In this study, the reaction between humus acids (humic acid and β-humus fulvic acid fraction) and bentonite and illite was studied at a variety of pH values. The degree of reaction was determined from the specific surface area and molar energy of adsorption. Characteristic parameters of adsorption isotherms for the formation of a mono-layer of the adsorbent, such as the constant C from the BET equation, mono-layer capacity (Nm), and standard error square (R2), were also included in the study. After the addition of humus acids, the specific surface area of illite and bentonite decreased at all pH values, reaching a minimum at pH 4. This indicates different degrees of reaction of humus acids with the clays and, probably, partial hydrophobization of the materials. The degree of reaction of humus acids with the minerals depended on the pH and, for certain combinations, it was highest at pH 4. This value is relatively close to that at which the humus acid fractions in question precipitate from solution.

A b s t r a c t. The actual denitrification to N 2 O and denitrification capacity to N 2 O after flooding of different soil samples stored for over 25 years in air-dry conditions and fresh, air dried samples were compared in our study. Zero N 2 O release was observed from the stored soils but the fresh soil samples had very low actual denitrification to N 2 O. NO 3 -addition significantly increased the amount of N 2 O (denitrification capacity to N 2 O) released after flooding, which depended on the length of storage and type of soils and was much higher in stored soils. Prolonged exposure of the soils to drought conditions caused a greater decrease in the Eh value compared with the fresh soil. The total cumulative release of N 2 O from the stored and fresh soils was correlated with the reduced NO 3 -and organic C content in soils enriched with NO 3 -. Some soils showed the capability of N 2 O consumption. CO 2 release depended on the length of storage and type of soils under flooding after prolonged drought. On average, CO 2 release was higher from the stored rather than fresh soils. The organic C content in the stored soils was generally lower than in the fresh soils, probably due to the storage effect. The cumulative CO 2 release from the stored soils was well correlated with the organic C while no correlation was observed for the fresh soil samples.K e y w o r d s: actual denitrification to N 2 O, denitrification capacity to N 2 O, long-and very short-storage time, soil respiration, archived soil INTRODUCTIONBiological activity in soil can be represented by several different parameters such as respiration, enzyme activity, ammonification, nitrification, denitrification, and emission of gaseous metabolites as well as oxidation-reduction processes (Bieganowski et al., 2013;W³odarczyk et al., 2011).Soils are subjected to temporal variations in temperature and moisture that can cause changes in physicochemical properties. Soil dry/wet cycles result from natural variations in soil moisture driven by environmental and biophysical processes such as precipitation, evapotranspiration, and drainage. Management factors such as irrigation, tillage and land cover (ie. vegetation type) can moderate or accentuate the amplitude of these natural cycles (Oliveira et al., 2005).Under in situ conditions, denitrification rates depend on oxygen availability, soil moisture, soil type, pH, NO 3 -concentration, but also on the availability of labile carbon compounds in soil (Burford and Bremner, 1975; Senbayram et al., 2009). Nitrate (NO 3 -) is a key node in the network of the assimilatory and respiratory nitrogen pathways. For bacteria, it is both a nitrogen source and an electron acceptor (Hayatsu et al., 2008). In agriculture and wastewater treatment, NO 3 -respiration by microorganisms is an important process in respect to economics, greenhouse gas emission, and public health. Several microbial processes compete for NO

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