Nitrogen and Phosphorus Mass Balances in Natural and Sewage-Enriched Cypress Domes

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Abstract
N and P mass balances were determined on 2 small cypress swamps in Florida [USA]. Nutrient inputs from secondary sewage effluent applied to 1 swamp for a 3-yr period (1975-1977) were 14.9 g m-2 yr-1 for total N (TN) and 11.4 g m-2 yr-1 for total P (TP). These values represent 90 and 99% of the TN and TP inputs to this system. Input rates measured on an undisturbed swamp were 0.051 g P m-2 yr-1 and 2.19-4.26 g N m-2 yr-1, of which bulk precipitation contributed 67% of the P and 20-39% of the N. Runoff from uplands supplied on additional 33% of the P and 14-26% of the N. N fixation was an important source of N to the natural dome: 34-66% of the TN input, or 0.75-2.81 g m-2 yr-1 (depending on the significance of nitrogenase activity measured on excised roots of woodly species). Mass balance calculations indicate that without N2-fixation there would be inadequate N to support measured growth rates of woody biomass. Application of effluent inhibited N2-fixation; less than 2% of the TN loading to the sewage-enriched dome was accounted for this process. Denitrification was more prominent in the sewage-enriched dome than in the natural dome both in absolute amount. Because of higher nutrient levels in shallow groundwater and higher infiltration rates, the sewage-enriched dome lost 6 times more TN and 16 times more TP by infiltration than did the natural dome. Higher uptake rates of N and P into above-ground cypress trees were calculated for the sewage-enriched dome than for the natural dome, but the fraction of nutrient inputs stored in the above-ground cypress biomass was much higher in the natural dome than in the sewage-enriched dome. Annual inputs exceeded outputs by 11.0 g N m-2 yr-1 and 10.4 g P m-2 yr-1 in the sewage-enriched dome, indicating 74% retention of TN inputs and 92% retention of TP inputs. Most of the nutrients remaining in the dome were retained in below-ground biomass and peat (90% of the TP input and 56% of the TN input). Cypress domes can serve as efficient natural tertiary treatment systems, and they should be considered as alternatives to conventional tertiary treatment methods and to other wetland and upland disposal sites.