Canopy gaps accelerate soil organic carbon retention by soil microbial biomass in the organic horizon in a subalpine fir forest
Introduction
Canopy gaps are small openings that form in the canopy of various forest types following the injury or death of one or more canopy trees; they generally occupy <0.1 ha in area (Yamamoto, 2000). Canopy gaps are a key component of the disturbance regime and old-growth character of subalpine coniferous forest in the upper reaches of Yangtze River (Taylor and Zisheng, 1988). According to gap dynamics theory (Yamamoto, 2000), mature forest communities are composed of a “mosaic” of patches at different stages of successional and compositional maturity. By increasing environmental heterogeneity and altering the distributions of abiotic and biotic resources, canopy gaps affect not only aboveground ecological process but also forest floor and belowground processes (Zhang and Zak, 1995, Bach et al., 2010, Wu et al., 2014). Consequently, gaps must be considered when examining ecological processes in old-growth forests.
The organic horizon is recognised as an important source of carbon (C) and nutrients for plants and plays an important role in maintaining forest productivity (Grayston and Prescott, 2005). According to the degree of litter decomposition (Takeda, 1988, Feng et al., 2006), the organic horizon can be divided into different components: of a fresh litter layer (LL), a fragmented litter layer (FL), and a humified litter layer (HL). The decomposition of leaf litter always starts with a high concentration of organic C, which decreases as the leaf material is incorporated into the FL and HL as well as the mineral soil layer (Grüneberg et al., 2013, Xu et al., 2014). The organic horizon plays crucial roles in nursing invertebrate and microbial biodiversity and conserving soil and water (Yang et al., 2007, Grüneberg et al., 2013). Soil microbial biomass, as a sensitive indicator of forest management practices (Bauhus and Barthel, 1995b), can be susceptible to microclimate variation such as thinning and canopy removal. Therefore, an understanding of biological and biochemical properties in the organic horizon is fundamental to understanding nutrient cycling in conjunction with associated microclimate variation.
Canopy gaps facilitate dramatic top-down trophic interactions between vegetation and soil microbe-mediated processes (Bauhus and Barthel, 1995a, Schmidt et al., 2007). Many studies of canopy gaps have concentrated on aboveground processes (Wu et al., 2013), whereas relatively few studies have addressed the belowground effects of canopy gaps, such as the influences of tree species (Kanerva and Smolander, 2007), treefall gap size or soil properties on soil microbial biomass (Arunachalam et al., 1996). However, whether gap creation alters microbial nutrient pools in surface organic horizons and how these pools relate to the total organic carbon (TOC), total nitrogen (TN) and total phosphorus (TP) are unknown. We hypothesized that gap creation will alter microbial nutrient pools and accelerate soil organic C retention by soil microbial biomass in the organic horizon. Limited information is presently available on the spatial and temporal dynamics of microbial pools and their influences on the responses of soil organic matter and nutrient flux to canopy gaps in subalpine fir forests. This study was designed (1) to determine the effects of canopy gaps on microbial C, microbial nitrogen (N) and microbial phosphorus (P) and their stoichiometric relationships in the organic horizon and (2) to investigate the microbial pools for TOC, TN and TP and the responses of stored TOC, TN and TP to canopy gaps.
Section snippets
Site description
The study site is located at the long-term subalpine forest ecosystem research station, Miyaluo Nature Reserve (31°14′–31°19′N, 102°53′–102°57′E, 2458–4619 m above sea level (a.s.l.)), Li County, Sichuan, Southwest China. The subalpine forest is located along the upper reaches of the Yangtze River on the eastern Tibetan Plateau, which is characterised by seasonal snow cover for half the year (Wu et al., 2010) and freeze–thaw cycles in the autumn and spring (Yang et al., 2007). The annual
Dynamics of temperature and snow cover
Based on our analysis of five seasons of monitoring data, annual average temperature and average snow depth each exhibited a decreasing pattern in the order gap center (5.0 °C, 21 ± 5 cm), canopy gap (4.6 °C, 13 ± 3 cm), expanded gap (2.8 °C, 7 ± 1 cm), and closed canopy (2.9 °C, 3 ± 1 cm) (Fig. 2, Fig. 3). There was no snow cover during the growing season.
Dynamics of MBC, MBN and MBP
Based on the three-way ANOVA results (Table 2), canopy gap position, season and organic horizon significantly affected MBC and MBP;
Microbial biomass response to canopy gaps
Our findings support the ideas that canopy gaps increased the MBC of the organic horizon and that they significantly affected MBC and MBP but not MBN (Table 2). If the pool of readily available C increases following forest disturbances such as cutting gaps, the microbial biomass could potentially retain C and nutrients, consistent with the findings of Bauhus and Barthel (1995b). The organic matter retention by soil microbial biomass was highest in the gap center, which can be expected to
Conclusions
Canopy gap creation promoted soil organic C retention by the soil microbial biomass in the organic horizon in a subalpine fir forest. Gaps accelerated C, N and P releases from the LL but inhibited their output from the HL. The microbial biomass of the LL responded rapidly and sensitively to canopy gaps, suggesting that soil temperature and snow cover, as governed by the canopy gap, are the most important factors affecting the microbial biomass pools in the organic horizon. The regeneration of
Acknowledgments
This study was financially supported by the National Natural Science Foundation of China (31570605 and 31570445). We would like to thank Min He, Qingui Wu, Wei He, Yeyi Zhao and other graduate students at the Institute of Ecology and Forestry, Sichuan Agricultural University, for assistance with field sampling and laboratory analyses.
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