Coexistence of vascular plants and biocrusts under changing climates and their influence on ecosystem carbon fluxes

Vascular plants and biocrusts are highly vulnerable to climate change in drylands wherein they control carbon (C) cycling. In drylands, these organisms are known to thrive alone or coexisting with each other. Yet, how multiple combinations of biocrusts and vascular plants influence C cycling remains poorly understood. Here, we conducted a mesocosm field experiment in the Chinese Loess Plateau to investigate the influence of six contrasting microsites (bare soil, biocrust, shrub alone, shrub with biocrust, grass alone, and grass with biocrust) on ecosystem C fluxes, including changes in net ecosystem exchange (NEE), ecosystem respiration (R_e), and gross primary productivity (GPP). We also analyzed the influence of environmental factors (e.g., water and solar radiation) on ecosystem C fluxes across different microsites. Finally, the annual (2022.6–2023.6) NEE was simulated and estimated based on a random forest model to quantify the contributions (net C uptake or emissions) of biocrusts and vascular plants and their coexistence to C budgets in drylands. Our results showed that biocrusts and vascular plants largely regulate C fluxes in this dryland, and more importantly, that the manner in which these biotic components are combined, strongly influence the outcomes for C fluxes. In particular, we showed that microsites of biocrusts, vascular plants, and their coexistence increased GPP and R_e by 1.2–6.1, 1.5–56.2, and 1.1–50.0 times, respectively, compared to bare soil microsite. All these microsites supported a net C uptake (–0.31 to –10.84 μmol m^–2 s^–1) except from bare soil, which was net C emission (+1.39 μmol m^–2 s^–1). However, we also found that compared to vascular plant microsites, biocrust-vascular plant coexistence reduced NEE, R_e, and GPP by 21 %–29 % (closer to zero), 39 %–40 %, and 12 %–33 % respectively, suggesting some sort of competition among biotic components. Also, annual NEE was 37 %–159 % (closer to zero) lower at biocrust-vascular plant coexistence compared to biocrusts or vascular plants thriving alone. Correlation analysis results showed that temporal variation in C fluxes of biocrusts, vascular plants, and their coexistence were mainly driven by soil water content and photosynthetically active radiation. In summary, our work showed that vascular plants and biocrusts are key drivers of C cycling in this dryland, and further provide novel insights that considering the different biotic components of these drylands alone and in combination is critical to finetune our measurements for C fluxes in a context of climate change.