The symbiosis between mycorrhizal fungi and plants drive carbon and nitrogen cycles. Fungi demand carbon exudate from plants in exchange for nitrogen and other nutrients retrieved and transported from the soil. The “liquid carbon” exuded from plant roots feeds mycorrhizal fungi and many other soil microbes, while also becoming stabilized in soil aggregates and humus. Jones explains that when this mycorrhizal exchange is inhibited by N fertilizer, which allows plants to absorb nitrogen “for free” (without providing liquid carbon in exchange), this reduces the flow of carbon into the soils, which in turn diminishes fungal networks and their delivery of micronutrients to plant hosts, and results in carbon-depleted soils.
Despite its abundance in the atmosphere, nitrogen is frequently the most limiting element for plants. There is a reason for this. Carbon, essential to photosynthesis and soil function, occurs as a trace gas, carbon dioxide, currently comprising 0.04% of the atmosphere. The most efficient way to transform CO2 to stable organic soil complexes (containing both C and N) is via the liquid carbon pathway. The requirement for biologically-fixed nitrogen drives this process.
If plants were able to access nitrogen directly from the atmosphere, their growth would be impeded by the absence of carbon-rich topsoil. We are witnessing an analogous situation in agriculture today. When inorganic nitrogen is provided, the supply of carbon to associative nitrogen fixing microbes is inhibited, resulting in carbon-depleted soils.
Reduced carbon flows impact a vast network of microbial communities, restricting the availability of essential minerals, trace elements, vitamins and hormones required for plant tolerance to environmental stresses such as frost and drought and resistance to insects and disease. Lowered micronutrient densities in plants also translate to reduced nutritional value of food [Jones 2014: 2-3].
Jones further explains how to modify agricultural practices to protect and build the soil: maintain year-round living ground cover, limit nitrogen and phosphorus fertilizer input, promote plant and microbial diversity, and integrate livestock into crop production systems.
Jones, Christine, 2014, Nitrogen: the double-edged sword, retrieved on July 5, 2018 from http://www.amazingcarbon.com/PDF/JONES%20%27Nitrogen%27%20(21July14).pdf