Impacts of urbanization on humus forms in inner-city forests of natural origin: a case study of Yekaterinburg

Within Yekaterinburg (Middle Urals, Russia), remnants of old-growth pine forests of natural origin have been preserved. In these forests, traditional urban management practices (such as landscape redevelopment, tree felling and planting, grass cutting, mechanical soil disturbance, and forest litter removal) are not applied. The case of Yekaterinburg creates a unique opportunity to study the impact of urbanization in its “pure” form. Our study analyzed humus forms, i.e., types of morphological structure of topsoils, which are determined by the composition and abundance of decomposers (primarily macro- and mesofauna). We compared humus forms in urban forests within the city to those in their suburban counterparts. The identification of humus forms was based on the European morpho-functional classification of humus systems and forms. The direction of changes in humus forms depends on the balance of various urbanization factors at different scales. These factors include city-wide influences (thermal pollution, eutrophication from atmospheric deposition and other sources, invasion of deciduous trees and shrubs into coniferous forests) and local factor (trampling due to recreational activities). With low recreational impact, the spectrum of humus forms shifts towards more zoogenic variants, with Oligomull and Mesomull replacing Dysmull. In contrast, high recreational pressure leads to a shift in the spectrum towards less active humus forms. In this case, Hemimoder, Eumoder, Hemimor, Humimor, and Eumor replace Dysmull. These transformation pathways can have opposite consequences for the carbon cycle under urbanization: in the first case, carbon is primarily accumulated in the organic-mineral horizons, while in the second case, it is deposited in the forest litter.