Abstract

Re-established riparian wetlands used to mitigate nitrogen (N) loss from agricultural soils to surface water may lose phosphorus (P) from the top soils that often have received fertilizers. This could lead to eutrophication of lakes and estuaries. For a 2-year period we established mass balances of N and Pin two restored riparian wetlands of similar to 0.6 ha situated on mineral soil. Monitoring began 5 years after restoration. Both wetlands received drainage water from upland agricultural fields rich in nitrate (1.5-12.3 mgN L-1) and low in total P (TP) (0.016-0.04 mg P L-1). Water balances were reasonably accounted for (15% imbalance at most). Water passed the wetlands as sheet flow without exchange with groundwater because of clay horizons in sub-soils, and sheet pilings along the stream banks allowed continuous measurements of inflow and outflow. The Egeskov riparian wetland (wetland:upland ratio 0.13) removed 121 and 28 kg N ha(-1) yr(-1) (43 and 75% of the load) and retained 0.08 kg P ha(-1) (6% of the load) in year one and had a net release of 0.15 kg P ha(-1) (25% of the load) in year two. The Stor A riparian wetland (wetland:upland ratio 0.02) removed 229 and 158 kg N ha(-1) yr(-1) (32 and 26%). Net releases of P were 0.33 and 0.90 kg P ha(-1) yr(-1) (22 and 127%). Nitrogen removal rates are on par with published rates for similar wetlands, while the P release rates appear surprisingly low. Phosphate outlet concentrations resembled the equilibrium concentrations (EPC0) where no phosphate exchange occurred between top soils and drainage water, suggesting that P release or retention was controlled by phosphate adsorption. This value was 0.015 mg P L-1 for Egeskov and 0.047 mg P L-1 for Stor A. The high phosphate affinity was probably governed by high ratios between oxidized iron and iron-bound P. The top soils (10 cm) contained 87 and 201 kg P ha(-1) as iron-bound P and herbaceous vegetation accumulated 10.7 and 16.5 kg P ha(-1) yr(-1). These figures are 55-136 and 8-11 times higher than the annual P-load to the wetlands, and we suggest that annual harvest of vegetation could maintain or even improve the P retention capacity of these wetlands. (C) 2012 Elsevier B.V. All rights reserved.

Keywords

Wetlands; Nutrients; Soil phosphorus; Phosphate adsorption; Mass balance; Macrophytes

Published in

Ecological Engineering
2012, Volume: 46, pages: 75-87
Publisher: ELSEVIER SCIENCE BV

UKÄ Subject classification

Oceanography, Hydrology, Water Resources


Publication identifier

DOI: https://doi.org/10.1016/j.ecoleng.2012.04.039

Permanent link to this page (URI)

https://res.slu.se/id/publ/88666