Using ion exchange resins to monitor nitrate accumulation and leaching in remote semiarid ephemeral stream beds
Description
This data was collected to test the applicability and efficiency of resin samplers and resin bags to monitor nitrates (NO3-N) in two small semi-arid intermitent flow catchments in Northwestern Mexico. Resin samplers were installed below the river bed and remained there for five months to capture NO3-N leachate during the summer rains. Resin bags were anchored in rock outcrops upstream of the resin samplers before the onset of the summer rainfall season. Bags were replaced every two weeks for four months to capture pulses of NO3-N concentrations in ephemeral streams. Results of the resin samplers found a difference of up to 12 kg ha-1 season-1 between the two catchments. The resin bags showed a higher accumulation of NO3-N concentrations in the catchment with lower vegetation cover (160.35 mg L-1 season-1) compared to the one with higher vegetation (67.88 mg L-1 season-1). Measured nitrate fluxes at both sites responded to rainfall pulses recorded during the monitoring period. Resin samplers and resin bags demonstrated an efficient way of determining spatio-temporal nitrate fluxes in semi-arid ecosystems in remote areas that are difficult to access, monitor, and collect data.
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Soil sampling and analysis In both experimental catchments (“El Guayabo” and “El Halcon”), one soil profile per site was excavated following the soil description protocol described in Siebe et al., (2006) and classified under the standard guidelines of the World Reference Base for Soil Resources (IUSS, 2007). Soil samples were taken using a soil auger (5 cm in diameter) in each identified horizon for physical and chemical analyses. Bulk density (g cm-3 dry soil) was carried out by cylinder method (Blake, 1965), pH was measured with a portable pH-meter (Hanna, HI8424, Washington, USA) on a 1:2.5 (soil:solution) soil-water extract. Soil organic matter (OM) was determined by wet oxidation (Walkley & Black, 1934), inorganic N was extracted with 2M KCl and determined using a segmented flow chemical analyzer (SEAL Analytical Inc. Mequon, Wisconsin, USA). The tone and intensity of the colors of the horizons were determined using the Munsell color system (Nickerson, 1940; Schmidt & Ahn, 2019). The soil profile descriptions are presented in the results section in Table 3. Resin and silica sand preparation Lewatit® MonoPlus M 600 (Lanxess AG, Leverkusen, Germany), a strong anion resin, was used in the Cl- form (table 2) to fill samplers and bags. The resins were washed with a solution of NaCl (1 M) for one hour and then rinsed five times in deionized water prior to installation. Silica sand, a neutral dilution medium, was washed with a solution of HCl (0.1 M) for one hour followed by thorough rinsing with distilled water three times. Resins and sand stayed moist prior to installation in order to maintain high ion exchange capacity. Resin sampler construction and installation PVC cartridges with a diameter of 0.1 m and a height of 0.1 m were used to construct the resin samplers which contained a mixture (2:1) of silica sand and anion exchange resins (Fig. 3). The mixture was prepared under laboratory conditions, packed densely into the cartridges and stored until installation in ice coolers. IER samplers were installed in “El Guayabo” and “El Halcon” catchments on May 26, 2016, and removed on October 12, 2016, which encompassed the entire length of the monsoon season.