Review

The global soil community and its influence on biogeochemistry

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Science  23 Aug 2019:
Vol. 365, Issue 6455, eaav0550
DOI: 10.1126/science.aav0550

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Microbes' role in soil decomposition

Soils harbor a rich diversity of invertebrate and microbial life, which drives biogeochemical processes from local to global scales. Relating the biodiversity patterns of soil ecological communities to soil biogeochemistry remains an important challenge for ecologists and earth system modelers. Crowther et al. review the state of science relating soil organisms to biogeochemical processes, focusing particularly on the importance of microbial community variation on decomposition and turnover of soil organic matter. Although there is variation in soil communities across the globe, ecologists are beginning to identify general patterns that may contribute to predicting biogeochemical dynamics under future climate change.

Science, this issue p. eaav0550

Structured Abstract

BACKGROUND

Soil is the largest repository of organic matter on land, storing ~1500 Gt carbon, which is at least as much as the vegetation (~560 Gt) and atmosphere (~750 Gt) combined. The turnover of this organic material (the rate at which it enters and leaves the soil) is governed by the most diverse community on Earth. By determining the rate and biochemical pathway of organic matter processing, fungi, bacteria, archaea, animals, and protists regulate soil fertility, plant growth, and the climate. Given their roles in regulating the exchanges of elements between terrestrial and atmospheric pools, the effective management of this soil community is among our most powerful weapons in the fight against the global threats of biodiversity loss and climate change. However, despite the critical importance of these organisms, the hyperdiverse nature of local soil communities has traditionally obscured efforts to identify general global patterns. As such, environmental factors have traditionally been used as proxies to represent the variation in soil functioning across landscapes. But it is the organisms—not only the environment—that directly drive the turnover of organic material. Given that different organisms have varying impacts on elemental cycling, exploring the functional biogeography of soil communities is likely to be critical for improving confidence in global biogeochemical model predictions.

ADVANCES

Over the past decade, a growing body of evidence highlights that regional differences in the soil community drive considerable variation in biogeochemistry. Just as the transition from forests to grasslands drive vast differences in ecosystem functioning, differences in the structure of soil communities can drive enormous variation in elemental cycling. By expanding our horizons to see beyond the complexity of local soil communities, ecologists have begun to identify general patterns in the biomass, composition, and diversity of soil communities. Despite the immense diversity of these organisms, the global soil community appears to be dominated by a manageable number of groups, which are likely to play a prominent role in the regulation of soil biogeochemistry. The metabolic activity and species richness of most soil organisms generally increase toward warm, moist tropical regions, where rapid elemental cycling depletes soil carbon relative to the higher latitudes. In addition, the huge accumulation of organic matter stocks in cold Arctic and sub-Arctic regions leads to huge abundances of soil microbes and animals at high latitudes. These global trends reveal key insights into the biological mechanisms that drive the distribution of organic matter on land as well as the vulnerability of different carbon stocks to future global change. Each new layer of global ecological information reveals distinct biogeographic patterns that provide insights into the fundamental distribution and dynamics of organic matter on land.

OUTLOOK

The field of soil ecology continues to uncover critical mechanisms that govern the turnover of organic matter at local scales. But placing these mechanisms into context necessitates that we continue to expand our understanding of the global biogeography of soil organisms. These communities can be viewed at multiple levels of ecological resolution, starting from the biomass of overall communities, which can then be divided into different functional groups, taxa, and functional traits. As we move down this list, we gain mechanistic detail at the expense of predictive understanding. While we continue to refine our detailed understanding of microbial taxa and trait compositions, we also need to step back to characterize the biomass distributions of the major functional groups of soil organisms, which reflect considerable differences in biogeochemical processing rates. As we generate this global ecological data, sensitivity analyses will then be necessary to identify the mechanisms that are most critical for improving biogeochemical model performance. These insights have the potential to improve predictions of soil fertility, plant production, and the climate. Ultimately, this emerging perspective of the most diverse and abundant community on land will provide fundamental insights into the organization of life on Earth.

Latitudinal trends in organic matter across terrestrial ecosystems.

(A) The latitudinal patterns of terrestrial carbon stocks, both aboveground plant biomass (green) and soil carbon stocks (brown). (B) The same latitudinal trend in soil microbial biomass, revealing similar patterns to that observed in soil carbon. (Data sources are provided in Fig. 2 in the main text.)

Abstract

Soil organisms represent the most biologically diverse community on land and govern the turnover of the largest organic matter pool in the terrestrial biosphere. The highly complex nature of these communities at local scales has traditionally obscured efforts to identify unifying patterns in global soil biodiversity and biogeochemistry. As a result, environmental covariates have generally been used as a proxy to represent the variation in soil community activity in global biogeochemical models. Yet over the past decade, broad-scale studies have begun to see past this local heterogeneity to identify unifying patterns in the biomass, diversity, and composition of certain soil groups across the globe. These unifying patterns provide new insights into the fundamental distribution and dynamics of organic matter on land.

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