4.3 Community shift of CO2 and
N2 fixation bacteria and driving factors during 20 year
of IM
The CO2 and N2 fixation bacterial
communities responded differently to IM. The CO2fixation bacterial community changed gradually with IM duration, with
the structures associated with CK and IM20 differing the most. However,
a relatively similar structure for soil diazotrophs was observed between
CK and IM20, indicating that community of N2 -fixing
bacteria responded to the disturbance at earlier stage of IM but
returned to resemble the original structure at IM20. These results
suggest that the community structure of soil diazotrophic bacteria is
more resilient than that of CO2 bacteria, despite
declines in abundance. Soils from all treatments had six predominant
species of CO2 fixation bacteria but these varied in
their relative abundances. The relative abundance of the most
predominant T-RF 177 bp was lowest (P < 0.05) in CK,
indicating IM improved the species represented by T-RF. RDA revealed
that numbers of this T-RF in IM topsoil samples were positively related
to AP and δ 13C, indicating soil AP might favor
the associated species. In addition, soil P content has also been shown
to be a major regulator for cbbL -containing algae composition
(Yuan et al., 2015). The most dominant group was composed of several
groups of α, β ,γ -Proteobacteria − Bradyrhizobium
sp. , Rhodospirillum centenum , Thioflavicoccus mobilis ,Stappia meyerae , Mesorhizobium ciceri , and Starkeya
novella. In the contrast, the relative abundance of T-RFs 44bp and
360bp in topsoil decreased sharply after 6 or 10 years of IM, suggesting
that the associated species may have been inhibited by rich soil
nutrition.
Proteobacteria
are fast-growing copiotrophs that thrive in environments with high
carbon availability (Fierer, Bradford, & Jackson, 2007). Soils in this
studied contained five dominant types of diazotrophic bacteria, with the
highest relative abundance observed (22.1-54.4% for T-RF 180bp) being
reached in the middle stages of IM. These higher abundances of T-RFs
exhibited an interesting relationship of ‘growth and decline’ between
groups of CK and IM20 vs the rest group of IM treatments. This
phenomenon suggested that different species of diazotrophic bacteria
have unique environmental preferences. It was difficult to recognize
taxa were favored under IM because several T-RFs identified to same
species. The number of factors driving changes within the diazotropic
community greater than for CO2 fixation bacteria and
included AK, SOC, C : N and AN content in topsoils, and AP and C:N in
the subsoils. Two taxa of diazotrophic bacteria, Rhizobium sp.
and Azorhizobium doebereinerae , were the most dominant of those
detected in the soils sampled.
4.4 Alteration of diversity of CO2 and
N2 fixation bacterialcommunity
and driving factors during 20 year of IM
Just as the abundances of CO2- and
N2-fixing bacteria responded differently to IM, so too
did the corresponding measures of diversity used in this study. A dip in
Shannon and Evenness index values detected for CO2-fixing bacteria at IM10 in both topsoils and subsoils was opposed to
the opposite trend observed for the Simpson index. Lower values for
Shannon and Evenness indices after 10 years of IM may reflect a
collective effect of large applications of manure. The Shannon and
Evenness indices, which reflect the total numbers and evenness of
microbial species, was decreased by manure input, which may have
stimulated certain groups of CO2-fixing bacteria
bacteria. Then, as the effect of manure declined with further duration
of IM, the indices recovered gradually. On the contrary, the Simpson
index reflects the primarily the dominant species and so may have
increased significantly at IM10 due to the dominance of TRF 177 bp. The
recovery of Shannon and Evenness index after 10 years of IM may be
attributed to an increase in the number of species, but these were quite
different from those in the CK and IM6. Overall, these results indicate
that the communities of CO2 fixation bacteria were
altered by IM practice. Conversely, both the Shannon and Evenness
indices for the diazotrophic communities indicated a return to the
original state (CK) suggesting that their communities was not impacted
significantly by IM practice. The above results implied that
CO2 -fixing bacteria was more sensitive to IM practice
with application of MCM than diazotrophic bacteria.