4.2 Effect of fertilization strategies on microbial community
structure
All fertilized treatments increased microbial PLFAs (Table 1), showing a
fertilizer-induced increase in microbial abundance. Large amounts of N
from mineral and organic fertilizers could benefit microbes through a
reduction in nutrient competition with rice plants (Jackson, Burger, &
Cavagnaro, 2008; Zhu et al., 2018). Moreover, the increase of plant
biomass caused by N fertilization would have a direct impact on
rhizodeposition and the high availability of labile C to microbes
(Christopher &Lal, 2007; Zhu, Vivanco, & Manter, 2016). G− bacteria
are usually considered to benefit more from rhizodeposits (Fanin et al.,
2019). However, the G+ to G−bacteria ratio was enriched
(0.55~0.83 after fertilizers vs.
0.45~0.53 in control) at 0−30 cm soil depths (Figures 2b
and c, P <0.05). Moreover, the abundance of G+ bacteria
exceeded that of actinomycetes at 0−30 cm. This indicated that both
mineral and organic fertilizers preferentially promote G+ bacteria. Pure
manure has been demonstrated to induce G−bacteria and reduce G+ bacteria
(Peacock et al., 2001). Wang et al. (2014) also found more C
incorporation by G−bacteria, which had originally been dominant in the
soils. In parallel, they found that litter addition would reduce the G+
to G− bacteria ratio, but its combination with mineral N could buffer
this decrease. This indicated that complex substrates induced more
G−bacteria and mineral N promoted G+ bacteria, thereby reducing the G+
to G−bacteria ratio. Relatively increased G+ bacteria levels due to
mineral N fertilization have also been reported in grassland soils
(Denef et al., 2009) and corn fields (Peacock et al., 2001). Consistent
with these observations, all treatments with mineral N fertilizers in
the present study induced G+ bacteria, even when the soil was
predominated by G−bacteria. This indicates a positive response of soil
G+ bacteria to mineral N fertilizers.
After partial replacement of inorganic fertilizer with straw, PLFA
biomarkers increased at 10–20 cm compared with those under NPK addition
alone (Table 2, P <0.05). The fungi to bacteria and G−
bacteria ratios were also relatively increased at this soil depth
(Figures 1a and d). This indicated that fungi were especially promoted
at 10–20 cm, probably owing to more suitable anaerobic conditions for
straw decomposition. Simultaneously, qCO2 was observed
to be the lowest at this soil depth. Some studies have suggested that
higher qCO2 means higher stress or disturbance for
microorganisms (Wardle & Ghani, 1995). Under the same level of
fertilizer input comparable to straw addition, MBC and PLFAs were
largely induced by chicken manure throughout the soil profile (Tables 1
and 2, P < 0.05). Manure addition further increased the
G+ to G− bacteria and G+ bacteria to actinomycetes ratios (Figures 1b
and c), which may indicate that manure induced more G+ bacteria. In
addition, the NPK + OM treatments showed a consistently low
qCO2 throughout the 0−40 cm soil profile (Figure 2).
Plaza et al. (2004) observed that qCO2 initially
decreased after pig manure amendment to soil and then increased with
further manure addition. Hence, qCO2 was not negatively
correlated with soil fertility or organic fertilizer input. The findings
above may preferentially support the connection between low
qCO2 and favourable soil conditions for microorganisms.
In addition to its effect on microbial communities through nutrient
availability, fertilization can indirectly alter microbial communities
through its effect on pH at 0–20 cm soil depths (Figures 3a and b).
More organic acids such as oxalic acid (Keiluweit et al., 2015) and
acetate (Farrar, Hawes, Jones, & Lindow, 2003) released from
rhizodeposits could partly cause a reduction in soil pH, especially in
the rooted layers compared with the deeper layers (Table 2, Fig 3a-b,P <0.05). In addition, N fertilizer has been reported to
cause soil acidification during N cycling (Bolan, Headley, & White,
1991; Geissler & Scow, 2014). However, in contrast to general
assumptions, higher SOC contents were observed in the control compared
with those in NPK at 0–10 cm. The soil respiration rate and
qCO2 were higher in NPK in this soil layer (Figures 2a
and b, P <0.05), and carbon use efficiency was also
found to be lower in NPK (Zhran et al., 2020). A greater amount of
respired than accumulated C in biomass can partially explain the
decrease in SOC. In addition, Loeppmann et al. (2016) found that maize
rhizodeposition can decrease the proportional activity of C- to
N-cycling enzymes. Mineral NPK fertilizers can increase root biomass and
then increase rhizodeposition, but it can also stimulate microorganisms
that need both C and N. More C-cycling enzymes will be released by
microbes to decompose SOC to maintain a certain microbial C:N ratio
(Devi & Yadava, 2006) if microorganisms need more C than rhizodeposits
can supply.