Projecting urban heat island effect on the spatial-temporal variation of microbial respiration in urban soils of Moscow megalopolis

Urbanization coincides with remarkable environmental changes including vegetation, soil and climate. An urban heat island is a well-known urban mesoclimatic anomaly, however its effect on functions of urban soils considering their spatial-temporal variability remain overlooked. The research aimed to explain the urban heat island effect on soil microbial respiration by linking meso-climatic CCLM and TERRA_URB models and the digital soil mapping approach for the case of Moscow megalopolis. Moscow megalopolis is the biggest city in Russia, which experiences on-going urbanization within the New Moscow project. As a result, a clear urbanization gradient was observed in land-use structure, climate and soil properties. For the study period (May–October 2019), the increase in average monthly temperature of urban soils compared to rural reached 1.7 and 2.0 °C in June and September with the maximal values obtained in the city center. The significant effect of soil temperature, carbon content and pH on soil microbial (basal) respiration was estimated based on the lab experiment with a representative sample of urban topsoils (n = 140). The resulting equation was integrated with the mesoclimatic data in two ways: i) considering urban heat islands (based on the modelled 500 m grid soil temperature maps) and ii) non-considering urban heat islands (based on the average monthly rural soil temperature). Comparison of the results showed that urban heat island increased microbial respiration by 5–10% on average and up to 25% in the most affected locations. The additional amount of CO2 which could be potentially emitted by soil microbes as a result of the urban heat island in Moscow megapolis was on average 0.2 Mg C ha−1 (0–10 cm layer; 1.1 g cm−3 bulk density). Small scale green zones in the city center (public gardens, residential court yards and green lawns along the roads) where high soil C contents coincided with the maximal urban heat island effect were the hotspots of microbial respiration. Although the estimated absolute values could be rather uncertain, the research highlights the vulnerability of urban topsoil C stocks to mineralization intensified by mesoclimatic anomalies.