Mitigating agricultural nitrogen load with constructed ponds in northern latitudes : A field study on sedimental denitrification rates

Constructed agricultural ponds and wetlands can reduce nitrogen loading from agriculture especially in areas where warm climate predominates. However, in cold climate temperature-dependency of microbiological processes have raised the question about the applicability of constructed wetlands in N removal. We measured in situ denitrification rates in a constructed agricultural pond using N-15-isotope pairing technique at ambient light and temperature throughout a year as well as diurnally. The field IPT measurements were combined with a wide set of potentially important explanatory data, including air temperature, photosynthetically active radiation, precipitation, discharge, nitrate plus other water quality variables, sediment temperature, oxygen concentration and penetration depth, diffusive oxygen uptake and sediment organic carbon. Denitrification varied, on average, diurnally between 12 and 314 mu mol N m(-2) h(-1) and seasonally between 0 and 12409 mu mol m(-2) h(-1). Light and oxygen regulated the diel variation of denitrification, but seasonally denitrification was governed by a combination of temperature, oxygen and turbidity. The results indicated that the real N removal rate might be 30-35% higher than the measured daytime rates, suggesting that neglecting the diel variation of denitrification we may underestimate N removal capacity of shallow sediments. We conclude, that by following recommended wetland:catchment - size ratios, boreal agricultural ponds can efficiently remove nitrogen by denitrification in summer and in autumn, while in winter and in spring the contribution of denitrification might be negligible relative to the loading, especially with short residence time.Peer reviewe

Organic carbon causes interference to nitrate and nitrite measurements by UV/Vis spectrometers : The importance of local calibration

Compared with sporadic conventional watersampling, continuous water-quality monitoring with opti-cal sensors has improved our understanding of freshwaterdynamics. The basic principle in photometric measure-ments is the incident light at a given wavelength that iseither reflected, scattered, or transmitted in the body ofwater. Here, we discuss the transmittance measurements.The amount of transmittance is inversely proportional tothe concentration of the substance measured. However, thetransmittance is subject to interference, because it can beaffected by factors other than the substance targeted in thewater. In this study, interference with the UV/Vis sensornitrate plus nitrite measurements caused by organic carbonwas evaluated. Total or dissolved organic carbon as well asnitrate plus nitrite concentrations were measured in variousboreal waters with two UV/Vis sensors (5-mm and 35-mmpathlengths), using conventional laboratory analysis re-sults as references. Organic carbon increased the sensornitrate plus nitrite results, not only in waters with highorganic carbon concentrations, but also at the lower con-centrations (< 10 mg C L−1) typical of boreal stream, river,and lake waters. Our results demonstrated that local cali-bration with multiple linear regression, including bothnitrate plus nitrite and dissolved organic carbon, can cor-rect the error caused by organic carbon. However, high-frequency optical sensors continue to be excellent tools forenvironmental monitoring when they are properly calibrat-ed for the local water matrix.Peer reviewe

High Nitrogen Removal in a Constructed Wetland Receiving Treated Wastewater in a Cold Climate

Constructed wetlands provide cost-efficient nutrient removal, with minimal input of human labor and energy, and their number is globally increasing. However, in northern latitudes, wetlands are rarely utilized, because their nutrient removal efficiency has been questioned due to the cold climate. Here, we studied nutrient retention and nitrogen removal in a boreal constructed wetland (4-ha) receiving treated nitrogen-rich wastewater. On a yearly basis, most of the inorganic nutrients were retained by the wetland. The highest retention efficiency was found during the ice-free period, being 79% for ammonium-nitrogen (NH4+-N), 71% for nitrate-nitrogen (NO3--N), and 88% for phosphate-phosphorus (PO43--P). Wetland also acted as a buffer zone during the disturbed nitrification process of the wastewater treatment plant. Denitrification varied between 106 and 252 mg N m(-2) d(-1) during the ice-free period. During the ice-cover period, total gaseous nitrogen removal was 147 mg N m(-2) d(-1) from which 66% was removed as N-2, 28.5% as N2O through denitrification, and 5.5% as N-2 through anammox. Nearly 2600 kg N y(-1) was estimated to be removed through microbial gaseous N-production which equaled 72% of NO3--N and 60% of TN yearly retention in the wetland. The wetland retained nutrients even in winter, when good oxygen conditions prevailed under ice. The results suggest that constructed wetlands are an efficient option for wastewater nitrogen removal and nutrient retention also in cold climates.Peer reviewe