There is increasing evidence that ecosystems are threatened by an increasing number of co-occurring multiple anthropogenic pressures worldwide. For instance, agroecosystems simultaneously encounter nutrient eutrophication, warming, drought, synthetic chemical pollutants and soil compaction, or at least some combination of these pressures¹. It is urgent to know the effects of multiple pressures on ecosystem functions. However, it is difficult to set up a factorial experiment with multiple pressures. If ten pressures were measured and each pressure had two levels (ambient and pressure), a factorial design will lead to 2¹⁰ = 1024 experimental units. This is an incredible number of units for a single experiment.

Prof. Matthias C. Rillig, my former postdoctoral tutor, was inspired by classic plant diversity experiments². Following this experiment, we created a gradient of the number of multiple pressures by randomly selecting pressures from a pool of ten anthropogenic pressures. This design allows us to investigate a general consequence of how the number of multiple pressures influences ecosystem functions. Employing such an experimental design, we found that the combination of multiple simultaneous pressures exerted negative effects on soil functions³. Interestingly, these effects were stronger than the sum of effects of single pressure, which are called “ecological surprises” or synergistic effects. This conclusion was then confirmed by a study testing plant growth and survival under a gradient of multiple pressures⁴.

After the publication of our previous study³, we had a brainstorm to see what we could do next. Since my doctoral study, I have been interested in how soil microbes stabilize plant biomass production in grasslands, with the main focus on the stabilizing effects of plant and soil microbial diversity. Therefore, an idea came to mind: whether and how does biodiversity stabilize ecosystem function under multiple pressures? Biodiversity is critical for underpinning ecosystem functions, yet we do not know whether biodiversity can promote ecosystem functions under multiple anthropogenic pressures.

Everyone liked this idea, and then we started to prepare this experiment. First, which model system will we use? We decided to use a soil microcosm system, just like our previous studies. Microsystems are easy for us to manipulate and conduct disturbance. Second, we need a gradient of biodiversity in order to determine the biodiversity effect. Because we focused on soil systems, the dilution-to-extinction approach was used to create the high and low soil microbial diversity treatments. This approach will lead to the first loss of rare species during dilution, which can simulate a realistic loss of biodiversity. How many levels of soil microbial diversity do we need? If there were three levels, the total experiment units would be three times more than that in our previous study. With important suggestions from our coauthors – Matthias and Masahiro, two levels (low and high soil microbial diversity) were used, as we may compare the response trend of variables along the number of global change factors within the high and low soil microbial diversity treatments. In other words, we replicated our previous study in the high and low soil microbial diversity treatments. Fourth, how many replicates should we use? Since there was more variance in the combined multiple pressure treatments, we increased replicates, which led to a total of 370 experimental units. It took us seven days to set up this experiment, with help from Tingting Zhao, Yun Liang and Yaqi Xu.

We measured soil respiration rate, microbial abundance, enzyme activity related to nutrient cycling, physical properties, bacterial and fungal community composition and diversity. How could we show the effect of soil biodiversity treatment on ecosystem functions along an increasing number of global change factor? We may use classic ANOVA analysis, while biodiversity effects could not be directly and nicely compared and visualized. Effect size of soil biodiversity treatment was calculated by Masahiro. When I came back to China, Dr. Xuefeng Zhu help to do microbiome analysis on the construction of soil microbial communities. Although these results were not presented, we were inspired by this analysis. Prof. Jing Xin and me completed the SEM analysis to show how an increasing number of global change factor affected soil functions, as required by one referee. Finally, our efforts were admired by referees. I am proud to organize this study and extend our collaborations with coauthors.

Here we find that soil microbial diversity contributed to ecosystem functions only when a few pressures (e.g., < 2) are active, and the positive effect of soil microbial diversity was decreased by numerous simultaneous pressures via altering soil microbial abundance and community composition. In other words, soil microbial diversity cannot buffer the negative effects of the co-occurrence of multiple pressures.

Our study highlights that reducing the number of anthropogenic pressures should be considered for achieving ecosystem sustainability, in addition to biodiversity conservation. One goal of biodiversity conservation is to sustain the provision of ecosystem functions (services). However, it is still less known the context dependency of effects of biodiversity on ecosystem functions. Here, we show that biodiversity effects could be reduced or even eliminated when there are many global change factors, because of the low abundance of organisms. Therefore, reducing the number of global change factors should be another goal for ecosystem sustainability. However, our study was conducted in small and simple ecosystems. It is interesting to test whether our response trend could be confirmed in complex ecosystems.

 References

1. Riedo, J. et al. Widespread occurrence of pesticides in organically managed agricultural soils—the ghost of a conventional agricultural past? Environ. Sci. Technol. 55, 2919-2928 (2021).
2. Weisser, W. W. et al. Biodiversity effects on ecosystem functioning in a 15-year grassland experiment: Patterns, mechanisms, and open questions. Basic and Applied Ecology 23, 1-73 (2017).
3. Rillig, M. C. et al. The role of multiple global change factors in driving soil functions and microbial biodiversity. Science 366, 886-890 (2019).
4. Zandalinas, S. I. et al. The impact of multifactorial stress combination on plant growth and survival. New Phytol. 230, 1034-1048 (2021).