The Roots of the Matter

the 2017 mAGazine

David McNear studies the rhizosphere-the area where plant roots, microbes, and soil interact. With a $500,000 U.S Department of Agriculture grant, he is exploring how phosphorus moves through the rhizosphere in a no-till cover crop system.

David McNear demonstrates the use of rhizotron, which allows him to view the root system within a mesocosm.

David McNear demonstrates the use of rhizotron,
which allows him to view the root system within a mesocosm.

Phosphorus is a mined, finite resource only found in a few regions of the world. Some scientists expect, at current usage rates, phosphorus production will peak or plateau within the next 100 years. Therefore, it is important for farmers to efficiently use the mineral.

In the most cropping systems, a lot of applied phosphorus doesn't make it into the crop, affecting use efficiency and the farmers' bottom lines.

McNear, UK associate professor in soil science, is studying ways plants can get the most from phosphorus in the soil, which should help farmers better capitalize on their fertilizer investment while at the same time preserving the environment.

For his study, winter wheat is the cover crop, followed by corn.

"Plants only use inorganic phosphorus. However, between 30 and 80 percent of all phosphorus in the soil is organic," he said. "In no-till systems, where winter cover crops are used, the plant roots and the phosphorus within the cover crop remain in the soil, but we don't have a good idea how much of this phosphorus is accessible by the next crop. We want to try to quantify that and try to optimize the system by selecting wheat cultivars that are efficient at extracting phosphorus from the soil."

McNear, with the help of UK wheat breeder Dave Van Sanford, will screen wheat populations spanning 100 years to look for root attributes that could be favorable for phosphorus uptake. McNear will select the most promising varieties and, with the help of UK soil scientists John Grove and Josh McGrath, grow them under simulated field conditions in the new UK Agroecosystem Mesocosm Facility. The mesocosms, equipped with unique root viewing tubes called rhizotrons, allow him to photograph the roots during plant growth. This provides a unique look into the interaction of the remaining wheat roots with the newly planted corn roots.

As corn grows through the cover crop roots, it starts a process called rhizosphere priming, during which the new crop pushes carbon into the soil, feeding soil microbes. The microbes begin to consume soil organic matter including the wheat roots. They retain some of the nutrients, including phosphorus, for themselves, but they release some of it into the soil, where it could be available for the corn.

"Scientists have thoroughly explored this process with nitrogen and carbon, but not much has been done with phosphorus," McNear said. "There's little doubt that the process is similar with phosphorus. Until now, we just haven't had the tools to quantify just how significant a contribution this process makes to P fertility."

Knowing this could help improve fertilizer and cover crop recommendations to farmers, which could result in fewer fertilizer applications.