Nitrogen removal and greenhouse gas fluxes from integrated buffer zones treating agricultural drainage water.

Integrated buffer zones (IBZ) are novel mitigation measures designed to decrease the loading of nitrogen (N) transported by subsurface drainage systems from agricultural fields to streams. In IBZ, drainage water flows into a pond with free water surface followed by an inundated, vegetated filterbed. This design provides an environment favorable for denitrification and thus a decrease in nitrate concentration is expected as water flow through the IBZ. However, due to the establishment of anaerobic conditions, there is a risk for increasing emissions of the greenhouse gases nitrous oxide (N2O) and methane (CH4). In this year-long study, we evaluated the N removal efficiency along with the risk of N2O and CH4 emissions from two pilot-scale IBZs (IBZ1 and 2). The two IBZs had very different yearly removal efficiencies, amounting to 29% and 71% of the total N load at IBZ1 and 2, respectively. This was probably due to differences in infiltration rates to the filterbed, which was 22% and 81% of the incoming water at IBZ1 and 2, respectively. The site (IBZ2) with the highest removal efficiency was a net N2O sink, while 0.9% of the removed nitrate was emitted as N2O at IBZ1. Both IBZs were net sources of CH4 but with different pathways of emission. In IBZ1 CH4 was mainly lost directly to the atmosphere, while waterborne losses dominated in IBZ2. In conclusion, the IBZs were effective in removing N three years after establishment, and although the IBZs acted as greenhouse gas sources, especially due to CH4, the emissions were comparable to those of natural wetlands and other drainage transport mitigation measures.

Efficiency of mitigation measures targeting nutrient losses from agricultural drainage systems: A review.

Diffusive losses of nitrogen and phosphorus from agricultural areas have detrimental effects on freshwater and marine ecosystems. Mitigation measures treating drainage water before it enters streams hold a high potential for reducing nitrogen and phosphorus losses from agricultural areas. To achieve a better understanding of the opportunities and challenges characterising current and new drainage mitigation measures in oceanic and continental climates, we reviewed the nitrate and total phosphorus removal efficiency of: (i) free water surface constructed wetlands, (ii) denitrifying bioreactors, (iii) controlled drainage, (iv) saturated buffer zones and (v) integrated buffer zones. Our data analysis showed that the load of nitrate was substantially reduced by all five drainage mitigation measures, while they mainly acted as sinks of total phosphorus, but occasionally, also as sources. The various factors influencing performance, such as design, runoff characteristics and hydrology, differed in the studies, resulting in large variation in the reported removal efficiencies.

Reducing adverse side effects by seasonally lowering nitrate removal in subsurface flow constructed wetlands.

Subsurface flow constructed wetlands with wood chips (SSF-CWs) have proven to effectively reduce the loss of nitrogen (N) from agricultural fields to surface water, however in some cases production of negative side effects such as methane and phosphate occur. We examined if these side effects can be avoided by decreasing the hydraulic retention time (HRT) from on average 82 h to 11 h during summer to autumn in two pilot SSF-CWs. Furthermore, we investigated the potential of the SSF-CWs to reduce phosphorus (P) loss from agricultural drainage systems. The influent and effluent concentration of total N (TN), nitrate-N, total P, phosphate-P, suspended sediment, and sulphate were monitored for five years (2013-2017). Methane concentrations were measured during two periods in 2014 and 2017. Flow was measured continuously by electromagnetic flow-meters. The nitrate-N removal was reduced from 98-100% to 27-32% and the sulphate reduction from 32-53% to 1-2% when decreasing HRT. Concurrently this resulted in a considerable decrease in the difference between the effluent and influent concentration of phosphate-P and methane concentration compared to similar periods in the preceding years. The SSF-CWs retained 67-85% of the annual loading of particulate P, but acted as both a sink and source of phosphate-P, thus further initiatives are therefore required to prevent phosphate-P release from SSF-CWs. Although during the entire monitoring period the SSF-CWs retained 29-33% of the total P loading. In summary, this study stresses how important a holistic approach is when implementing and designing new N mitigation measures.