Soil phosphorus flux from emergent marsh wetlands and surrounding grazed pasture uplands

Phosphorus (P) release from wetland soils to overlying waters is important to consider when restoring wetland hydrology. Soil physicochemical characteristics influence P dynamics between underlying soil and overlying water. Our study initially characterized wetland and surrounding upland soils prior to flooding. Deep marsh wetland soils had greater moisture content, soil organic matter, nitrogen (N), P, and lower bulk density than surrounding upland pasture soils, which indicates a nutrient concentration gradient between wetland and upland soils. To determine the short-term P dynamics between soils and overlying water, we conducted four laboratory soil water core studies during a 15-month period. Surface soils (0-10 cm) collected October 2005, February 2006, October 2006 and December 2006 from wetlands and their surrounding uplands within cow-calf grazed pastures were flooded for 7 days, and we measured P release from soil to overlying water. Phosphorus release rates from wetland (deep marsh and shallow marsh) and upland soils were similar. Values ranged between −20 mg m−2 d−1 (retention) and 77 mg m−2 d−1 (release). There was a significant, although weak, negative linear relationship between P release from deep marsh soils and hydroperiod. Thus, it may be important for land managers to consider increasing hydroperiod of wetland soils to decrease P release and increase retention. In addition, there was a significant negative exponential relationship between P release and days since deep marsh soil inundation. This suggests that to decrease P release from soils, soils should be wet rather than dry for prolonged periods, prior to flooding. We found significant relationships between P release from upland soils and their nutrient content (N, P and carbon). Reducing nutrient content in upland soils may help reduce the magnitude of P release from soil.