Morton's book Eating the Sun, about photosynthesis and other plant matters, has provided a starting point for my explorations. It's a pretty good introduction, but I still find myself struggling, both to grasp and to retain.
Plants need many elements to survive and thrive, to make proteins and other chemical material. They get carbon, oxygen and hydrogen from the surrounding air. They also require nitrogen - oxidized in the form of nitrates, reduced in the form of ammonia - and phosphorus [oxidized phosphate]. Nitrate and phosphate fertilizers are, of course, much-used in modern, non-organic agriculture.
Plants are eukaryotes, of course, and one of their deficiencies, if you can call it that, is that they can't 'fix' nitrogen. That's to say they can't transform, via electron transfer and enzyme action, nitrogen gas into reduced ammonium ions. This nitrogen fixing is done by bacteria, including some cyanobacteria. Up until recently, eukaryotes have been dependent, for the two billion years or so of their existence, on nitrogen fixed by bacteria.
Now to look at a little of the complex history of nitrogen fixation. The experts divide earth history into four periods, the Hadean, the Archaean, the Proterozoic and the Phanerozoic. The nitrogen-fixing machinery evolved in the iron-rich oceans of the Archaean. Nitrogen-fixing occurs through the electron transfer chains of proteins, and they require iron and molybdenum. However, in the Proterozoic, the longest period of earth history, iron and molybdenum levels dropped substantially. We're talking here about the oceans, and in the very stable and 'boring' Proterozoic, what evolved and was maintained for a long time was what has been called a 'Canfield ocean', named after earth scientist Don Canfield.
The Proterozoic lasted nearly two billion years, half of the lifetime of life. Its beginning was marked by the 'Great Oxidation Event' and the snowball earth, its end was marked by what Morton calls 'isotopic wildness' and global glaciation leading to the Cambrian explosion.of complex life forms. The middle Proterozoic period has been described as the 'boring billion', due in part to its flat, unchanging carbon isotope record.
Eukaryotes had evolved by the early Proterozoic, and the atmosphere was oxygenated, though not to today's extent. This allowed the slow development of complexity, and the evolution of sexual reproduction, but the fossil record shows little change during this long period. Canfield and others argued that the atmospheric changes at the beginning of the Proterozoic not only oxygenated the oceans but, perhaps more importantly, changed their sulphur chemistry. In fact, oxygen levels in the atmosphere were still too low to affect the oceans much. To quote Morton:
The oxidized surface of the planet would have provided the oceans with a greatly increased supply of sulphate, which microbes in the oxygen-free depths of the oceans would reduce into sulphides. Something similar can be seen in the poorly aerated waters of the Black Sea.The point is that this 'Canfield ocean' is distinct from the previous Archaean ocean and the later oxygen-rich Phanerozoic with its 'dissolved oxygen available even at depth'. This view of things disrupts ideas of a smooth transition to today's oxygen-rich world.
So, during this period, 'the sulphides would have precipitated out any iron' [and I can't pretend to really understand what this means], and they would also have 'got rid of the soluble molybdenum oxides which provide today's bacteria with their supply of the metal' [ditto]. Nitrogen fixation in such an environment would not have been easy, and eukaryotic algae were basically starved of usable nitrogen. This explains, probably, the flatness of the carbon 13 record, which usually fluctuates according to the dumping of phosphates into the ocean through erosion and tectonic plate movements. In modern times, the limits to the growth of oceanic life are set by the phosphate levels - in the Proterozoic it was probably set by the usable nitrogen levels [ammonia, essentially].
None of this is set-in-stone science, however, and I'll leave it there and look at the nitrogen cycle from a more contemporary perspective. The nitrogen cycle [as well as the sulphur cycle] is driven by bacteria. So is the carbon cycle, if you allow that chloroplasts were once bacteria, now harnessed to eukaryotes. Basically, usable nitrogen is fixed through a two-stage process, first of oxidation of ammonia into nitrites, and second the oxidation of nitrites into nitrates. Ammonia oxidation is performed by bacteria and archaea, nitrite oxidation by Nitrobacter, mainly. This two-stage process is called nitrification. I don't want to go into too much detail, because I'll probably get it wrong. Ammonia is available through waste material, animal and vegetable. Importantly some of the bacteria are endosymbiotic, attached to root nodules and thus directly providing plants with usable nitrogen. Denitrification completes the process by returning nitrogen to the atmosphere.
Soils become 'depleted' if there isn't enough nitrogen-fixing bacteria to keep plants healthy. In the nineteenth century and early twentieth a lot of work was done to develop artificial nitrogen-fixing, culminating in the Haber-Bosch process, which led to a massive production in chemical fertilizer in the twentieth century [and a near four-fold increase in soil yields in the course of that century]. Morton tells the story nicely; the great pioneering geologist James Hutton was one of the first to recognize the importance of compost and manure for healthy productive farmland. He himself was a model farmer, utilizing the 'Norfolk rotation' to greatly enrich the soil. By the nineteenth century, such rotation systems and an increased use of manure had trebled the yield of wheat on English farm lands - a massive boon to the rapidly growing population. Of course it wasn't always understood that nitrogen-fixing was the key to increased productivity, but once this was established, interest was raised in the possibility of artificial nitrogen fixing. By the 1930s, almost a million tonnes of nitrogen was being fixed annually into fertilizer. Nowadays, the figure is more like a hundred million tonnes a year. This helped enable humanity to feed itself, but it has a down side. The excess nitrates go into river systems and out to sea, with various negative consequences, or back into the atmosphere, sometimes as nitrous oxide, a very powerful greenhouse gas.
Artificial fertilizers also break the cycle between manure and bacteria naturally enriching the soil and diversity of growth producing various endosymbiotic or otherwise mutually beneficial organisms. They have enabled the production of high-yield monoculture, which in turn requires pesticides and other inputs to be maintained at a constant level. To abandon the use of such intensifiers, to return to organic farming, would inevitably mean giving up a great deal more land to crops, with all sorts of attendant consequences. Thorny problems abound, and on that note I shall abandon this post.