The expansion of agriculture worldwide has led to significant environmental problems associated with nutrients leaching and the pollution of surface waters (Figure 1). This often results in eutrophication and degradation of water resources.
In Denmark, the expansion of agriculture was also accompanied by intensive drainage of waterlogged areas, such as wetlands and peatlands, which naturally reduce the nutrient loads from higher to lower areas through particles sedimentation and biogeochemical processes. This considerably reduced the land’s capacity to control the high discharge of nutrients from agricultural fields. In addition, the Danish agricultural lands are highly drained (>50%), which speeds up the nutrient transport to coastal and inland waters in comparison to other runoff pathways.
In Denmark, the use of drainage filters such as surface-flow constructed wetlands are recognized as a means to reduce nitrogen loads by increasing the hydraulic residence time and allowing denitrification to occur (Figure 2). Two new studies, however, assessed their potential as phosphorus sinks (Mendes et al., 2018b, 2018c). It was primarily questionable whether phosphorus retention in these systems would be representative and/or constant due to the large variability in systems receiving event-driven drainage discharge according to previous studies, and doubtful biogeochemical stability of the phosphorus retained in the soil/sediments and saturation of the sorption sites. A previous study in Denmark had shown that lowland anoxic and iron-rich soils tended to release phosphorus due to the reduction of ferric iron to ferrous iron (Forsmann and Kjaergaard, 2014). In addition, preliminary results demonstrated that the soils of these wetlands were quite reduced, which could favour dissolution of phosphorus bound to redox-sensitive iron forms and release to the water column.
To properly address these research questions, three surface-flow constructed wetlands with a similar design but variable phosphorus loads and dominant forms (particulate or dissolved) and differing geology, were selected and systematically monitored (Figure 3). The wetlands consisted of a sedimentation pond (1 m deep), followed by a basin with two shallow and three deep zones (0.3 and 1 m deep), where the total area represents 1% of the catchment area.
It was firstly found that the wetlands worked as net phosphorus sinks (0.3-10.5 g m−2 yr−1). The phosphorus loads explained 52-72% of the variation in mass retention, i.e. a major explanatory variable, which agreed with previous studies. The phosphorus loads, however, had little effect on the percent retention, whose variation was partly attributed to the dominant phosphorus forms, as particulate forms are more easily retained through settling than dissolved forms, and to the inputs of iron (the dominant phosphorus sorbent in the study) in relation to phosphorus. Therefore, the study showed that the percent retention was likely associated to the biogeochemical stability of the phosphorus retained.
Despite the initial belief that the phosphorus stability would be compromised by a consistently reduced soil and dissolution from iron compounds, the percent retention was still representative (41-51%). These findings led to a second study (Mendes et al., 2018c), where the focus was on the intrinsic mechanisms, which support phosphorus stability and retention in the soil/sediments (Figure 4). The experimental setup included the analysis of soil/sediments geochemistry, redox conditions and the concentration of phosphorus in the water column across the wetlands.
The new findings demonstrated that phosphorus was mainly retained in the sediments associated with iron. Moreover, despite the continuous loading of phosphorus, the deposited sediments presented generally a high sorption capacity and availability of iron in relation to phosphorus, which decreases the likelihood of phosphorus release to the water column, according to Forsmann and Kjaergaard (2014).
But the question about the stability of the iron-bound phosphorus, given the consistent soil reducing conditions still remained. Measurements of dissolved oxygen concentration near the sediment-water interface demonstrated not only clear annual variations, but a predominantly aerobic environment. This strengthened the hypothesis that any release of iron-bound phosphorus from the reduced soils would rebind to iron as soon as it reaches aerobic conditions in the water phase following precipitation to the top sediments. Negative correlations between dissolved reactive phosphorus concentration in the water column and dissolved oxygen concentration near the sediment-water interface also supported this hypothesis.
These studies suggested that the efficiency of surface-flow constructed wetlands as phosphorus sinks are strongly connected to the inputs of phosphorus sorbents (e.g. iron, aluminium and manganese) at the drainage discharge in relation to the phosphorus loads and the maintenance of aerobic conditions near the sediment-water interface. This would allow a consistent availability of phosphorus sorption sites in the deposited sediments and stability of phosphorus bound to redox-sensitive iron compounds, thus extending the time of efficient operation.
The studies also highlighted the importance of a sedimentation pond as a primary treatment stage, mainly in wetlands receiving a large load of phosphorus in particulate forms, as most of the phosphorus retention occurred there. Thus, maintenance operations could have a particular focus on excavating the excess deposited sediments and accumulated phosphorus in the sedimentation pond in order to extend the wetland’s operational lifetime.
The experiments and findings are described in the articles entitled Phosphorus retention in surface-flow constructed wetlands targeting agricultural drainage water, and Phosphorus accumulation and stability in sediments of surface-flow constructed wetlands, recently published in the journals Ecological Engineering and Geoderma, respectively. The works were conducted by Lipe R.D. Mendes, Bo V. Iversen and Charlotte Kjaergaard from Aarhus University, and Karin Tonderski from Linköping University.
The research projects were funded by the Danish Strategic Research Council, GUDP, and CAPES Foundation.
- Forsmann, D.M., Kjaergaard, C., 2014. Phosphorus release from anaerobic peat soils during convective discharge — Effect of soil Fe:P molar ratio and preferential flow. Geoderma 223–225, 21–32.
- Mendes, L.R.D., 2018a. Surface-flow constructed wetlands retaining phosphorus from agricultural drainage water. Hydrological and biogeochemical factors controlling the retention of phosphorus and its stability in soils/sediments. PhD Thesis, 144 pages. Science and Technology, Dept. of Agroecology, Aarhus University, Denmark.
- Mendes, L.R.D., Tonderski, K., Iversen, B.V., Kjaergaard, C., 2018b. Phosphorus retention in surface-flow constructed wetlands targeting agricultural drainage water. Ecol. Eng. 120, 94–103.
- Mendes, L.R.D., Tonderski, K., Kjaergaard, C., 2018c. Phosphorus accumulation and stability in sediments of surface-flow constructed wetlands. Geoderma 331, 109–120.