On a walk through the countryside, the living beings that attract the most attention are animals large enough to make them visible to the naked eye: insects, reptiles, birds or some mammal. Plants go more unnoticed, but they are an inescapable part of the landscape, always present. However, in the soil , where not much attention is usually paid, there is a lot of biodiversity and hosts some of the most important processes in the maintenance of ecosystems.
Soil as a vital part of the ecosystem
The ecological term “soil” differs from the meaning it has in colloquial language. The soil, from the point of view of ecology, is a complex and dynamic system, made up of several layers or horizons , which fulfills various functions in the ecosystem.
The top layers of the soil are organic. The first, called the H horizon , presents residues of organic matter deposited by the living beings that inhabit the ecosystem. The dry leaves of plants, the remains of fallen branches and bark of trees and carcasses of dead animals are found in this horizon. Beneath this layer is the O horizon , which has the same composition, except that the remains are much more fragmented, forming humus .
Below are the so-called mineral layers, which sit on top of the bedrock. The first of these is called the A horizon , which accumulates decomposing organic matter mixed with highly fragmented mineral remains. In the next layer or E horizon , the organic matter is already very scarce, and it is composed mainly of sand and silt. At the next level is the B horizon , dominated by gravels from the fragmentation of the parent rock. It is followed by the C horizon with much larger fragments. The roots of trees and shrubs usually penetrate to the C horizon, which provides them with support and a suitable growth medium. The last layer is the one formed by the unfragmented parent rock, it is the R horizon .
Soil formation happens in both directions. On the one hand, from the bottom up, the rock is fragmented by the activity of plants, through their roots, and by erosion processes. Horizon after horizon, its fragments become finer and finer. On the other hand, from top to bottom, the organic matter deposited by living beings fragments, decomposes, infiltrates the lower layers, and mineralizes. Thanks to these processes, plants can obtain, through their roots, the mineral salts necessary for their metabolism.
Through this process, phosphorus, nitrogen, carbon and other components of organic matter are recycled in the ecosystem, closing their cycles. But for this to happen, the activity of certain living beings that live inside the soil, under the litter and moss, is necessary; they are the decomposers, which are grouped into two types: the detritivores and the saprophytes .
Detritivores: breaking up organic remains
As mentioned, all organic matter that falls to the ground is assimilated, from the top down, by the soil. For this, the first necessary step is that the remains of living beings are fragmented into smaller pieces, and if possible, simpler.
Faced with the carcass of a large animal, the first task of fragmentation is in the hands of scavengers , which are not considered part of the decomposers. The detritivores act later , on animal remains, dry leaves, feces and other debris that accumulate on the surface.
Among the usual detritivores, myriapods such as millipedes, crustaceans such as isopods — known colloquially as ball bugs — beetles such as springtails, tenebrionids or dung beetles, slugs, earthworms, etc. stand out. Some animals are specifically responsible for fragmenting the remains of wood, a material that is very difficult to decompose; they are the xylophagous . There are also detritivores in marine ecosystems, such as sea cucumbers, many species of crabs or polychaete worms.
The detritivores feed on this accumulated waste in the superficial layers of the soil, leaving much smaller fragments and their own feces, which after several digestive processes, are already in a very simple state in molecular terms. It is at this point that the saprophytes come into play.
Saprophytes, remineralizing the soil
While the detritivores act by fragmenting the organic matter mainly mechanically, the saprophytes are the true decomposers of the ecosystem, and they work the organic matter in its chemical aspect .
Unlike detritivores, saprophytic organisms do not eat, but rather absorb nutrients directly . They are therefore not animals, but mainly fungi and, secondly, bacteria. During their metabolic processes, saprophytes convert organic matter back into simple mineral salts, which can be recovered by plants for metabolism. This process is called remineralization .
As was the case with detritivores, only some species of fungi, called xylosaprophytes , are capable of decomposing lignin, the main component of wood. Without these organisms, plant debris would accumulate in the soil without decomposing.
References:
FAO. 2006. Guidelines for soil description (4th ed.). Food and Agriculture Organization of the United Nations.
Godbold, DL et al. 2006. Mycorrhizal Hyphal Turnover as a Dominant Process for Carbon Input into Soil Organic Matter. Plant and Soil, 281(1), 15-24. DOI: 10.1007/s11104-005-3701-6
Schmitz, OJ et al. 2015. Chapter Ten – Functional Traits and Trait-Mediated Interactions: Connecting Community-Level Interactions with Ecosystem Functioning. In S. Pawar et al. (Eds.), Advances in Ecological Research (Vol. 52, pp. 319-343). Academic Press. DOI: 10.1016/bs.aecr.2015.01.003
