Hey Brooke,
Your initial question about fungal vs bacterial dominant soils opens up a whole interesting world of understanding why soils at different levels of ecological succession are good at growing entirely different types of plants. We first have to unpack succession a bit, so I will post a short snippet of my notes for teaching the
PDC:
Succession
Ecological Succession is the biological process whereby there is a change in the species composition of an ecological system over time.
After a disturbance event which opens up new soil, the first species to appear are generally pioneer species (both plants and animals) whose life strategies allow them to take advantage of the disturbance.
Over time, a wider array of different species will tend to displace the initial pioneers to create more stable and longer-lasting ecosystems.
A climax ecosystem is the final product of the successional process, although it may take decades or even hundreds of years for an ecosystem to reach its climax state following a disturbance event.
R-selected vs. K-selected species
Although ecologists today say the designations of species as R-selected (from rate) or K-selected (from the German term for carrying capacity) is over-simplified, the basic concepts help build a basic understanding of successional dynamics.
R-selected species are adapted to grow quickly and take advantage of disturbances in the landscape. Pioneer species that we think of as “weeds” often fall into this category.
K-selected species have a longer-term strategy, spending more time building structures that allow them to live in the environment over the long-term.
Trees fall into this category.
As an example, the ideal successional process in forested humid temperate climates is:
Bare ground -->
Pioneer plants (weeds, high Nitrate, lack of oxygen) -->
Early Successional Grasses (Bermuda grass, Brassicas) -->
Mid-Succession (Rye, Solenacea, Vegetables) -->
Mature Grasses (Corn,
Perennial Grasses, around 1:1 fungal:bacterial biomass ratio) -->
Bushes and Shrubs (becoming fungal dominated) -->
Deciduous Trees -->
Conifers (highly fungal dominate)
The shifting mosaic:
Real landscapes don’t actually follow this ideal pattern uniformly. Disturbances over time reset the successional clock on certain parts of the landscape, and feedback loops from microclimates, animal impacts, and
local topology drive certain parts of the landscape through the successional progression faster or slower than surrounding areas.
The balance between bacterial bio-mass and fungal bio-mass in the soil shifts over time as succession progresses.
* Early succession is dominated by bacterial life, and many R-selected species have symbiotic relationships with certain species of bacteria.
* By the time you reach mid-succession, the bacteria/fungal bio-mass ratio is nearly 1:1.
* As fungi begin to dominate, the plant communities tend to shift more toward shrubs, bushes, and trees. Mature forests tend to be dominated by fungi.
Here is another snippet of notes on how biological changes over succession affect soil pH and help determine which families of plants want to grow:
The Biological Dimension of pH
Assigning a single number to soil pH implies that the actual pH of the soil is pretty much the same across an entire region with basically a single soil type. This ignores the fact that the pH of a specific bit of soil will be determined by a combination of the baseline pH of the inorganic components of the soil along with the pH modifying influence of the organic matter and the exudates being released by the soil life.
The baseline pH of the soil is created by the ratio of sand/silt/clay and the exact ratio of minerals that the various kinds of sand, silt, and clay bring to the mix.
This baseline is then modulated by the presence of organic matter (humus) and soil life. The bacterial/fungal ratio is the largest determining factor to how the soil microbiology modulates the soil pH because it determines which major form of plant-available Nitrogen will be the predominate exudate.
Bacterial-dominate soils will tend to have a lot of Nitrate because this is the most common form of Nitrogen created by bacteria and because nitrifying bacteria tend to break down Ammonium into Nitrate.
Fungal-dominate soils will be much richer in Ammonium because this is the form of Nitrogen most fungi produce.
Real soils tend to be a mosaic of various pH values since the mosaic of soil life creates niches of different pH according to the bacterial/fungal ratio being fed by the plants.
Soils in pioneer ecosystems may have almost no fungal biomass and around 1000 micrograms of bacteria per gram of soil. Soils in climax forests may have as much as 100,000 micrograms of fungi per gram of soil and around 300 micrograms of bacteria per gram of soil.
This means that microbial exudates in pioneer ecosystems can have a pH as high as 9 while the microbial exudates in a climax forest can be as low as 5.
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