Phosphorus storage and release in response to flooding: implications for Everglades stormwater treatment areas

Abstract

As part of the Everglades restoration program, 16 000 ha of constructed wetlands will be reestablished on land presently in agricultural production. These wetlands will be used to remove Phosphorus (P) from agricultural runoff before it enters the Everglades. Histosols, organic soils, are the predominant soil type in the Everglades Agricultural Area (EAA), and the conversion of these soils from drained to flooded conditions has important implications for P storage. Phosphorus storage in organic soils has been shown to be both positively and negatively affected by anaerobic conditions. In this study, P storage and release was followed in a 146 ha area during its conversion from farmland to wetland. The development of a productive biological community, as evidenced by strong diel dissolved O 2 and pH cycles, occurred within 3 weeks of flooding at one site and 2 months at a second site. This biological community was considered influential in maintaining the low concentrations of both N and P in the water column relative to soil porewater concentrations. Maximum total P (TP) and total Kjeldahl N (TKN) concentrations of 0.3 and 5 mg l −1, respectively, were recorded in the water column following flooding. These concentrations declined to background levels within 2–3 months. Soil porewater TP and total dissolved Kjeldahl N (TDKN) concentrations increased to maxima of 4 and 24 mg l −1, respectively, 2 months following flooding. Nutrient profiles across the soil–water interface were used to estimate flux rates. Calculated NH 4-N flux rates ranged between 0.18 and 0.74 μg cm −2 d −1 and P fluxes ranged between 0.03 and 0.15 μg cm −2 d −1. Phosphorus fluxes from the soil to the overlying water are a function of the mobility of different P fractions. Phosphorus fractions within soil cores, collected immediately upon flooding and again 1 year after flooding, were identified using bicarbonate (labile), sodium hydroxide (Fe- and Al-bound) and hydrochloric acid (Ca- and Mg-bound) extractions. Labile inorganic P increased, while labile organic P decreased in response to flooding. Phosphorus associated with Ca and Mg increased in the surface 0–45 cm soil profile in response to flooding. These data suggest that immediately following flooding the reestablished wetlands will act as a source rather than a sink for P, and P concentrations in the water column will not meet discharge requirements. Although this only occurs for a short time period, steps need to be taken to contain or recycle the water during this initial start-up. Soil fractionation data indicate that while organic P is the primary means of P retention within these soils, Ca-phosphates may play a significant role in P storage. Therefore, the reestablished wetlands should be operated to enhance Ca phosphate formation in addition to biological P uptake.