Tag Archives: Soil

Fungus governs soil’s carbon content


By Tim Radford

The soil stores the greater part of the Earth’s carbon. Just how much it stores is determined largely by what sort of fungi live in the roots of plants and trees, researchers have found.

LONDON, 28 January – Most of the planet’s carbon is neither in the forests nor the atmosphere. It is in the soil under your feet. US scientists think that they have identified the mechanism that keeps most of this awesome treasury of carbon locked away in the soil – or surrenders much more of it back to the atmosphere. The answer is: a fungus.

This answer matters because what happens to soil carbon is critical to predicting the planet’s future climate, according to Colin Averill of the University of Texas at Austin.

He and colleagues from the Smithsonian Tropical Research Institute in Panama and Boston University in Massachusetts report in Nature that the storage of carbon in soils is influenced by the mycorrhizal fungi that live in symbiotic relationships with plants.

In a symbiotic relationship, creatures benefit from each other, and in this case the fungi extract nitrogen from the soil, and make it available to the roots of the growing plant. Plants take carbon from the air to make their tissues; when a tree falls, or a branch breaks, or a shrub dies, most of the carbon gets back into the atmosphere through decomposition. But some gets buried, and stays in the soil

Averill and colleagues decided to look at the respective roles of two kinds of mycorrhizal fungus: one group known as ecto- and ericoid mycorrhiza (EEM), and another called arbuscular mycorrhiza (AM). The first produce enzymes that degrade nitrogen.

Out-competing microbes

That means that whenever there is organic nitrogen in the soil, the fungi take the greater share: they compete with soil microbes for the soil nutrients.  So the scientists predicted that if the EEM type was dominant, then there would be greater proportions of carbon conserved in the soil.

They then looked at all the known data about soil carbon and nitrogen in various ecosystems: the boreal forests of the north; the temperate woodlands, the tropical forests and the grasslands.

An Amanita mushroom, the fruiting body of an ectomycorrhizal fungus associated with the roots of a Hemlock tree in Harvard Forest, US Image: Colin Averill

Amanita mushrooms, part of an ectomycorrhizal fungus in the roots of a Hemlock tree in Harvard Forest, US
Image: Colin Averill

Where the proportions of arbuscular mycorrhiza were highest, the levels of soil carbon tended to be lower. In an EEM world, there could be 70% more carbon stored in the soil. Unexpectedly, they found that the relationship was independent of, and mattered far more than, the effects of net primary production, temperature, rainfall and levels of soil clay. What mattered most was the type of fungus dwelling in the roots of the forest trees, or the savannah grasses.

“Natural fluxes of carbon between the land and atmosphere are enormous and play a crucial role in regulating the concentration of carbon dioxide in the atmosphere and in turn, the Earth’s climate”, said Averill.

“This analysis clearly establishes that the different types of symbiotic fungi exert major control on the global carbon cycle, which has not been fully appreciated or demonstrated until now.”

Complex relationships

The research, once again, is a reminder that climate models depend on an understanding of how the world works, and that there is still much more to understand about planetary workings. Fungi are mostly invisible. Ceps, morels, chanterelles, truffles and field mushrooms are edible prizes that pop up from the soil, but most of the fungal action is below the soil.

The biggest single creature on the planet is not the blue whale but a fungus that covers 10 square kilometers of soil in the Blue Mountains of Oregon, in the US.

The research is a reminder of a secret kingdom buried in the first metre or so of the world’s soils, a kingdom with profound influence on the machinery of the planetary carbon cycle.

“The research is not only relevant to models and predictions of future concentrations of atmospheric greenhouse gases, but also challenges the core foundation in modern biogeochemistry that climate exerts major control over soil carbon pools,” said Adrien Finzi, of Boston University, one of the authors. – Climate News Network

Rainy mountains speed CO2 removal


The rainswept Southern Alps are young mountains and growing fast Image: Philip Capper from Wellington NZ via Wikimedia Commons

The rainswept Southern Alps are young mountains and growing fast, providing new rock for weathering
Image: Philip Capper from Wellington NZ via Wikimedia Commons

By Tim Radford

The speed at which soil is produced by rain falling on mountain slopes proves to be much faster than science had realised – with significant implications for carbon in the atmosphere.  

LONDON, 19 January – US scientists have measured the rate at which mountains make the raw material for molehills – and found that if the climate is rainy enough, soil gets made at an astonishing speed. And in the course of this natural conversion of rock to fertile farmland and forest loam, carbon is naturally removed from the atmosphere.

Isaac Larsen of the University of Washington in Seattle and colleagues from California and New Zealand took a closer look at rates of weathering on the western slopes of the Southern Alps in New Zealand. They report in Science that, according to their measurements, rock is being transformed into soil more than twice as fast as previously believed.

On the ridge tops of the NZ mountains, soil was being manufactured by chemical weathering (which is scientific shorthand for rain splashing on rock) at the rate of up to 2.5mm a year.

“A couple of millimeters a year sounds pretty slow to anyone but a geologist”, said David Montgomery, one of the authors. “Isaac measured two millimeters of soil production a year, so it would take just a dozen years to make an inch of soil. That’s shockingly fast for a geologist, because the conventional wisdom is it takes centuries.”

The research matters because – once again – it throws new light on one of the dark regions of the climate machine: how carbon dioxide is removed from the atmosphere, at what rate, and where it goes and where it all ends up.

Temperature drop

The Southern Alps of New Zealand are in geological terms young, and still going up in the world: they include some of the fastest-uplifting mountains on the planet. They are also among the rainiest: more than 10 metres of precipitation a year, on average.

Uplift – the process of mountain-building – provides fresh new rock for weathering to work on. Rainclouds arrive on the prevailing winds from the Tasman Sea, hit the mountain sides, rise, condense and release their burden on the western slopes, to generate colossal run-off, lots of silt and rock fragments and dissolved silica, and to nourish dense, vigorous forests at the bottom of the slope.

And along with all this trickling water and new soil is a steady delivery of carbon, removed from the atmosphere’s carbon dioxide.

The hypothesis that mountains play a role in chemical weathering, carbon dioxide removal and climate change is not new. Decades ago scientists argued that when the continent of India slammed into Asia and lifted up the Himalayas and the Tibetan plateau more than 50 million years ago, this process generated conditions for monsoon rainfall that accelerated the removal of carbon dioxide from the atmosphere at such a rate that global temperatures dropped dramatically and ushered in the Ice Ages.

Such an argument is difficult to clinch, but the latest research from NZ certainly lends support to the reasoning that new mountain chains are influential components in the climate machine.

Strenuous research

Larsen and colleagues calculate that the young, wet mountain chains of the world make up only 14% of the land area that drains into the ocean, but account for 62% of the sediment, 38% of the total dissolved solids and 60% of the dissolved silica delivered down the rivers and into estuaries and deltas and ultimately to the sea, where huge quantities of this run-off settle to become carbonate rock.

Mountains, in effect, are agencies that turn carbon dioxide from the air into limestone beneath the sea, and the evidence from the Southern Alps is that this happens more speedily than anyone first thought.

To complete the research, the scientists had repeatedly to take helicopter rides to the highest ridges, hike down to collect a burden of new soil, and then climb the steep mountain slopes again to await the return flight.

Back in Washington, they tested their soil samples for levels of beryllium-10, an isotope made at the Earth’s surface by cosmic rays, and therefore an indicator of the newness of the soil, and the rate at which it formed.

“I’ve worked in a lot of places,” said Larsen. “This was the most challenging fieldwork I have ever done.” – Climate News Network