Abstract:
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 280mmyr 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.
