The Agha Jari Formation: A Potential Source of Ammonium and Nitrate Nitrogen Fertilizers

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.