Estimation of the adenylate energy charge in unamended and amended agricultural soils

To determine relatively low concentrations of adenine nucleotides in agricultural soils a NaHCO₃-based extradant was developed and compared with the trichloroacetic acid-paraquat-phosphate extradant. The new medium, consisting of chloroform, sodium bicarbonate, phosphate and adenosine (pH 8.0) gave soil extracts which could be investigated without further neutralization and dilution. ATP was measured directly in the soil extracts by the luciferin-luciferase system. ADP and AMP were estimated after their enzymatic conversion to ATP by standard methods. The quantities of nucleotides corrected for recovery of standards were used to calculate the adenylate energy charge (AEC) from the formula AEC = ATP] + 1/2ADP]/ATP] + ADP] + AMP], The AEC was estimated in six unplanted soils from agricultural fields. A very similar energy charge of 0.3-0.4 was found in all soils sampled which indicates a low metabolic activity of the soil population. Two other soils with a pronounced difference in biomass-C content were used to investigate the influence of different amendments on the AEC. In an experiment with low glucose supplements up to 500 μg C g^?1 soil, the soil with the low biomass-C (a cambisol) showed a distinct increase of the AEC from 0.34 to 0.50, whereas the soil with the high biomass-C content (a phaeozem) increased its AEC only slightly from 0.32 to 0.37. In another experiment with high glucose supplements the phaeozem reached its maximum AEC value of 0.56 after the addition of 4000 μg Cg^?1 soil. An amendment with 8000 μg C g^?1 soil gave no further increase. In the combisol the addition of 1000 μg C g^?1 soil increased the AEC to 0.61. Higher supplements gave only a slight further increase to a maximum value of 0.67 after the addition of 8000 μg C g^?1 soil. The same AEC value was reached when the cambisol was amended with a mixture of organic substrates at a concentration of 10,000 μg C g^?1 soil.