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Several studies have reported increased MBC under diversified crop rotations
compared to monocropping. Soil MBC increased under a diversified crop rotation
[spring wheat–barley (Hordeum vulgaris L.) hay–corn–pea] compared with a con-
tinuous spring wheat system (Sainju et al. 2012 ). Some studies have indicated that
monocultures select for less diverse microbial communities. For example, wheat
monocultures had less microbial diversity than a crop rotation with red clover or
field peas indicating that legume-based crop rotations support the diversity of soil
microbial communities and may affect the sustainability of agricultural ecosystems
(Lupwayi et al. 1998 ). A long-term maize monoculture decreased the diversity of R.
leguminosarum biovar viciae populations and favored a specific subgroup of geno-
types. A shift in the distribution of the symbiotic genotypes within the populations
under corn monoculture was also observed (Depret et al. 2004 ). Meriles et al. ( 2009 )
reported a decline in fungal (Trichoderma and Gliocladium) populations under con-
tinuous soybean (Glycine max L.) compared with corn–soybean and soybean–corn
rotations. In a study which included a 12-year sorghum–castor–sorghum rotation
and a 10-year continuous castor crop, a differential distribution of fungal genera
was noted. More diversity was recorded in rotation plots than in monoculture. In the
monoculture plots, one or two genera (mostly Streptomyces) predominated repre-
senting 80 % of the actinomycetes (Venkateswarlu and Srinivasarao 2005 ). These
studies indicated the negative effects of the monoculture on soil biota over crop
rotations.
Rotations including winter cover crops—sorghum grain and forage with winter
rye (Secale cereal L.) and cotton (Gossypium hirsutum L.)–winter rye–grain sor-
ghum—returned higher plant biomass, and other types of root exudates in the soil
to promote microbial biomass and metabolic diversity as indicated by several enzy-
matic activities, than grain sorghum–cotton and continuous cotton after only 3 years
(Acosta-Martínez et al. 2011 ). An evaluation of microbial communities in the top
soil of dryland cropping systems (grain sorghum–cotton, cotton–winter rye–grain
sorghum, and a rotation of forage sorghum with winter rye) under different tillage
practices for 5 years identified lower fungal:bacterial ratios under cotton-based
cropping systems than under forage sorghum–winter rye (Acosta-Martínez et al.
2010 ). Soil under forage sorghum–rye had higher population densities of
Bacteroidetes and Proteobacteria but lower Actinobacteria compared to grain sor-
ghum–cotton and cotton–winter rye–grain sorghum. The tilled soil had more
Chloroflexi, Gemmatimonadetes and Verrucomicrobiae than the NT plots. This
study demonstrated that differences in microbial communities are more affected by
crop rotation than tillage management.
However, contrasting results on bacterial and fungal relative abundances and the
dominant members of the soil microbial community were observed in two cropping
systems (continuous wheat and sorghum–wheat–soybean rotation) when commu-
nity qPCR with 16S rRNA gene sequencing was used (Ng et al. 2012 ). The qPCR
assays revealed a reduced bacterial copy number in the crop rotation. Continuous
wheat had more relative bacterial numbers and diversity than the rotation treatment.
Continuous wheat had more abundant Cyanobacteria probably due to the longer
unplanted period between crops when the soil surface was exposed to sunlight while
the rotation treatment had more Actinobacteria probably due to the incorporation of
M. Grover et al.