3.5 The cbbL- and nifH-containing bacterial
diversity and their relationship with soil properties
Soil microbial diversity was employed as a third indicator of microbial
response to IM. The diversity of CO2 and
N2 fixation bacteria responded similarly to IM (Table
2). It was observed that IM10 was unique in both topsoils and subsoils
with respect to diversity indices of CO2 fixation
bacteria, with both Shannon and Evenness values being lower level
compared with the remaining treatments (P < 0.05),
while the Simpson index displayed an opposite trend. Similar trends were
observed for diversity indices of diazotrophic bacteria in topsoils,
while no differences were observed in subsoils. Two-way ANOVA analysis
of variance revealed that the diversity of cbbL - andnifH -containing bacteria was significantly affected by IM time
and even more so by soil depth (Table 2). Correlation analysis between
diversity index and soil properties based on same layer revealed that
evenness and Shannon indices of cbbL -containing bacterial were
positively correlated only with AK (P = 0.016) in the topsoils
and with AP (P = 0.015) in the subsoils (Table 3), respectively.
However, more soil factors were positively correlated with diversity
index of nifH -containing bacterial, including evenness index (E)
with SOC (P = 0.021), TN (P = 0.033) and AN (P =
0.040) in topsoils, and with SOC (P = 0.020), TN (P =
0.014) and AN (P = 0.023) in subsoils. Soil TN and AN were also
positively correlated with Shannon index in subsoils.
- Discussion
- Alteration
of soil properties during 20 year of IM
Soil pH usually decreases as a result of long term fertilization with
high rates of mineral fertilizer
(Schroder et al.,2011). It is
encouraging that soil pH was stable in response to MCM management in
this study. Soil nutrients usually accumulate with increasing duration
of IM because of annual fertilizer applications (Tan et al.,2013). This
study demonstrated
that
application of MCM resulted in enhanced soil fertility with the
exception of NH4+-N. The increased
NO3--N and decreased
NH4+-N over time might be the result
of enhanced nitrification, a commonly observed response to intensive
agriculture (Bi et al., 2017). This improved fertility is mainly
attributed to direct input of mineral fertilizers and indirect
supplement of organic fertilizers decomposition. Soil OC varied over
time, increasing sharply at IM6, returning to a lower level similar to
CK for more than 10 years, and finally recovering somewhat at IM20. It
has been shown that organic
fertilizer usually improves SOC in
long-term studies (Han et al.,2018). We also detected an increase of
SOC after 20 years of IM, although levels did fluctuate during the
preceding 15 years. The dynamics of SOC depends on the balance of OC
input and its mineralization. The sharp increase at the first stage was
reflected the large amount of manure applied that did not decomposed
rapidly. However, mineralization accelerated with the improvement of
soil microbial community during the second stage, so that SOC then
dropped to the same level as CK. Soil
δ13C
is closely related to
SOC
dynamics and is an important index to study the history of reconstructed
plant communities, determine the SOC source, soil quality, and soil
C
sequestration rates (Mendez-Millan et
al., 2014; Zhang et al.,2015). Soil δ13C increased
with duration of IM and reached the maximum at IM15 (Fig. 1). This
increase in δ13C may be attributed two factors. One
was that the lighter isotopes of 12C were easier to
volatilize via organic matter decomposition compared to those containing13C, which resulted in the increased relative
abundance of 13C (Guillaume, Muhammad, & Kuzyakov,
2015). It is possible that the
chemical bonds containing 12C are easier to break down
than 13C in enzymatic reaction (Powers & Schlesinger,
2002). Another one was that input pig manure was rich in13C because pig food contain C4 plant of maize having
higher δ13C (ranged from-17‰ to -9‰) than C3 plant
(ranged from-32‰ to -22‰) (Bai et al., 2012;
Farquhar, Ehleringer, & Hubick,
1989).