| Muck and Mystery Loitering With Intent |
blog - at - crumbtrail.org |
One of the themes discussed many times here is that life is a group effort. An often discussed aspect of this is that soil isn't a granular medium suffused with chemicals, it is alive and must be alive to function. Many people have what can be described as a hydroponic metaphor for life. They think of plants dangling naked roots into circulating water doped with mineral salts. All that is needed to grow plants in this metaphor is light. This simple metaphor is then applied to reality in the field, imagining soil as simply being something plant roots can grasp so they don't fall over in the wind, while soil moisture circulates the nutrients.
How? There are no pipes and pumps in the soil to do this circulation. Much of the time soil is far too dry to support any such metaphor. Several earlier posts 1,2,3 discussed Vesicular Arbuscular Mycorrhizal fungi (VAM, or just AM) for its role in phosphorus transport as well as sequestration of massive amounts of carbon in the durable form of glomalin which is the threadlike remains of dead VAM lacing undisturbed soil. Phosphorous isn't the only thing VAM transports.
It seems a mighty feat for a microscopic fungus built from threadlike filaments. But collectively, these spindly mushroom relatives help move several billion tons of nutrients out of the soil and into plants each year. . .Even cooler than we thought. VAM is not only the chief phosphorous transport system it also moves major amounts of nitrogen. How?"Ignorance (about the movement of nitrogen) limits our understanding ... in what is arguably the world's most important symbiosis," says Yair Shachar-Hill, the lead MSU author on the study. . .
The fungus-plant partnership is one of the planet's oldest and dates back more than 400 million years, when plants began to move out of the oceans and onto land. Plants trade a bit of their sunlight-made sugars for building block nutrients that fungi wring from the soil. Scientists have understood broad outlines of this evolutionary bargain for years, but specific details remained fuzzy, especially those related to nitrogen.
To learn more about nutrient uptake, MSU researchers led by Shachar-Hill, along with collaborators at New Mexico State University and the U.S. Department of Agriculture research center near Philadelphia, tagged nitrogen with easy-to-spot atomic markers and then watched as it traveled from soil to fungus to plant roots.
Many had assumed that the fungus would play a modest role. The team found, however, that the fungus acts more like a four-lane highway than a two-track country road in shuttling the nitrogen into plant roots. More than a third of the total nitrogen taken up by the plants came by way of the fungus,
"The really fascinating part is the mechanism underlying the transfer process," says Maria Harrison, a plant biologist at Cornell University's Boyce Thompson Institute and an expert on fungus-facilitated movement of other soil nutrients into plant roots. "Dr. Shachar-Hill and his colleagues were able to show that the fungus acquires the nitrogen from the soil and then links it to carbon and moves this combination molecule towards the plant. Then just before delivery to the plant cell, it unhitches the carbon and releases only the nitrogen to the plant."It's long been known that soil carbon, aka organic matter, greatly increases soil fertility (see CEC) by making soil nutrients more available to plants. Several processes have been identified related to soil chemistry and texture, the size and shape of soil particles is important too in supporting chemical processes. This is another way that soil carbon is important for fertility since a shortage would reduce the ability of VAM to transport nitrogen.
There are many implications. Agronomic practices that impede or destroy VAM hugely impact the functional fertility of soil. If the only nutrients available to plants are those in the immediate vicinity of their roots then they can starve in the midst of plenty. This leads to excessive use of fertilizer which is not only an expense that is increasing it is a pollution hazard since excess nutrients end up in ground and surface water.
Plowing is a major culprit since it slices and dices the delicate threads of VAM in the soil. Exposing the soil to atmosphere has other negative impacts on soil chemistry and texture as it oxidizes and emits gasses to the atmosphere. The carbon dioxide, methane and nitrogen oxides emitted not only contribute to green house effects, they impoverish the soil.
Pesticides matter. Fungicides intended to combat plant diseases may harm VAM too. This is similar to problems with insect control since some insects are beneficial while others are harmful. Pesticides must be well targeted and always understood to be risky business that has costs as well as benefits. Anti bacterials are similar. The harm to human digestive flora from antibiotic use is analogous to the issues with soil fungi. Some way to sort the good guys from the bad guys is useful.
The muck and mystery of dirt farming has long been ringed by fetishes and superstition because we don't understand soil science. Though unsettling and difficult to defend intellectually, many traditional superstitions work! We can't dismiss them simply because they are superstitions, and can benefit from taking them seriously while investigating the detailed mechanisms involved. They are clues to good science.