Nitrogeneous photodegradation from dissolved organic matter in natural water systems

Organic nitrogen is a dominant form of terrestrial N exported to the coastal waters of the southeastern United States. It has been previously considered unavailable to coastal plankton. Recent research shows that photochemistry contributes significantly to the terrestrial N availability to microorganisms. In nitrogen-limited ecosystems, photodegradation of natural organic matter to release small biological available nitrogenous compounds from humic substances may be a key process regulating microbial turnover of dissolved organic matter in these systems. Humic substances are organic acids (of molecular weight between 500 and 10,000), operationally defined on the basis of their retention on hydrophobic resins and further classified as humic acids or fulvic acids based on their solubility at low pH.;The chemical reaction of ammonia with humic substances is referred to as ammonia fixation. It is well known from laboratory studies that the nitrogen incorporated into humic substances is not subsequently released upon acid hydrolysis. It is also recognised that the nitrogen incorporated into soil organic matter upon fertilisation of agricultural soils with ammonia is not readily remineralized and made available to plants. In other words, the ammonia is bound by organic matter in a form which is resistant to microbial attach or chemical degradation. Recent research shows the nitrogenous photodegradation in humic substances.;Photochemical release of nitrogen may have significant impact on nitrogen cycling, on the other hand, changes in humic chemical composition may change the reactivity of humics with pollutants; Ammonia can be photochemically released from humic substances, though the mechanism of this process is unclear. Through assessing the kinetics of ammonia photorelease from natural waters collected from Louisiana the preliminary mechanistic assumption was made.;Photochemically exposures were carried out using a Suntest CPS+ solar simulator (Atlas Electric Devices, Chicago). Ammonia was measure by an ion chromatograph (Dionex DX-500). Solar irradiation of dissolved organic matter (DOM) results in substantial conversion of recalcitrant dissolved organic nitrogen (DON) to ammonia. Based on this study, 6% of DON from Suwannee River humic acid and 8% of DON from Suwannee River fulvic acid converted to ammonia per day photochemically.;The photorelease of ammonia from humic substances is a multistep reaction; intermediates may form during irradiation to form ammonia. Acid could accelerate the ammonium photoproduction rate by prohibiting the ammonia uptake processes in which the uptake species is ammonia instead of ammonium.*.;Amino acids were also observed to be photoreleased from Suwannee River fulvic acid and Bayou Trepagnier water (site 56). The average amino acid photorelease rate is in the order of nM h-1. The amino acids photochemical released may further undergo degradation to release ammonia. The photodegradation of several amino acids mediated by humic acid and fulvic acid were investigated. Methionine, tyrosine and DOPA degraded much faster in the presence of fulvic acid or humic acid than in pure water. The reactive transients photochemical produced from humic acid or fulvic acid play significant roles in the photodegradation of amino acids. Hydroxyl radical photoproduced from humic acid or fulvic acid controls the photodegradation of methionine. Other reactive species besides hydroxyl radical contribute significantly to the photodegradation of tyrosine. Average about 50% of the degraded amino acids investigated release ammonia.;*Please refer to dissertation for graph.