AoB Interviews: Hans Lambers on soil phosphate acquisition in impoverished soil

This week guest author Charlie Haynes is AoB Blog’s roving reporter at the EPSO/FESPB plant biology Europe conference.

 

Hans Lambers Hans Lambers is the Winthrop Professor at the University of Western Australia. He competed his PhD in 1979 at the University of Groningen in the Netherlands and since then worked at Melbourne University, Australian National University and Utrecht University. His research focuses on mineral nutrition of native Australian plants and crop and pasture legumes. He very kindly agreed to talk to me about some of the challenges of soil phosphate impoverishment.

Why is phosphate impoverishment so significant?
It’s of less importance in Europe which imports food and animal feed from parts of the world where phosphates passing the problem on. There it is an issue of excess of phosphate, dumped on the land and ending up in waterways. Europe could stop fertilising now and still have crops for the next 20 years. But when you go to other parts of the world; Australia, South America, Africa and South East Asia, phosphate insecurity is a real issue. This may be because the amount in the soil is too low for effective crop production, or it may be that it is there but it’s not readily available. So it’s an issue for crop production and thus food security. What we can do though is instead grow plants that can use that phosphorus in the soil much more effectively. There is a tremendous opportunity.

How does this limit these countries in what they can grow and the yields they can produce?
In Africa phosphorus is the key limiting factor, even some the driest areas of Saharan Africa. People who worked in barchenener discovered that simply by adding phosphorus you could get a higher yield. The dry soil significantly reduces the mobility of phosphorus in the soil and it becomes a significant limiting when you have a dry soil (Lambers H, Raven JA, Shaver GR, Smith SE. (2008) Plant nutrient-acquisition strategies change with soil age. Trends in Ecology and Evolution 23: 95-103).

So why do these communities not buy fertiliser to increase their yield?
Fertiliser is very expensive for these groups. It has to travel vast distances to the harvest, and these groups simply don’t have the money for it. So there instead we are working towards crops that are more efficient at using the existing phosphorus or are better at getting it out of the soil. This is however a bit of a risky business – if you have soils that are very nutrient poor to begin with then plants that extract it more effectively will make the soil even more phosphorus deficient. Whatever you take out of soil have to replace in order to be sustainable.

What makes phosphates accessible?
Phosphates in soil are readily available at a neutral pH. Calcareous soils with their more alkali pH lock phosphates up in calcium complexes. The phosphate is there but not readily available to crops. More acidic soils also lock up phosphates – but this time not in calcium complexes but instead, as complexes of oxides and hydroxides of iron and aluminium. Chile has very acidic soil, with a pH of almost 4, and lots of these metal oxides and hydroxides so all the phosphate isn’t readily available. However the plants have special adaptations that allow them to access it in these conditions.

What are these adaptions?
These plants have a special structure which works in combination with the plant biochemistry. What they produce is massive quantities of carboxylates. These are molecules with a negative charge – like phosphate. These exchange for one another, releasing the phosphate ions into the soil solution, whilst carboxylates anion take the place of phosphate in the soil. You’re effectively mining the phosphate that is in the soil out of it’s tight bindings. It’s then in the solution and anybody can take it up (Lambers H, Bishop JG, Hopper SD, Laliberté E, Zúñiga-Feest A. (2012) Phosphorus-mobilization ecosystem engineering: the roles of cluster roots and carboxylate exudation in young P-limited ecosystems. Annals of Botany 110: 329-348).

Could this be put into another crop either by breeding or genetic modification?
I would take one step back and ask ‘what crops do we have now at already can do that’? White Lupin is an excellent example and there are a few other Lupin species that do exactly the same. There are also some Lupin species that don’t have these wonderful structures but instead something close to it, and some without a structure at all that still release carboxylates. So we actually already have a lot of species that can already played this trick. Rather than engineer this in Soybean, it’s important to get a thorough understanding of the technology. Understanding is and farming it in crops with the gene is an obvious first stage. We already have crops with the this ability in lupins – which are much better than wheat and barley at this stage. I don’t think it’s impossible but it’s important to take it one step at a time (Lambers H, Clements JC, Nelson MN. (2013) How a phosphorus-acquisition strategy based on carboxylate exudation powers the success and agronomic potential of lupines (Lupinus, Fabaceae). American Journal of Botany 100: 263-288).

