Abstract:
Nitrogen deficiency ranks right behind water shortage as the second-most constraint to
crop production in the coarse-loamy sand of the Gareh Bygone Plain (GBP) in southern
Iran. Since better resource utilization is essential for wise energy management, contrary
to reported cases, surface water pollution by N maybe a boon in the GBP. The Agha Jari
Formation (AJF), in which the Bisheh Zard Basin (BZB) has been formed, contains anomalously
high concentrations of NO3
− and NH4
+ in its calcareous sandstone, siltstone
and marl components. As about 60% of our irrigation water is supplied through underground
resources and are heavily over-exploited, the artificial recharge of groundwater
(ARG) is actively pursued in Iran. Therefore, a major proportion of the ephemeral discharge
of the BZB is spread over 1365 hectares of sedimentation basins and recharge
ponds of the ARG system in the GBP. These basins and ponds are covered with improved
rangeland, and eucalyptus (Eucalyptus spp.) and acacia (Acacia spp.) plantations. Therefore,
it is expected that some of the geological-N dissolved in floodwater, and also carried
by the suspended load, will reach the groundwater and supply the plants coming in contact
with the water and/or sediment. Moreover, as both NO3
− and NH4
+ are considered
pollutants of groundwater, the relative abundance of each should be determined, so that
possible remedial measures can be designed and implemented. To study the origin of N
in the BZB, and to investigate the fate of the dissolved and adsorbed N as it travels from
the watershed to the root zone, 13 rock samples, 5 floodwater samples and 81 soil samples
were collected and analyzed for NO3
−and NH4
+. The NO3
−-N concentration was lower than
that of NH4
+-N in most of the samples: 5.1 vs. 9.6 mg kg-1 in rocks, 13.4 vs.17.4 mg L-1 dissolved
in floodwater, 1.3 vs.4.3 mg kg-1 carried by the suspended load, and 10.6 vs.15.5 mg
kg-1 in the 0-125 cm soil profile. Assuming that the mean annual inflow of the GBP-ARG
system is 7 million m3 ,we receive 89.4±52.4 kg ha-1 yr-1 of NH4
+-N and 68.7± 24.6 kg ha-
1 yr-1 of NO3
−-N, of which some furnishes the vegetative cover with N. A part of what
remains may reach the watertable after passing through the vadose zone.
