Plant Roots and Soil Life: Mutual Dependence in the Extreme

        Root exudates create a booming economy in the root zone, the area right around the roots of plants and their mycorrhizal associates. Only a few millimeters to a few centimeters thick, the root zone contains the most life of any part of the soil: ten to fifty times the numbers of bacteria and many times the amount of fungi of soils outside the root zone. Between eleven thousand and fifteen thousand species of bacteria live in the root zone per teaspoon of soil, compared to the several hundred per teaspoon outside it, and there are several miles of mycorrhizal threads per teaspoon of root-zone soil, compared to several meters per teaspoon outside. This commitment by plants of their hard-earned solar energy to support a bunch of freeeloaders down there in the soil must have a payoff, right? How important are these little critters to the trees?
      Because of this energy transfer, “nutrient retention and cycling, soil physical structure, and the composition of the soil community are directly tied to the types of plants. This bioregulation of soil processes and properties by plants benefits plant growth, and plants and soils become tied together by reinforcing positive feedback. When trees or ecological equivalent plants are removed from forest ecosystems for a long enough period, the ability of soils to support those plants deteriorates.
     In the 1960’s, loggers clear-cut a number of forested areas in the Siskiyou Mountains of southwestern Oregon. Most clear-cuts regenerated well, but every one of four or five replantings at the “Cedar Camp” clear-cut failed. Twenty years after the cutting, ecologists studied the soil structure and biology at Cedar Camp soils were structureless, “like beach sands,” while nearby forests contained well-structured and aggregated soils with diverse pore sizes. Greater numbers of bacteria and harmful fungi, and fewer mycorrhizal fungi, inhabited the Cedar Camp soils. Compounds called siderophores, which are created by beneficial microbes and which help plants resist pathogens and gather iron, existed in much lower numbers. Populations of mites and other tiny arthropods that graze on bacteria and fungi were also low.
    The scientists replanted the site, this time adding about 3/4 cup (150ml) of soil from established forests to the tree holes at planting time. The results were dramatic. “Seedlings given soil from young forests grew roots faster, formed more mycorrhizas and survived and grew better than seedlings receiving no soil transfers. Soil transfers from older forests improved seedling growth, but not survival and mycorrhiza formations. Forest soils (especially those from younger forests) contained something (or ‘somethings’) that had been lost from the clearcut, and that loss effectively destroyed system resiliency.
    The researchers aren’t sure exactly what happened, in either the problem or the solution. They hypothesized that the loss of energy from root exudates so changed the food web structure of the soil community that nutrient cycling caused by grazing soil organisms pooping out digested microbes effectively increased. With no mycorrhizas, the trees had no other mechanism for gathering nutrients. The loss of soil aggregates compounded the problems by altering the physical and chemical structure of the soil. These and many other factors, including the harshness of the site, prevented reestablishment of trees. We don’t quite know how generally we can apply the results of this work. Some things do become clear, though.
    The mutual interdependence of plants and the soil community, especially in extreme environments, is obvious. Soil biology is key to nutrient cycles and plant health, and plants are key to soil biology and health. The worse your soils, the more attention you should pay to soil biology, but even those of us with decent soils should not garden in complete ignorance. We disregard the soil community at our peril, and our lack of respect can come back to haunt us. Not only pollution and chemical additives can damage the soil community, but also the “simple” change from forest to grassland modifies the soil community immensely, affecting crop production, crop survival, and our workload.
    If the trees are paying so much attention to the soil microbes, why aren’t we? We should not pant trees. We should plant ecologies. This means polycultures, not only of plants, but also of all the living elements of a healthy soil food web.
                             
                      - Dave Jacke, Edible Forest Gardens Volume 1: Ecological Visions and Theory for Temperate Climate Permaculture