Pore water transport mechanisms and soil diagenesis in an impacted isolated wetland within the Lake Okeechobee drainage basin: Implications for internal nutrient loading.

摘要

The deteriorating water quality of Lake Okeechobee due to excess phosphorus (P) associated with beef cattle and dairy operations from surrounding basins have been an environmental concern for many decades. During this period, isolated wetlands within these basins have been extensively ditched and drained to improve farmlands and facilitate agriculture. Restoring the wetlands has been proposed as one of the strategies to reduce overland P loss by retaining it in the soils. Phosphorus fluxes associated with soils and shallow pore water from agricultural lands can contribute to internal loading which could undermine the effective use of these wetlands. Soil properties of deeper horizons within impacted isolated wetlands are rarely investigated due to the assumption that these wetlands are flooded, and that biogeochemical interactions occur at the soil-water interface. However, prolonged droughts and periodic draining can leave the wetlands dried for most of the year, and water tables can drop over a meter below the ground surface.;As part of my doctoral research, I investigated soil properties of surface as well as deeper horizons from an impacted isolated wetland located on a beef farm and its surrounding upland pasture. The upland soils (Alfisols) showed pedologically formed horizons compared to the stratified wetland soils (Entisols). The soils were sandy up to a depth of 100 cm, and showed increasing clay content further down core (>100 cm). This can have implications for higher P sorption capacity, perched water tables and slower hydraulic conductivity. Net P reserves were highest at the surface 28 kg ha-1 (46%), however, non-trivial quantities were also found in the deeper horizons associated with the clay horizons at 15 kg ha-1 (25%). Due to the transient hydrology, one of the hypotheses tested was that internal loading of P from diffusive and advective processes was a significant component of the P budget to the wetland. Diffusive fluxes of SRP to the wetland water column were estimated at 1 mg m-2 d-1 , while advective fluxes associated to water table fluctuations were 8 (+/-10) mg m-2 d-1. The wetlands also showed very low annual hydroperiods with multiple drawdown events, this can have a negative effect on the P loading to the water column due to fast changes in soil redox conditions. In addition, P release and storage capacity of the soils was evaluated as a function of depth, based on batch equilibrium experiments, and amorphous Fe and Al content. There was a good correlation between Smax and Fe+Al (r = 0.84), and between P and Fe+Al (r = 0.91), suggesting that Fe and Al can be used as a good indicator for P sorption capacity and P content. The EPC0 values at the wetland and upland surface (0--2 cm) was 1.8 (+/-1.3) mg L-1 and 8.9 mg L-1 respectively, suggesting that the surface horizons could behave as a potential source of P to the wetland. Based on the soil P storage capacity experiments both the wetland and the upland soils showed positive storage capacities. Results from this study adds to our overall understanding of P dynamics within impacted wetland soils in the Okeechobee basin, which will help develop sound P management strategies in the future.