Quality monitoring of urban green areas of Pisa

The European Union, in the “Thematic Strategy on the Urban Environment” (2006), has pointed out the importance of identification and monitoring urban environment quality indicators. In this scheme, the quality of 31 urban green areas in Pisa was assessed by monitoring topsoil physical, chemical and biological features and by using the plant bioindicator Taraxacum officinale Web. Results were compared with an extra-urban area (near S.Rossore-Migliarino-Massaciuccoli Natural Park) and with the quantitative limits fixed by Italian Law (DLgs 152/2006). The soils of the green areas of Pisa were mostly sandy, sub-alkaline, and lightly calcareous, with rather high organic matter content. They showed a widespread pollution by hydrocarbons, probably caused by road traffic incomplete combustion and by domestic heating. Only in few sites the total amounts of Cd, Cr and Hg were significantly higher than the minimum value established by the Italian Law. Probably due to the vehicular traffic, pollution by Pb, Cu and Zn was instead widespread. A sequential extraction (BCR method) was performed for determining the forms of heavy metals within the soil matrix. Instead of Zn and Pb, Cr and Cu showed the absence in soil of the labile forms. As far as Cr forms were concerned, the most represented one was the residual, while the anthropogenic addition of the element was found in the oxidizable form. For Cu forms, the most represented was the residual, while reducible and oxidizable forms were enriched by anthropogenic addition of the element. Regarding Pb and Zn forms, the most represented one was the reducible, while the anthropogenic additions of the elements were evenly distributed among the four fractions. Principal component analysis revealed the existence of four different patterns of distribution of the elements, whose differences have been attributed at the origin, each composed of a different number of variables: the first one, consisting of As, Cd, Cr, Cu, Pb, Sb, Sn, Sr, and Zn, whose main source was fit to identify in the traffic; as to the second factor, consisting of Fe, Mn, and Ni, the main source was identified by pedogenic substrate. Each of the third and the fourth factor was composed of a single element, respectively Cr and Hg, which had a particular distribution, different from that of all other HMs. Even the geostatistical analysis has identified the previous four patterns of distribution of HMs, bringing back, through interpolation techniques, also their coarse spatial distribution; according to this analysis, the central area of the city was identified as the one having the most polluted green areas. Hierarchical clustering analysis identified different groups of areas with different levels of contamination, both from the qualitative and quantitative point of view. Platinoids (Pt, Pd), “new generation” pollutants, released from recently introduced three way catalytic converters, absent in the substrate, showed a beginning of accumulation in a limited number of areas. As to soil biological features, cumulative respiration of monitored areas was evaluated during a 25 day incubation period. Results showed that evolved CO2-C was different among the monitored areas and influenced by soil carbon total content and pollution level. Community level physiological profiling of soil microbial population of the monitored areas was performed by using Biolog Ecoplates. The relative data of AWCD, CMD and H (Shannon-Weaver index of biodiversity) showed a limited variability among the monitored areas. Soil antioxidant capacity was evaluated and showed changes among monitored areas, being correlated with organic carbon content and, in particular, with phenolic substances. Soil enzymatic activities vary among monitored sites and appear to be influenced by organic matter and pollutants’ contents. The control site has shown the lowest rate of activities if compared to all the other sites. Positive correlation was detected among all enzymatic activities except for dehydrogenase. In spite of the presence of various pollutants, vegetal bioindicator Taraxacum officinale Web. did not show any alteration in the photosynthetic process with values of Fv/Fm, and FPSII, representing respectively the efficiency of PSII in conducting photochemical events and photochemical yield of PSII, typical of the leaves of healthy plants. Even photosynthetic pigment contents and qNP, representing mechanisms aiming at dissipating excess excitation energy, did not show any alteration among the monitored areas. That trend was confirmed by other parameters, monitored in order to understand if the polluted urban environment is not dangerous for the plant or if vegetal put into repairing actions. On the dissected aboveground and radical portions of the biomarker, antioxidant capacity, phenolic content, heavy metals contents and metal chelating capacity of vegetal tissues did not reveal any differences among the monitored sites, probably showing the dandelion ability in non-absorbing pollutants. We can finally conclude that the degree of pollution of the green areas in the city of Pisa, although present, does not cause major problems to the utilized bioindicators. We need more sensible biomarkers to effectively detect this degree of pollution.