Thirdly, as to toxic stress, a wide range of N concentrations over which the effect occurs has been proposed, the stress being much higher in chronic (e.g., 10 mg L 1 NHþ 4 e N and NO 3 e N, 5-month, >105 mg L 1 NHþ 4 eN, 21 days,than in acute experiments (5 mg L 1 NHþ 4 eN, 2 days).