Linking soil microbial community traits and organic carbon accumulation rate under long-term conservation tillage practices
Introduction
Soil organic carbon (SOC) plays a major part in sustaining soil fertility (Oldfield et al., 2019) and soil biological health (Alhameid et al., 2019). However, through several mechanisms, agriculture cultivable cropping has caused a reduction in SOC stocks around the world (Lal, 2004). Firstly, soil disturbance during tillage and other management operations reduces the physical protection of soil carbon (C) through breaking macroaggregates into microaggregates (Six et al., 2000, Du et al., 2015). The exposed SOC is more easily accessible to microorganisms, resulting in decreasing SOC stocks (Razafimbelo et al., 2008). Secondly, cultivated land is used to maintain close to neutral soil pH levels and prevent water accumulation, thereby stimulating microbial oxidation of SOC. Finally, cropland is generally covered with vegetation for a short time relative to natural ecosystems and part of the vegetation is removed through harvest, causing lower soil C input rates (Guo et al., 2017). The resulting decline in soil C stocks reduces soil quality and adds to anthropogenic CO2 emissions (Li et al., 2019). These issues prompted researches into management practices that slow down or reverse soil C losses in croplands (Post and Kwon, 2000, Smith, 2016).
By minimizing soil disturbance and retaining crop residue, conservation tillage practices may increase soil C stocks (e.g. Chavarria et al., 2018). However, soil C gains under these management practices varies widely between studies, ranging from positive (Jha et al., 2020, Li et al., 2021a) to negligible (Niu et al., 2019, Gao et al., 2021). These inconsistent effects on SOC content may be due to soil microorganisms, which are involved in the formation, transformation and decomposition of SOC (Fan and Liang, 2015, Kallenbach et al., 2016). However, the relationship between soil microbial communities and SOC change is still not fully understood, limiting our ability to predict soil C storage in terrestrial ecosystems, including croplands (Wieder et al., 2015). Thus, their relationship needs further study (Malik et al., 2016, Anthony et al., 2020).
Previous studies assessed the relation between soil C storage and various soil microbial community traits (Li et al., 2021b, Duan et al., 2021), including diversity, composition and keystone taxa (e.g. Fontaine and Barot, 2005; Kallenbach et al., 2016; Zheng et al., 2021). Li et al. (2020a) found that microbial diversity is positively correlated with SOC stocks and total N under no-till management, suggesting that more diverse soil microbial communities may be conducive to soil C storage. However, the impact of tillage practices on soil microbial diversity differs among previous studies. For instance, Legrand et al. (2018) reported that bacterial diversity was more responsive to tillage disturbance than fungal diversity, whereas Wang et al. (2020) found the opposite. As microbial diversity is linked to C transformation rates (Wertz et al., 2006, Wagg et al., 2014) and nutrient availability (Maron et al., 2018), it plays a crucial role in predicting ecosystem functioning and soil C storage potential under different management practices.
Besides microbial diversity, the composition of the soil microbial community also affects soil C dynamics. In this context, microbial taxa are usually divided according to their functions and the difference between soil bacteria and fungi is especially important (Waldrop et al., 2000, Lin et al., 2020). Fungi have a greater capability to degrade lignin, cellulose, and other recalcitrant organic C (Crowther et al., 2012), whereas bacteria generally prefer labile organic matter such as sugars and fats (Paterson et al., 2008; Zheng et al., 2018). These preferences are reflected in the extracellular enzymes released by either microbial group; bacteria are the main producers of cellulase (e.g. β-1,4-glucosidases, β-1,4-xylosidases and β-1,4-D-cellobiohydrolases) targeting labile C pools, whereas fungi are the main producers of non-specific ligninase (e.g. peroxidases and polyphenol oxidases) depolymerizing recalcitrant C pools (Chen et al., 2020a). Recent evidence suggests that soil C stocks across tillage practices are correlated with the abundance of bacterial groups, but not fungal groups (e.g. Sun et al., 2018). Whereas a recent meta-analysis showed that conservation tillage practices do not affect fungal-to-bacterial biomass ratios on average (Chen et al., 2020b). Hydbom and Olsson (2021) found conservation tillage had a positive effect on arbuscular mycorrhizal fungi but not on saprotrophic fungi and bacteria. This suggests that treatment effects are context dependent, and underline the need to study the relation between SOC and microbial composition across a range of management practices and environmental conditions.
