| Abstract: | The Ultuna long-term soil organic matter experiment in Sweden (59′82° N, 17′65° E) was started in 1956 to study the effects
of different N fertilisers and organic amendments on soil properties. In this study, samples were taken from 11 of the treatments,
including unfertilised bare fallow and cropped fallow, straw with and without N addition, green manure, peat, farmyard manure,
sawdust, sewage sludge, calcium nitrate and ammonium sulphate, with n = 4 for each treatment. Samples were taken from topsoil (0–20 cm) and subsoil (27–40 cm depth) and analysed for concentrations
of phospholipid fatty acids (PLFAs), organic C, total N and pH. The results showed that the subsoil samples reflected the
total PLFA content of the topsoil, but not the microbial community structure. Total PLFA content was well correlated with
total organic C and total N in both topsoil and subsoil. Total PLFA content in topsoil samples was highest in the sewage sludge
treatment (89 ± 22 nmol PLFA g dw−1). This contradicts earlier findings on microbial biomass in this sewage sludge-treated soil, which indicated inhibition of
microorganisms, probably by heavy metals added with sludge. A switch towards microbial growth and faster decomposition of
organic matter occurred around 2000, coinciding with lowered heavy metal content in the sludge. According to the PLFA data,
the microbial community in the sewage sludge treatment is now dominated by Gram-positive bacteria. A lack of Gram-negative
bacteria was also observed for the ammonium sulphate treatment, obviously caused by a drop in pH to 4.2. |
