Soil in the city or on a field for farming is subject to a variety of changes, including changes related to human influence. How many natural and anthropogenic factors might act on soil? There might be increasing temperature, drought, the presence of microplastics, various pesticides, heavy metals, salinity, atmospheric nitrogen deposition – a wide range of factors with different modes of action and different effects. The circumstances are very difficult to capture in experiments because such a study would have to include numerous factors and all combinations of those factors. For example, an experiment examining the effect of ten interacting factors would have over a thousand different treatment combinations. Such experiments are not possible in ecology.

For this reason, soil ecologists have until now mostly studied the effects of one or two factors at a time. “About 99% of the more than one thousand papers we screened for this study only examined effects of one or two factors on soils, with very few experiments looking at more factors,” explains Dr. Anika Lehmann, a member of Professor Rillig’s team at Freie Universität Berlin.

Rillig and his team examined the effects of ten factors of global change in a microcosm experiment. First, they examined each factor on its own. Then, the team looked at the effects of including more and more factors, and they did this by randomly adding factors from the set of ten factors. This way they could ask: what is the effect of just the number of factors, rather than what they actually are. The results were quite surprising given the wide range of factors used. For all the responses measured using parameters related to soil carbon storage and biodiversity of fungi, there was a clear trend with the number of factors included: soil functions and biodiversity declined when the number of factors increased. This means that the direction of effects could be predicted simply by knowing how many factors were acting on the soil and its organisms, irrespective of what those factors actually were. In order to predict the actual effect, it is still helpful to know which factors were involved, but as more and more factors came into play, it was nearly impossible to predict responses precisely. Nevertheless, the direction of effects was clear.

“The consequences of these findings are a bit sobering, but there is also some good news here,” explains Rillig. First, it means that we are currently still quite ignorant of what might actually happen with global change, with increasing factors acting on ecosystems. In addition, there may be some surprises in store: for example in the experiment, soils became water-repellent when many factors were taken into account, something that was not at all apparent from looking at the single-factor results. On the other hand, perhaps this also means that every little bit helps. Every factor that we eliminate or reduce could potentially help soils and ecosystems. Moreover, some factors are interrelated, such as climate change factors or pesticide use, which means that by reducing at least some portion of the factor (by changes in behavior or suitable policy measures), others could be reduced as well. “What has become very clear from our study: we need to rethink global change biology with a focus on the multitude of factors and their interactions,” says Rillig.

Publication

Rillig MC, Ryo M, Lehmann A, Aguilar-Trigueros CA, Buchert S, Wulf A, Iwasaki A, Roy J, Yang. 2019. The role of multiple global change factors in driving soil functions and microbial biodiversity. Science  DOI: 10.1126/science.aay2832

Contact

Prof. Dr. Matthias C. Rillig, Institute of Biology, Freie Universität Berlin, Tel.: +49 30 838 53165; Email: matthias.rillig@fu-berlin.de