Soils contain the largest pool of phosphorus on Earth. Yet, farmers need to add phosphorus to their fields to help crops grow. This is called the Phosphorus Paradox: there is so much phosphorus in soils, yet plant growth is limited by this nutrient. The tricky part is that most of the phosphorus in soils is locked away in forms not available to plants. How can plants access the phosphorus they need to grow?
Phosphorus minerals in soil can be broken down and released, physically or chemically. This is called weathering. Plants can accelerate the amount of weathering that occurs in soils, especially thanks to the hard work put in by their root systems. This work comes at a cost, though. When a plant puts energy into growing below-ground, that energy is not being used for other types of growth. For example, if cash crops have to put in extra effort to get at hard-to-reach phosphorus, the crop yield may ultimately be lower.
Cover Crops to the Rescue!
Cover crops are already valued by farmers for their benefits to soils – protecting them from erosion, improving structure, and enriching them with organic matter. There is some evidence that cover crops may even help with phosphorus!
I was interested in studying how cover crops might team up with microbes to access soil phosphorus and make this nutrient available to cash crops. The idea is to see if cover crops can influence the amount of phosphorus available in the soil. To better understand this process, it is useful to see how cover crops interact with microbes known for taking up phosphorus. Perhaps the most famous of these microbes are the arbuscular mycorrhizal fungi (AMF). AMF colonize the roots of plants and navigate the soil in search of nutrients, like phosphorus. Think of this like a barter system. The plant has food (tasty carbon molecules) to offer the fungi as a reward for finding some of the nutrients in the soil that the plant most desperately needs to survive. Hence, cover crops and AMF might cooperate to get phosphorus from hard-to-reach places.
My Experiment at the KBS LTER
The KBS LTER Main Cropping System Experiment provides a great opportunity to test these questions. I used soils from plots managed with two different agricultural treatments. T1 represents a conventional corn-soy-wheat rotation with fertilizers and T4 is organically managed corn-soy-wheat with cover crops.
After over 30 years of continual treatments, the soils and phosphorus in these plots look very different from one another. T1 soil has much higher available phosphorus for plants – much like what farmers add to their fields today. T4 is much lower in available phosphorus but generally has more phosphorus stored in organic matter. This creates a realistic comparison of soil phosphorus scenarios that might be faced by different types of farmers based on the history of practices used on their fields.
So far, I have used these soils in potting experiments to grow different types of cover crops. I suppress the naturally occurring AMF, and then reintroduce specific AMF to my plant pots to compare how much phosphorus the cover crops take up when AMF is present or absent. Trials are ongoing, but preliminary results suggest that different types of cover crops vary in their response to AMF colonization and the relative availability of soil phosphorus. Some cover crops appear to benefit greatly from AMF when it comes to their phosphorus uptake, while others do not. More research is on the way!
My goal is to learn more about when, where, and how cover crops and AMF are able to team up to process soil phosphorus. With this knowledge, growers might be able to justify phosphorus cycling as a reason to plant particular cover crop species. Also, understanding the importance of soil biology, such as AMF, might incentivize growers to adopt management practices that support these native species. Conservation practices, like cover crops, offer benefits to both the farm and to the environment.