Summary The stability of biochar in soil is of importance if it is to be used for carbon sequestration and long‐term improvement of soil properties. It is well known that a significant fraction of biochar is highly stable in soil, but carbon dioxide (CO2) is also released immediately after application. This study investigated the nature of the early release of CO2 and the degree to which stabilizing mechanisms protect biochar from microbial attack. Incubations of 14C‐labelled biochar produced at different temperatures were performed in soils with different clay contents and in sterilized and non‐sterilized soils. It emerged that carbonate may be concentrated or form during or after biochar production, resulting in significant carbonate contents. If CO2 released from carbonates in short‐term experiments is misinterpreted as mineralization of biochar, the impact of this process may be significantly over‐estimated. In addition to the CO2 released from carbonates, there appears to be a labile fraction of biochar that is oxidized quickly during the first days of incubation, probably by both abiotic and biotic processes. Later in the incubation, biotic mineralization appears to be the primary cause of CO2 evolution. Finally, we found that both production temperature and clay content affect biochar mineralization. As protective mechanisms hypothesized to prevent degradation of organic matter in soil usually implicate clay, we conclude that biochar is likely to be protected from mineralization during the early stages of incubation by its own recalcitrant chemical and physical nature as well as by physical protective mechanisms.

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Demková L., Jezný T., Bobuľská L. (2017): Assessment of soil heavy metal pollution in a former mining area -before and after the end of mining activities. Soil & Water Res., 12: 229−236.Toxicity and persistence of heavy metals, which are accumulated in the environment as the result of diverse industrial activities, represent serious environmental problem worldwide. The intense mineral extraction in mining areas has produced a large amount of waste material and tailings, which release toxic elements to the environment. The aim of the study was to determine in two time horizons (1997, 2015) the heavy metal contents of samples derived from ten sampling sites located in the former mining area of Central Spiš (Slovakia). In order to compare the level of contamination, the contamination factor (C i f ), degree of contamination (C d ), and pollution load index (PLI) were computed. Spearman's correlation coefficient was used in order to detect the relationships among heavy metals. A serious situation was found for Hg, Zn, and Cd, which exceeded limit values at all sampling sites within both studied years. In 1997, the average values of contamination factor have shown very high contamination with all studied heavy metals, and moderate contamination with Co. In 2015, the study area was classified as very highly contaminated with As, Hg, Zn, Cu, considerably contaminated with Ni, Cr, Pb, and Cd, while Co contamination was not detected. Since 1997 till 2015 the pollution load index decreased by about 38%, nevertheless even then almost all sampling sites were classified as heavily polluted. Despite the fact that mining activities were stopped or limited at the beginning of the 21 st century, the presence of heavy metals in soils remains at a serious level. The high level of contamination is a result of heavy metal persistence and non-biodegradability.

Heavy metals (Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, and Zn) in soils and plants of four different ecosystems (forest, grassland, agro and urban ecosystem) at different distances from the source of the pollution were analyzed in order to assess and compare soil contamination in the various ecosystems and determine the potential accumulation of plants depending on the place they inhabit. Correlation relationships among heavy metals in soils differ depending on the ecosystem, and between soil and plant, the heavy metals showed significant correlation for Cu, Mn, Ni, Pb and Zn. Contamination factor (C), degree of contamination (C) and pollution load index (PLI) were used in order to determine the level of environmental contamination of the study area. All studied ecosystems were rated as moderately contaminated (except agroecosystem, which was found as low contamination ecosystem) according to C and extremely polluted according to PLI. The highest pollution in both cases was found in urban ecosystem, and Cd, Cu and Fe were determined as the biggest pollutants.

Relationships between alien plant species and their aboveground effects have been relatively well studied, but little is known about the effects of invasive plants on belowground faunal communities. Nematodes are abundant, ubiquitous and diverse soil biota, and alterations of their community compositions can illustrate changes in belowground ecosystems. In 2016 and 2017, we determined the response of species diversity, community composition and trophic composition of the soil nematode communities to invasion by the alien plant Solidago gigantea in two ecosystems, forest and grassland, where invasion takes place. Nematode abundance was higher and number of identified nematode species was lower at invaded than uninvaded sites, indicated by lower species diversity, regardless of ecosystem. Herbivorous nematodes were the most affected trophic group. Herbivore abundance was higher at invaded than uninvaded sites and in grassland than forest. The herbivorous species Boleodorus thylactus, Geocenamus sp., Helicotylenchus spp., Paratylenchus bukowinensis, Pratylenchoides crenicauda and Rotylenchus robustus were more abundant at the invaded sites. Abundances of nematodes in the other tropic groups were limited or not affected. The invasion did not significantly affect the ecological and functional indices, except for the Channel Index in 2016. Differences were observed in values of Enrichment Index (indicator of resource availability), Channel Index (indicator of ascendant bacterial/fungal decomposition channel) and Basal Index (indicator of depleted-perturbed soil food webs) between grassland and forests. We can thus conclude that invasion by S. gigantea significantly alters nematode community indicators (abundance, species diversity and specific trophic groups); however, this effect seems to be significantly influenced by the type of ecosystem where invasion takes place.