Driving cars, flying across the country, and runoff from agriculture fields are all sources of excess nutrients that can contaminate natural ecosystems. Although an extra supply of nutrients may sound like a good thing, the overabundance of nutrients is actually a global concern because many organisms have special adaptations to low nutrients. Arid (or dry) ecosystems are of particular concern because even atmospheric dust increases the amount of nutrients in these environments, which means that the source of excess nutrients can be hundreds or thousands of miles away.
Normally, nitrogen becomes available in dry environments, such as deserts, through the conversion of nitrogen in the atmosphere to a form that plants can use, a process called “nitrogen fixation”. These tasks are often performed by leguminous plants (like peas and beans) and microbes. However, besides the challenge from nutrient deposition, dry ecosystems are also vulnerable to excess nutrients caused by encroachment (or taking over of the landscape) by nitrogen-fixing shrubs. These leguminous shrubs can eliminate space for native plants to grow and survive. “This landscape transformation is a serious problem in drylands” says Baldarelli. “These shrubs tend to be less palatable than the native perennial grasses and the loss of biodiversity can create ecosystem-level challenges, not to mention the loss of aesthetic value” she adds. Specifically, excess nutrients through nutrient deposition or nitrogen-fixing shrubs can shock microbes, and may alter their typical ecological functions such as fixing nitrogen.
Two researchers at Kent State, Lauren Baldarelli, PhD Candidate and Dr. David Ward, Art and Margaret Herrick Endowed Professor of Plant Biology in Kent State University’s Department of Biological Sciences, along with Dr. Scott Collins in the Department of Biology at the University of New Mexico, determined how nitrogen-fixing plants and soil microbes contribute to the overall nitrogen availability in the Chihuahuan desert.
In their recently published paper in Plant and Soil, "How encroaching shrubs and nutrients affect N2-fixation in the Chihuahuan desert", Baldarelli, Ward and Collins determined the effects of nutrient deposition and shrub encroachment on nitrogen fixation of soil microbes. They found that microbes fixed less nitrogen when nitrogen was readily available. This suggests that microbes would rather benefit from the existing nitrogen rather than use the great amount of energy it takes to fix it, or that the excess nitrogen could have negative effects on microbial communities. This is something these researchers hope to address in the future.
They also found that, when encroaching, shrubs contribute more available nitrogen to the landscape than soil microbes. However, it’s important to note that this study was a snapshot in time and only suggests what is happening in the springtime before the start of the monsoons. These researchers predict a potentially different outcome in other seasons. “It’s possible that microbes may contribute more fixed nitrogen than vascular plants after the monsoon season as microbes are quickly activated and are ready for nutrient transformation by even the smallest pulse of precipitation”, says Baldarelli. Nonetheless, soil microbes are incredible nitrogen fixers based on the large proportion of the soil surface that they occupy. This work is extremely relevant as nutrient deposition continues to rise due to growing cities and the expansion of development into our natural lands.
For more information about this research, contact Lauren Baldarelli at lbaldare@kent.edu
The full research article in the journal Plant and Soil can be viewed here https://rdcu.be/coBsx.