Agriculture Reference
In-Depth Information
13.3.3 PATTERNS OF FAMES
Overall, the relative abundance of indicator FAMEs (Fig. 3) was less
variable across fields than soil enzyme activities, as reflected by smaller
F-statistics. Based on relative abundance, markers for Gram+ bacte-
ria showed more variation across fields (F-statistic = 11.5) than that of
Gram− bacteria (F-statistic = 5.4) or actinomycetes (F-statistic = 8.2).
Markers of saprophytic fungi were generally similar across the fields
(F-statistic = 3.7) while that of AMF showed more variation (F-statistic
= 15.2) with the highest relative abundance in fields 8, 10, 11, 12, and
13. A marker for soil micro/mesofauna showed the most variation of the
FAME indicators (F-statistic = 19.4). The sum of all biomarkers for fun-
gi (i.e. total fungi) was relatively consistent across the fields (F-statistic
= 3.5), while the sum of all markers for bacteria (i.e. total bacteria) was
more variable (F-statistic = 16.9). Biomarkers for total fungi and total
bacteria accounted for a mean of 30.1 and 32.2% of the total FAMEs,
respectively, which reflected a fungi:bacteria FAME ratio close to one
(mean = 0.94) for most fields, except in fields 12 and 13, where it was
appreciably lower.
When soil physicochemical factors were used to constrain the ordina-
tion of the relative abundance of 14 indicator FAMEs with RDA (Fig. 4),
the full RDA model accounted for only 29.3% of the variation (p < 0.001).
The fi rst two RDA axes accounted for 13.1 and 11.5% of the overall varia-
tion and 36.7 and 32.1% of the fi tted model ( Fig. 4a). All explanatory
variables retained in the model were signifi cant (p < 0.05) in constrain-
ing indicator FAMEs based on permutation tests. Olsen P, GWC, and pH
played a role in the dispersion of fi elds along the fi rst axis, while soil tex-
ture variables (clay and silt) and EOC varied mainly along the second axis
( Fig. 4b). While it was retained in the selection procedure and signifi cant,
MBC did not appear to play a strong role in the ordination of indicator
FAMEs, based on the length of its vector. Higher Olsen P was associated
with bacterial markers while higher GWC and pH were associated with
fungal and micro/mesofaunal markers. EOC was correlated with the gram-
positive bacterial marker i17:0.
