Interference from plant roots in the estimation of soil microbial ATP,C, N and P

Excised, solution-grown roots of maize or ryegrass added to two pasture soils at the rate of 6.0mg g^?1 and 13.8 mg g^?, respectively, increased the flush (fumigated minus control values) of CO₂-C by up to 1.89-fold, KCl extractable N by up to 1.88-fold, and NaHCO₃ extractable P by 3.28-fold. The ATP content of the soil was increased by up to 1.42-fold. Because of high variability the effect of the roots on the C and N flushes was not significant at P < 0.05.Incubation of the root-amended soils for 7 days at 25°C prior to fumigation much decreased the contribution from the roots to the C and N flush, and to the ATP content. There was, however, still a large significant effect of the roots on the P-flush, this being up to 3 times greater than the equivalent soil without roots.In soil samples with a high viable root density (> 6mg g^?1) such as may occur in dense pastures, greenhouse pot experiments or rhizosphere soil samples, it is recommended that they be incubated for 7 days prior to fumigation and analyses. Without such prior incubation there is the risk that root material may be included in the “microbial” biomass estimations.