Nutrient Assimilative Capacity of an Alluvial Floodplain Swamp

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Abstract
Nitrate, ammonium, and phosphate were added to the forest floor of an alluvial swamp at a weekly rate of 1 g of N or P/m2 for nearly 1 yr. The experimental design allowed examination of nutrient assimilation and accumulation under field conditions that cannot be duplicated in the laboratory and produced results relevant to management of these swamps for wastewater treatment. Nitrate loss by denitrification was rapid and persistent; only slight accumulation occurred in surface water, and soil water accumulation was undetectable. Ammonium accumulated on cation exchange sites but was transformed to nitrate during drydown of sediments in summer and autumn. As nitrate did not accumulate, a tight coupling of nitrification and denitrification is inferred. Phosphate accumulated in sediments principally in acid extractable form with little evidence of loss after the additions ceased. At these high loading rates, uptake of nitrogen and phosphorus by vegetation and accumulation in tree stem-wood was small in comparison with disappearance by denitrification and accumulation in sediments. The capacity of the swamp for nutrient removal was highest for nitrate, intermediate for ammonium, and lowest for phosphate. Annual drydown of sediments would be required for sustained ammonium removal in swamps with prolonged flooding, as in this case. It appears that swamps of this type could be managed for inorganic N removal from sewage effluent, but their usefulness for tertiary treatment of phosphate is limited by the capacity of sediments for P storage. Mangement considerations not evaluated by this study, but deserving further investigation, are possible health impacts related to sewage effluent and competing ecological services such as wildlife production.