Microbial fluctuations after soil heating and organic amendment

Abstract

Microorganisms in heated (200°C, 1 h) soil not inoculated (H) or inoculated with 0.5% of fresh soil (Hi) and amended with wheat straw (Hi+WS) or poultry manure (Hi+PM) were determined during a 3-month soil incubation. Heating completely sterilised the soil, although the normal soil manipulation in the laboratory contaminated H with a few microbes. In the inoculated soil samples, whether amended or not, microbes were able to actively proliferate, but soil inoculation was essential to develop field microbial conditions. Organic amendment (WS or PM) neither inhibited microbial proliferation nor changed the order of the sizes of the various subgroups. Saprophytic bacteria predominated over the other microbial groups, followed by fungal propagules and actinomycetes; fungal mycelium was relatively well developed, whereas the numbers of photoautotrophs, cyanobacteria and algae were low. With regard to the N-mineralizers, there was a relatively high number of ammonifiers, while chemoautotrophic nitrifiers, both NH4+- and NO2−-oxidizers, were scarce. Microbial groups, however, differed in their response to treatment. In regard to the heterotrophic microbes, amendment with WS increased counts of fungal propagules and hyphae length, while bacteria, particularly actinomycetes and ammonifiers, were lowered; conversely, PM treatment favoured bacterial, actinomycetal, ammonifier and fungal mycelium development and propagule generation. All the autotrophic microbes, whether photo- or chemo-autotrophs, were decreased by the addition of organic residues and, in general, the effect was more negative with PM than with WS. The results indicated that WS could restore the numbers of fungi in the heated soil, but it could cause a reduction in other microbial groups, whereas PM increased the numbers of all the taxonomic groups and the ammonifiers. However, neither residue favoured the recovery of autotrophic C- and N-fixers and nitrifiers in the heated soil.