This methods section is mostly duplicated from a manuscript currently under review for the Journal of Environmental Quality
Description of research sites
We measured nutrient leaching along topographic transects, each of which included depressional and adjacent upland soils, at several sites within the Des Moines Lobe region of Iowa during 2018, 2019, and 2020. Not all transects could be measured each year due to logistical and site access constraints. All transects had subsurface tile drainage, and most did not have surface inlets. Crops included conventional corn and soybean, corn and soybean with a winter rye cover crop, and corn and soybean fields where depressions were planted to miscanthus. Water levels were previously monitored in a subset of these depressions and all were shown to pond water intermittently during the growing season, even those with surface inlets (Martin et al., 2019a). Depression boundaries and morphological characteristics were calculated using a digital elevation model following McDeid et al. (2018).
In all years, we monitored transects in fields within a 260-ha section located approximately 4 km southwest of Ames, IA, (41.96 °N, 93.69 °W), where surface water ponding dynamics and water chemistry were studied previously (Martin et al., 2019a,b). Following the conventions in this previous work, transects were playfully named according to their shapes. These fields were managed for corn and soybean production for many decades prior to spring 2019, when three transects were planted to miscanthus, with the rest remaining in corn or soybean. The transects planted to miscanthus were surrounded by fields planted to corn or soybean, and one miscanthus transect (denoted “Moorhen”) contained four sampling plots under corn or soybean at the upland end of the transect.
In 2019, we also measured 7 additional transects that spanned two 260-ha sections located 10 km northeast of Ames, IA (42.13 °N, 93.50 °W), which had also long been managed for corn and soybean production, and where a winter rye cover crop had been planted during our sampling period; these transects are designated with the prefix “RS”. In 2018, we measured four additional transects located 10 km northeast of Emmetsburg, IA (43.21 °N, 94.62 °W); these transects are designated by the “DD15” prefix. In all transects, corn and soybean were managed within regional norms, with N fertilizer applied prior to corn planting or as side dress; cattle manure was applied to all of the northeast Ames transects and was historically applied to some of the southwest Ames transects.
Transects spanned a distance of 100 to 150 m along a hillslope from the bottom (or local minimum) of each depression to an adjacent upland, placed perpendicular to elevation contours when possible to maximize the difference in relative elevation among sampling plots. Ten plots were established at equidistant intervals along each transect, where plot 1 was located at the bottom of the transect and plot 10 was located at the top. At each plot, three replicate lysimeters were installed 60 cm apart as described below, and locations were recorded by GPS.
Depressional soils were typically mapped as Okoboji series (fine, smectitic, mesic Cumulic Vertic Endoaquolls) or Harps series (fine-loamy, mixed, superactive, mesic Typic Calciaquolls). Uplands were mapped as Clarion (fine-loamy, mixed, superactive, mesic Typic Hapludolls) or Nicollet (fine-loamy, mixed, superactive, mesic Aquic Hapludolls). Intermediate soils were often mapped as Canisteo (fine-loamy, mixed, superactive, calcareous, mesic Typic Endoaquolls) or Webster series (fine-loamy, mixed, superactive, mesic Typic Endoaquolls).
Resin lysimeter construction and deployment
We used ion exchange resin lysimeters (Susfalk & Johnson, 2002) to measure nutrient leaching. Each lysimeter was deployed for one year, with installation in mid-April and retrieval in mid-April of the following year. Lysimeters were constructed from 5 cm diameter PVC tubing, unions, and couplers, which contained cation/anion exchange resin (IONAC NM-60 H+/OH-, J.T. Baker, Phillipsburg, NJ) placed between nylon mesh screens (153 µm Nitex). The space above and below the mesh was filled with quartz sand and the bottom of the lysimeter was covered with hardware cloth affixed by a cable tie. The horizontal area of each lysimeter was 28.6 cm2.
Lysimeters were installed underneath undisturbed soil columns by excavating a 50 cm tunnel at a 45° angle relative to the soil surface with a 10 cm diameter bucket auger. At the bottom of each installation tunnel (35 cm depth), a horizontal pocket was excavated with a metal spatula and the lysimeter body was tightly pressed into the overlying soil. Brightly colored nylon rope was tied around each lysimeter and threaded through the excavation tunnel to the soil surface to assist with locating the units. Installation tunnels were backfilled with excavated soil, and a 10 cm length of rebar was added to each tunnel at 20 cm depth to enable location using a magnetic locator.
The 35 cm lysimeter installation depth was chosen to balance multiple considerations. First, this depth was shallow enough to allow placement of lysimeters by hand at the end of the narrow installation tunnel, thus minimizing soil disturbance. Second, 35 cm is above typical groundwater levels for these cropped depressions, which most commonly pond from above due to surface and shallow subsurface runoff, rather than from groundwater rise (Schilling et al., 2018). Third, 35 cm is below dominant zone of corn and soybean biomass in this region (Nichols et al., 2019) and is below the dominant zone of denitrification, which is greatest at the surface due to microbial C limitation in deeper soils (Cambardella et al., 1999; Yeomans et al., 1992). Therefore, accumulation of nutrients in the lysimeters was likely correlated with losses to field tile at approximately 100 cm depth, acknowledging that additional biological transformation or sorption may have occurred in deeper soils.
After one year of deployment, lysimeters were located using GPS, a magnetic locator, and by searching for the colored ropes, and were excavated by shovel. In the lab, resin was removed from each lysimeter, weighed, immersed in 2 M potassium chloride in a 1:5 ratio of resin mass (g) to solution volume (mL), and shaken for one hour. The supernatant solution was decanted to plastic bottles and stored at -20° C. Each resin sample was extracted twice in this manner to increase nutrient recovery (Langlois et al., 2003).
Chemical analyses
Nitrate, ammonium, and P in the lysimeter extraction solutions were measured using colorimetric assays on a 96-well microplate reader (Biotek Synergy HT, Winooski, VT) (D’Angelo et al., 2001; Doane & Horwáth, 2003; Weatherburn, 1967). The P measured in the lysimeter extraction solutions might approximate a “soluble reactive” pool but cannot be strictly defined as such because samples were not filtered prior to analysis. However, the sand at the top of the lysimeter prevented direct mixing between resin and soil. Masses of nitrate, ammonium, and P recovered in the second extraction were 20 (8)%, 20 (15)%, and 17 (18)% of the masses recovered in the first extraction, respectively (values in parentheses are standard deviations). Nutrient masses from the first and second extractions were summed and expressed on an area basis by dividing by the area of each lysimeter. Three samples were excluded from subsequent analyses because they had unusually high ammonium or P.
To provide additional context, the 2018 lysimeter extraction solutions were also analyzed for Na, Mg, Mn, Al, and Fe by inductively coupled plasma optical emission spectroscopy (Perkin Elmer Optima 5300 DV, Waltham, MA). Samples were analyzed in radial mode with three technical replicates.
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