Year of Soils column by Wietse de Boer

It was Antonie van Leeuwenhoek who first made visible the world of bacteria in the 17^th century using his own, handcrafted microscopes. Van Leeuwenhoek thought he was looking at ‘animalcules’, tiny animals. Little did he know that his observations would open the door to discovering the greatest conceivable diversity of life forms. Bacteria are small – about 1/1000^th of a millimetre – and there is not much to set them apart in terms of appearance: some are shaped like balls, others like straight rods and that's about it. What is it then that makes them so diverse?

If a man is known by his deeds – as the old saying goes – the same is definitely true for bacteria. Except that being ecologists, we like to call them 'functions' rather than deeds, and we like to refer to the wide range of different functions in bacteria as 'functional diversity'. These functions are absolutely essential to life on earth: without bacteria, there quite simply wouldn’t be any.

Crops wouldn’t be able to grow if groups of bacteria didn’t make mineral nutrients available to plant roots. Thousands of different types of bacteria are involved in breaking down soil organic matter, most of which is made up of decaying plant remains. This gives them the energy they need to grow. As a result of their activity, mineral nutrients are released that plant roots can absorbed.  It’s a closed cycle: plants feed on nutrients made available by bacteria from remains of previous plants.

And then there are bacteria that enter into a more direct symbiotic relationship with plants. A well-known example are Rhizobia, which fix nitrogen after establishing themselves in the root nodules of legumes such as peas. Rhizobia enter the roots of legumes from the soil and are fed sugars by the plants. This gives them energy to do something extraordinary. They capture nitrogen from the air and turn it into a plant-usable form: ammonium. Feeding plants on nitrogen from the air, supplies of which are unlimited: wouldn’t it be enough to make anyone jealous?

We humans eventually managed to pull off the same trick, albeit under extreme temperatures and using extreme pressure. The results - fertilizers - have proved very useful indeed, but by now we’re all aware that their large-scale use also causes damage to the environment. So as agriculture becomes more and more sustainable, it’s likely that Rhizobium-bacteria will once again be playing a key role.

In addition to these, soil bacteria have many other functions that contribute to the overall wellbeing of the soil ecosystem. To develop more sustainable forms of agriculture, we need to put our insights to practical use: it’s only by managing microbes in the soil that we can really grow crops sustainably.

Soil bacteria microcolony photograph: Olaf Tyc.