Agriculture Reference
In-Depth Information
were obtained from additional locations in each greenhouse, and from ad-
ditional greenhouses owned and cultivated by different farmers. In addition
to the VMS that monitored the vadose zone, each site was instrumented with
an observation well that penetrated the upper phreatic groundwater with
screens to 5m below the water table (Fig. 1). In order to validate the results
obtained by the VMS, nitrate profi les in the vadose zone pore water were
compared with the nitrate concentrations in sediment samples from three
additional boreholes in each greenhouse. Samples were collected at a 0.5m
depth resolution from the top 3m of the profi le using a standard hand auger.
14.2.3 CHEMICAL AND ISOTOPIC ANALYSIS
Nitrate concentrations in the water samples were determined using ion
chromatography (DIONEX, 4500I). Soil samples were extracted by KCL
and analyzed for ammonium by the Nesslerization method (APHA, 1989),
for nitrite using the colorimetric method, and for nitrate using the second-
derivative method (APHA, 2005). Total nitrogen in the soil was analyzed
following the Kjeldahl method (Benton, 1999). The isotopic composition
of
15
N and
18
O of nitrate in the water samples was determined through ni-
trate reduction to nitrogen dioxide, which was then analyzed using a gas
mass spectrometer (McIlvin and Altabet, 2005).
Throughout the study, the following data were collected: (1) crop type
and growing cycle, (2) irrigation quantity and quality, (3) fertilization re-
gime, (4) temporal variation of the vadose zone water-content profi le, (5)
chemical composition of the vadose zone and underlying groundwater,
and (5) isotopic composition (
15
N-
18
O) of nitrate obtained from the vadose
zone pore water.
14.3 RESULTS AND DISCUSSION
14.3.1 AGROTECHNICAL REGIME
A comparison of agrotechnical regimes implemented in the organic and
conventional greenhouses showed very similar general inputs. For example,
