HIGH FLYERS THINK TANK

Innovative technical solutions for water management in Australia

University of Adelaide, 30 October 2006

Plant and soil sciences
by Dr David Chittleborough, School of Earth and Environmental Sciences, University of Adelaide

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I want to talk to you today about soil and water. It is usually the water in the soil that has not really been figuring much in the debate about water at the moment, so I will try to redress that. And here I acknowledge Pichu Rengasamy's work. Pichu is a research fellow at the University of Adelaide and I will quote his figures in the first part of this talk.

I am going to talk about the 'hidden' resources – water resources that are stored in soils – and ways in which we might be able to achieve a greater water use efficiency; and then move on to talk about the other aspect of water, specifically regarding water quality and how soils impact greatly on water quality.

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To look at Australia's hidden water resources: there is a total annual rainfall of 3,226,533 gigalitres (GL), according to the National Land and Water Resources Audit. We have here the annual runoff and the annual aquifer recharge, so the water stored in soils across Australia is quite large indeed.

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To look at water use in Australia: we see here the amounts used for irrigation and for urban/industrial and rural uses. The total water use per year is 24,000 GL.

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Let us consider water storage in soils in dryland grain growing areas, just as an example. On the map, this area is shown about here (like these areas on the west coast of South Australia and in Western Australia). The total rainfall over that area is 120,000 GL, the water stored in those soils is nearly 115,000 GL, and the available water that is actually unused is 28,700 GL.

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So there is a very large amount of water that is stored in soils and is not being used: 1.2 times the water used by the water industry and rural and urban uses. If that water was all used efficiently for, say, wheat production – we are using the cropping zone as our example here – that would translate into many tons of extra grain and considerable economic benefit.

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In view of the large amount of water that is not being used, this is some work from John Passioura a couple of years ago. He published this graph where you can see the available water stored in the soil in megalitres per hectare, versus the grain yield in tons per hectare. You can see the potential line if we were efficiently using the water that was stored in that soil; and you can see that our actual yields are quite a bit less than that potential. Passioura looked at reasons for this, as we have also been doing at the University of Adelaide, and clearly a large amount of this discrepancy is caused by soil constraints, particularly subsoil constraints, which I will touch on in a minute.

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This is just one example of a subsoil constraint. You see the soil water content versus the energy required by plants to actually get that water into their systems. You can see that as the soil water content decreases, so the plants need to exert a lot more effort, expressed here in kilopascals, to try and get that water up. Plants will tend to wilt at about 1500 kilopascals, and you can see on the graph the effect of even minor amounts of salinity on the effort required by plants to take up that water.

At the right of the figure you can see the actual effect of even modest amounts of salinity in soils. In megalitres per hectare of water actually used, there is quite a decrease from the non-saline through to the electrical conductivity of 1.0 deciSiemen.

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What are the ways in which we can manage soils to achieve a greater water use efficiency? There are three ways shown on this slide: modifying the available energy (altering the canopy), modifying the available water within the soil (removing subsoil constraints, in particular), and modifying the energy exchange rate between the soil and the atmosphere (surface modification). For this talk I am going to focus on removing subsoil constraints.

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This slide shows a variety of Australian cropping soils and some typical prairie soil out of the United States. It is a great pity that we don't have more of the prairie-type soils. In these soils the plant available water is very high relative to that of the texture-contrast soils which are so much a part of our cropping area. Marked in black on the map are the sorts of soils with strong texture-contrast horizons and very dense, very often impermeable and often sodic subsoils that are inimical to plant root exploration. On the map, notice where that distribution of these texture contrast soils is coincident with our cropping zone.

So that is what we have got to cope with here, rather than the US prairie soils.

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How can we manage these soils to achieve a greater water use efficiency, in view of this subsoil constraint? It does place a significant limitation on water use efficiency. We have a paucity of knowledge, however, of the soil and root processes causing this subsoil limitation and we know even less about how we can overcome it. Despite this recognition over the last 20 years or so, we still haven't advanced our knowledge very greatly on overcoming this difficulty.

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Here is some desktop work that was done by the late Richard Payne, in the South Australian Department of Water, Land and Biodiversity, together with David Maschmedt. They looked at the proportion of land in South Australia with potential for subsoil amelioration. We think more than two-thirds of the soils have subsoil constraints that could perhaps be overcome. These soils have subsoil properties that are not intractable. For example, indurated or cemented horizons, chemical toxicities that would obviate any reasonable possibility of physico-chemical or biological intervention.

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This slide illustrates some of the subsoil constraints. The salinity debate has been focused pretty much on the seepage type of salinity in the lower parts of the landscape (see slide).

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But in fact a much greater effect in terms of productivity and water use efficiency is generated in the recharge zone, from the texture-contrast soils. We do not have the dramatic effect of visible salt on the land surface, but in fact it has a tremendous effect on plant growth and the use of water in the soils. Its focus is at the boundary between the A horizon and the B horizon.

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The lighter coloured areas covering most of Australia are those with the sorts of subsoil where constraints are present, while those marked in the darker colour (eg south-west) are areas that are subject to seepage salinity.

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Let's move on to thinking about how it is that we can effect a physico-chemical transformation – and, I should say, biological transformation – in soils like those on the left here to produce soil like that on the right. On the right, we see extensive root exploration of the total soil volume. Water stored in the profile is more readily available, and to a greater depth, than in the soil on the left. What are the possibilities of effecting such dramatic transformation on the intractable B horizons of texture contrast soils?

