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.