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Comparison of Laboratory and Quick-Test Methods for Forage Nitrate

USDA-ARS, Grazinglands Research Lab., 7207 W. Cheyenne St., El Reno, OK 73036

View larger version (34K): [in a new window]   Fig. 1. Scatter plots with linear regression lines (orthogonal fit) and paired t-test plots of NO–3–N (µg g–1) extracted from oven-dried winter wheat samples (Feekes growth stage 7 and 10.1) and measured by flow injection analysis (FIA), laboratory microplate nitrate reductase kit (M-NaR), ion specific electrode card (ISE-card), test strip reflectometry (TSR), and field nitrate reductase kit with spectrophotometer detection (F-NaR abs). In the panels on the right, mean differences are shown as the horizontal solid lines, with the 95% confidence interval above and below depicted as dashed lines.

View larger version (25K): [in a new window]   Fig. 2. Laboratory microplate nitrate reductase kit (M-NaR, •), ion specific electrode card (ISE-card), test strip reflectometry (TSR, ), and field nitrate reductase kit (spectrophotometer detection; F-NaR abs, ) determinations of nitrate using fresh tissue extracts compared with measurements obtained by flow injection analysis (FIA) using fresh tissue extracts. Winter wheat samples were collected at Feekes growth stage 7 and nitrate concentrations are expressed on a dry weight basis. Diagonal dashed line corresponds to nitrate measurements by alternate methods that would be equivalent with the FIA method (y = x).

View larger version (35K): [in a new window]   Fig. 3. Comparison of measurements of NO–3–N (µg g–1) extracted from oven-dried and fresh winter wheat samples (Feekes growth stage 7 and 10.1) using the field nitrate reductase kit with either visual (F-NaR vis) or spectrophotometer (F-NaR abs) quantification of the Greiss-Ilosvay reaction color. Inset plots depict results of paired t-test; the mean difference is shown as the horizontal solid line, with the 95% confidence interval above and below depicted as dashed lines.

View larger version (26K): [in a new window]   Fig. 4. Relationships between nitrate extracted from fresh and oven-dried winter wheat samples collected at Feekes growth stage 7 and analyzed by flow injection analysis (FIA) and laboratory microplate nitrate reductase (M-NaR) methods. Tissue nitrate concentrations are expressed on a dry weight basis.

View larger version (38K): [in a new window]   Fig. 5. Linear regression and matched pairs statistical analyses of nitrate additions to an oven-dried tissue extract. For each method (FIA, flow injection analysis; M-NaR, laboratory microplate nitrate reductase kit; TSR; test strip reflectometry; ISE-card, ion specific electrode card), the nitrate concentration of untreated extract was subtracted from the treated extracts and the adjusted nitrate concentrations of the extract regressed against results obtained for nitrate in deionized H2O.