Drought of early time in growing season decreases community aboveground biomass, but increases belowground biomass in a desert steppe

Background: Increasing drought induced by global climate changes is altering the structure and function of grassland ecosystems. However, there is a lack of understanding of how drought affects the allocation and trade-off of above- and belowground biomass in desert steppe. We conducted a four-year (2015-2018) drought experiment to examine the responses of community above-and belowground biomass (AGB and BGB) to manipulated drought and natural drought in the early period of growing season (from March to June) in a desert steppe. We compared the associations of drought with species diversity (species richness and density), community-weighted means (CWM) of five traits, and soil factors (soil Water, soil carbon content, and soil nitrogen content) for grass communities. Meanwhile, we used the structural equation modeling (SEM) to elucidate whether drought affects AGB and BGB by altering species diversity, functional traits and soil factors.Results: We found that drought reduced the species richness, and species modified the CWM of traits to cope with drought. Manipulated drought had the effect on soil water content, but not on soil carbon and nitrogen content. We also found that the experimental and natural drought decreased AGB, while natural drought increased BGB. AGB was correlated with species richness, density, plant height and soil water, while BGB was correlated with CWM of plant height, CWM of specific leaf area, CWM of leaf dry matter content, CWM of leaf nitrogen content, soil water, soil carbon and nitrogen content. The SEM results indicated that the experimental and natural drought indirectly decreased AGB by reducing species richness and plant height, while natural drought and soil nitrogen content directly affected BGB. Conclusions: These results suggest that species richness and functional traits can modulate the effects of drought on AGB, however natural drought and soil nitrogen determine BGB. Our findings demonstrate that the long-term observation and experiment are necessary to understand the underlying mechanism of the allocation and trade-off of community above-and belowground biomass.