Water and nitrogen budgets under different production systems in Lisbon urban farming

• Water and N budgets are presented for typical urban vegetable gardens in Lisbon.
• RZWQM predicted soil water storage for a 1-year cycle with an average RMSE of 5.5%.
• N surplus associated with non fertiliser sources and inadequate organic mixture C:N.
• Leaching, denitrification, volatilisation and runoff were the pathways for N losses.
• N availability per t of organic material varied from 9.2 to 20.9 kg year−1.

Public concern is growing over soil and groundwater contamination from the use of agrochemicals in urban farming. Heavily used nitrogen (N) fertilisers are converted to nitrates that can be a health hazard. In this study, water and N budgets over a 1-year period are presented for typical urban vegetable gardens in Lisbon. A conceptual analysis supported by an integrated methodology of field experiments and modelling identified the N surpluses associated with conventional and organic gardens. It is concluded that the gardening systems are continuously cropped using high N and water application rates. For all of the case-study allotments, the N inputs, mainly from organic amendments with diverse N release rates, were higher than the crop uptake generating surpluses that were lost by different processes. On one study site a drainage flux of 280 mm yr−1 was calculated, with a mean concentration of 295 mg NO3− l−1. On another site N accumulated in the lower soil depths at a rate of 420 kg NO3− ha−1 yr−1. The cumulative impact of N surpluses on the environment and human health must be considered. To minimise adverse impacts, we propose the selection of organic fertilisers with N release rates close to the crop N uptake, the prevention of excess irrigation to minimise N leaching and gaseous losses and the inclusion of the non-fertiliser N sources in the fertiliser calculations. It is shown how an integrated model can be used to predict the N release dynamics from the organic fertilisers as affected by the moisture conditions.