With a growing population and ever more devastating consequences of climate change, we face one of humanity’s biggest challenges: to feed the world while nourishing soils well into an uncertain future. Perhaps we can partner with soil microbes to provide crop plants with just the right nutrients at just the right time, building up our soils while also getting the most out of them. As well, we can try to cultivate soils and soil microbes that are resilient to the stresses of climate change. My research mission is to determine how much nitrogen soil microbes can free from resources already available in the soil, like soil organic matter and decomposing plant debris, and how their capabilities are enhanced or suppressed by biological and climate factors.
While results are still forthcoming, here are the main projects I have been working on during my PhD.
Do roots encourage microbes to release nitrogen from organic matter?
I examined the nitrogen-mobilizing properties of microbes living in the rooting zone of growing plants, comparing it to soil in which no roots are present. I surrounded soil with mesh of two different sizes to permit or block roots from entering. I installed these mesh-encased soil samples between the roots of corn plants growing in agricultural plots under high-intensity and low-intensity management*. After 42 and 84 days of burial I retrieved soil from within the mesh and measured: a number of soil properties; amount of carbon and nitrogen present and their distribution between different chemical forms; nitrogen-acquiring enzyme activity; the rate of plant-available nitrogen production; and two different measures of the microbial biomass and identity of soil microbes.
* high-intensity = chisel plow tillage, inorganic fertilizer addition, and bare fallow / low-intensity = ridge tillage, no added fertilizer, ryegrass cover crop
Does drought change how microbes break down complex nitrogen?
I exposed soils to normal or drought-stressed water conditions and determined how microbes and nitrogen cycling respond to these water dynamics. I kept soil collected from organically- and conventionally-managed agricultural plots under either ideal moisture conditions or repeated drying and rewetting to simulate droughts likely to occur with climate change. After eight weeks (and four dry-rewet cycles) I measured similar soil and microbial properties as in the root project described above. Here I also used isotopic techniques to determine with greater accuracy than standard methods the rate at which microbes generate plant-available nitrogen, as well as the rate of a key preliminary step in this process in which microbes break proteins into amino acids.
How does the identity of soil microbes matter for nitrogen cycling?
In both experiments I use high-throughput sequencing of DNA to identify the particular mixture of microbial species present, thereby elucidating how roots and drought shape microbial communities. I also look to connect these nitrogen and microbial observations to begin pinpointing which microbes—or groups of microbes—most efficiently and effectively mobilize soil nitrogen. I perform all of these experiments in soils exposed to two different agricultural regimes (e.g. conventional vs. organic), allowing me to determine whether agricultural management strategies can hinder or promote plant-microbe relationships that nourish crops.
Agricultural management decisions shape soil microbial communities
I am gathering results from studies that sequenced microbial genes in agricultural soils under contrasting management. Genetic sequencing technology is still relatively new, and the results of high-resolution snapshots of soil microbes have not yet been gathered together to discover generalizable patterns about how agricultural management shapes their communities. By uniting and interpreting the results of studies comparing microbial communities under contrasting fertilization, tillage, and cropping practices, I am developing a more integrated picture describing how soil microbes respond to agricultural management.