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.

Quantifying soil surface photolysis under conditions simulating water movement in the field: a new laboratory test design.

Soil surface photolysis can be a significant dissipation pathway for agrochemicals under field conditions, although it is assumed that such degradation ceases once the agrochemical is transported away from the surface following rainfall or irrigation and subsequent drainage of soil porewater. However, as both downward and upward water movements occur under field conditions, relatively mobile compounds may return to the surface, prolonging exposure to ultraviolet light and increasing the potential for degradation by photolysis. To test this hypothesis, a novel experimental system was used to quantify the contribution of photolysis to the overall dissipation of a new herbicide, bicyclopyrone, under conditions that mimicked field studies more closely than the standard laboratory test guidance. Soil cores were taken from 3 US field study sites, and the surfaces were treated with [(14) C]-bicyclopyrone. The radioactivity was redistributed throughout the cores using a simulated rainfall event, following which the cores were incubated under a xenon-arc lamp with continuous provision of moisture from below and a wind simulator to induce evaporation. After only 2 d, most of the test compound had returned to the soil surface. Significantly more degradation was observed in the irradiated samples than in a parallel dark control sample. Degradation rates were very similar to those observed in both the thin layer photolysis study and the field dissipation studies and significantly faster than in the soil metabolism studies conducted in the dark. Thus, for highly soluble, mobile agrochemicals, such as bicyclopyrone, photolysis is not terminated permanently by rainfall or irrigation but can resume following transport to the surface in evaporating water.