Eli5 why plants which need nitrogen from the ground do not take it from the air which is 70% Nitrogen
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Nitrogen, as used by plants, is usually in the form of the nitrate ion. This ion is relatively difficult and expensive to form, so most plants cannot create it from atmospheric nitrogen.
Most of the nitrogen in the air is diatomic, pairs of nitrogen atoms bound together. That’s a very strong bond, which is difficult to break apart with the kind of energy plants have available to them, so the nitrogen they consume mostly comes from compounds which contain nitrogen and are held together by much weaker bonds.
The nitrogen in the air is N2, two nitrogen atoms attached with an extremely strong bond. Plants need single nitrogen atoms to build stuff.
It’s like if you were surrounded by pairs of LEGO bricks superglued together, and to survive you needed to build structures requiring single bricks. Even if they’re pairs of bricks of the type you need, you’re gonna have to get them elsewhere.
Because evolution is not a design. Why can’t animals photosynthesise? Why can’t mammals extract oxygen from water? Why do complex organisms not have the ability to asexually reproduce? Species evolve alongside each other and develop niches. Some evolve in high organic nitrogen niches. Others rely on symbiosis with nitrogen fixing bacteria.
A useful analogy here is burning wood.
Why don’t dry forests burst into flame when there’s all those dry needles or leaves and wood around? The answer is because it takes a certain amount of energy to activate the process of chemically converting wood into carbon dioxide and water vapor gases, so you need to invest that energy to start the process.
Nitrogen has the same initial problem. In order to use it, plants need to bring it out of the air and convert it into a more useful form. But plants don’t have the high amounts of energy required to perform that process.
Some plants have developed a relationship with certain bacteria that can do this special conversion for them. Clover is a good example – it has nodules on its roots that these bacteria live in. They specialize in grabbing nitrogen and converting it into a more useful form that the plant can take advantage of. And that’s why farmers like to plant clover to let a field “rest” for a season – it’s effectively recharging the soil.
Chloroplasts evolved separately to plants and were absorbed into plants at a later time as eukaryotes. Could have something to do with it.
Some plants do though (beans for instance) but they Do it themselves, they kinda have a symbiotic relation with some organism in the soil that do it for them)
As people have mentioned: nitrogen in the air is N2, which is extremely difficult to break apart into single nitrogen atoms – and no plants can currently do this.
Instead, plants rely on soil bacteria to do this for them. This is a slow and difficult process; legumes help by providing cosy root nodules for the bacteria to live in, and the plants reap the rewards.
Farmers used to rotate their crops, planting legumes in a field every few years, then leaving it to lie fallow the next season for the leftover root nodules to rot down and release the nitrogen compounds into the soil.
The most enormous advance in agriculture of all time was the Haber process – a system for combining atmospheric nitrogen with methane, and producing ammonia to use as fertiliser. Something like 80% of the nitrogen in animal life these days is as a result of artificial fertiliser produced this way.
However, the Haber process is not very environmentally friendly – it takes a whole lot of energy, and produces greenhouse gases. And on top of that, runoff from the actual application of fertiliser (since the water in the soil leaks into the water table, rivers, etc), causes a whole bunch of other problems down the line.
As such, there’s some really interesting work going on to genetically engineer the nitrogen-fixing enzymes from the soil bacteria directly into plants, which would then indeed be able to get their nitrogen directly from the air.
It’s a major challenge, though – bacterial DNA is not like plant DNA, so you can’t just insert the relevant chunk into a plant nucleus and expect it to work. Instead, they’re looking at inserting it into the DNA of the mitochondria within the cell.
Mitochondria used to be independently-living bacteria, but they formed a symbiotic relationship with other cells about a billion and a half years ago, trading some neat metabolic tricks for an ongoing supply of nutrients. The relationship is complex and the lines between the two have become blurred – but mitochondria are a hell of a lot more similar to the nitrogen-fixing bacteria than plants are, and a lot of the research is based around translating the necessary genes into ones that will run within the mitochondria.
If they get it right, it will be *huge*.
I don’t know man. Why don’t you take oxygen from water?
It’s not the abundance, it’s the ease of use
Plants absorb nitrogen from the soil as both NH₄⁺ and NO₃⁻ ions; Ammonium & Nitrate ions respectably.
Ions are a lot easier to use than pure N₂ (nitrogen gas), because the bond between the 2 nitrogen ions in N₂ is really strong.
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