NEWS & VIEWS ECOLOGY

Good dirt with good friends An analysis of data from forests across the planet reveals that the types of beneficial fungus with which tree roots associate determine the amount of carbon stored in soils. See Letter p.543 MARK A. BRADFORD a

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n 1936, US President Franklin D. Roosevelt signed an act to conserve the “natural resources of the land”, commenting1: “The history of every Nation is eventually written in the way in which it cares for its soil.” Decaying organic matter improves soil health because it binds the soil, preventing erosion, and serves as a sponge that retains nutrients and water for plant growth. The amount of organic matter in soil is therefore an important determinant of its fertility and — because organic matter in the soil contains around three times as much carbon as the atmosphere2 — of the magnitude of climate change3. Our understanding of what regulates the amount of soil organic matter is currently undergoing a conceptual upheaval 4. On page 543 of this issue, Averill et al.5 show that commonly assumed controls, such as tem­ perature, do not explain why stores of organic matter differ across soils in temperate, tropi­ cal and boreal forests. Instead, the authors propose that a primary control is the types of fungus with which trees form mutually benefi­ cial relationships. Our health is increasingly seen to depend on the microorganisms that live in intimate association with us. It is the same for plants. Most species of land plant form relationships with soil microorganisms known as myco­ rrhizal fungi, which grow in and around their roots. The plants provide the fungi with simple carbon compounds (such as sugars) in exchange for nutrients such as nitrogen. The sugars fuel fungal activity, helping them to spread out from the plant into the soil, forming what is essentially an extended root network. The threads (known as hyphae) of this fungal web dramatically increase the surface area for nutrient uptake and exude enzymes to catalyse the decay of organic matter, releasing nutrients for plant growth. However, as with our own friendships, myco­rrhizal relationships vary in their costs and benefits to both partners. The relation­ ship between arbuscular mycorrhizal (AM) fungi and trees, for example, could be likened to Facebook friendships in that both invest and receive little (they exchange small amounts of carbon and nitrogen). Ectomycorrhizal (EM)

b

Nitrogen flow Carbon flow

EM fungi

Free-living microbes

↑ Soil organic matter

Inorganic nitrogen

AM fungi

Free-living microbes

↓ Soil organic matter

Figure 1 | Fungal types and soil organic matter. Some tree species, such as oaks, associate with ectomycorrhizal (EM) fungi (a), whereas others, such as maple, associate with arbuscular mycorrhizal (AM) fungi (b). In both relationships, the plants provide the fungi with carbon in exchange for nitrogen, but AM fungi primarily obtain inorganic soil nitrogen and EM fungi access nitrogen from organic matter. Averill et al.5 show that the soils of forests dominated by EM associations have greater stores of organic matter than those dominated by AM associations. The authors propose that this difference arises because the use of organic nitrogen by EM fungi reduces the nitrogen available to free-living microbes that feed on organic matter, thereby slowing their activity and reducing the breakdown of organic matter.

fungi and trees are more like best friends, however, demanding a lot but giving much in return. This key difference arises because EM fungi use the extra carbon they demand to access nitrogen from soil organic matter that is otherwise unavailable to plants, whereas AM fungi, like roots, primarily take up inorganic nitrogen from the soil (Fig. 1). Averill et al. studied the effects of these dif­ fering relationships on soil organic-matter stores by assembling a data set of the carbon and nitrogen content of soils around the world. The data revealed that ecosystems dominated by trees that form relationships with EM fungi store 1.7 times more carbon per unit of nitro­ gen than systems in which AM fungi dominate. The authors propose, in line with a previous hypothesis6, that these richer carbon stores result from competition for nitrogen between EM fungi and free-living soil microorgan­ isms that feed on organic matter — the EM

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fungi outcompete these microbes for access to nitrogen, retarding their activity and hence reducing losses of organic matter (Fig. 1). The authors suggest that the consequences of these differences extend to the global scale. Climate warming is predicted to ramp up the metabolic activity of free-living soil microbes, increasing the carbon dioxide respired from soils to the atmosphere and thus creating a positive feedback loop that will amplify warm­ ing3. But the competition hypothesis suggests that this effect will be smaller in EM-domi­ nated forests than in AM-dominated forests, because EM fungi will limit the organicmatter-degrading activity of free-living soil microbes. Alternatively, the main reason for the higher organic matter in EM-dominated forests might be that trees in these systems allocate more carbon below ground to satisfy the greater demands of EM fungi6. This explanation is

