The heavy metal budget of an urban rooftop farm.

Urban rooftop agriculture is a growing enterprise in the US with the goal of providing high quality, healthy, locally grown produce for city dwellers. However, air pollution abatement and the purification of stormwater are among the ecosystem services emphasized in studies of conventional green roofs. If rooftop farms actually capture pollutants, then accumulation of heavy metals in the soil could pose a problem over time. This study reports the heavy metal concentrations in soil, atmospheric deposition, and drainage output of 8 metals from the Brooklyn Grange Navy Yard Farm, rooftop vegetable farm in New York City, USA. Drainage of Pb and Mn were 6% and 14% of atmospheric bulk deposition, respectively, meaning that the Grange could be a net sink for Pb and Mn. Although there were small scale hotspots in the soil, farm-wide averages for heavy metal concentrations never exceeded guideline levels, and relatively low concentrations of Pb and Ba in the soil suggest that rooftop soils may be less vulnerable to contamination related to traffic and construction. In comparison to the growing seasons, we found relatively high concentrations of Pb and Cr in the soil during fallow periods when the soil was bare. To reduce the atmospheric deposition of heavy metals to soil, it is important to cover the soil with mulch, and discard the used mulch and unmarketable portion of vegetables, instead of recycling them via composting for soil amendments.

On the occurrence of adventitious branch roots on root axes of trees.

Based on positive results for 11 of 17 species included in an anatomical survey of tree roots, we concluded that the origin of adventitious branch roots (ABR) on established, undisturbed woody parental root axes is a widespread occurrence. ABR were morphologically indistinguishable from branch roots formed in primary tissues of a parental axis, and they occurred without increase in branch root density. We concluded that ABR are an unrecognized component of undamaged root systems. Using continuous serial sectioning to perform a census of branch roots within parental root axes, we obtained definitive evidence that ABR participate in root turnover. Comparisons of older and younger axes, and the chronology of root formation in older axes fulfilled the expectation that with greater age of parental axis, ABR become increasingly predominant among intact branch roots. We confirmed this trend for one of the species scoring negative in the survey, which means that our survey results were probably influenced by age variation. Thus, any of the six negative results obtained in the survey may have been dependent on the young age and slenderness of the axes that were available for examination in those six species.