Nitrification and Urea Hydrolysis in Amended Calcareous Saline Soils

This study was conducted to evaluate the nitrification rates and urea hydrolysis in calcareous saline soils amended with alfalfa and chicken manure. Nitrification was carried out in an air-dried soil/sand (1 : 1 w/w) mixture incubated in a leaching tube at 35 degrees C. Leaching was repeated every 2 weeks for a total of 10 weeks. The cumulative amounts of NO3--N in each of the 6 soils treated with ammonium sulfate or urea was linear with time of incubation. Results indicated that the zero-order nitrification rate constants varied significantly between soils. No significant differences (p = .05) in nitrification rate constants were observed whether ammonium was derived from urea or from ammonium sulfate. The total amounts of NO3--N produced from saline Al-Khasib and Dair soils were significantly increased (p = .05) with the addition of organic materials (alfalfa and chicken manure) compared with two other N sources (ammonium sulfate and urea). Studies were also conducted to test the effect of NaCl, CaCl2, and Na2SO4, applied at rates that produced EC of 10, 15, and 20 dS m(-1), on urease activity in two soils (Zubair and Missan soils) amended with organic materials (alfalfa and chicken manure). Urease activity was measured after the amended soil samples were allowed to equilibrate at 30 degrees C for 15 days. Results showed that urease activity increased significantly (p = .05) with the application of organic materials. The stimulation was greater in salt-amended soils than in nonsaline soils. The stimulation of soil urease activities by salt amended treatments declined in the following order when compared at the same EC level: NaCl > CaCl2 > Na2SO4.