Biogeochemistry of the Brooklyn Grange, an urban rooftop farm

Background/Question/Methods

Urban agriculture could be a vital part of a strategy to increase urban sustainability. Cities typically have limited open space available for production agriculture. However, flat rooftops are abundant. If structural and logistical challenges can be overcome, rooftop agriculture could be viable. The New York City recently developed a funding partnership to promote rooftop agriculture in order to improve stormwater management, compost-based waste management, and local food production. This presents an ideal opportunity to study the dynamics of a coupled human-natural system. A first step is to develop quantitative water and nutrient budgets for the Brooklyn Grange, a 0.6-ha vegetable farm atop an 11-story building in the former Brooklyn Navy Yard in New York City. We will address the following. How is N from food-waste compost partitioned into crop yield and leachate into combined sewers? How can improved management increase nutrient retention and reduce irrigation demand? What is the budget for heavy metals entering the site in compost and atmospheric deposition and leaving the site in leachate and vegetable produce? We have installed instrumentation at the farm to monitor precipitation, irrigation, evapotranspiration, atmospheric deposition, and leachate. Management records are used to estimate the input and output of nutrients via fertigation and crop harvest.

Results/Conclusions

During the 2014 growing season, the average water applied per irrigation was 9480 liter / day, of which 8650 liter / day was lost to evapotranspiration. The soil-less growing medium was not especially effective at retaining water, indicating an opportunity to improve water use efficiency by adjusting the components of the medium, thereby reducing irrigation demands. The preliminary analysis of KCl extractable N of the media,  and the sap analysis of the vegetable indicated sufficient to excessive levels of ammonium, but also nitrate deficiency. Based on this finding, the fertigation regime was adjusted and subsequent tissue analysis of harvested vegetables did not show any deficiency or atypical concentration of nutrients. Analysis of resin extracts of atmospheric deposition and leachate is in progress and will continue through the winter. A new growing medium with improved water holding capacity is being tested in the greenhouse during winter, 2015 and will be tested on the farm during the 2015 growing season.