Table 1. N₂-fixation (%Ndfa) in poplar plants from five wild poplar trees.

Sample	% Ndfa	Rate of N2-Fixation (mg/kg/day)
1–1	0.087	2.70
1–2	0.089	3.01
1–3	0.031	0.75
1–4	0.102	2.21
1–5 UNDOSED	BDLa	BDL
2–1	0.292	13.2
2–2	0.101	4.55
2–3	0.081	3.04
2–4	0.040	1.48
2–5 UNDOSED	BDL	BDL
4–1	0.103	5.74
4–2	0.077	2.91
4–3	0.761	20.6
4–4	0.529	14.3
4–5 UNDOSED	BDL	BDL
5–1	0.168	5.54
5–2	0.689	14.6
5–3	0.241	11.4
5–4	0.276	11.1
5–5 UNDOSED	BDL	BDL
6–1	0.041	2.08
6–2	0.038	1.21
6–3	0.218	10.0
6–4	0.474	16.9
6–5 UNDOSED	BDL	BDL

Nitrogen derived from a ¹⁵N-enriched atmosphere, %Ndfa represents the proportion of total N content originating from atmospheric N-fixation, calculated using the average of the undosed samples as a reference value. Rate of N₂ fixation is measured as mg dry weight of newly fixed N per kg of total plant tissue per day. Data were from Expt. 1.

^a BDL, Below detection limits