Leaching Export of Dissolved Organic Carbon and Dissolved Nitrogen from Agricultural Soils in Southern Michigan.

Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) are two closely linked components of dissolved organic matter leaching from soils. DOC can be significant to soil C balance and affects the bioavailability of trace metals, and DON may be significant to soil N balance and a source of reactive N to surface waters. while leaching losses of nitrate (NO₃^-) are often studied because of eutrophication and public health concerns, leaching of DOC or DON from agricultural soils is rarely reported. This study examines leaching of DOC, DON, ammonium (NH₄⁺), and NO₃^- from no-till maize, perennial herbaceous crops (switchgrass, miscanthus, native grasses and restored prairie), and a tree plantation (poplar). Pore-water was sampled below the root zone (1.2 m soil depth) in non-frozen periods (March-November) between 2009 and 2015. Leaching was estimated on a crop-year basis from concentrations in pore-water and modeled drainage rates. DOC leaching rates and volume-weighted mean concentrations were similar in all crops, averaging 12.1 kg ha^-1 y^-1 and 3.6 mg L^-1, respectively, with the exception of poplar, which was lost more DOC (21.5 kg ha^-1 y^-1 and 6.8 mg L^-1). DON averaged 9.8 kg ha^-1 y^-1 and 2.5 mg L^-1 for maize, which was higher than perennial herbaceous crops (mean: 1.1 kg ha^-1 y^-1 and 0.35 mg L^-1) and poplar (1.8 kg ha^-1 y^-1 and 0.6 mg L^-1). Nitrate-N leaching averaged 13.0 kg ha^-1 y^-1 and 4.5 mg L^-1 while ammonium-N leaching from these crops was comparatively small (less than 5% of NO₃^-). Nitrogen leaching occurred in all crops with or without N fertilization but higher N rates led to increased leaching in maize. Perennial crops leached more nitrogen in first few years after planting, but N leaching significantly decreased as the crops matured. Our study shows how perennial crops can minimize N leaching and consequent movement into aquifers and surface waters, but mainly via reduced NO₃^- leaching rather than affecting DON. DON was a significant component constituting ~20 percent of the total N leaching. Fertilization of annual crops resulted in enhanced NO₃^- and DON leaching with little effect on DOC or NH₄⁺ leaching.