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Effect of urea fertilizer on leaching in some forest soils Otchere-Boateng, Jacob

Abstract

The study examined, by means of a series of field and laboratory experiments, the impact of urea fertilizer on soil nutrient leaching from four forest soil sites including a coarse-textured soil with little organic matter, a soil with substantial nitrification ability, a soil with relatively high organic matter content, and one with a thick forest floor. Duplicate tension lysimeters, installed near the surface and below the root zone in control and fertilized plots on each site, were used to obtain soil leachate solutions. After soil sampling and analyses, a year of lysimeter installation, and about seven months of pre-fertilization solution monitoring and analyses, urea (448 kg/ha urea-N) was applied in the fall. Soil - water sampling was continued for 246 days following fertilization. Some additional field soil sampling and analyses and laboratory soil - column leaching studies and soil analyses were conducted to confirm the field lysimeter results, and study the physico-chemical reactions and mechanisms responsible for those results. Field lysimeter soil-solutions were analysed for pH, electrical conductivity, Na, K, Ca, Mg, NH₄-N, NO₃-N, total N and P, while in the laboratory experiments, solution analyses included pH,electrical conductivity, Na, K, Ca, Mg, Fe, Mn, Al, organic matter and urea. Soil analyses included pH, cation exchange capacity, exchangeable cations and extractable P. The results indicated that the mechanism underlying leaching following urea application is not dominated simply by cation exchange reactions and mass ion effect. There were no serious losses of Na, K, Ca, and Mg immediately following a fall urea application. On the contrary, urea fertilizer caused a decline in Ca and Mg leaching. This was confirmed by laboratory leaching studies at 4°C. Leachate concentration of Al, Fe, Mn and organic matter, however, increased substantially (especially at 22°C) following urea treatment. Increases in CEC (caused by a rise in pH associated with (NH₄)₂CO₃ production by urea hydrolysis), the Donnan distribution, and organic colloid dispersion appear to define the behaviour of these cations. Increased leaching losses of bases (e.g. Ca and Mg), however, occurred in the field during the warmer months. The leaching was found to relate to pH decreases associated with increased nitrification (nitric acid formation). Decreased pH resulted in subsequent decreased CEC and promoted metal cation leaching because of the mobile nitrate anion and the strongly adsorbed hydrogen ion. Studies in the laboratory showed that urea fertilizer may cause increases in the effective CEC and exchangeable Na, K, Ca and Mg, and decreases in "exchangeable" Al, Fe and Mn. Increases in CEC were larger at 22°C than at 4°C. At only one of the sites, dissolved N concentration increased to more than 10 mg/l in the soil solution below the root zone, after urea application. The soil at that site is characterized by very coarse texture, extremely low organic content, and limited root distribution. Subjective evaluation of such morphological features in the profile of nitrogen-deficient soil might be adequate to predict the likelihood of serious N contamination of ground water, caused by urea at conventional rates. Results from all sites and consideration of climate and geography suggest that fall urea application in southwestern British Columbia forests is unlikely to cause sufficient metal cation leaching to be detrimental to water quality.

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