Did you and your grade school friends ever find yourselves in intricate negotiations around the lunch table, trading that boring snack your mom packed you with the sweeter and more enticing dessert in your friend’s lunchbox? Well, similar to you and your childhood friends, plants also partake in such a trading of commodities around their own ‘lunch table’.
Plants use their packed lunch — newly photosynthesized carbon — to build biomass, such as new leaves, taller stems, and a larger root system, while also pumping some of these carbon compounds into the soil. In some cases, more than 10% of the carbon photosynthesized by the plant flows from plant roots out into the soil! This begs the question: why do some plants release such a significant amount of carbon to the soil? What advantage does this process (rhizodeposition) confer to the plant? Part of the answer lies in understanding the challenge plants face in acquiring nutrients such as nitrogen from the soil.
In contrast to our extensive knowledge of how plants capture carbon via photosynthesis above-ground, scientists know much less of how plants take up nutrients below-ground – in part because it’s simply more difficult to study the hidden world below our feet! Also, while the atmosphere is relatively homogeneous and CO2-rich, soil is much more heterogeneous and lacking in nutrients. In fact, most nutrients in the soil are bound up in complex organic compounds that aren’t directly accessible to plant roots; rather, soil microbes must first decompose this organic material, releasing nutrients into the soil water where plant roots can more easily slurp up these nutrients. 
This relationship between plants, microbes, and soil nutrient availability gets at one reason plants appear to release carbon into the soil. Specifically, carbon released from plant roots stimulates the growth and activity of the soil microbial community which can then enhance how much organic matter microbes decompose and increase soil nutrient availability for the plant. That’s right, there’s no free lunch in nature – plants must use the carbon compounds they build by photosynthesis to acquire nutrients, and one way they do this is by exuding carbon to feed microbial decomposers!
My research in Rich Phillips’ Ecosystem Ecology lab at Indiana University is aimed at better understanding this negotiation at the lunch table — that is, the relationship between plant carbon allocation and nutrient acquisition below-ground. I’m trying to understand how the carbon ‘cost’ of acquiring nutrients varies among different plant species and across ecosystems that vary in soil nutrient availability and are host to different types of soil microbes. For example, we expect that plants should feed the microbial community little carbon if there is already a lot of nitrogen readily-available to the plant. The plant is also less likely to actively release carbon into the soil if the microbes present in the soil don’t efficiently enhance nitrogen availability for the plant, which could happen either because the microbes aren’t good at decomposing the specific organic compounds in the soil or because the microbes keep most of the nitrogen for themselves.
A recent greenhouse study I conducted shows tree species that associate with different types of microbes vary in the amount of carbon they release into the soil. However, there doesn’t seem to be a predictable relationship between the amount of carbon released by roots and plant nutrient uptake. It appears that trees use a suite of strategies to acquire soil nutrients, including associating with different types of microbes and varying the amount of carbon released by roots; surprisingly, some species are able to take up significant amounts of nitrogen without releasing much carbon into the soil. I am planning additional studies to better understand when and where trees employ different strategies to access soil nutrients and to tease apart this tripartite interaction between plants, microbes and soil nutrient availability.
Our lab’s research is starting to tackle this ecological research challenge as we recognize that being able to better quantify how much carbon plants are putting into the soil and how this affects plants’ ability to take up essential nutrients is critical to predicting how our natural ecosystems will respond to human-induced changes in element cycles, such as increased atmospheric CO2 concentrations and nitrogen deposition.
Differences in plant root characteristics, the soil microbial community and soil nutrient availability all play a role in dictating how much carbon a plant must spend below-ground to take up a given quantity of nitrogen. Just as trading snacks at your lunch table might not have always been a simple give and take and likely varied depending on what was in your lunch box that day, the relationship between plants and microbes trading carbon and nutrients is clearly complex.
Edited by Maria Tiongco and Katherine VanDenBurgh
Looks like you are on to something big! And as a compliment, IU has a bbc video on the Science of Soil