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