Water samples from the vadose zone and groundwater were
collected at both sites every 3 to 6 weeks for 19 months (total
of 262 water samples). A comparison of nitrate concentrations
in the vadose zone pore water at the two sites throughout
all sampling campaigns exhibited a striking difference
(Fig. 3). Nitrate concentrations below the root zone (>1 m)
under the organic greenhouse exhibited a very high average
concentration of 357 mg L−1, with a peak average concentration
of 724 mg L−1 at a depth of 2.5 m. Nitrate concentrations
below the root zone of the conventional greenhouse
were much lower, with an average value of only 37.5 mg L−1.
A closer look at the upper part of the unsaturated zone of
both sites exhibited an opposite concentration pattern. Nitrate
concentrations under the conventional greenhouse showed a
high average concentration of 270 mg L−1 in the root zone
(<1 m) that quickly reduced in the deeper part of the vadose
zone. This pattern is preferable for both agronomic and environmental
reasons as nitrogen is available for root uptake
in the shallow zone with minimal down migration of nitrate
to the deeper parts of the vadose zone. On the other hand,
under the organic greenhouse, the root zone suffered from a
relative shortage of nitrate with an average concentration of
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