Researchers leveraged the unique land management gradient available at the LTER to better understand how microbe diversity impacts nitrogen fixation rates in the soil. They looked at an understudied group of soil microbes, diazotrophs, which are responsible for a key source of nitrogen in agricultural soils.
Increasing aboveground biodiversity is known to promote beneficial ecosystem services. However, less is known about the benefits of biodiversity in the soil, particularly in the microbe communities that facilitate critical nutrient cycling.
New research from the KBS LTER provides valuable insight into the biodiversity of an important, yet cryptic, group of soil microbes: diazotrophs. Why are they important? Diazotrophs can convert atmospheric nitrogen into ammonium through a process called free-living nitrogen fixation, providing a sustainable nitrogen source in agricultural soils. If we can identify land management practices that optimize this beneficial nitrogen source from diazotrophs, farmers could reduce their reliance on chemical fertilizers.
Researchers, led by MSU graduate student Brandon Kristy from Sarah Evans’ lab, wanted to determine if sustainable land management practices could boost soil diazotroph biodiversity, and if higher diazotroph biodiversity would promote greater nitrogen fixation. To do this, they took advantage of the extensive land-management gradient available in the Main Cropping System Experiment (MCSE) at the KBS LTER.
They compared diazotroph diversity and free-living nitrogen fixation activity of annual cropping soils, perennial soils, and forest soils sampled from the MCSE. They also took it a step further, manipulating diazotroph biodiversity in the lab by fumigating soil microcosms.
In the field, they found that higher diazotroph diversity did not increase nitrogen fixation. For example, annual row-crop soils harbored the highest diazotroph diversity, but perennial soils had the highest nitrogen fixation, and soil abiotic factors like moisture and temperature best predicted high nitrogen fixation.
However, the lab experiment showed a stronger correlation between diversity and nitrogen fixation, the strength of which varied by land management practice. Notably, when they fumigated soils in the lab, perennial soils maintained diazotroph diversity, suggesting that they’re more resilient to extreme disturbance.
The research provides a new window into soil microbial communities, which are often understudied but play a central role in sustainable agriculture. Brandon Kristy noted, “This was a fantastic collaboration that further demonstrates the importance of our long-term field sites for understanding how specific management practices, like integrating perennial crops into agricultural landscapes, can boost beneficial ecosystem services. “