Soils under threat

09 Nov, 2013 03:00 AM
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IT'S difficult to imagine Australia's rangelands soils teeming with microbes, especially in the current severe drought, but they are - and if they go, the productivity of the rangelands goes with them.

This may happen under a warming, drying climate, a new global study has shown, changing the soil nutrient balance and pushing extensive areas of land now carrying livestock towards desertification.

Less moisture not only inhibits plant growth, it makes life harder for soil microbes - and microbes are key to building and cycling reserves of soil nitrogen and carbon.

Dozens of researchers looked at soil-plant interactions on 224 dryland sites across 16 countries, and wrote in the journal Nature that diminishing interaction between plants and soil life could be a major factor in pushing semi-arid lands into full desert aridity.

Plant-microbe interactions are vital for building nitrogen (N) and carbon (C) in the soil, through biological processes like nitrogen fixation and breakdown of organic matter into simple carbon. Less biological activity because of warmer, drier conditions inevitably leads to less N and C, two key drivers of plant productivity.

On the other hand, soil reserves of phosphorus (P), that other key element, will increase under a warmer, drier climate. Phosphorus is not dependent on biology: it is a product of the physical breakdown of weathering rock, which will continue unabated.

In the Nature article, the researchers wrote this "de-coupling" of the two drivers of nutrient development in rangelands soils will lead to nutrient imbalances liable to push currently semi-arid regions into full desert aridity.

It's not the only factor driving desertification, said University of New England soil scientist Matthew Tighe, one of three Australian researchers involved in the paper, but modelling of the desertification process shows it to be an important one.

How fast this process might happen is unknown, but Dr Tighe believes it could happen quickly if poor land management adds to the effect of adverse climatic conditions.

But the reverse is also true: sensitive land management can do much to counter the effects of adverse climate trends.

Dr Tighe has worked extensively in the rangelands, in places like Cobar and Nyngan in western NSW, where he has seen graziers like the Mosely family of "Etiwanda" restore the productive capacity of degraded landscapes using grazing management.

On these properties, he has measured increases in soil biology, and in soil carbon and nitrogen.

As these biological systems mature, they move from a bacterial dominance, which enables fast cycling of nutrient, into a more stable fungal-dominated state that contains higher levels of carbon.

It's the nature of a semi-arid environment that building groundcover and managing the soil for better rainfall infiltration and fertility is a slow process - it can take 10-40 years to see significant change, Dr Tighe said.

The process will get harder and slower as the climate shifts, so Dr Tighe's advice to any rangelands landholder looking to build resilience against future climate shifts is: start now.

Starting in 20-30 years, when climate change may begin to bite, could be too late to stall ecological collapse.

"Decoupling of soil nutrient cycles as a function of aridity in global drylands". Nature Research Letter

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