Table 1.

Summary of total ¹⁵N-N₂ production in oxic incubations with ¹⁵NH₄⁺ or ¹⁵NO₂⁻. Mixed-effects models were used to estimate overall rates of total ¹⁵N-N₂ production for the incubations in Fig. 1a. Treatments 1 to 6 were applied to sediments from the first set of 4 rivers, and then just treatments 1 and 2 for the subsequent set of 12 rivers. Model fitting was carried out in the lme4 package in R⁴⁵ and rate estimates, standard errors (s.e.) and 95% confidence intervals derived using emtrends from the emmeans package (see Methods). Significant production (bold) of ¹⁵N-N₂ was only measured with treatments 1 and 3.

Code, Treatment	Rivers (replicates)	Total 15N-N2 (nmol N g−1 h−1)	s.e.	Lower 95% C.I.	Upper 95% C.I.
1, 15NH4+ + ATU	4 (5)	0.110	0.337	−0.667	0.886
2, 15NH4+	4 (5)	1.855	0.326	1.078	2.631
3, 15NH4+ + 14NO2− + ATU	4 (5)	0.152	0.337	−0.625	0.929
4, 15NH4+ + 14NO2−	4 (5)	1.941	0.326	1.165	2.717
5, 14NH4+ + 15NO2− + ATU	4 (5)	0.314	0.326	−0.462	1.091
6, 14NH4+ + 15NO2−	4 (5)	0.279	0.326	−0.497	1.055
1, 15NH4+ + ATU	12 (5)	0.129	0.178	−0.249	0.506
2, 15NH4+	12 (5)	1.465	0.176	1.091	1.839