Glomalin changes in urban-rural gradients and their possible associations with forest characteristics and soil properties in Harbin City, Northeastern China

Glomalin-related soil protein (GRSP) is a glycoprotein from the hyphae and spores of arbuscular mycorrhizal fungi. Despite urbanization being the leading cause of present-day land-use changes, there is limited information available on the effects of urbanization on GRSP. We sampled soil from 257 plots in Harbin City, China, and surveyed forest characteristics, soil properties, and urbanization gradients related to ring road development, urban history, and land use. Two glomalin components (easily extracted glomalin, EEG; and total glomalin, TG) and their relative contributions to soil organic carbon (SOC: EEG/SOC, TG/SOC) were measured in the laboratory. We found exponential increases in EEG/SOC and TG/SOC from the most urbanized to the most rural regions, indicating that urbanization sharply reduced glomalin-related SOC sequestration. In general, 1.3-1.4-fold higher glomalin levels were found in the newly urbanized, previously rural areas, while glomalin contribution to SOC sequestration was lower by 38-59% for EEG and 74-85% for TG in the most urbanized regions compared to rural regions. Accompanying these recorded changes in glomalin, linear decreases in soil pH and electrical conductance were observed in all three urban-rural gradients from the urban center to the rural area, and steep decreases in conifer ratio and shrub richness were seen in two of the gradients. The complex associations among glomalin and forest characteristics, soil properties, and urbanization gradients were decoupled and cross-checked using redundancy analysis variation partitioning and structural equation model analysis. Urbanization indirectly changed glomalin features by altering soil properties, with soil properties accounting for over 60% of the glomalin variation. Forest characteristics and urbanization gradients contributed to 10-15% of the glomalin variation. With rapid urbanization occurring in China and on a global scale, glomalin variation should be considered when evaluating soil carbon sequestration and in developing effective forest management strategies, with the aim of ameliorating soil degradation in urbanized regions by rehabilitating glomalin accumulation.