Introduction
Soil stores twice carbon (C) more than the atmosphere (Jobbágy and Jackson, 2000; Lal, 2004; Scharlemann et al. , 2014), and the decomposition of soil C (also called as soil microbial respiration, MR) releases 6-10 times more CO2 into atmosphere than the current levels of fossil fuel consumption every year (Boden et al. , 2009; Friedlingstein et al. , 2021). Thus, soil C cycling plays critical role in regulating global C budget and atmosphere CO2 concentration (Bond-Lamberty et al. , 2018; Friedlingstein et al. , 2021). Moreover, large amount of studies has confirmed that MR is critically sensitive to current climate change, especial rising temperature (Davidson and Janssens, 2006; Bond-Lamberty and Thomson, 2010; Bradford et al. , 2021). The ongoing global warming may potentially accelerate soil C loss (Friedlingstein et al. , 2021) and thus makes soil a large C source to atmosphere in the future (Davidson and Janssens, 2006; Bond-Lamberty and Thomson, 2010; Bond-Lamberty et al. , 2018). Quantifying the variations and drivers of MR and its response to rising temperature (also called temperature sensitivity, Q 10) is a high priority in order to better model and predict terrestrial C cycle under global warming (Zhou et al. , 2009).
Elevational gradients are ideal platform to study the response of soil C cycling under climate warming (Kong et al. , 2022). Different elevational gradients cause various climate levels, such as temperature and precipitation (He et al. , 2021; Kong et al. , 2022), but with similar soil parent material and plant species pool. Therefore, elevational gradients could provide more realistic insight in the underlying mechanism driving MR and Q 10 (Conantet al. , 2011; Longbottom et al. , 2014). Various studies found that MR declined along the elevational gradients (Garten and Hanson, 2006; Gutiérrez-Girón et al. , 2015), since warmer soils in low elevation contribute to more active soil microorganisms and high decomposition rates (Gutiérrez-Girón et al. , 2015). In contrast, there is also study suggesting that elevation positively affects MR (Kong et al. , 2022), due to high soil C concentration at high elevation that offset the negative effect of low temperature. Therefore, the net elevational effect on MR depends on the tradeoff between climate and respirated substrate along the elevation. More studies are still needed to figure out this question and its regional characteristic.
Lab incubation is a commonly used method to determine the microbial respiration, which usually incubates multiple soil samples at the same time under the same temperature or temperature range (Ding et al. , 2016; Liu et al. , 2017; Li et al. , 2020; Zhanget al. , 2022). The unified incubation temperature might be too high to samples from cold sites, while too low to samples from warm sites (Li et al. , 2020). However, it’s critical difficulty to set a specific incubation temperature for each soil samples in the lab incubation. Instead, recent studies used an adjusted MR by the field temperature of each site based on the unify incubation temperature, which easily solve the difference between field temperature and incubated temperature (Li et al. , 2020).
Temperature sensitivity (Q 10) of MR also serve as a reference for how regional C pools may respond to future warming (Davidson and Janssens, 2006). To date, there is still no consistent elevational trend of temperature sensitivity along the elevational gradients. Several studies suggest that high elevation increasesQ 10 (Gutiérrez-Girón et al. , 2015; Konget al. , 2022; Okello et al. , 2022; Zeng et al. , 2022), while others reported higher Q 10 at lower elevations (Lipson, 2007), or no significantly elevational trend (Schindlbacher et al. , 2010; Xu et al. , 2014; Wanget al. , 2016; Zuo et al. , 2021). This indicates that the current understanding of the elevational effect onQ 10 is not comprehensive enough.
To further quantify the underlying mechanism in driving MR andQ 10, we applied a standardized sampling along 9 elevational gradients from 400 m to 1100 m in a subtropical forest in South China. These soil samples were incubated in the lab to determine MR and Q 10. Our objectives were to examine the response of MR and Q 10 to the environmental change induced by elevational gradients in the subtropical forest, and then quantify their main drivers. We hypothesized that: 1) high elevation reduces MR but increase Q 10, and 2) the varied MR and Q 10 along elevation would be largely explained by the soil and plant community structure and environmental change induced by the elevational difference.