Recent studies have used network analyses (Weiss et al., 2016) to identify taxonomic levels and keystone microbial groups in soils (e.g. Lupatini et al., 2014). These analyses suggest that keystone microbial groups are affected by tillage frequency and straw management, with possible consequences for soil C dynamics. For instance, Banerjee et al. (2016) found that straw amendment favor keystone taxa such as Acidobacteria, Frateuria, and Gemmatimonas in bacteria and Chaetomium, Cephalotheca, and Fusarium in fungi, which had strong associations with organic matter decomposition rates. Lin et al. (2019) identified Thermogemmatisporales as the most important keystone taxa across various straw and manure treatments and found that this taxon decreased with an increase in SOC content. Li et al. (2021b) indicated that several bacterial taxa (e.g. Acidobacteria and Bacteroidetes) were closely related to turnover of specific organic components during straw decomposition under different tillage practices. Using a similar approach, several studies linked specific fungal and bacterial taxa to organic matter decomposition and transformation in agricultural soil (Banerjee et al., 2016, Li et al., 2017). These preferences of bacteria and fungi for specific residue compounds may affect long-term soil C dynamics (Frey et al., 2003, Fontaine et al., 2011). However, the role of keystone taxa in SOC accumulation itself remains to be explored.
The Loess Plateau, a semi-arid region in northwest China, suffers from severe soil erosion and soils in this area are not conducive to soil C sequestration (Zhang et al., 2014a). In addition, conventional tillage practices have caused a decline in SOC stocks across the Loess Plateau (Chen et al., 2009, Hou et al., 2013). Conservation tillage practices have often been proposed to stimulate soil C sequestration in this area (Zhang et al., 2014b, Wang et al., 2018), but the relation between soil microbial traits (i.e. diversity, composition and keystone taxa) and SOC accumulation is still unclear. Thus, our objectives in this study were: (i) to evaluate the impact of tillage managements on the soil environment (e.g. C and N properties), microbial community traits, microbial activity, and SOC accumulation rate (SAR); (ii) to identify keystone taxa and assess the links between soil microbial community traits and SAR.
Section snippets
Site description and experimental design
A long-term field experiment was established in 2003 at the Dryland Farming Experimental Station, which is located in Shou Yang, Shanxi Province (113°10′’E, 37°90′’N, 1100 m. ASL), on the Loess Plateau in northeast China. The site has a continental monsoon climate, with mean annual evaporation of 1700–1800 mm (Wang et al., 2019), mean annual precipitation of 483 mm, and a mean annual temperature of 7.4 ℃ (Li et al., 2021c). Fig. S1 showed the detailed rainfall and mean daily temperature data.
Soil properties and SAR
Soil properties differed among the three treatments (Table 1). Compared to CT-RR, both RT-RI and NT-RM significantly increased SOC and MBC contents at 0–10 cm, whereas only RT-RI increased SOC and MBC contents at 10–25 cm. The TN contents were similar for the three treatments at 0–10 cm, and the highest TN contents were observed under RT-RI at 10–25 cm. Soil concentrations also differed significantly among treatments at both soil depths and ranged from high to low as follows: RT-RI
SAR in response to tillage practices
Soil C accumulation reflects the net balance between organic matter inputs (e.g. crop residue and rhizodeposition) and soil C decomposition by soil microbes (e.g. Meng et al., 2017). Compared with CT-RR, RT-RI increased SAR at both soil depths, and NT-RM increased SAR at 0–10 cm (Table 1). These findings corroborate numerous studies suggesting that conservation tillage practices with residue retention stimulate SOC sequestration compared to conventional tillage without residue retention,
Conclusions
In summary, our study showed that RT-RI and NT-RM significantly increased SAR in the 0–10 cm soil layer relative to CT-RR, likely due to a combination of reduced soil disturbance and increased input of crop residue. Network analysis suggested that NT-RM resulted in a more stable bacterial network compared to conventional practices, and both RT-RI and NT-RM produced a more stable fungal network. Moreover, several bacterial and fungal keystone taxa and microbial diversity correlated positively to
Declaration of Competing Interest
The authors declare that there are no competing interests
Acknowledgments
This work was supported by the following items: the Ministerial and Provincial Co-Innovation Centre for Endemic Crops Production with High-quality and Efficiency in Loess Plateau (SBGJXTZXKF-02), the Science and Technology Project (2015BAD22B03), the National Key Research and Development Program of China (2018YFD0200408) and the China Scholarship Council (File no. 202003250127). We also thank the editors and all anonymous reviewers for improving the manuscript.
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