Agriculture Reference
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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 exhib-
ited 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 environ-
mental 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 suf-
fered from a relative shortage of nitrate with an average concentration
of a similar amount of N-fertilizers and use a similar amount of water.
Nevertheless, the nitrate concentration profiles appear to be very different
(Fig. 3). Identification of the nitrate sources in the vadose zone might be
examined through the isotopic composition of ∂ 15 N and ∂ 18 O in the nitrate
molecules. An isotopic analysis of nitrate in the water samples from the
vadose zone exhibited significant differences in isotopic composition in
the upper ~5m of each site (Fig. 4). ∂ 15 N values in the upper part of the va-
dose zone underlying the organic greenhouse are rather heavier (average
15 N= 15.6 ‰; STD = 2.12) in comparison with the depleted ∂ 15 Nvalues
observed under the conventional greenhouse (average ∂ 15 N= 6.26 ‰; STD
= 0.98). The differences in isotopic composition in the upper part of the
vadose zone correspond well with the expected isotopic values of pro-
cessed manure versus synthetic fertilizer and natural soil nitrogen (Fig. 5)
(Kendall and McDonnell, 1999).
One direct implication of the isotopic composition is the accurate iden-
tifi cation of the penetration depth of the agricultural leachates. Though
the nitrate concentration profi les are very different (Fig. 3), the isotopic
fi ngerprint shows that after four years cultivation in the new established
greenhouses agricultural leachates penetrated to a depth of ~5-7m in both
greenhouses (Fig. 4). Although the impacted depth in both greenhouses is
similar, the concentration pattern beneath the root zone in the conventional
greenhouse shows signifi cantly lower concentrations as compared with
those of the organic greenhouse.
 
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