Drivers of decoupling in drylands

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A study reveals that increasing aridity alters the balance of carbon, nitrogen and phosphorus in dryland soils, providing insight into how global climate change will affect soil fertility and ecosystem services. See Letter p.672

In all terrestrial ecosystems, the cycling of chemical elements is driven by both biotic and abiotic processes, but the ways in which these factors affect cycling vary for different elements. This is in part because carbon and nitrogen inputs to ecosystems are driven largely by biological processes such as photosynthesis and nitrogen fixation, whereas phosphorus inputs are driven mostly by weathering of rocks. The different driving processes can result in these cycles showing decoupled responses to environmental influences1, as has been most convincingly demonstrated for long-term chronosequences (gradients of soils of different ages) in which the availability of phosphorus relative to nitrogen diminishes sharply over time2,3. On page 672 of this issue, Delgado-Baquerizo et al.4 provide compelling evidence for a similar type of decoupled response to increasing aridity for dryland ecosystems worldwide.

The authors analysed soil samples collected from 224 plots in dryland ecosystems that vary greatly in aridity, from all continents except Antarctica. They found that, as aridity increases, there is a decline in concentration of both the total and the most biologically available forms of carbon and nitrogen, but an increase in the most biologically available forms of phosphorus. The decline in carbon and nitrogen most probably emerges through impairment by moisture limitation of biological processes that drive their ecosystem inputs and fluxes, whereas the increase in available phosphorus results from greater weathering of phosphorus-containing rocks and reduced uptake by plants (Fig. 1).

Figure 1: Aridity causes elemental cycles to decouple.

Aridity is predicted to increase in many dryland ecosystems worldwide because of global climate change. Delgado-Baquerizo et al.4 report that, as aridity increases, available soil carbon (C) and nitrogen (N) decline, whereas available soil phosphorus (P) increases. This is a consequence of the impairment of biological processes that contribute to the C and N levels, and of an increase in the relative importance of abiotic processes that contribute to P availability.

Delgado-Baquerizo and colleagues' results reveal that, because the response of soil phosphorus to increasing aridity is decoupled from that of carbon and nitrogen, there are sharp increases in the ratios of both carbon to phosphorus and nitrogen to phosphorus. Using a statistical approach for estimating causative pathways, known as structural-equation modelling, they also provide evidence that aridity has a direct negative effect on soil organic-matter content and phosphatase enzyme activity (an indicator of biological phosphorus demand), but a positive effect on total and inorganic phosphorus. These lines of evidence suggest that, as aridity increases, the growth of plants and other biota should become more limited by nitrogen (because it is cycled in organic matter), but less limited by phosphorus.

There are distinct parallels between the findings of this study and those of previous work that examined ecosystem development on newly formed land surfaces (that is, primary succession). It has been shown for several long-term chronosequences that such new surfaces have a high availability of phosphorus relative to nitrogen, and that as the significance of biological processes increases over time, organic matter in soil develops so that phosphorus becomes increasingly limiting relative to nitrogen3,5,6. The changes that occur with increasing aridity therefore operate largely in the opposite direction to those that occur during ecosystem development. It would thus be interesting to ascertain whether shifts in the balance of nitrogen to phosphorus resulting from aridity have knock-on effects on vegetation characteristics, ecosystem processes and above-ground and below-ground biota, as has been shown in previous studies of ecosystem development3.

The finding that aridity leads to increasing availability of phosphorus relative to nitrogen may at first sight seem paradoxical, especially given that some of the world's driest regions, such as much of Australia, are also among the most phosphorus-limited7. Severe phosphorus limitation is especially apparent for very old soils (including those in arid regions) from which phosphorus has been lost over long periods and cannot be replenished. In drylands, as in all ecosystems, there are likely to be multiple drivers of the availability of phosphorus relative to nitrogen, with soil age reducing it3,8 and, as shown by Delgado-Baquerizo et al., aridity enhancing it. Although there is some evidence that rainfall patterns influence how nutrient balances respond to soil age9, the extent to which the balance between soil nitrogen and phosphorus is driven by soil age rather than by aridity, or by the interactive effect of these two factors, remains a largely open question.

“The balance between carbon, nitrogen and phosphorus will become increasingly disrupted as ecosystems become drier.”

Delgado-Baquerizo and co-workers' findings offer fresh insight into the consequences of the increased aridity that is projected to occur because of human-driven global climate change10. Specifically, they reveal that, for dryland ecosystems worldwide, the balance between carbon, nitrogen and phosphorus will become increasingly disrupted as ecosystems become drier. This will occur through widespread losses of soil organic matter (and therefore of biologically available pools of carbon and nitrogen), and through an increased role of abiotic factors (and therefore of available phosphorus). Of particular concern are the authors' data showing that this decoupling of elemental cycles accelerates in a nonlinear manner as aridity increases, suggesting that, as global climate change progresses, the ecosystem properties of many drylands could pass a tipping point that will be difficult or impossible to reverse.

The study highlights the fact that, as aridity increases, adverse ecological consequences will arise not only through the direct effects of moisture limitation, but also through the indirect effects of decoupled elemental cycling and reduced organic matter in soil. This could have far-reaching consequences. For example, dryland ecosystems will be able to store less carbon both above and below ground, thus compromising their ability to mitigate increased levels of atmospheric carbon dioxide and climate change. More immediately, reduced soil carbon and nitrogen may impair the supply of nutrients from the soil and therefore the productivity of crops and livestock, with potentially dire consequences for many of the more than 2 billion people who inhabit dryland regions. This study underscores the fact that increased aridity associated with global change, and its effects on soil nutrient balances, could greatly affect the capacity of drylands to deliver key ecosystem services upon which human well-being depends.


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Correspondence to David A. Wardle.

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Wardle, D. Drivers of decoupling in drylands. Nature 502, 628–629 (2013) doi:10.1038/502628a

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