Ammonium photoproduction from aquatic humic and colloidal matter

It has recently been shown that nitrogen contained in humic substances can be released as low molecular weight species through photochemical processes or reactions. Nitrogen released in this manner may then become available for biological use and chemical reactivity. This report provides additional kinetic data for nitrogen photochemistry of organic matter in natural waters from Louisiana and for Suwannee River humic and fulvic acids. Natural water samples were fractionated into colloidal (> 0.2 mu m, < 1000 Dalton) and dissolved (> 1000 Dalton) fractions and the kinetics of these fractions as well as 0.2 mu m filtered water samples were measured. For irradiations of less than 10 hours of simulated noon-time sun, samples showed linear production of ammonium and first-order fading at 350 nm. Ammonium photoproduction was enhanced as much as three-fold by lowering pH, but linear kinetics were maintained. Light-dark cycling showed continued release of ammonium immediately after irradiation, followed by uptake at longer post irradiation times. A two step mechanism was proposed to explain the observed kinetics. Calculated net daytime release of ammonium from natural surface waters ranged from 0.33 to 1.9 mu M h(-1) in the samples studied, corresponding to a photic zone conversion rate of 9-20% of the total organic nitrogen per day into ammonium.