Table 1.

Effects of the N supply on respiration and N concentration of first- and second-order roots of 13-year-old P. tremuloides in the field experiment (SE, standard error; n.s., not significant; ***P < 0.001).

Treatment	First sampling date			Second sampling date
	N−	N+	P	N−	N+	P
Root age (days)
Mean	15	17		45	46
Root N concentration (%)
Mean	1.66	1.71	n.s.	1.37	1.64	***
SE	0.06	0.07		0.04	0.03
Respiration per unit biomass (nmol O2 g−1 DW s−1)
Mean	11.20	9.13	n.s.	5.70	5.92	n.s.
SE	1.08	1.10		0.50	0.48
Respiration per unit N (nmol O2 g−1 N s−1)
Mean	683.5	533.0	n.s.	410.2	351.3	n.s.
SE	65.5	66.8		34.3	32.8