Agriculture Reference
In-Depth Information
TABLE 4:
Partitioning variation in soil potential enzyme activities as a function of soil
physicochemical factors and indicator FAMEs at the 13 organic tomato fields in Yolo
County, California, USA in the 0-15 cm surface layer.
Percent variation
explained
a
Explanatory
variables
b
Variables retained
Overall model
64.9***
16
Soil physicochemical
factors
37.7***
11
MBC, total N,
−
3
NO-N, GWC, total
C, MBN,
+
4
NH-N, C:N, Olsen P,
DOC, DON
FAME indicators
6.1***
10
20.4ω6c, 16.1ω5c, 10Me18.0,
i15.0, a15.0, 10Me16.0, a17.0,
cy17.0, cy19.0, 10Me17.0
a
Variation explained by the full model (i.e. soil factors and FAMEs) consists of both the
respective contributions of each set of explanatory variables as well as their covariation.
Significance was assessed using 1000 permutations of either the full RDA model or the
appropriate partial redundancy analysis model, with ***P < 0.001.
b
A parsimonious set of explanatory variables was derived using forward selection based
on a double stopping rule of both an alpha criterion and an adjusted R
2
within each set of
explanatory variables individually (Blanchet et al., 2008 and Borcard et al., 2011).
The fi rst axis of a PCA of relative abundances of these indicator FA-
MEs showed a similar pattern as the RDA, with three out of four fungal
markers as well as the micro/mesofaunal marker grouping away from all
other markers, while axis 2 did not yield any clear pattern (Supplemental
Fig. 2). In general, the fi elds were more dispersed in the RDA ordination
relative to the PCA, reinforcing a role of soil physicochemical factors in
explaining variation among fi elds in the microbial community.
The pattern of FAME biomarkers was also somewhat related to the type
of primary organic amendment that was applied in the past year in both the
RDA and PCA (Fig. 4b and Supplemental Fig. 2). Fields using manure as
the primary organic amendment and several fi elds using composted green
waste formed separate clusters (except fi eld 12), with both groups contain-
ing fi elds that also used a vetch cover crop. The only fi eld using a vetch
cover crop without compost or manure clustered with compost.
