Abstract
Fertilization is a common approach to increase or sustain soil
fertility, but its impact on microbial biomass and community structure
remains controversial, particularly in paddy soils. In this study,
we investigated the effect of
different long-term fertilization strategies, beginning in 1986, namely
no fertilization, mineral fertilization, mineral fertilization combined
with rice straw or chicken manure, on microbial biomass and community
composition at four soil depths (0–10, 10–20, 20–30, and 30–40 cm).
The extracted soil phospholipid fatty acids (PLFAs) were pooled into
gram-positive (G+) bacteria, gram-negative (G−) bacteria, fungi, and
actinomycetes groups. Results showed that irrespective of the
fertilization type, the abundance of PLFAs decreased with soil depth in
the following order due to nutrient decrease along soil profiles: fungi
> G− bacteria > G+ bacteria >
actinomycetes. Mineral fertilization induced G+ bacteria more than G−
bacteria and actinomycetes, which suggested that the inorganic nutrients
in mineral fertilizers are utilized more by G+ bacteria than by other
microbial groups. Partial replacement of mineral fertilizer with manure
further stimulates G+ bacteria at all depths. Redundancy analysis showed
obvious microbial separation at the 0−20 and 20−40 cm soil depths due to
the rhizodeposition effect and also revealed that the microbial
communities were significantly correlated with nutrient content (soil
organic carbon and available N) and pH.
Overall, our findings highlight
microbial community shifts due to different fertilizer types, which
provides basic information for understanding how substrate availability
controls the structure of soil microbial communities in paddy soil
systems.
Key words : mineral fertilizer –rhizodeposition effect –straw
addition – soil depth – organic manure – paddy soil.
INTRODUCTION
Paddy soil is estimated to cover a total area of approximately 161
million ha worldwide, with 18.9% of this area in China (FAOSTAT, 2016).
Mineral or organic fertilizers, or their combinations, are usually
applied to paddy fields to maintain soil fertility and increase rice
crop yields. Organic fertilizers commonly consist of crop residues or
animal manure. Crop residue and manure addition can increase soil
organic carbon (SOC) content (Yan et al., 2007; Liu et al., 2014; Zhang
et al., 2016), but they can also be a source of CH4emissions in paddy soil (Shen et al., 2007; Tang et al., 2016).
Application of manure or rice straw combined with mineral fertilizer in
paddy soils was demonstrated to be effective in increasing soil
fertility (mainly due to increased microbial biomass and SOC) (Xu et
al., 2018). Similarly, a mixture of 70% manure in combination with 30%
chemical fertilizer improved rice yields and bacterial diversity, and
also alleviated soil acidification (Chen et al., 2017).
Soil microorganisms play a key role in nutrient cycling, and maintaining
or increasing microbial diversity is crucial for soil health (Keeler et
al., 2009; Whiteside et al., 2012; Fierer, 2017). Microbial diversity is
affected by several factors, such as soil physical and chemical
properties (Lynn et al., 2017; van Leeuwen et al., 2017), fertilization
(Huang et al., 2018), and irrigation (Azziz et al., 2016; Das et al.,
2016). Most previous studies on microbial communities focused on topsoil
(based on tillage or direct fertilization)because the composition is
more variable in the surface horizons (Eilers et al., 2012), which
generally refers to depths of 0−20 cm for paddy soils. However, a
previous study suggested that nutrients can translocate from the topsoil
to the deeper layers in maize and wheat fields (Kramer et al., 2013).
Another previous study determined that up to 30% of microbial biomass
could still be found in the C horizon (55−65 cm in grassland and
cropland) (van Leeuwen et al., 2017). Hence, soil depth should be
considered as an important environmental gradient that structures
soil-microbial communities (Eilers et al., 2012), especially the subsoil
close to the plough layers. Mineral fertilizer plus manure addition has
been demonstrated to increase different taxa of bacteria and archaea for
topsoil (0–20 cm) and subsoil (below 20 cm) compared to mineral
fertilizer alone in long-term paddy fields (Gu et al., 2017).
Previous studies have shown that different fertilization strategies
could alter different microbial-community structures. Mineral N
fertilizer was reported to increase the gram-positive to gram-negative
bacteria (G+ to G–) ratio in paddy soil (Zhang et al., 2007) and
temperate grassland soil (Denef et al., 2009), while no ratio difference
was observed in other studies (Zhang et al., 2012; Dong et al., 2014).
It has been suggested that G–bacteria preferentially use labile
plant-derived C and G+ bacteria that are able to utilize recalcitrant
compounds from soil organic matter (SOM) (Kramer & Gleixner, 2008;
Fanin et al., 2019). Complex litter addition was found to greatly
promote fungi than bacteria and also increased the G– to G+ bacteria
ratio in subtropical forest soil (Wang et al., 2014); the straw residue
was demonstrated to promote more G– bacteria than G+ bacteria (Tang et
al., 2018). Manure was reported to promote G–bacteria in rotation
cropping (Peacock et al., 2001) and paddy soils (Zhang et al., 2012).
However, there is still limited information regarding the
microbial-community change along the soil profiles of paddy soil
supplied with equal amounts but different types of fertilizer based on
long-term observation. It has been demonstrated that, unlike inorganic
fertilizer, straw and manure addition increase the relative abundance ofGemmatimonadetes and Planctomycetia (Tang et al., 2019).
In the present study, we investigated the microbial biomass and
responses of specific microbial groups (the fungi to bacteria, G+ to
G–, G+ to actinomycetes, and fungi to G– bacteria ratios) to different
fertilizer types (mineral and organic combination with equal amounts of
mineral fertilization [NPK]) at 0–10, 10–20, 20–30, and 30–40 cm
soil depths in paddy soil systems. The microbial biomass and community
composition were measured using chloroform fumigation extraction and
phospholipid fatty acid(PLFA) analyses, respectively. We hypothesized
that 1) the microbial abundance and biomass would decrease with soil
depth; 2) fertilization would increase microbial abundance and biomass;
and 3) mineral fertilizer would promote G+ bacteria and thereby increase
the G+ to G– bacteria ratio, while more complex organic fertilizer
(straw and manure) would stimulate more G– bacteria.
MATERIALS AND METHODS