While humans are busy looking up in the sky to find other extraterrestrials on other planets, one look down is all it would take to find an entire new world awaiting our discovery. Soil organisms live in the soil and are usually not seen by most of us, which may also seem to be aliens to people because of their invisibility. But those little alien-like creatures have a significant impact on biogeochemical cycles, and consequently, the climate.

The organisms living in the soil are different and various, from fungi to snails, and from cyanobacteria to nematodes. One classification for them is based on their sizes, namely microflora (e.g. bacteria), microfauna (e.g. protozoa), mesofauna (e.g. small arthropods) and macrofauna (e.g. earthworms) (Blume et al., 2016). They can also be classified by other attributes, for instance, their function in biology and ecology. The one thing they have in common is that they help with decomposition in the soil organic matter cycling.

But before we dive into those amazing aliens who live closest to us, there is a fundamental question to be asked: why is there soil? How does the soil, where those organisms live, come to be? The latest emerging and widely accepted explanation comes in the form of the soil continuum model (Lehmann & Kleber, 2015).

In the soil continuum model, organic matter exists as a spectrum of organic fragments that are continuously processed towards smaller molecular size. Molecular structure does not control long-term decomposition of soil organic matter. The model focuses on the ability of decomposer organisms to access soil organic matter and on the protection of organic matter from decomposition provided by soil minerals and soil aggregates (Lehmann & Kleber, 2015; Schmidt et al., 2011). That means, in principle, soil organic matter would be decomposed by soil (micro)organisms given the appropriate conditions and sufficient time, but the matter is distributed on the spectrum of different decomposition phases, and because of the protection from soil minerals and aggregates, soil comes into the place where soil is.

As there might (potentially) be endless species and variations under the umbrella term extraterrestrial aliens, so are the soil organisms. In this article, two cases would be discussed to provide a closer look at how they live, how they interact, and how they impact.

Those little aliens know how to use their own talents to trade for food. The example for this would be mutualism between legumes and nitrogen-fixing bacteria. Nitrogen fixation is the process that changes nitrogen gas into ammonia, which can be used by living organisms. This process is mediated in nature only by N-fixing rhizobia bacteria. In legumes and a few other plants, the bacteria live in nodules, which are the small growths on the roots. Within these nodules, nitrogen fixation is done by the bacteria (Flynn & Idowu, 2015; Sørensen & Sessitsch, 2007). The plant gives space for those bacteria to live and provides carbohydrates to them; in return, the ammonia they produce is taken up by the plant.

Not only do they have the tricks to get nutrients for themselves, but they also play a very important role at a global scale. Those little aliens’ strong impact can be found in the positive feedback loop of Arctic greening, which is essentially the advance of the treeline in the Arctic zone, turning tussock tundra to shrub tundra, then to woodland, and finally to forest (Wilmking et al., 2006). While this is still in debate in academia, it is widely accepted that the tree line advance can cause a positive feedback loop to global warming (e.g., Parker et al., 2015; Wilmking et al., 2006; Zhang et al., 2013). It seems counterintuitive at first sight because with more trees it should be beneficial to mitigate global warming. The behind-the-scenes reason has something to do with soil organisms.