Abstract <p>An old arable plot and a 32-year-old fallow in the Botanical Garden with Southern Federal University (Rostov-on-Don, Russia) are studied to assess the seasonal changes in biological properties Haplic Chernozem. As part of the biological activity analysis, we examine the intensity of soil respiration, rate of organic matter decomposition, and soil enzyme activities, including catalase, dehydrogenase, phenol oxidase, and fluorescein diacetate hydrolysis. The arable plot displays a considerable decrease in all indicators of biological activity as compared with the fallow plot. The intensity of soil respiration, more dependent on current hydrothermal conditions than on long-term meteorological parameters, shows the highest seasonal variation. The rate of litter decomposition also considerably varies throughout the year and is closely related to average monthly temperatures. Statistically significant differences in these characteristics between the examined plots are mainly recorded in the growing season. Characteristic of the soil enzyme activity is relatively low seasonal variation; however, statistically significant differences between the plots remain throughout the year. This suggests a high potential of soil enzyme activity in biodiagnosing the farmlands to assess the long-term consequences of different land use types. The observed patterns can serve as the basis for predicting the dynamics of soil biological processes under climate changes.</p>