The contributions of soil mesofauna to leaf and root litter decomposition of dominant plant species in grassland
Introduction
In terrestrial ecosystem, most net primary production is returned to soil as plant litter (Wardle et al., 2004). Litter decomposition is a key ecosystem process that regulates carbon sequestration and nutrient supply (Swift et al., 1979). Therefore, a better understanding of the relative contribution of the drivers of litter decomposition is important in predicting ecosystem process and function (Aerts, 2006). Litter decomposition is widely known to be driven by climate, litter quality and soil organisms (Bradford et al., 2016). Higher quality litter (i.e. lower C/N ratios and higher initial N concentration), for example, often decomposes faster (Swift et al., 1979). Litter quality has been considered the primary controlling factor of litter decomposition on a regional scale, while climate may be the predominant factor on a global scale (Cornwell et al., 2008). Recent models showed that climate and litter quality together explain about ~60–70% of litter decomposition rates (Parton et al., 2007). However, the extent to which soil fauna might contribute to litter decomposition remains unclear (Kutsch et al., 2009). The soil fauna may be play a key role in litter decomposition because, besides the direct effect (e.g., fragmentation and litter consumption), soil fauna can modify the structure and activity of microbial communities (Hättenschwiler & Gasser, 2005) and affect litter decomposition indirectly. Therefore, a better understanding of the total effects of soil fauna on litter decomposition will improve our capacity to predict changes in decomposition.
Recent studies showed that the effects of soil fauna on litter decomposition are generally positive (Garcia-Palacios et al., 2013; Kampichler and Bruckner, 2009; Lin et al., 2019), and the soil fauna might interact with litter quality to affect litter decomposition at the local scale (Bradford et al., 2002; Carrillo et al., 2011; Smith and Bradford, 2003). Empirical studies have shown that the higher litter quality, the greater the contribution of soil fauna to litter decomposition rates (Fugii et al., 2016; Fujii et al., 2018; Schädler and Brandl, 2005). Other researches found that soil fauna can facilitate the decomposition of recalcitrant litter (Garcia-Palacios et al., 2013; Milcu and Manning, 2011; Yang and Chen, 2009). Furthermore, litter decomposition exhibits not only spatial heterogeneity but also temporal heterogeneity. Litter decomposes rapidly in the beginning, and as it ages the decomposition rate declines (Harmon et al., 2009). The predominant controls over decomposition change with time (Chapin III et al., 2011), thus, the contribution of soil fauna to decomposition may also vary with decomposition stages (Fujii et al., 2016; Garcia-Palacios et al., 2016).
Our understanding of soil fauna effects on litter decomposition in grassland is largely based on studies of aboveground leaf litter (García-Palacios et al., 2013; González and Seastedt, 2001). However, grassland are strongly subjected by human management (e.g. grazing or mowing), resulting in higher carbon (C) inputs from roots than that of above-ground inputs (Freschet et al., 2013; Gill and Jackson, 2000; Robinson, 2007). Meanwhile, the difference in litter quality and decomposition environment between above-ground leaf and below-ground root may result in the disparity in litter decomposition between different organs (Freschet et al., 2012; Zhang et al., 2008). Thus, it is necessary to examine whether soil fauna contribute differently to root and leaf litter decomposition, however, the related understanding in grassland ecosystem is fewer. In addition, Ma et al. (2016) showed that the mass losses for leaf and root litter were positively correlated only in the early stage, and became decoupled over time. Such inconsistent pattern of leaf and root litter decomposition may partly derive from the differential role of mesofauna in leaf and root litter decomposition over time. For example, in temperate forest, the presence of soil mesofauna increased leaf microbial respiration rates in the early decomposition stage, but did not affect root microbial respiration rates during the experiment (Fujii et al., 2016). Therefore, it is necessary to simultaneously examine the mesofaunal contribution to leaf and root at different decomposition stages in grassland.
Previous studies have examined the independent effects of plant litter quality, plant organ and decomposition stage on litter decomposition and faunal contribution thereto, but few field studies have considered the interactions among these factors in grassland. In our grassland, perennial dominant species are the most productive organisms, thus the litter decomposition of perennial dominant species is often a main determinate in ecosystem processes (Grime, 1998; Koukoura et al., 2003). To fill this research gap, a field leaf and root litter incubation experiment at different decomposition stages was conducted with soil fauna exclusion treatments, using two dominant species from different functional groups in an upland grassland in Southwest China. We focused on mesofauna (e.g. Collembola and Oribatida), the most abundant soil fauna in most terrestrial ecosystems (Bardgett and Cook, 1998; Petersen and Luxton, 1982). Litterbags with different mesh sizes were used to control the accessibility of the litter to soil mesofauna (Bao et al., 2015; Wang et al., 2009). Here, we test the following hypotheses: (1) higher annual decomposition rates and mesofaunal contribution thereto in relatively higher-quality litter species and leaf (lower in C:N), leading to amplification of effects of litter quality and organ on decomposition due to the preference of soil fauna for high-quality food resources. (2) Mesofaunal contribution to litter decomposition are relatively more important in the early litter decomposition stage with more labile substrates stimulate faunal activity, and the temporal disparity will influence the roles of mesofauna in leaf and root litter decomposition.
Section snippets
Site description
The experimental site was selected in the Nanshan artificial grassland, Guangxi Province, P. R. China (23°48′-25°58′N, 110°26′-110°40′E). The mean altitude is 1760 m a.s.l. The local climate is subtropical humid monsoon with a mean annual temperature of 11.7 °C. The diurnal temperature difference is 5–10 °C. Mean annual precipitation is 1862.5 mm with mainly concentrated in May–June. Mean relative humidity is 87% (Nanshan Climate Station, Hunan Province; Li et al., 2012). Soils at the site are
Effects of litter quality, litter organ and soil mesofauna on litter mass loss
The initial litter qualities were largely different between litter organ (i.e., leaves and roots) and litter species (i.e., L. perenne and T. repens) (Table 1). T. repens litter exhibited relatively higher quality (i.e. lower C/N ratio and higher nutrient content) for both leaf and fine root, and the leaf litter quality was generally higher than root litter for both L. perenne and T. repens (Table 1). Annual litter mass loss was significantly higher in higher-quality T. repens than in
Discussion
The goal of our study was to examine the contribution of soil mesofauna to leaf and root litter decomposition for different substrate quality. We found the presence of mesofauna amplified the effect of litter quality on decomposition, but reduced the differences in leaf and root annual litter decomposition due to the stronger contribution of mesofauna to root litter, leading to a partly contradiction to our hypothesis 1. Furthermore, our results support the hypothesis 2 that the mesofaunal
Conclusion
We demonstrated that the presence of soil mesofauna made a significant positive contribution to litter decomposition and the contribution was larger in higher-quality litter than in lower-quality litter, indicating that soil fauna enlarged the contribution of litter quality to decomposition because of their preference for high-quality, palatable food resources. The larger contribution of mesofauna to decomposition rate in fine root litter and higher inputs of root in grassland, suggesting there
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgments
This project was supported by the National Undergraduates' Innovation and Entrepreneurship Training Program (201910596011), and the Scientific Research Foundation of Guilin University of Technology (GUTQDJJ2018055; GUTQDJJ2018007).
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