Extracellular DNA in soil: how long can it persist?

The aim of this work was to obtain extracellular DNA (eDNA) molecules pure enough to estimate their longevity in soil by an isotope-based approach. Extracellular DNA molecules were extracted from all the horizons of a forest soil and purified according to two different ways: eDNA1 was obtained by the procedure proposed by Davis (1986), and eDNA2 was obtained by a “combined” procedure that add to the previous method the use of binding resins following the “DNA bind-wash-elute” method. The two differently purified eDNAs were compared for their purity index (A260/A280 ratio), PCR amplification, and natural abundance of radiogenic (14C) and stable isotopes (13C and 15N). The purity index and the PCR appeared not able to clearly assess the efficiency of the purifying procedures. The eDNA isotopic signature resulted more sensitive and was strongly affected by the purification procedures. The isotopic measurements have shown that for the eDNA1 the major contaminant, mainly in the upper horizons, was the soil organic matter (SOM), even if we cannot exclude that the similar 13C, 15N and 14C of DNA and SOM could be due to the use of SOM-deriving C and N atoms for the microbial synthesis of DNA; for the eDNA2, we obtained extremely low values of 14C, and this was ascribed to the presence of fossil fuel-derived substances used during the purification, although in amounts not revealed by GC-MS analysis.