Isotopic composition (δC and δN) in the soil-plant system of subtropical urban forests.

This study brings information on the dynamics of C and N in urban forests in a subtropical region. We tested the hypothesis that C and N isotopic sign of leaves and soil and physiological traits of trees would vary from center to periphery in a megacity, considering land uses, intensity of automotive fleet and microclimatic conditions. 800 trees from four fragments were randomly chosen. Soil samples were collected at every 10 cm in trenches up to 1 m depth to analyze C and N contents. Both, plants and soil were assessed for δC, δN, %C and %N. Physiological traits [carbon assimilation (A)], CO internal and external pressure ratio (Pi/Pa) and intrinsic water use efficiency iWUE were estimated from δC and Δ δC in leaves and soil ranged from -27.42 ‰ to -35.39 ‰ and from -21.22 ‰ to -28.18 ‰, respectively, and did not vary along the areas. Center-periphery gradient was not evidenced by C. Emissions derived from fossil fuel and distinct land uses interfered at different levels in δC signature. δN in the canopy and soil varied clearly among urban forests, following center-periphery gradient. Leaf δN decreased from the nearest forest to the city center to the farthest, ranging from <3 ‰ to <-3 ‰. δN was a good indicator of atmospheric contamination by NO emitted by vehicular fleet and a reliable predictor of land use change. %N followed the same trend of δN either for soils or leaves. Forest fragments located at the edges of the center-periphery gradient presented significantly lower A and Pi/Pa ratio and higher iWUE. These distinct physiological traits were attributed to successional stage and microclimatic conditions. Results suggest that ecosystem processes related to C and N and ecophysiological responses of urban forests vary according to land use and vehicular fleet.