| Summary: | Clinoptilolite zeolite has a theoretical cation exchange capacity of 2.25 moles of charge kg⁻¹, and a rigid three-dimensional lattice riddled with angstrom-sized tunnels, and interconnected voids, in which water and exchangable cations are held. The hypothesis was that clinoptilolite had the facility to preferentially and internally sorb NH₄⁺, where it would be physically protected from microbial nitrification. Hence nitrification rates would be decreased and plant N-fertilizer use efficiency increased. Exchange capacities of clinoptilolite determined at 30°C by saturation/desorption for NH₄⁺, K⁺ and Na⁺ were approximately 2.00 moles of charge kg⁻¹, while capacities for Ca²⁺ and Mg²⁺ were 1.53 and 0.97 respectively. On this basis three site groups were identified: those accessible to all cations studied, sites accessible to all cations but Mg²⁺ and sites only accessible to NH₄⁺, K⁺ and Na⁺. Equilibrium isotherms were used to determine selectivity of site groups at 30°C. Consideration of site accessibilities and selectivities indicate an overall preference of clinoptilolite of: K⁺ > NH₄⁺ > Na⁺ = Ca²⁺ > Mg²⁺. Notably, the plant macronutrient cations, K⁺ and NH₄⁺, are preferentially sorbed. Nitrification of NH₄⁺ on clinoptilolite amended sands incubated at 20% volumetric moisture capacities, was studied in the laboratory. Treatments were washed mortar sand amended with 0, 5 and 10% clinoptilolite by volume and 2.38 and 3.57 moles of NH₄⁺ m⁻³ of sand-clinoptilolite mix. Nitrification was evaluated by monitoring NH₄⁺ loss. Rates of nitrification decreased with increasing clinoptilolite amendment and decreased with N-fertilizer initially applied. The effect of clinoptilolite in slowing nitrification was more pronounced at higher initial NH₄⁺-fertilizer application. The hypothesis that internally sorbed NH₄⁺ in clinoptilolite is physically protected from microbes resulting in decreased nitrification rates was confirmed. The effect of clinoptilolite on N-use efficiency of creeping bentgrass was studied in a field trial. Factorial treatments included washed mortar sand amended with 0, 5 and 10% clinoptilolite by volume and 25, 50 and 75 kg of N ha⁻¹ growing month⁻¹. Approximately 45% of applied N was harvested in clippings from 10% clinoptilolite amended sand in contrast to 36% N recovery on 100 % sand. This supports the hypothesis of improved plant N-fertilizer use efficiency on clinoptilolite amended sand.
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