Vacuum Trough Extractors for Measuring Drainage and Nitrate Flux Through Sandy Soils

Vacuum trough extractors have been used in a number of studies for quantifying NO⁻₃ leaching, despite limited data verifying their performance. In two studies, extractors were evaluated for measuring drainage, NO₃-N concentrations, and NO₃-N flux under irrigated corn (Zea mays L.). In the first, extractors were placed within four large (2.4 by 2.4 by 2.3 m deep) drainage lysimeters. Extractors were placed at either 1.4 or 1.8 m below the surface of a reconstructed Hecla loamy fine sand. Quality and quantity of monthly outflow from extractors and lysimeter tile drains were directly compared over a 5-yr period. Drainage and NO₃-N flux from the extractors were significantly correlated to tile drain measurements during the entire study (r = 0.90*** and 0.69***, respectively). Overall, extractors underestimated drainage and NO₃-N flux (6 and 13%, respectively). Excluding the first year's data, extractors underestimated tile drainage by 3% and NO₃-N flux by 1%. Nitrate concentrations from tile drains appeared to lag 2 and 10 months behind the 1.8 and 1.4 m deep extractors, respectively. Nitrate-N concentrations (flow-weighted for duration of study) were 27 mg L⁻¹ from tile drains and 25 mg L⁻¹ from extractors. Vacuum levels at which the extractors were operated had very minor effects on quality and quantity of effluent collected. Extractors buried in narrow trenches 1.5 m below field plots were evaluated over a 5-yr period in the second study. These extractors were placed under a Maddock sandy loam receiving annual N rates of 0, 112, 224, and 224-split (3-way) kg ha⁻¹. Drainage measured by these extractors was 70% less than predicted by a water balance, which used the Jensen-Haise equation to calculate evapotranspiration. Although the extractors were found incapable of supplying drainage flux, NO₃-N concentrations were statistically the same as determined by nearby suction cups at the same depth.