Fig. 2

Effect of added spikes before and after DNA isolation on microbial quantitation. P synthetic spike DNA was added to samples prior to DNA isolation (a–c) in a concentration gradient (a–b) of 14 (circle), 34 (square), 70 (upward triangle), 140 (downward triangle) and 340 (diamond) pg per Rhizobium culture (1 ml) or soil (1 g) and (c) 39 (square) pg per soil (1 g) and then co-isolated with microbial DNA. Spikes were also added to purified microbial DNA (d–f) in a concentration gradient of 2 (circle), 4 (square), 8 (upward triangle), 20 (downward triangle), 40 (diamond), 80 (hexagon) and 200 (cross) pg per isolation microbial DNA (μg). a–c The synthetic spike plasmid DNA may be cleaved during DNA purification. Horizontal lines represent the average no of 16S rRNA for each group of samples. Bawburgh soil was used throughout