Response of bacteria in particle size fractions to organic
amendments. In previous studies, the effects of the different
treatments in the long-term field experiment in Ultuna were
shown to vary significantly with respect to organic matter turnover
(19, 21). The specific layout of the experiment, relating
manure applications to equal amounts of organic carbon, allows
a direct comparison of treatments with respect to breakdown
of organic matter by soil organisms. Referring to carbon
turnover, treatments could be ranked by decreasing digestibility:
GM . AM . SS . peat. Particularly the latter two treatments
showed a considerable accumulation of organic carbon
(Corg). For peat, it was demonstrated that turnover is slow, and
therefore Corg derived from peat has accumulated in soil. This
finding was supported by the large amounts of Corg derived from peat in the silt fraction compared to those in clay size
particles (21). Our results of the microbial population analysis
reflect what has been reported for Corg turnover. In general,
diversities were highest in soils treated with GM and AM and
lowest in soils that received no or inorganic fertilizers and thus
corresponded to the digestibility of the fertilizer applied.
Amendments with SS and peat as fertilizer resulted in quite
unique bacterial population structures that were due to different
compositions rather than to increased or decreased diversities.
Both treatments exhibited very low pH values (pH 5.8),
which were probably responsible for some community changes.
Soils amended with peat were also characterized by a high C/N
ratio. Some T-RFs were not detected in soils amended with SS
and peat despite their presence in other treatments. They
included members of the Holophaga/Acidobacterium division
(95, 236, and 242 bp), Prosthecobacter (236 bp), or low G1C
gram-positives (242 bp). These bacteria may not have been
able to withstand the acidic environment. Other bacteria belonging
to the Holophaga/Acidobacterium division and the genus
Prosthecobacter were able to colonize most treatments,
except soils containing SS. The sensitivity of some microbial
groups to the generally high heavy metal content of SS may
explain this finding. The high abundance of a bacterial group
with a T-RF of 205 bp in SS-treated soils was striking. Two
clones with a matching T-RF were found: one fell into the
Holophaga/Acidobacterium division, whereas the other showed
the highest similarity to Proteobacteria. The high diversity of
bacteria in sand size particles of soils amended with GM may
have been responsible for the fast turnover of organic matter.
In addition, a particularly high abundance of some gram-positive
bacterial species, including Arthrobacter globiformis and
Bacillus species, was found in the GM treatment.