The Web of Life (23-2) Soil


Soil lies at the heart of Earth’s ‘critical zone’ – the thin veneer extending from the top of the tree canopy to the bottom of aquifers. ~ American environmental engineer Steve Banwart

For terrestrial ecosystems, soil is the fundamental substrate. Its quality is a key resource for life aboveground.

Soil starts as relatively large lumps that are identical to parent rock in chemical composition. Young soils are variable in their nutritional base.

Microbes are ever the initiators of every ecosystem. Their success and diversity determine the support base for other life. Soil quality reflects this. The smell of rich earth emanates from geosmin (C12H22O) produced by Streptomyces soil bacteria. As an evolutionary adaptation to appreciate soil conditions, the human nose is extremely sensitive to geosmin: able to detect it at concentrations as low as 5 parts per trillion.

The pioneer plants that invade a new soil must tolerate severe conditions. To ease the arduousness, these plants often associate with nitrogen-fixing prokaryotes. Many lichens harbor cyanobacteria which help them establish themselves. Some flowering plants have root nodules that nestle microbial assistance.

In establishing their residence pioneers significantly change the soil. Carbon dioxide from root respiration produces carbonic acid which accelerates chemical weathering.

Dead plants become a substrate for less hardy microbes than those that first landed. Humus forms, greatly increasing a soil’s capacity for holding water. As roots grow and penetrate bedrock, they create cracks, breaking rock apart.

Soil matures into 3 layers, or horizons, unimaginatively labeled by convention as A, B, and C. The uppermost horizon (A) is a leaching layer: organic debris breaks down and is washed downward into the middle layer by rainwater.

Nutrients accumulate in the middle horizon (B), which contains humus and clay. The deepest horizon (C) comprises parent rock and fragments, resembling soil’s earliest times.

Soils become chemically less diverse as they mature. As rock weathers, roots absorb essential elements which become trapped in the plant body. Plant parts slowly recycle these elements into the soil. Nonessential elements leach away.

In processing soil over time, plants are the drivers of ecological chemistry. The composition of the plant community is a harbinger for the fate of a terrestrial ecosystem.

In good soil that is a largely homogeneous substrate, vegetation patterns itself upon water need and availability. As rainfall decreases, plants become sparser until aridity collapses into desert.

Though winds often blow rain clouds from the ocean to deposit their liquid load on lands near shore, there is correlation between soil moisture and locally generated rainfall inland. The process is intricate, but the upshot is that areas naturally tend to retain their humidity level via a self-reinforcing feedback loop.

During the day, the Sun warms the Earth, evoking evaporation from inland lakes and rivers, and from the soil itself. Water vapor rises until it meets colder layers of air and condenses. Then it starts to rain.

The more moisture in the soil, the more water that evaporates, increasing the likelihood of an afternoon shower. In a humid region, areas with lower soil moisture produce the warmest air, facilitating water vapor to rise the highest and meet cooler air layers the soonest. It rains most frequently in these places.

Afternoon precipitation events tend to occur during wet and heterogeneous soil moisture conditions, while being located over comparatively drier patches. ~ Swiss climatologist Benoit Guillod et al

Soil quality and Earth’s atmospheric gas composition are intricately intertwined via several dynamics.

Nitrogen is the nutrient that most often limits rates of plant growth. ~ American ecologist Christine Goodale

Even during summer dry spells, some patches of forest soil remain waterlogged. These catchments are hot spots of microbial activity that removes nitrogen from groundwater and releases it into the atmosphere.

Denitrification (nitrogen removal) typically reduces plant growth and thereby lessens forest productivity. Water retention and drainage affect soil quality and the nitrogen cycle in a surprising way: too much soil moisture during dry times reduces nitrogen, and thereby limits plant vitality.

The acidity of soil determines how much nitrous acid outgasses into the atmosphere or is retained as nitrite. Via nitrification by soil microbes, nitrite (NO2) turns into nitrate (NO3), which is a usable form of nitrogen for plants.

Nitrous acid (HNO2) plays a key role in regulating atmospheric processes. Sunlight breaks nitrous acid down into nitric oxide (NO) and a hydroxyl radical (OH). OH controls the atmospheric lifetime of gases which affect air quality, including catalyzing the chemistry that forms ground-level ozone, a primary component of smog.

Although hydrogen is an abundant element throughout the universe, H2 is present in only trace amounts in Earth’s atmosphere. Certain soil bacteria scavenge it out of the skies for fuel. Soil actinobacteria are the main sink for atmospheric hydrogen. This in turn influences the concentrations of other atmospheric gases, including methane (CH4) and nitrous oxide.

Small invertebrates – especially earthworms, ants, and termites – help keep soil healthy, allowing soil to absorb decayed plant organic matter and thus nourish vegetation. These critters also maintain an ecological balance which limits the incursion of detrimental species.

In contrast, monoculture agriculture destroys natural biodiversity, reducing soil productivity, and allowing infestation of unwelcome pests. The typical response by modern farmers has been to apply chemicals which further degrade soil quality. From a perspective of maintaining a sustainable or depletable resource, human agriculture is stunningly inept.