Fig. 3. Ratios of ²⁹N₂ and ³⁰N₂ production consistently below those predicted.

a Consistent ²⁹N₂ production (nmol g⁻¹ h⁻¹) from ¹⁵N-ammonia added to oxic sediments, against each corresponding measure of ³⁰N₂ production at each time-point (>0.5 h < 10 h) in each incubation in Fig. 1a presented here as the partial residuals from mixed-effects models (n = 100 and n = 300, for the 4- and 12-river datasets, respectively). b The corresponding measured values for R from a, for the first 4 rivers incubated with either ¹⁵NH₄⁺ (95% CI for R = 2.01 to 2.64) or ¹⁵NH₄⁺ and additional ¹⁴NO₂⁻ (95% CI for R = 2.12 to 2.74), against those predicted for denitrification of porewater NO₂⁻. c Measured R values for the 12 river sediments incubated with only ¹⁵NH₄⁺ (95% CI for R = 1.41 to 2.20), against predicted R values for denitrification, anammox, and a cryptic coupling. See main text and Table 2. Upper and lower box boundaries are 75th and 25th percentiles, respectively, upper and lower whiskers are 90th and 10th percentiles, respectively, the extreme outliers the maxima and minima and the horizontal line the centre, median value.