So are some parts of the world focusing on the wrong crops for their soil type and climate?
Yes absolutely! In chile they used to grow Andean Lupin. When the Spanish invaded they forbade the natives from growing these lupins as they weren’t Spanish crops. The natives switching to foreign crops is a daft idea when they already had a crop suited to their environment! Quinoa is an example of another crop where this happened, and the Spanish stopped that. They arguably had better crops than the foreign spanish ones then introduced. One thing one can do though is intercropping. This is where you grow plants concurrently interspersed between one another. If you want to grow wheat, it cannot grow particularly well in some South American environments. If you intercrop it with Lupin, can mobilise that phosphorus and the neighbours can benefit from that. You can also do crop rotation. A group in Germany has actually done this, working with rotations of soybean and maize. Maize is not so good at accessing phosphate, soybean – depending on the cultivar you use, is. The good soybean cultivars show a real benefit for the next crop – a phosphorus benefit. You can grow them at the same time or you can grow them in rotation to access this phosphate. Both of these techniques have tremendous benefits.

What stops people in phosphorus poor environments from doing this already?
That’s an interesting question. If you go to china, intercropping has been done for hundreds of years and you can demonstrate that with the right intercropping combinations you can have a 40-50% higher yield – which is pretty impressive! A British or Irish farmer with an increase in yield of that kind of level would be ecstatic! So the Chinese already have done that, and Europe is exploring it. I’m certain it could be done in other parts of the world, but it’s not happening on a large scale and that’s because a lack of education. It’s important to educate local farmers about this from Africa to Australia! I’m working with a group in Germany Andreas Burgutts, who is screening sorghums for better phosphate accessibility, using leaf manganese levels as a marker. These are taken up by the plant in the same as as phosphates and so used as a marker. Work like this requires going to Africa, and selecting the right cultivar for the conditions there, not in our lab field. This is about doing research and then making use this research reaches farmers, and doesn’t stay in a scientist’s ivory tower. Work needs to be done and go beyond journals, into places where we can make a difference,

Who else is working on taking this knowledge into the field?
I had a visit from someone from ICRISAT. They are based in India and work on major crop draught and salinity. They are now keen to work on phosphorus, and they had heard of my work and were interested in developing something together. These big international institutes have the links with the grassroots farming communities in the parts of the world where you can truly make a difference. I may be able to do high end science but without the connections I’m not able to have much of a real world impact.

Who else is involved?
The big international institutes are doing good work, IRRI in the Philippines, ICARDA in Aleppo and ICRISAT in Hyderabad. These large international institutes aren’t just interested in the science, but also applying it, and I think that is really important.

Do these plants have potential in any other key areas?
Yes, for instance where you have soil contaminated with heavy metals you could use them in the process of photoremediation. Here plants as used for their ability to remove heavy metals an ‘clean’ soil. There are areas in Belgium that have been heavily polluted with zinc or copper. Chemical or physical cleaning of this soil is almost impossible. You need a species that accumulates these metals to a very high concentration, but is also a fast grower, producing a lot of biomass, or else the process takes along time. There is serious potential in this. In addition to this these plants can be used in phytomining or prospecting, accumulating small amounts of metals that act as an indicator for a larger deposit of metal in the earth. This can act as a pretty good indicator of gold and some other metals to allow groups to commence mining.
Hans’ book “Plant Life on the Sandplains in Southwest Australia, a Global Biodiversity Hotspot” will be out in September, and is now available online at the UWA Publishing website.