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DNA buoyant density was resolved in primary CsCl gradients, and total DNA was quantified in each gradient fraction. DNA was extracted from soil incubated in the presence of either artificial air (○) or artificial air containing 99.8 atom% 15N2 (•). The shaded region corresponds to fractions of buoyant density of 1.727 to 1.733 g ml−1 which were selected for further analysis in secondary gradients containing bis-benzimide.
