4.1. Direct effects of DWC on soil carbon mineralization
As we hypothesized, the strong DWC with intensified drought had a
stronger impact on C emission, showing that compared to the
constant-moisture control treatment (60% WHC), the 100-20% WHC
treatment decreased cumulative C mineralization during the DWC period in
OF and BF soils and increased it in BF+ soil, while the 80-40% WHC
treatment did not significantly change the C emission of all three soils
(Fig. 4a). This is because the 80-40% WHC treatment might not cause as
much osmotic shock and cell lyses as the strong DWC treatment (100-20%
WHC in this study) during the drying period
(Guo
et al., 2012). Also, the 80-40% WHC treatment might not reach the
drought threshold, and the accessible C from physical and physiological
mechanisms after the mild drought (to 40% WHC) may not contribute much
to the subsequent CO2 pulse
(Barnard
et al., 2020; Slessarev et al., 2020).
Although the 100-20% WHC treatment decreased cumulative C
mineralization during the entire DWC period (0-90 d) in OF and BF soils,
this reduction in C emission of the 100-20% WHC treatment only occurred
in two cycles and did not significantly affect C mineralization in other
cycles (Table S2). This result was consistent with a meta-analysis of
Zhang
et al.
(2020),
which showed that the rewetting-driven CO2 pulse can
fully compensate the reduced CO2 emission during the
drying period.
For BF+ soil, the 100-20% WHC treatment increased cumulative C
mineralization by 16% relative to the mean constant-moisture control
treatment (Table S2). Since few active SOC in the BF+ soil was left
after 25-year BF treatment and 815-d laboratory incubation, the
microbial community in the BF+ soil is more dominated by fungi (Fig.
5c). Fungi are considered more resistant to drought and osmotic stress
than bacteria due to their multicellular hyphal networks allowing remote
access to water in the soil
(Barnard
et al., 2013). Therefore, the reduction in C mineralization of the soil
with more fungi (BF+ soil) during the drying period may be smaller, and
the cumulative C mineralization may be higher during the whole DWC
period compared to the soil dominated by bacteria (OF soil). Moreover,
DWC can result in a microbial composition shift toward more desiccation
tolerant. For example, sensitive microorganisms die or become inactive
during drought and are difficult to recover, thereby supplying
substrates for more drought-tolerant surviving microorganisms
(Meisner
et al., 2021).