Jun 27, 2012
From forestry to geoengineering, silicate weathering counts
Watch out for rocks. From harvesting trees to growing crops – or even geoengineering the Earth's climate – many of our activities have an intimate relationship with the weathering of the Earth's crust. Now research shows how important it is to obtain good estimates of local rock-weathering rates if we are to avoid upsetting these finely balanced natural cycles.
Over 90% of the Earth's crust is made from silicate rocks. Without them plants and trees would not grow. "The weathering of silicate rocks is a key process in soil formation and a source of essential plant nutrients," said Martyn Futter, from the Swedish University of Agricultural Science.
Lashing rain, freeze–thaw cycles and howling gales all do their bit to make rocks crumble and, over millions of years, turn to soil. For plants, those rocks add a host of essential nutrients to the soil including phosphorus and base cations such as calcium, magnesium, sodium and potassium.
So how quickly does a rock turn to soil? Measuring silicate weathering rates is no easy task because rocks weather slowly and experiments take decades. Meanwhile, models of silicate weathering are highly dependent on the input parameters – the local conditions that influence the weathering rate. As a result, estimates of silicate weathering tend to vary wildly.
To assess that variability, Futter and his colleagues scrutinized 394 silicate weathering estimates, taken from 82 sites on three continents. They show that different methods of estimating the weathering rate from the same site can produce differences of several hundred per cent (Environmental Research Letters).
"These estimates can be useful in a qualitative sense, to rank susceptibility to weathering at different locations, but it is very risky to use a single estimate in a quantitative sense," Futter told environmentalresearchweb. "Our results suggest that you need at least three independent estimates when making management decisions related to silicate mineral-weathering rates."
So who needs to think about silicate weathering rates? One industry where soil quality has serious implications is forestry. Traditionally foresters have harvested the trunks of trees but left the brash – twigs, needles and leaves – behind. However, in recent years the demand for renewable energy led some foresters to start harvesting the brash too, to fuel the latest biomass power stations.
Now it seems that using every last needle, leaf and twig for bioenergy may not be as good for the environment as it first appears. "Compared to the trunk, the leaves and needles are the most nutrient rich part of the tree," explained Futter. Removing the brash means that those nutrients do not get recycled back into the soil. In regions where soils are poor and silicate weathering generally slow (as is often the case for forestry locations), the soil quality will decline rapidly.
It is not just foresters who need to take note of rock dust production. Many geoengineering proposals are also intimately linked to silicate weathering rates. Seeding our skies with sunlight-reflecting sulphate aerosols for example, will increase the acidity of rainfall. In areas where silicate weathering is slow the acid rain could leach cations out of the soil faster than weathering can replenish them, leading to poor quality soils.
Meanwhile, the idea of consuming carbon dioxide from the atmosphere by grinding up minerals and spreading them over the landscape sounds plausible, but depends heavily on silicate weathering rates.
"We are not saying that these projects should not be done but that we need to understand the processes better in order to measure the benefits and assess the trade-offs we are likely to encounter," said Futter.
About the author
Kate Ravilious is a contributing editor for environmentalresearchweb.