4.1 Microbial groups along soil depth
The decline in the abundance of microbial PLFAs with soil depth (Table
1, P <0.05) could arise from the reduction in energy
(carbon) and nutrient availability along the profile. A reduction in the
ratio of fungi to bacteria with soil depth (Figure 1a) is also an
indication that the fungal population declined much faster than that of
bacteria along soil profiles. Similarly, the decreasing trends of the
fungi to G−bacteria and G+ bacteria to actinomycetes ratios, as well as
the increasing G+ to G−bacteria ratio with soil depth could mean that
microbial dependence on nutrients adhered to the following order: fungi
> G−
bacteria>
G+ bacteria > actinomycetes. Fungal abundance and activity
are considered low in paddy soil because of prolonged anaerobic
conditions. However, in paddy soils, rice plants could release oxygen
from the root through aerenchyma (Frenzel et al., 1992) and created an
aerobic environment in the rhizosphere. Previous studies have
demonstrated that fungi have the highest capacity for assimilating
rhizodeposits in paddy soils (Ge et al., 2016). Such translocation to
fungi can be rapidly detected in PLFAs due to the direct connection of
fungal hyphae and mycorrhizal fungi with rice roots (Yuan et al., 2016).
Therefore, oxygen limitation caused by root biomass reduction with soil
depth could largely suppress fungal abundance. G− bacteria have been
reported to preferentially use labile compounds such as rhizodeposits
and exudates from plant biomass, and G+ bacteria are able to use
recalcitrant compounds from SOM (Kramer and Gleixner, 2008; Fanin et
al., 2019). Zhu et al. (2017)also found that G+ bacteria increased after
rhizo- and micro-C addition. Labile substrates decrease with increasing
soil depth, thereby causing G− bacteria to decline faster than G+
bacteria. As actinomycetes are well known for their key role in
degrading complex compounds (Acosta-Martínez et al., 2008), they are
relatively the least sensitive to labile substrate reduction with soil
depth. Thus, the sensitivity trends of the PLFAs with soil depth were
not altered by fertilization type.
The decrease in MBC and MBN with soil depth corresponded with the
decrease in SOC and available N content along the soil profile (Table
2). This indicates that the growth of microorganisms is related to the
availability of C and N nutrients (Loeppmann et al., 2016; van Leeuwen
et al., 2017). SOC levels are fundamentally determined by the balance
between organic matter inputs and their losses through decomposition
(Six, Frey, Thiet, & Batten, 2006). In our current study,
rhizodeposition and fertilization were the two main sources of microbial
C nutrient availability. Zhu et al. ( 2017) found that up to 45% of
rice rhizo-C was stabilized within SOC. In paddy soils, more than 50%
of the total root biomass is allocated to the first 5 cm of surface
soil; therefore, the upper soil layer can receive more C input than the
soils below (Li et al., 2004; Li & Yagi, 2004). The decrease in the
δ13C isotopic signature of soil and PLFA from
C4 plants with soil depth after
C3−C4 vegetation change in a previous
study also demonstrated the decreasing influence of plants on deeper
soil (Kramer & Gleixner, 2008). Higher MBC and MBN contents were
observed at 0–20 cm in the control, indicating that soil layers with
fresh C input can increase microbial biomass. RDA results also showed an
obvious separation of the 0–20 and 20–40 cm soil layers, even in the
control without fertilizer, confirming the rhizodeposition effect on the
0–20 cm soil layer (Figure 3a).