The Prairie Soil Forming Engine is Running on Empty

My attention drifted from my field notebook to the cow. She was standing about 50 feet away in the field just beyond the wetland, motionless and very curious. I brushed the muck from my jeans and stood up. The cypress dome around me was one of several wetlands interspersed throughout this West-Central Florida ranch. Apparently, this cow wanted to know what I was doing there. I had come to this site to study the soil organic carbon, to learn about the ways it supports the above-ground plant community and the rates of accumulation and loss. I hadn’t been expecting to come face to face, literally, with agriculture.

But there we were.

People have been transforming natural landscapes to ranches and farms for decades, ditching and draining marshes, like those in Florida, and plowing under tallgrass prairies, like those near the Chicago region. The expansive prairies that occupied most of the Midwest US had some of the deepest and richest soils in the world, and so those prairies have been largely lost to wide stretches of fertile agricultural lands that feed millions. In order to add to the few prairies that remained after the agricultural boom, efforts to conserve or restore this habitat continue. However, in addition to the desire to save a disappearing habitat, there’s a compelling need to bring back the prairies, and the natural processes that produced so much value in the first place. A high-quality tallgrass prairie is bio-diverse, resilient, and generates deep, nutrient-rich soil. The type of soil which is now desperately in short supply.

Soil exists as an interface between the living and the dead at the Earth’s surface. This vibrant ecosystem is a recycling boundary layer, an amalgamation of weathering minerals, decomposing plant matter and animal waste, and a community of fungi, insects, worms, and microorganisms. Healthy soil enables the exchange of water, nutrients, and gases that support living organisms - above and within the soil. Animals, like my leering Florida cow, participate in this process by eating grasses and then excreting waste, worms and microorganisms turn this manure into soil that supports the growth of more grasses, which will in turn feed more animals and their offspring.

To make a soil fertile to plants, there must be a sufficient supply of water and nutrients. The big three, nitrogen-phosphorus-potassium (NPK), are the nutrients that are essential to life. Nitrogen and phosphorus are elemental components of DNA and other essential organic compounds in living organisms, and potassium is important in the flow of water and nutrients within plants. All of these nutrients may be found in a healthy, productive soil because of the activity of soil organisms. Phosphorus is available from certain minerals and is also biologically cycled back into the soil from the decomposition of organisms and plants. The geologic processes that turn rock into clay minerals add potassium to soil because they release the potassium that was bound in the mineral structure of the rock. This is why some clay forming minerals, like vermiculite, are great in gardening.

Frequently, the most limiting element to plant growth is nitrogen. Most nitrogen is obtained from the atmosphere, where there is a huge pool available in gas form. However, the two nitrogen atoms that make up the gas molecule (N2) are held together by one of the strongest chemical bonds found in nature, a N-N triple bond. Before a plant can use this source of nitrogen, those bonds must be broken. The nitrogen that is released must then be made soluble in the soil medium. All of this is accomplished chiefly through specialized soil bacteria, like rhizobia. These bacteria use the enzyme nitrogenase to “fix” the nitrogen into ammonia (NH3), which is then converted to a soluble ammonium ion (NH4+) which is available to plants in the soil. Legumes are often associated with symbiotic forms of the nitrogen fixing bacteria, so these plants are beneficial to the soil fertility. The decay of animal waste will also produce soluble, active nitrogen in the soil. The most dramatic nitrogen producer is lightening, which literally has the power to break the N2 bond and deliver nitrogen into the soil in the form of nitrates (NO3-).

When we consider that healthy, nutrient-rich soils are a valuable but limited resource, the time it takes to replenish the supply becomes significant. The rate of soil formation, or pedogenesis, varies greatly across biomes. Forest soil develops under a canopy of trees and associated vegetation, much of which is usually decades old or older. Wetlands generate their deep, rich peat by a continuous addition of dead plant material onto a mat of saturated and slowly decomposing organic matter. And prairies balance a succession of productive grasses and forbs throughout the growing season with the decomposition of the previous growing season’s cast-offs. Each of these different natural habitats promotes the development of a unique set of soil organisms that complete the complex ecosystem interactions and cycles that require decades or more to reach optimal soil productivity. And the most fertile and desirable soils for agriculture may take hundreds, if not thousands, of years to form in nature.

Unfortunately, soil building processes are much slower than the rate at which soil is lost once land has been converted to agriculture. When crops are planted and harvested on a large scale, soils can lose their sustaining ecosystems and any remaining soil forming processes usually fail to keep pace with the extraction of nutrients. When the original vegetation is removed, soil erosion may increase due to a loss of supporting ground cover and root systems. As upper layers of soil are lost to wind or water erosion, underlying soil is exposed to weathering, increasing decomposition and oxidation and loss of this soil, causing a feedback that may accelerate soil loss over time. This is all being made much worse by climate change. Intense, heavy rainfall patterns are projected to continue in the Midwest, driving soil erosion rates on farms that is unsustainable.

However, the climate change debate has also led to discussions about the potential for soil to sequester some of the excess atmospheric carbon. Soil, as it turns out, may sequester considerable stores of carbon for long periods of time (the details are a topic for a future newsletter). International and national conferences and agreements have set into motion policies that are directed at creating incentives to sequester soil carbon and there are already several countries quantifying and utilizing their soil resources in this way. All of this renewed interest means the benefits to the soil and dependent ecosystems are now measurable in a whole new way. For example, the US federal government produces a USDA National Greenhouse Gas Inventory Report, regularly updating it to reflect the changing science.

Back at the ranch in Florida, my cow friend lost interest in me and joined a caravan of other cows walking in a line towards a beckoning bull in the distance. I glanced back down at the wetland muck and began taking samples. In the years to follow, I would take more samples elsewhere, in prairies and forest ecosystems, and wonder – how much of this rich soil, and all the life connected to it, will still be here in the future?

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