Purpose The application of bio-fertilizers is one of the management practices that can help to maintain or increase the content of organic matter (OM) and improve soil fertility in arable soils. While some results have been obtained in relation to the influence of bio-fertilizers on organic matter content, less in known about the fractional composition of humus. Materials and methods The aim of this study was to determine the effects of the bio-fertilizer UGmax on soil total organic carbon (TOC), dissolved organic carbon (DOC), and the fractional composition of organic matter (C of humic acids (CHAs), C of fulvic acids (CFAs), and C in humins) in the humus horizon of an arable field. Measurements were taken in 2005 before the application of UGmax and in 2008, 3 years after its application, which was done in 2005, 2006, and 2007. Forty soil samples were taken in 2005 (the control year without UGmax), while 20 samples were taken after UGmax treatment and 20 from the control in 2008. Samples were always collected after the plants were harvested. Results and discussion After the 3-year period of the experiment, the TOC content was 6.3 % higher in plots on which UGmax was applied in comparison to the control, while the DOC content was 0.19 percentage points lower after 3 years of bio-fertilizer use as compared to the initial year of the experiment. The contribution of DOC to TOC decreased significantly after the application of UGmax in comparison with the control. The content of CFAs and its contribution in the TOC pools in soil without UGmax was higher at the end of the experiment compared to the beginning, while there was an inverse relationship in the soil with the bio-fertilizer. In comparison with the control, organic matter in the soil treated with UGmax had a higher content of C of humic acids, C in humins, and higher CHAs/CFAs ratio. Conclusions We conclude that the use of a bio-fertilizer that increases the stable fractions of organic matter provides evidence of an increase in the soil OM stability. In turn, the contribution of the organic matter fractions that are more resistant to decomposition is crucial for increasing soil carbon sequestration.

Artificial soil sealing in urban areas has attracted increasing attention because of its potential hazard to urban ecosystem. Covering soils with impervious materials has a significant impact on their properties and is essentially an irreversible process. In contrast to natural, open soils, sealed soils undergo a significant alteration of their physicochemical properties, and in turn, negatively influence microbial biomass and enzymatic activity. In general, 33 soils from different parts of the city of Toruń (NW Poland) were sampled and divided into 3 groups according to the degree of soil sealing: (1) soils sealed with semi-pervious concrete paving slabs (A), (2) soils sealed with impervious surfaces, such as asphalt and concrete (B), and (3) non-sealed soils (the reference group—C). Soil samples were assayed for (1) microbial biomass carbon (MBC), and nitrogen (MBN) and soil respiration activity (RESP), (2) the activity of soil enzymes, and (3) physicochemical properties. Soil sealing significantly reduced the content of carbon and nitrogen (both total and microbial), soil respiration, the activity of urease, and fluorescein diacetate hydrolysis compared to non-sealed soils (C), while the degree of soil sealing did not significantly affect these properties. Soil moisture and nitrate reductase activity were the only properties significantly differentiated by the degree of soil sealing. Canonical correlation analysis indicated that soil biological activity was caused mainly by the variation in MBC and MBN content, as well as dehydrogenase, catalase, and cellulase activities. The highest correlation was obtained between the soil moisture and the first canonical variable for microbial biomass and enzymatic activity. The results showed that the artificial sealing in urban areas can significantly alter the soils by reducing their carbon and nitrogen content as well as microbial biomass and its activity compared with open soils. The analysis of variance showed that the degree of soil sealing did not affect most of the studied soil properties, although the differences in raw data between impervious and semi-pervious sites were remarkable.