Plant diversity legacy interacted with drought legacy and
recurrent drought
Our results showed that both the BE and CE were greater in plant
communities with a drought legacy (Fig. 4d and e), and the CE was the
most pronounced in the Drought-Ambient treatments and in the highest
plant richness level, while there was an opposite pattern for the SE,
reflecting the interaction between drought and plant diversity legacy
and subsequent drought. There were two potential, and not mutually
exclusive, mechanisms for these drought legacy effects on the CE.
Firstly, drought stress can favor specific soil pathogens and lead to a
breakout after rewetting (van der Putten et al. 2016; Barneset al. 2018). Increasing susceptibility of plants to soil
pathogens during drought stress allows the pathogen population to
accumulate in monocultures rather than high-richness mixtures (Preeceet al. 2019). Some soil pathogens might go dormant when facing
unfavorable conditions and such a strategy allows them to quickly
recover and occupy empty soil niches after rewetting (Schimel 2018;
Crawford & Hawkes 2020). Secondly, post-drought soil has higher
nutrient concentrations than soils without historical drought due to a
pulse of mineralization following rewetting through the reactivation of
microbial activity and enhanced decomposition rates of dead materials
(Bloor & Bardgett 2012; Leitner et al. 2017), thereby promoting
plant growth. Our results likely support the first hypothesis, because
we added soil inoculums into background soils (1: 10 in volume ratio)
that reduced the effects of nutrient differences, and plant monoculture
yields were lower in the Drought-Ambient than Ambient-Ambient treatments
(Fig. 3b). The negative SE occurred in the Drought-Ambient treatments
and the highest plant richness level, and this was due to increasing
dominance of a small-size species (i.e., D. aegyptium ) in
mixtures (Fig. S3). Previous work suggest that grass species have more
negative PSFs than forbs (Kulmatiski et al. 2008). In our system,
grass species might exhibit negative PSFs in monocultures but largely
escape it in mixtures due to efficient dilution effects of
species-specific soil pathogens.
We found that drought soil legacy was diminished by subsequent drought.
For example, the CE and SE were less pronounced in the Drought-Drought
than Drought-Ambient treatments, and the CE and SE were consistent in
the strength and direction between the Ambient-Drought and
Drought-Drought treatments (Fig. 4e and f). Subsequent drought simply
slowed down soil processes and limited microbial activities (Bever 2002;
Schimel 2018), thereby diminishing the soil legacy effect. Our findings
illustrated that historical and subsequent drought events had different
directional effects on plant diversity-productivity relationships, and
they can interact to regulate net diversity effects and its components
(i.e., complementarity and selection effects). Future rainfall changes
are predicted to generate complicated patterns in biodiversity-ecosystem
functioning (i.e., community productivity, ecosystem carbon cycling,
soil mineralization and others) relationships through shaping soil
microbial community structure and activity.