1. Introduction
Global climate models predict increases in atmospheric carbon dioxide (CO2) concentration and surface temperature (IPCC, 2013), which are likely to alter regimes of global hydrologic cycling, including an increase in the number and severity of droughts and heavier precipitation between drought periods (Dai, 2013; Donat et al., 2016). Soil moisture is a key factor controlling the microbial decomposition of soil organic carbon (SOC) (Moyano et al., 2013). When the soil is dry, microbial activity and substrate diffusion will be restricted, and soil respiration will be reduced (Schimel, 2018). The rapid precipitation after drought usually increases mineralization of SOC, creating a large pulse of CO2(Birch, 1958; Kim, et al., 2012). The CO2 pulse has been attributed to the rapid consumption of microbial necromass as available substrates and released microbial osmoregulatory substances in response to water stress (Blazewicz et al., 2014; Chowdhury et al., 2019; Warren, 2016). In addition, the sudden influx of water at rewetting can expose previously inaccessible carbon (C) to microorganisms through aggregates breakdown (Denef et al., 2001; Najera et al., 2020; Schimel et al., 2011). This rewetting-driven CO2 pulse can be sustained for more than 20 days (Canarini et al., 2017) and elevated by as much as 475% relative to a constant-moisture soil (Fierer and Schimel, 2003). Therefore, soil drying-rewetting cycles (DWC) are considered an essential environmental factor regulating C cycle in terrestrial ecosystems (Borken and Matzner, 2009; Fierer and Schimel, 2002; Muhr et al., 2008; Schimel, 2018; Zhu and Cheng, 2013).
Although a large number of studies have examined the effects of DWC on soil C decomposition, DWC increased (Butterly et al., 2009; Miller et al., 2005; Shi and Marschner, 2014; Xiang et al., 2008; Yemadje et al., 2017), decreased (Shi and Marschner, 2014), or did not change (Yemadje et al., 2017) cumulative C decomposition relative to constant-moisture control in previous studies. Zhang et al. (2020) indicated in a recent meta-analysis that relative to a constant-moisture control (with the same mean value), the rewetting-driven CO2 pulse can fully compensate the reduced CO2 emission during the drying phase, thus DWC did not change cumulative C decomposition. They also commented that the changes in cumulative C loss depend on DWC intensity. Intensified drought usually triggers a stronger CO2 pulse after rewetting (Barnard et al., 2015; Li et al., 2018), because a severer drought may lead to a higher frequency of microbial death and accumulate more compatible solutes, which can contribute largely to the CO2 pulse (Barnard et al., 2020; Guo et al., 2012). Moreover, there may be a drought threshold, beyond which the inaccessible C will be accessed, thereby contributing to the subsequent CO2 pulse (Canarini et al., 2017; Homyak et al., 2018). Moreover, DWC can promote the release of old C that had been occluded in soils for more than 600 years (Schimel et al., 2011). However, due to the difficulties in separation of decomposition of SOC pools with different turnover times, few studies have evaluated the vulnerability of different SOC pools to DWC. Therefore, due to the large proportion of stable C in the total soil C pool, clarifying its response to climate change is essential for predicting future changes in the global C cycle.
Soil organic carbon can be divided into three pools with different physiochemical characteristics and turnover times (Davidson and Janssens, 2006; Lin et al., 2015). The most active fraction is annually cycling, active SOC, which mainly consists of microbial biomass and plant detritus, and has a fast turnover time from weeks to years, contributing to approximately 0~5% of the total SOC pool (Parton et al., 1987; Semenov et al., 2018). It hardly contributes much to the global C cycle due to its small size and very fast turnover time. The most inert fraction is millennially cycling, passive (inert) SOC, which is mainly composed of black carbon or humic substances and has a turnover time even more than centuries, accounting for about 10~40% of the total SOC pool (Schmidt et al., 2011). Furthermore, the more critical pool is the decadally cycling, relatively resistant (intermediate, slow) SOC, which is the dominant component of the total SOC pool, accounting for around 60~80% of the total SOC pool, and has a turnover time of decades. It is generally believed that different microbial communities use different soil substrates (Berg and McClaugherty, 2008; Xu et al., 2015), and bacteria are considered more responsible for consuming readily available substrates (Moore-Kucera and Dick, 2008), while fungi are believed to have the ability to decompose more stable SOC (Xu et al., 2015). Fungi display an overall higher resistance to drying and subsequent rewetting processes than bacteria (Barnard et al., 2015; Yuste et al., 2011). As a consequence, relatively resistant SOC may be more susceptible to DWC due to the higher resistance of fungi to drought and thus the smaller reduction in C decomposition during the drying process (de Vries et al., 2018; Zhang et al., 2020).
Furthermore, previous conditions can affect current processes, which is called the legacy effect (Monger et al., 2015). The DWC-driven legacy effect can compensate for 14% of the decrease of cumulative C decomposition during the DWC period (Li et al., 2018). However, the legacy effect of DWC on the decomposition of different SOC pools remains unclear. Thus, the direct and legacy effects of DWC on SOC pools with different turnover times remain uncertain and require further investigation.
To fill this knowledge gap, we use soils from three plots in a long-term experimental field to explore the direct and legacy effects of DWC on active and relatively resistant soil C decomposition. In our study, the direct effect represents the effect of DWC on C decomposition in the DWC period, and the legacy effect denotes the effect of previous DWC on C decomposition in the post-DWC incubation period with constant moisture. We conducted a laboratory incubation experiment of 128 days, combining three water regimes, nine 10-d DWC (in a row) and one 28-d extended period, to investigate the direct and legacy effects of DWC on active and relatively resistant soil C decomposition. We hypothesized that (i) compared to the constant-moisture control, the DWC with intensified drought would stimulate more C emission during the entire drying-rewetting period due to the strong CO2 pulse after rewetting; (ii) the relatively resistant SOC (the bare-fallow and the bare-fallow+incubation soils) would be more vulnerable to DWC than the active SOC (the old-field soil) due to the higher resistance of fungi in the relatively resistant SOC to DWC; and (iii) the DWC could create a legacy effect on soil C decomposition due to the changes in microbial biomass and labile substrates.
2. Materials and methods