3.4. Land use change impact on NO3-N concentrations in the
groundwater
Our monitoring data collected in 2002 and 2007 showed that
land use type and irrigation water quality were altered at some
sampling sites over time. The sample wells were grouped into
general irrigation and crop-rotation types. Table 4 shows the
relationships between the irrigation resource and cropping
practices at each of the sampling sites for the data collected in
the two years.
Fig. 4 demonstrates the temporal dynamics associated with
using well irrigation. As can be seen in Fig. 4, the NO3-N
concentrations in shallow groundwater varied significantly with
land use changed. A general increase of NO3-N concentrations in shallow groundwater was observed as a result of land use change
from wheat–maize rotations to wheat–vegetable, vegetables and
cotton. The highest increases occurred when land use changed to
cotton (9.60 mg L1) and vegetables (7.52 mg L1), and the
smallest increase occurred when land use was changed to
wheat–vegetable rotations (4.33 mg L1). However, under the
same irrigation conditions, shallow groundwater NO3-N concentrations
decreased when land use was changed to fast-growing
trees.
The highest concentration of nitrate in the groundwater in 2002
was found in cotton (8.33 5.63 mg L1), followed by vegetables
and wheat–vegetable rotation, which had concentrations of
7.13 6.22 and 4.65 2.37 mg L1, respectively. When land use
was changed to wheat–maize rotations in 2007, the greatest increase
or nitrate concentration occurred when the wheat–vegetable
rotation, which had previously resulted in low nitrate concentrations,
was abandoned. Nitrate leakage may increase as land use changes to
wheat–maize rotations. Additionally, shallow groundwater flow may
also enhance the accumulation of nitrate.