Stream acidification is a serious problem affecting water quality and ecosystems around the world, particularly in areas with a large amount of agricultural activity.

Stream acidification is caused primarily by excess nitrogen either in the air or in the water. This excess nitrogen may get into the atmosphere from ammonia or nitrogen oxide emissions (these gases are also thought to contribute to global warming as well as to stream acidification).

At first glance, this may seem a bit ridiculous. After all, as we all learn at school, nitrogen is one of the most common elements in the atmosphere, and oxygen is very obviously present in the atmosphere as well – we’d suffocate without it. So how on earth or off it can a combination of nitrogen and oxygen contribute to global warming and stream acidification?

This is a good question, and some people are skeptical of these claims. Whether or not atmospheric nitrogen actually contributes to stream acidification and global warming is uncertain. Possibly the form of the nitrogen (e.g. as ammonia) has an effect. Incidentally, it is this sort of atmosphere (lots of ammonia) that we are told was required for the "primordial soup" that they say everything evolved from. Food, if not soup, for thought.

But excess nitrogen does indeed contribute to stream acidification. Now, under normal circumstances, it is rather hard to get nitrogen from the atmosphere into the soil. And we need to get nitrogen into the soil, as plants need it for growth! Usually, nitrogen can get "fixed" in the soil by lightning, by certain plants (beans, clover, gorse and legumes). And once it’s in the soil and is taken up by the plants, it goes around in a merry little cycle, whereby animals eat the plants and then excrete the nitrogen via urine.

Normally, although animal (and human) urine contains a high amount of ammonia (if you’ve ever mucked out a stable, you’ll have smelt the horse urine breaking down into ammonia), it does not contribute to stream acidification in the normal way. If an animal happens to be standing in the water when it urinates, this may raise the pH of the water very slightly, but the quantity isn’t great.

But human beings didn’t take long to notice that nitrogen in the soil led to better plant growth. In the past, farmers would practise crop rotation to make sure that enough nitrogen would be fixed in the soil for later crops: one year, they’d plant beans, peas or clover as a crop for one field, then in subsequent years, they’d use that field for wheat or pasture, and when the nitrogen seemed to be running out, they’d plant beans again. There was no chance of stream acidification here.

The problem started when people started adding artificial fertilisers to the soil. Most commercial fertilisers have a good percentage of nitrogen in their make-up, and some, such as urea, have a very high percentage indeed. And when too much is put on and can’t sink in, or rainfall washes the fertiliser into the groundwater systems, this is when stream acidification occurs.

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