review we compile the most important literature on pedogenesis of Central European Chernozems since the 1920s, according to the soil forming factors climate, time, vegetation, relief and man. Our review demonstrates that there is no consensus on the factors controlling the formation, conservation and degradation of Central European Chernozems in published literature. We found that (1) no absolute time of formation could be stated so far, and that (2) Central European Chernozems formed not only under steppe but also under forest vegetation; (3) the spatial distribution of Chernozems and Phaeozems did not correlate with climate conditions or topographic position, and (4) until now no other factors were considered to be responsible for Chernozem development. Recent studies showed that these unknown factors could include anthropogenic activity and vegetation burning as they could form black soils or strongly affect the composition of soil organic matter. We concluded that not all soils classified as Chernozems in Central Europe are steppe soils and thus, as they do not necessarily reflect past climate, the classification may be misleading

Dating charred soil organic matter - comparison of radiocarbon ages from macrocharcoals and chemically separated charcoal carbon

Radiocarbon dating of charcoal in soils is commonly used to reconstruct past environmental processes. Also microcharcoal that is chemically isolated from soil organic matter by high-energy UV photo-oxidation can be dated with 14C accelerator mass spectrometry (AMS). We compared the 14C AMS ages of 13 pairs of hand-picked macrocharcoals and microcharcoal samples separated via the UV oxidation method; both charcoal fractions were taken from the same soil samples (prehistoric pit fillings). We found that in most cases, the microcharcoal fraction yielded older ages than the single macrocharcoal pieces, and that the differences between the ages are not systematic. A reason for these age differences might be that the microcharcoal fraction consists of more stable components than macrocharcoals and thus yields older ages. Dating of microcharcoal would give a mean age of charred organic matter in soil material and the ages of the more stable compounds. Thus, 14C data obtained from the microcharcoal fraction in soils is not comparable to macrocharcoal ages and should not be used to complement existing macrocharcoal data sets

Charred organic matter and phosphorus in black soils in the Lower Rhine Basin (Northwest Germany) indicate prehistoric agricultural burning

We investigated the properties of dark soils, namely Luvic Phaeozems in the Lower Rhine Basin (NW-Germany). We discovered clusters of hundreds of regularly shaped pits that were always connected to the Bht horizons of adjacent Luvic Phaeozems. The occurrence of anthropogenic pits in connection with Phaeozems was never reported before, and we called them Phaeozem pits. We conducted archaeobotanical (charcoal analysis) and geochemical (black carbon, carbon, nitrogen and phosphorus concentrations) investigations. We compared soil material from Phaeozem pits with clearly anthropogenic pits: (i) settlement pits, and (ii) slot pits, that can be found outside of prehistoric settlements and do not contain any visible artefacts, but are defined as anthropogenic by their shape. The main results of our study were: (1) Phaeozem Bht horizons and pits are man-made and archaeological off-site features. (2) Soil properties of the Phaeozem horizons could have been affected by inputs of soil amendments, perhaps to fertilize agricultural fields or gardens. (3) The high proportion of charred organic matter in the dark soil material of Phaeozem horizons and pit fillings results from vegetation fires ignited by man. (4) They were presumably formed in several periods between Mesolithic and the Middle Ages, mainly during Younger to End-Neolithic (4400-2200 BC). We concluded that the Phaeozem horizons could have been formed by agricultural burning, like slash-and-burn, during several (pre)historic epochs