Nitrate and Phosphate Removal Effects of Compost Amendments in Wetland Mesocosms

Abstract

Constructed wetlands can remove nitrate and phosphate from runoff and shallow groundwater, and are often used as treatment alternatives or components of riparian buffers to remove nonpoint-source pollutants. Reducing conditions and organic carbon are necessary for denitrification to occur in wetlands, whereas phosphate is primarily removed by adsorption onto mineral substrates. The objective of this research was to investigate the effectiveness of mineral soils amended with organic matter to remove both NO3--N and PO43--P from nutrient-enriched water. Studies were carried out in a greenhouse at 14.4 1.7C using columns constructed from PVC pipe (15.2 cm inside diameter 51 cm tall). Triplicate columns were filled with mineral soil amended with 0%, 5%, 10%, or 20% compost by dry mass to provide additional organic matter. Each column was saturated (25 h mean retention time) with water enriched with 20 mg L-1 NO3--N and 10 mg L-1 PO43--P. Nitrate removal, calculated as first-order removal rate constants, increased significantly (p < 0.0001) with increased organic matter from 0% to 20% compost treatments. Experimental runs were repeated over a 3-month period; the effect of time on nitrate removal was also significant (p < 0.0001) and was observed for all treatments in this study. Phosphate removal significantly decreased (p < 0.0001) with increased organic matter additions. The greatest reduction of phosphate occurred during the first 6 h after saturating the columns. Our research demonstrates that organic matter amendments can significantly enhance the nutrient removal functions of saturated soils.