NEWS & VIEWS RESEARCH consistent with the emerging idea that belowground plant inputs to soils are the dominant precursors for the formation of soil organic matter4,7. Pinpointing which mechanism explains Averill and colleagues’ results will require more data and involve challenges common to all large observational data sets, includ­ ing unobserved variables and spurious cor­ relations. Perhaps different mycorrhizal associations reflect adaptations to environ­ mental conditions, as opposed to being the cause of ecosystem differences. For example, in colder climates, where the cold slows the decay of organic matter and trees produce tough leaves that are hard to break down, EM fungi might dominate simply because of their ability to acquire nitrogen from organic matter8,9. In Averill and colleagues’ analyses, the strength of the mycorrhizal effect depends on the amount of soil nitrogen. To investigate this dependency, I used their model results to calculate organic-matter stores in tem­ perate and tropical forests, where the myco­ rrhizal types co-occur and where the authors conclude that EM-dominated forests have 1.3 times more carbon per unit nitrogen. At the low end of the authors’ nitrogen-content range (0.2 kilograms of nitrogen per square metre), EM-dominated forests actually have less (0.96 times) carbon than AM forests, and at 1.0 kg N m–2, below which many of the observations fall, they have 1.21 times more. It is not until soil nitrogen reaches values at the upper end of their observations (3 kg N m–2) that carbon stores are 1.3 times greater in EMdominated forests, a pattern consistent with the idea that the strength of the mycorrhizal effect is strongly dependent on soil-nutrient availability6. Despite the need to further explore such nuances, Averill and colleagues’ findings have important implications for the way we manage land resources in the face of a changing carbon cycle and climate. We depend on model projections to inform strategies to preserve our natural resources, yet the relevant models have been developed on the basis of an understand­ ing of soil dynamics that is increasingly shown to be wanting10. Climate, soil texture and plant productivity drive soil organic-matter storage in these models11 but were found by Averill et al. not to play a determining part in organicmatter levels. Their finding that it is instead the relative dominance of trees associating with different mycorrhizal fungi that corre­ lates with the amount of soil organic matter highlights the need to consider how local-scale biotic interactions shape global and regionalscale carbon dynamics. ■ Mark A. Bradford is in the School of Forestry and Environmental Studies, Yale University, New Haven, Connecticut 06511, USA. e-mail: [email protected]

1. Statement on Signing the Soil Conservation and Domestic Allotment Act, 1 March 1936. American Presidency Project http://www.presidency.ucsb. edu/ws/?pid=15254. 2. Jobbágy, E. G. & Jackson, R. B. Ecol. Appl. 10, 423–436 (2000). 3. Denman, K. L. et al. in Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (eds Solomon, S. D. et al.) Ch. 7 (Cambridge Univ. Press, 2007). 4. Schmidt, M. W. I. et al. Nature 478, 49–56 (2011). 5. Averill, C., Turner, B. L. & Finzi, A. C. Nature 505, 543–545 (2014).

6. Orwin, K. H., Kirschbaum, M. U. F., St John, M. G. & Dickie, I. A. Ecol. Lett. 14, 493–502 (2011). 7. Clemmensen, K. E. et al. Science 339, 1615–1618 (2013). 8. Phillips, R. P., Brzostek, E. & Midgley, M. G. New Phytol. 199, 41–51 (2013). 9. Johnson, N. C., Angelard, C., Sanders, I. R. & Kiers, E. T. Ecol. Lett. 16, 140–153 (2013). 10. Wieder, W. R., Bonan, G. B. & Allison, S. D. Nature Clim. Change 3, 909–912 (2013). 11. Bonan, G. B., Hartman, M. D., Parton, W. J. & Wieder, W. R. Global Change Biol. 19, 957–974 (2013). This article was published online on 8 January 2014.

SO L A R SYST E M

Evaporating asteroid The asteroid Ceres has been thought to contain abundant water. Observations acquired with the Herschel Space Observatory now show that this Solar System object is spewing water vapour from its surface. See Letter p.525 HUMBERTO CAMPINS & CHRISTINE M. COMFORT

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riting in this issue, Küppers et al.1 report that Ceres — a dwarf planet or the largest asteroid in the Solar System, depending on the definition used — is releasing water vapour from its surface at a rate of about 2 × 1026 molecules, or 6 kilograms, per second. The presence and abundance of water in asteroids2,3 are relevant to many areas of research on the Solar System, ranging from the origin of water and life on Earth to the largescale migration of giant planets such as Jupiter. Water has been suspected of being a sig­ nificant component of Ceres for more than 30 years4. But it is only now that observations obtained by Küppers et al., using the European

Space Agency’s Herschel Space Observatory, have allowed the direct identification of water molecules escaping from two regions on the surface of this object (Fig. 1). The authors’ result backs up previous indirect observational evidence5,6 for water in this planetary body, and is particularly timely given that NASA’s Dawn spacecraft7 will soon visit Ceres, fresh from its successful mission to another intrigu­ ing small world, the asteroid Vesta. One of the most puzzling questions about the origin and evolution of asteroids is why Vesta and Ceres are so different. They are both located in the main asteroid belt, between the orbits of Mars and Jupiter, and their orbits are quite close to each other: about 2.4 and 2.8 astronomical units from the Sun, respectively (1 astronomical unit is the mean

Figure 1 | Artist’s impression of the asteroid Ceres.  Küppers et al.1 have discovered water vapour emanating from two regions on the surface of the asteroid. (Figure adapted from an illustration by Chris Butler/SPL.) 2 3 JA N UA RY 2 0 1 4 | VO L 5 0 5 | N AT U R E | 4 8 7

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Ecology: Good dirt with good friends.

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