Carbon and nitrogen stock in soils of subtropical urban forests: Isotopic δ13C and δ15N indicators for nature-based solutions in a megacity

Abstract

Urban forest soils play a pivotal role in enhancing the environmental sustainability of cities, contributing to various natural processes, including plant–microbe interactions, microbial activity, and the decomposition of organic matter. Consequently, urban forest soils emerge as effective NBS, underscoring their potential to mitigate environmental challenges and foster sustainable urban ecosystems. In these sense, this manuscript aimed at evaluating how soil attributes of the urban forests of São Paulo, Brazil, with different adjacent land uses, influence the capacity to store the excess of C and N from anthropogenic emissions, making this ecosystem an important reservoir of urban anthropogenic emissions. Three hundred urban forest soil samples were collected from the surface to a depth of 50 cm. All samples were analyzed for C and N content (and their stable isotopes). In addition, granulometric tests were also carried out to classify the soils. It was found that the most central forest fragment has the highest C and N contents at all analyzed depths, probably due to the association with physical aspects of soil texture. For all layers, the most central fragment soil sample, the only one with a clay soil, presented approximately twice as many elements when compared to the soil samples of the other sites. In general, C and N stocks (and their stable isotopes, δ13C and δ15N, respectively) varied significantly in forests located in the center-periphery direction (%N - F = 24.58, p < 0.05; %C - F = 22.48, p < 0.05; δ15N - F = 4.27, p < 0.05; δ13C - F = 19.8, p < 0.05; C/N - F = 14.56, p < 0.05). This more central forest fragment with higher vehicle emissions showed greater potential to store these atmospheric elements and with greater neutralizing efficiency than the other forest fragments. δ13C and δ15N contents together with the C:N ratio indicated the efficiency of biogeochemical cycling, through decomposition, in different urban forest fragments. More peripheral fragments showed high efficiency in C and N cycling along the analyzed soil profiles, while in the most superficial layer, the most central fragment was highly efficient. These shed light results how integrating NbS principles into strategic urban planning and city-level climate policies can bolster the effectiveness of urban green areas. The integration not only promotes carbon sequestration and efficient nutrient cycling in the soil but also fosters sustainable practices, contributing to a more resilient urban landscape.