Extension growth [cm] of the apple trees on the rootstocks M7, M26, CG16, CG30 and CG210 in relation to their position in the grass lane or old tree row of the previous orchard. Means with standard errors of the mean, n = 20. 

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... evaluated with three-way analysis of variance (Statview 5.0, SAS Institute, Cary, NC). The DGGE banding patterns were compared using principal component analysis (PCA) (CANOCO, Microcomputer Power, Ithaca, NY). Each band was treated as an operational taxonomic unit (OTU). Band intensity indicated the relative abundance of the OTU under the applied PCR conditions (Gelsomino et al., 1999). To adjust for lane-to-lane differences, band intensities were normalized by dividing the intensity of each band by the average intensity of the corresponding lane. Redundancy analyses (RDA) were used to correlate DGGE banding patterns with environ- mental factors, such as soil treatments, plant growth, and position in the test orchard. Significance of these correlations was assessed with the Monte Carlo per- mutation test, based on 499 random permutations of the data ( P < 0 . 05). Tree growth during 2002 on the various rootstocks differed according to the scion-dwarfing influence of each rootstock genotype. However, trees on the rootstocks M7, M26 and CG16 remained smaller when growing in the previous tree rows compared with the previous grass lanes, while the growth of trees on CG210 and CG30 did not differ according to their position with respect to previous trees in the orchard (Figure 2). The preplant compost or fumigation soil treatments had no significant effect on tree growth with one exception: tree growth on CG16 was reduced in the compost treatment compared to all other treatments (Figure 3). Soil nutrient contents were generally similar for the compost and the compost/fumigation treatments, which were distinguished from the fumigation only and the control treatments by a significantly higher availability of B, Ca, Cu, P and K (Table 1). The availability of Fe and NO − 3 was lower in the compost treatments. The concentration of Mg and Zn in soil was not affected by preplant treatments. Both soil pH and soil organic matter content were higher in the compost treatments than in the fumigated-only soils and in the controls. Compared to the differences in nutrient availability and soil pH between the treatments, the differences between the two row positions were small (Table 1). Only the difference in soil organic matter content, which was considerably higher in the grass lane than in the tree row, was more pronounced between the row positions than between the treatments. Soil respiration was higher in the compost and compost/fumigation treatments, and lower in the fumigation only and control treatments (Figure 4). Colony counts of cultivable bacteria were also higher for both compost treatments than for fumigation only and control treatments (data not shown). Overall, bacterial community composition as assessed by PCR-DGGE was highly variable even between the replicates of individual treatments (Figure 5). However, differences in DGGE banding patterns were correlated with treatments. In all of the rootstocks whose roots and soil were tested – M7, M26, CG30 and CG210 – the composition of rhizosphere bacteria differed between trees grown in the previous grass lanes as compared to the old tree rows of the previous orchard (Table 2). This effect was often visible along the first axis in the principal component analysis (Figure 6). The effect of orchard position on bacterial rhizosphere community structure often masked the effects of soil treatments. When the fingerprints of the two orchard positions were analyzed separately, a significant effect of preplant soil treatment became recognizable (Table 2). Both fumigated and non-fumigated treatments constituted distinct groups. The separation between fumigated and non-fumigated treatments was also recognizable in the PCA plots of the M7 and the CG210 rootstocks (Figure 6). As observed for the rhizosphere bacteria, orchard position of replant trees had a strong impact on community composition of actinobacteria in the rhizosphere of M7, M26 and CG210, though this effect was not detected with CG30 (Table 2). In the PCA- plots of M7 and CG210, a differentiation of the old row samples from the grass lane samples was recognizable along the second principal component axis (Figure 7). The effect of preplant soil treatments on actinobacteria in the rhizosphere was obscured by the effect of old row position in the orchard, but became more evident when the banding patterns of the samples from old rows or grass lanes were analyzed separately (Table 2). The distribution of samples in the PCA plots for M7, M26 and CG210 indicated that the soil treatments could be roughly separated into distinct groups with the two fumigated treatments on one side and the control and compost-amended soil on the other (Figure 7). The fingerprints of rhizosphere bacteria associated with the four rootstocks differed strongly from each other (Figure 8). The two ARD-tolerant rootstocks CG30 and CG210 had similar fingerprints, as did the two conventional rootstocks M7 and M26. The species composition of rhizosphere actinobacteria of M7 and M26 differed from those of CG30 and CG210 (Figure 8). The rootstock-specific differences in rhizosphere actinobacteria were not as pronounced as the differences in general bacteria. The results of this study show a strong influence of: (1) replant tree location with respect to previous tree positions, and (2) rootstock genotype, both of which affected tree growth and bacterial rhizosphere community composition in the experimental orchard. In contrast, the preplant soil treatments had little effect on tree growth and only a slight impact on bacterial and actinobacterial rhizosphere community composition. The reduced growth of trees grafted on the rootstocks M7, M26 and CG16 in the old tree rows compared to the old grass lanes is in accordance with the findings of Buszard and Jensen (1986) that ARD was more severe in soil collected from under the can- opies than in soil from the alleyways. The growth of the rootstocks CG30 and CG210 was similar in both positions. This is not unexpected, as these two rootstocks had been rated as relatively tolerant to ARD in an experiment with a mixture of soils from New York orchards with a history of ARD problems (Isutsa and Merwin, 2000) and were selected for this trial accordingly. The composition of rhizosphere bacteria and actinobacteria differed clearly between the old rows and grass lanes. These differences support the observa- tions of Mazzola (1999) that bacterial community composition in the tree rows of a young orchard was significantly different from the microbial community in the adjacent uncropped soil. Our data show that these differences outlast the removal of the trees for more than a year. There are at least two factors that may have contributed to the observed differences between the bacterial communities in the two orchard positions. First, microbial communities in the rhizosphere of both trees and herbaceous plants often differ between plant species (Saetre and Bååth, 2000; Marschner et al., 2001). These differences are attributed to differences in the release of organic com- pounds from roots into the rhizosphere (Seatre and Baath, 2000), which are the main energy source for rhizosphere microflora (Whipps, 1990). Twenty ...