Effectiveness of denitrification bioreactors with woodchips, corn stover, and phosphate-sorbing media for simultaneous removal of drainage water N and P in a corn–soybean system

Millions of acres of farmland in the midwestern United States (US) are artificially drained, and this contributes to the export of nitrogen (N) and phosphorus (P) from agricultural land to surface water. Using a 36-acre tile-drained farm field, effects of P-sorbing media in combination with a denitrifying bioreactor system constructed with woodchips (WC) and corn stover (CS) on reducing nutrient export in drainage water were tested for 3 cropping years (2018–2020). The field was divided into three subfields as replicates. In each subfield, the drainage water was divided and separately channeled into three bioreactors, each of which contains one of the three different substrates: WC, CS, and CS–WC (1:1 v/v mixture of CS and WC), randomly assigned. The outlet of each compartment contained a 2.25 L flow-through chamber filled with activated iron (Fe) filings as P-sorbing material. Both WC and CS bioreactors were effective in removing drainage NO₃⁻ with a 77% (WC), 86% (CS), and 89% (CS–WC) reduction in mean NO₃⁻–N concentration. For the three cropping years, the WC bioreactor reduced the total drainage inorganic N (NO₃⁻–N + NH₄⁺–N) load by 72%, but the CS bioreactor increased the total inorganic N load in the drainage water due to the substantial release of NH₄⁺ with the decomposition of CS. The breakdown of CS also increased drainage P. The NH₄⁺ and P release decreased with the decrease in the proportion of CS; thus, not more than 10% of CS is recommended for blending with WC to enhance the performance of a bioreactor. The P-sorbing Fe filing media reduced the P loads in drainage by an average of 19% during the 2-year study.