Agriculture Reference
In-Depth Information
13.4.2 PATTERNS AND DETERMINANTS OF MICROBIAL
COMMUNITY BIOMASS AND COMPOSITION
Soil microbial community composition, as measured by FAMEs, was not
as strongly differentiated among individual fields as soil potential enzyme
activities, based on F-statistics that were predominately lower for FAMEs.
Rather, FAMEs formed weak clusters in both the PCA ( Supplemental
Fig. 2) and RDA ( Fig. 4) that were associated at least in part with the pri-
mary organic amendment used (manure, composted green material, and/
or vetch cover crop). The clusters in the PCA and the RDA, as well as the
relatively low proportion of FAME variation accounted for in the RDA
with the measured soil physicochemical factors (<30%), suggests that un-
measured attributes of the organic amendments may exert strong effects
on the microbial community, or that past management is still having ef-
fects. Microbial communities unique to the type of organic amendment
may also have an inoculating effect and contribute to the differentiation
of the microbial community composition ( Marschner et al., 2003 and
Lazcano et al., 2008).
Microbial community composition was associated with factors deter-
mined by parent soil type (e.g. clay and silt) as well as those infl uenced
by a combination of soil characteristics and management (e.g. Olsen P
and pH). Similarly, Bossio et al. (1998) showed that soil type followed by
specifi c management operations (e.g. cover crop incorporation or manure
application) were the primary factors in governing the composition of mi-
crobial communities in a cropping system experiment comparing organic
and conventional management on similar soils in the same landscape. Of
these 13 fi elds, those that used manure as a primary organic amendment
clustered together in the RDA and were associated with increased Olsen P
and increases in Gram+ and Gram− bacteria and with decreases in fungal
and micro/mesofaunal markers. Increased P availability has been shown to
negatively affect fungi in other agricultural landscapes (Lauber et al., 2008
and Williams and Hedlund, 2013) and can result from manure application
over time (Clark et al., 1998). Another example of possible management
effects is fi eld 12, in which the microbial community was strongly dif-
ferentiated from other fi elds. It had been in organic management more
