Variation in quality and decomposability of red oak leaf litter along an urban-rural gradient

This study tested whether urban land use can affect the chemistry and decom posability of Quercus rubra L. (red oak) leaf litter in forests within and near a large metropolitan area. Cities may affect the quality of leaf litte r directly through foliar uptake of atmospheric pollutants, and indirectly through alterations in local climate and changes in soil fertility caused b y pollutant loads and altered nutrient cycling regimes. Using a microbial b ioassay, we tested whether red oak leaf litter collected from urban and sub urban forests in and near New York City differed in decomposability from li tter of the same species collected from rural forests 130 km from the city. We found that oak litter from the urban forests decayed 25% more slowly an d supported 50% less cumulative microbial biomass in a laboratory bioassay than rural litter. Rural litter contained less lignin and more labile mater ial than urban litter, and the amounts of these chemical constituents were highly correlated with the decay rate coefficients and integrated microbial growth achieved on the litter. The specific causes of the variation in lit ter chemistry are not known. The results of this study suggest that decompo ser activity and nutrient cycling in forests near large cities may be affec ted both by altered litter quality and by altered biotic, chemical and phys ical environments. The sensitivity of the microbial bioassay makes it usefu l for distinguishing differences in within-species litter quality that resu lt from natural or anthropogenic variation in the environment.