Soil archive

The genetic diversity and culturability of bacteria in archived Broadbalk soil samples has been compared, in two plots under continuous wheat, one fertilized with farmyard manure (FYM), the other with inorganic N, P, K and Mg. The impact of soil drying process necessary for archiving was also assessed. Fresh soil samples (2003) were compared to freshly prepared archived soils (air dried and milled, 2003) and to archived soils taken from the same two plots in 2002, 1944 and 1914. Semi-selective media were used to assess different culturable populations, and soil community DNA was extracted and PCR-amplified to compare total 16S rDNA profiles using denaturing gradient gel electrophoresis (DGGE).

The FYM plot supported higher populations of enteric bacteria and pseudomonads in fresh soil, compared to the inorganically fertilized plot. The archived FYM soils also showed increased viability of culturable bacteria, in particular the 1914 soils, possibly due to the increased soil organic matter. DGGE analysis indicated that the continuous applications of FYM and N144 over time had selected for different dominant bacterial populations. The DNA profiles for 2002 and 2003 were similar in each plot but differed from those of the 1944 and 1914 samples.  The Rothamsted Soil Archive dates back to 1846, and our preliminary results indicate that further analyses will enable us to study changes in different microbial populations over time to assess the long-term impact of different agricultural practices.  (Ian Clark)