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