Salt of the earth
Examining the relationship between salinity and atmospheric CO2Most of us are aware of the dual problems of salinity in Australia’s soils and of rising carbon dioxide in the atmosphere. What fewer people realize is that the two are strongly interconnected. This salinity/carbon relationship formed the PhD topic of Dr Vanessa Ngar Lai Wong, a recent graduate of the Fenner School of Environment and Society. Dr Wong now works as a scientist at Geoscience Australia and continues to study the effects of salinity in Australia as part of her research there.
Dr Wong explains that the problem of salinity in Australia is compounded by its geology. Compared to much of the planet, Australia doesn’t have much geological activity such as volcanoes and earthquakes. This means that most of the landscape is very old. Over millions of years the minuscule amounts of salt in the air, rocks and rainwater accumulate and concentrate in the soil and are never removed. In the far distant past, some areas of the country were also under shallow seas which evaporated leaving their salt behind. Even those areas above the ancient sea level were affected by windblown salt from the dried seas. P> For many thousands of years nature achieved a fragile salinity balance. The native vegetation that covered the continent prior to European settlement was deep rooted and consumed a lot of groundwater. This had the effect of lowering ground water and keeping the salt below the root layer. When this vegetation was replaced by shallow rooted European crops, that require less water and are only in the ground for short periods, the effect was to raise the water table. With the rising water came salt - which in severe cases, can turn the land into sterile salt scalds.
All these factors combine to make salinity one of Australia’s biggest environmental problems. But the productivity of the land is not the only problem caused by salt. The heath of the soil is a key factor in sinking atmospheric carbon dioxide. Biologically active areas of vegetation are an important sink, but surprisingly, the soil itself contains three times as much fixed carbon as the vegetation on the surface. The combined carbon in all the world’s soils also amounts to double the total amount present in the atmosphere as CO2. This means that soil and the microorganisms in it, form a large and vital part of the earth’s natural carbon regulating mechanism.
In areas with rising salinity, the soil micro-organisms are stressed and as a result have a higher respiration rate which liberates more CO2 into the air than normal. Worse still, stressed soil organisms excrete far less carbon in the useful form of minerals and stable soil nutrient compounds for plants to use. The net result is that saline soils are far worse carbon sinks than healthy ones. Dr Wong explains that the whole process of carbon sinking is a highly complex web of interrelated factors. She doesn’t think that there’s likely to be a universal quick fix and solutions may well be the different between different parts of the country. Each area has its own environmental factors and so may require appropriately tailored action. But above all, she believes that it’s vital to understand the underlying science so that we can make appropriate decisions.
Armed with the right knowledge, it is possible to improve things. As part of her studies, Dr Wong was involved in a project to investigate the possibility of reclaiming moderately saline areas of farmland in the NSW tablelands. Various types of regenerative vegetation were investigated on land that was highly degraded and therefore sparsely vegetated. By digging soil pits about a meter deep, she was able to extract core samples at different depths to monitor the soil carbon content at various stages of the process. In this particular study it was found that tall wheatgrass (Agropyron elongatum) had the capacity to effectively colonize salty areas and over the course of a few years, increase the soil carbon level by up to three times – approaching that which would be expected under the original native vegetation. From a farmer’s perspective, this is an attractive proposition because the wheatgrass is palatable to grazing animals and with the emergence of carbon trading schemes it may also be a lucrative source of income.
Dr Wong has found most of the land owners she has worked with have been very enthusiastic and helpful about her research. She says this is what appeals to her most about her work. “It’s the application of science to real problems and helping to create genuine improvements in the world.”