Identifying Peak Intervals of Denitrification in Land-Applied Wastewater Using Automated Sampling Chamber.

The expanding vegetable food processing and cheese making industries have resulted in a large increase of wastewater generated. The most practical and cost-efficient way to dispose of this wastewater is through on-site land application, which is regulated by government agencies to protect water quality. The amount of nitrogen applied is a concern, as it can affect nitrate in drinking water. Current regulations in Wisconsin permit annual nitrogen applications equivalent to plant uptake plus demonstrable denitrification. However, denitrification rates on these land application systems have not been directly quantified. Accepted protocols exist for N₂O measurement in agricultural settings, which combined with the acetylene inhibition method can measure a significant portion of denitrification. However, the timing of the measurement after effluent application to adequately quantify N₂O emissions is important. An automated chamber sampling system was integrated with the acetylene inhibition method to profile soil N₂O emissions hourly from six wastewater application sites with a variety of soil textures and application systems. Three sites used center pivots to apply wastewater and three used a ridge and terrace system, similar to a constructed wetland. In each application system, two sites had a silt loam and one site had a sandy soil. A spike of N₂O gas was measured shortly after wastewater was applied, followed immediately after by increased N₂O flux. The flux of N₂O after wastewater application appears to be soil and application type dependent, with greater fluxes under ridge and furrow systems. In sandy soil N₂O flux returns to levels observed prior to wastewater application more rapidly than on silt loams. Rainfall appears to have little effect on N₂O flux, which may indicate that flux increases are a result of increased nitrogen and organic carbon additions with the wastewater. Using these data as a guide, multiple static chamber measurements should be taken within 48 hours of a wastewater application to accurately quantify denitrification.