Short-Term Dynamics of CO2 Emission and Carbon Content in Urban Soil Constructions in the Steppe Zone

Constructed Technosols are an important component of urban green infrastructure, whose role in the carbon balance of urban ecosystems remains poorly studied. The dynamics of carbon stocks and CO2 emission of soil structures depend on both bioclimatic conditions and the parameters of the construction: the substrates used and the composition and thickness of the layers. The carbon stock dynamics and CO2 emission were studied on constructozems of different compositions at the Experimental Station for Study of Soil Constructions in the Botanical Garden of the Southern Federal University (Rostov-on-Don). The station consists of 15 sites representing five different variants of soil constructions made of substrates traditionally used for landscaping and beautification tasks in cities of the steppe zone. Haplic Chernozem of the automorphic position in the vicinity of the Experimental Station was studied as a background (reference) soil. Monitoring studies during two seasons (from September 2020 to December 2022) enabled us to compare the seasonal dynamics of organic and inorganic carbon contents and CO2 emission for different soil constructions with the background. All constructions created on the basis of humus-accumulative horizons of chernozems demonstrated a pronounced dependence of CO2 emission on the temperature of air (r = 0.76, 0.82, 0.76, and 0.49 for constructions 2, 3, 4, and 5, respectively, at p < 0.05) and of soil (r = 0.58, 0.74, 0.75, and 0.68 for constructions 2, 3, 4, and 5, respectively, at p < 0.05). The biomass stocks of lawn grasses growing on the constructions were determined, and a positive correlation between the CO2 emission and the increase in the aboveground biomass was revealed: a moderate correlation for construction 2 (r = 0.48, p < 0.05), a pronounced correlation for constructions 3 and 4 (r = 0.5, p < 0.05; and r = 0.68, p < 0.05), and a strong correlation for construction 5 (r = 0.75, p < 0.05). The dynamics of the stocks of various forms of carbon in the first year of operation of the structures were studied. Based on the comparison of carbon fluxes and stocks, it is shown that lawn ecosystems at the early stages of constructozem development cannot be considered as net carbon stocks in the temperate continental climate of Rostov-on-Don.