TABLE 3.

Sugars (A), carboxylates (B), and amino acids (C) in the rhizosphere soil solution of lettuce (cv. Tizian), grown in soils with long-term organic (HU-org, BIODYN2) or mineral (HU-min, CONMIN) fertilization history.

	HUB-LTE				DOK-LTE
	HU-org	HU-min	HU-org	HU-min	BIODYN2	CONMIN	BIODYN2	CONMIN
	Basal (mature)		Subapical (young)		Basal (mature)		Subapical (young)
A. Sugars in the rhizosphere soil solution (nmol cm–1 root length)
Fructose	2.21 ± 1.04 a	2.02 ± 0.48 a	1.70 ± 0.51 a	4.34 ± 1.31 a	n.d. b	0.93 ± 0.09 a	1.47 ± 0.03 b	2.85 ± 0.03 a
Glucose	n.d. b	1.80 ± 0.49 a	1.62 ± 0.14 a	3.41 ± 0.58 a	n.d. b	1.23 ± 0.18 a	5.95 ± 1.75 a	1.22 ± 0.30 a
Sucrose	0.57 ± 0.06 a	1.53 ± 0.53 a	0.90 ± 0.20 a	1.19 ± 0.31 a	n.d.	n.d.	n.d. b	0.45 ± 0.03 a
Maltose	n.d. b	0.61 ± 0.03 a	0.47 ± 0.01 b	1.00 ± 0.10 a	n.d. b	0.36 ± 0.01 a	n.d. b	0.46 ± 0.02 a
Sum	2.79 a	5.97 a	4.70 a	9.95 a	n.d. b	2.54 a	7.43 a	4.97 a
B. Carboxylates in the rhizosphere soil solution (nmol cm–1 root length)
Malate	n.d. b	2.91 ± 0.53 a	n.d. b	14.87 ± 3.11 a	n.d.	n.d.	0.72 ± 0.35 a	n.d. b
Citrate	5.42 ± 1.83 a	7.75 ± 3.10 a	2.23 ± 0.57 b	5.73 ± 1.23 a	8.83 ± 3.87 a	3.91 ± 1.46 a	3.37 ± 0.62 a	5.97 ± 1.21 a
Succinate	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.	n.d. b	7.78 ± 1.77 a
Fumarate	n.d. b	0.29 ± 0.05 a	0.30 ± 0.05 a	0.34 ± 0.09 a	0.39 ± 0.07 a	n.d. b	0.56 ± 0.04 a	n.d. b
Benzoate	0.15 ± 0.05 a	0.06 ± 0.01 a	0.10 ± 0.01 a	0.08 ± 0.01 a	0.02 ± 0.005 b	0.11 ± 0.004 a	0.05 ± 0.01 a	0.09 ± 0.01 a
Sum	5.57 a	11.02 a	2.63 b	21.03 a	9.25 a	4.02 a	4.71 b	13.85 a
Lactate	59.47 ± 12.41 a	61.83 ± 20.71 a	20.95 ± 4.56 a	61.15 ± 17.22 a	35.83 ± 2.87 a	12.17 ± 3.28 b	35.20 ± 7.68 a	30.96 ± 5.07 a
Acetate	66.65 ± 0.64 a	25.61 ± 11.91 b	11.27 ± 4.99 b	34.20 ± 2.70 a	30.11 ± 21.88 a	11.33 ± 3.93 a	24.52 ± 2.23 a	n.d. b
Sum	126.12 a	87.45 a	32.22 b	95.35 a	65.95 a	23.51 a	59.73 a	30.96 a
C. Amino acids in the rhizosphere soil solution (pmol cm–1 root length)
Glutamic acid	8 ± 2 a	19 ± 9 a	21 ± 8 a	19 ± 5 a	90 ± 21 a	8 ± 3 b	294 ± 82 a	8 ± 2 b
Asparagine	12 ± 3 a	36 ± 21 a	15 ± 3 a	19 ± 6 a	612 ± 234 a	5 ± 1 b	1868 ± 867 a	6 ± 1 b
Serine	51 ± 6 a	40 ± 11 a	57 ± 5 a	55 ± 2 a	282 ± 18 a	42 ± 4 b	307 ± 14 a	50 ± 6 b
Glutamine	19 ± 10 a	40 ± 14 a	16 ± 6 a	77 ± 35 a	97 ± 16 a	10 ± 3 b	131 ± 9 a	12 ± 2 b
Glycine	56 ± 4 a	36 ± 10 a	48 ± 5 a	49 ± 2 a	249 ± 22 a	41 ± 1 b	290 ± 5 a	42 ± 1 b
Threonine	6 ± 1 a	6 ± 1 a	9 ± 2 a	7 ± 2 a	93 ± 11 a	11 ± 6 b	144 ± 27 a	7 ± 3 b
Histidine	n.d.	n.d.	n.d.	n.d.	15 ± 6 a	n.d. a	26 ± 3 a	n.d. b
Alanine	16 ± 4 a	9 ± 3 a	15 ± 2 a	13 ± 1 a	152 ± 17 a	6 ± 1 b	224 ± 20 a	9 ± 1 b
Proline	10 ± 2 a	6 ± 3 a	17 ± 4 a	18 ± 6 a	51 ± 5 a	6 ± 1 b	70 ± 9 a	7 ± 1 b
Cystine	2 ± 2 a	n.d. a	n.d.	n.d.	n.d.	n.d.	n.d.	n.d.
Thyrosine	n.d.	n.d.	n.d.	n.d.	108 ± 19 a	4 ± 1 b	94 ± 9 a	3 ± 2 b
Methionine	n.d. a	7 ± 1 a	n.d.	n.d.	119 ± 37 a	n.d. b	166 ± 23 a	n.d. b
Isoleucine	n.d. a	7 ± 2 a	n.d. b	8 ± 1 a	n.d.	n.d.	51 ± 15 a	n.d. b
Leucine	n.d. a	0.3 ± 0.3 a	0.4 ± 0.4 b	6 ± 0.4 a	16 ± 0.7 a	n.d. b	88 ± 32 a	n.d. b
Phenylalanine	19 ± 4 a	6 ± 1 b	14 ± 3 a	10 ± 0.6 a	18 ± 2 a	11 ± 2 a	54 ± 13 a	12 ± 3 b
Sum	200 a	212 a	212 a	284 a	1903 a	144 b	3808 a	156 b

The plants were cultivated in minirhizotron culture for nine weeks. Micro-sampling of soil solutions was conducted with sorption filters in 1–2 cm regions of subapical (young) roots and from basal (mature) root zones (8–9 cm). Means ± standard errors. Different lowercase letters indicate significant differences between organic vs. mineral fertilization tested separately for the sites DOK-LTE and HUB-LTE by t-test (p ≤ 0.05). n.d. = not detectable.