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A DELICATE BALANCE: SCIENTISTS SEEK WAYS TO INCREASE SOIL NITROGEN WITHOUT HARMING THE ENVIRONMENT
PULLMAN, Wash.--If ever there was a clinching argument for meandering evolution, it is the process of symbiotic nitrogen fixation. Beautiful though it is, such a process could not possibly be the result of straightforward design.
We need nitrogen desperately, as do all living things, for nitrogen atoms are a key component of many important biological molecules, including DNA, RNA and proteins. And proteins, as they say, are us.
We get most of our nitrogen by eating plants or animals that have eaten plants. Plants generally get their nitrogen from the soil, usually in the form of ammonia or nitrate released from animal wastes and rotting plants, both of which would seem to be endless sources. But ammonia and nitrate are constantly being removed from the system. Nitrate is very soluble and washes away, or leaches, below the reach of plant roots. Oxygen-starved bacteria in waterlogged soil will break down the nitrates for their oxygen, thus returning nitrogen into the atmosphere as a gas. And when we harvest a crop, we take away the nitrogen contained in the plant tissues. So for plants to grow, useful nitrogen must be constantly replenished.
This useful nitrogen comes ultimately from the atmosphere, which is 78 percent nitrogen gas. But nitrogen gas, which contains two nitrogen atoms bound to each other, is too stable to transform easily into the reactive forms that plants can use. It must be transformed, or "fixed." Fixation involves splitting the paired nitrogen molecules apart and converting each into ammonia, a more chemically reactive compound. This process requires an enormous amount of energy.
Some nitrogen is fixed by lightning and ultraviolet rays. But 80-90 percent of fixed nitrogen was originally put in the soil by one of the roughly 100 species of bacteria that can fix nitrogen gas by using the energy they get from the other nutrients they feed on. But since there is usually a limited amount of these other nutrients, the amount of fixed nitrogen these bacteria produce is relatively small.
Then there are the rhizobia.
By themselves, rhizobia are like many other soil microorganisms, searching for nutrients and growing slowly, very frugal in their metabolism. However, rhizobia have a double life--they can also associate with legume plants such as peas, lentils and alfalfa. Rhizobia invade the legume roots, and the infected roots subsequently develop growths called nodules that house the bacteria, whereupon the bacteria start fixing nitrogen for the plant.
Legume crops are major players in agriculture, for not only do they provide food containing large amounts of nitrogen, but some of the nitrogen they build up through their symbiotic relationship with the rhizobia can be returned to the soil where it can be used by other crops in succeeding years.
This "symbiotic" relationship is also enigmatic, as Michael Kahn and his collaborators will attest. As researchers in WSU's Institute of Biological Chemistry, Kahn's group is intrigued by the relationship between the bacteria and its host.
To convert one molecule of nitrogen gas into two molecules of ammonia takes the equivalent of about 50 ATPs. ATP (adenosine triphosphate) is the energy currency of the cell. Fifty ATPs is a biochemical way of saying "very, very, very expensive." In fact, nitrogen fixation is the most expensive reaction known. So why, asks Kahn, do bacteria invest so much energy to fix nitrogen and then just give the ammonia they produce to the host plant? Why don't they just stop when they have made what they need for themselves?
Kahn believes the ammonia is actually payment for something and that producing so much and letting it go only makes sense in a symbiotic environment. "The question becomes," he says, "what can you pay for that is worth as much energy as you had to put in to make this ammonia? That's really where my lab is focused, trying to understand whether the trade is for things like carbon sources or oxygen or protection from the environment."
Though his research crosses several disciplines, Kahn is essentially a microbial physiologist. And microbial physiologists tend to be pretty conservative in their economics.
"When what you are paying is obviously valuable, you've got to be getting something back," he says. "And you can't do a one-for-one thing, like trade an ammonia for a glucose (a simple sugar), because a glucose isn't worth enough. But that's the level we're trying to understand what's going on, so that maybe we can get better terms in the trade."
As intellectually exciting as it is by itself, Kahn's question is more than academic.
If world population growth fulfills projections from the Population Reference Bureau, another 2.4 billion people over the next 25 years will need to be fed. But these numbers are only part of the picture.
In the 1997 "State of the World," Gary Gardner warns that we are currently using nearly all of the cropland that is available: "...for the first time ever, the entire burden of increased grain production rests on yields alone. Not only is area expansion unable to assist in raising output, but net shrinkage in cropped area is a drag on production, increasing the pressure on yields still further."
Agriculture met its first population challenge this century by developing dwarf grain varieties and pouring on the nitrogen fertilizer. But now the "Green Revolution" seems to have reached a plateau. Besides, heavy synthetic nitrogen use has created a host of problems.
Leaching of nitrates causes enrichment of waters downstream, which leads to eutrification. Too much nitrogen acidifies the soil. Also, according to a recent report in "Science," excess nitrogen in ecosystems appears to favor weed plants at the expense of native species. After 12 years of nitrogen additions in a test plot, ecologists David Tilman and David Wedin report that "species richness declined by more than 50 percent."
There are also direct threats to human health. Nitrates can cause blue baby disease and are linked epidemiologically to some cancers.
So how to increase productivity, in which nitrogen availability is a major factor, without creating a further threat to the environment and human health?
One seemingly obvious way would seem be to "simply find the perfect symbiont, the ideal bacterial companion," says Kahn. Just finish what nature perhaps hasn't yet gotten around to, match a plant to the perfect bacteria.
Like so many things, however, it isn't that easy.
First of all, says Kahn, plants aren't all that picky when it comes to the bacteria they'll associate with. Rhizobia and legumes are adapted to each other--rhizobia that infect alfalfa will not infect pea, for example. But still, if a plant gets too choosy in who it will associate with, it may be left with no bacteria at all. "It's like looking for the perfect husband or perfect wife," says Kahn. "You may end up unmarried."
Finally, back to the basic fixation puzzle, Kahn wants to understand better how the trade-offs work in the symbiosis to see whether there might be a way of making it more productive for the plant. And that would mean higher productivity and increased available nitrogen without some of the problems associated with synthetic fertilizer use.
It might also be the key to understanding what Kahn calls the "holy grail" of plant symbiosis, "associative nitrogen fixation." There are a number of grasses, mainly tropical, that get a little of their nitrogen from a loose association with bacteria. If that relationship could be developed to make a sufficient nitrogen contribution to grain, it might replace one of the huge chemical fixes upon which modern agriculture so depends.
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