A very interesting experiment was carried out several years ago by Robin Graham, at the Waite Institute. He started by choosing about 15 sites on soils like this across Victoria and South Australia. He took the A horizon off and then got in further down with vigorous physical comminution and added organic matter and various sorts of amendments like gypsum and calcium, then put the A horizon back and watched what happened. There was double the grain yield and an 80 or 90 per cent increase in the biomass as a result of this. No other measurements were done apart from that surface, above-ground plant productivity. We don't know what actually happened to the soils in this case, but over 15 years the transformation on these sites could be seen from the road. So anecdotally there was a tremendous change in this subsoil.

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So what is the current method of trying to intervene here? We have spent many generations seeking information on how to manage the top 10 or 15 cm of soil. We have very little technology at all to effectively intervene below that and, at least as far as Australia is concerned, that would potentially be a major area of research and could have huge economic and environmental benefits.

Here are some of the techniques that are used: ripping – large amounts of energy are required; using gypsum or other calcium amendments; and techniques such as biological drilling, which CSIRO Plant Industry are looking at, where plants with vigorous root systems are employed to 'punch' holes into the B horizon. I would like to put it to you that there are some other ways that perhaps we can try, and I have two here. One is using plant roots as agents for amelioration, and the other is using nanoparticulates, both natural and designed, to effect a change in this area.

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To focus very briefly on the use of nanoparticulates: CSIRO has an emerging science initiative called environmental nanovectors. The idea is that we need to understand the nature of nanoparticulates in the environment, in particular in soils and water, and that we can perhaps modify them to utilise their potential for moving down the soil profile and effecting a change through the pore network of soils, for example, to change the physico-chemistry of that medium.

To return to plant roots as agents for amelioration: we need to understand better how these pores, and networks of pores, in soils develop and how roots interact with soils, particularly in the B horizons. You see on the right of the slide some CAT scanning work that we have been doing with medical CAT scanners to track the way in which roots move through these types of profiles. By studying a range of different root systems – we don't really understand very much about plant roots in this environment – we hope to understand how roots might be able to get into those sorts of profiles and modify them physically and chemically.

It has been proposed that ironstone soils occurring over vast areas of Western Australia and indeed other parts of Australia as well, and the calcrete horizons are in fact caused by plants. In particular, the exudates from native plants. According to this hypothesis, they are not so much the result of prolonged weathering over vast time periods since the Tertiary. Under mallee woodlands they have recorded subsurface alkaline horizons with calcrete and silcrete and under proteaceous health, acidic laterite profiles with pisolitic ironstone gravel have been identified. This controversial view is suggesting that in fact it is the plants that have developed these (highly weathered) soils, not the other way round.

Exudates from Proteaceae, can chelate iron, aluminium and manganese and effect dramatic changes in the soil chemistry. This has got a number of us thinking that it might be possible to utilise plants to effect a biological transformation of texture contrast soils. So whereas the idea in Plant Industry is to use plants for physical change (biological drilling), perhaps we can utilise plant roots as agents for chemical and physical change thereby converting them into a medium where much greater water use efficiency is possible.

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There are some plant transformations – I am not a plant biologist but John Passioura has put forward a number of possibilities – which might increase yield potential. For example, biochemical and physiological research to improve crop cultivars and molecular transformations to improve genetic make-up of plants are ways to effect transpiration, photosynthesis and conversion of biomass.

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I would like to finish my talk by looking at the other side of it, which is an active area of research in our laboratory: soils and their impact on water quality. I will just choose phosphorus and carbon. We have an awful lot of this washing around in the environment and causing difficulties.

On the left of the slide is a plot of phosphorus level in soil, and also the amount that is actually in runoff. You can see that, as compared with US soils, Australian soils for any particular level of phosphorus have a much greater amount of runoff P. Soils 'leak' phosphorus. We talk in agriculture in Australia about having a leaky agricultural system in relation to water. Well, soils also leak phosphorus.

Soils also have a huge effect on the amount of carbon that is in water draining from catchments and causing difficulty in treatment plants in this country. In this slide you can see here the clay content of the soil versus the dissolved organic carbon level. As the clay content increases, so the amounts of carbon in runoff are decreased. So soils have a very big effect on carbon and on phosphorus flux. Yet the amount of effort that has gone into trying to attenuate these components, in terms of modifying soils, has been quite modest.

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Here is one way that we are looking at doing that. Methods of attenuation include industrial by-products but these have limited value, not least because there are very often undesirable components like arsenic, cadmium. lead. Gypsum and buffer zones have been used as physical ways of doing it. We have achieved quite a marked reduction in carbon and phosphorus movement using some novel polymers, like poly (DADMAC). This is a positively charged compound which is water soluble, and its effect is to sorb onto the clays, changing the soil from negative to positive. In becoming a positively charged entity, the soil then is able to capture anions like phosphate and much of the dissolved organic carbon. We have achieved quite a marked reduction in small plot studies and in cores; we have now gone to a full experiment in a catchment with poly (DADMAC) to test its utility in the 'real world'.

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In conclusion: by improving water use efficiency via soil management and plant improvement, the water in Australian soils can be utilised for huge economic benefit. And there is considerable potential for improving the quality of runoff using new technologies.