There's life under all that snow
By Anna Gray
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| New research by URI soil microbiologist Patrick Sorensen reveals the vulnerable process taking place as snowpacks shrink. |
When snow blankets the landscape, it may seem like life slows down. But beneath the surface, an entire world of activity is unfolding. “Unlike many plants or some animals, which tend to go dormant or are much less active during winter, soil microbes are actually very active under winter snowpacks,” says Patrick Sorensen, a soil microbiologist and assistant professor of soil ecology and biogeochemistry in the University of Rhode Island’s Department of Natural Resources Science.
All winter, soil microbes decompose organic matter, releasing nutrients that are critical for plants. By spring, these nutrients are perfectly timed to fuel new growth. Warming winters and reduced snow cover can disrupt this timing, allowing nutrients to wash into streams, escape into the air, or leave plants short of what they need.
Microbial bloom
When snow melts, microbial activity surges. The soil microbial population size “blooms” as it seizes nutrients released by melting snow and rising groundwater, then eventually “crashes” when the soil is depleted of these resources. This cycle produces a pulse of nitrogen that shapes soil fertility. While scientists have observed this pattern globally, the mechanisms behind it have been unclear.
New research by Sorensen and colleagues, published in Nature
Microbiology, shows that distinct groups of microbes orchestrate this
seasonal nitrogen cycling. Winter-adapted microbes thrive at low soil
temperatures, snowmelt specialists peak for a short window of time when soils
are saturated with snow melt, and spring-adapted microbes dominate once soils
warm up. Each group plays a different role in recycling nitrogen: winter and
snowmelt microbes break down organic compounds like amino acids for both energy
and biomass, while spring microbes help convert nitrogen into forms plants can
use, sometimes retaining it in the soil and sometimes allowing it to escape.
Rethinking nitrogen in soil
The study challenges long-held assumptions about how
microbes build biomass. Traditionally, scientists focused on a few inorganic
nitrogen transformations, but Sorensen’s team found that soil microbes actively
transform thousands of organic nitrogen compounds. They also discovered that
some microbes coordinate both organic and inorganic nitrogen transformations
and even interact with other species to recycle nitrogen more efficiently.
By using organic nitrogen to grow and generate energy,
different microbial groups coordinate complex nitrogen transformations under
snowpacks, and understanding these trait-based interactions may help predict
how nutrient cycling responds to climate change.
These findings suggest that soil nitrogen cycling is far
more dynamic and complex than previously understood. Depending on which
microbial groups are active and when, microbes can either promote nitrogen loss
by converting nitrate into gases that escape to the atmosphere or enhance
nitrogen retention by recycling it into forms available to plants or re-used by
other microbes.
“We were surprised to see how central organic nitrogen
recycling was to fueling such large microbial population changes,” Sorensen
says. “It shows that nitrogen cycling during snowmelt is driven not just by
simple inorganic transformations, but by complex interactions among microbes,
organic matter, and changing environmental conditions.”
Timing matters in a warming world
These seasonal microbial dynamics are tightly linked to
plant nutrient needs. Nutrients released during microbial population collapse
often become available just as plants begin growing in spring. But climate
change is threatening this synchronization.
“We think microbial nutrient release during winter and plant
nutrient uptake in spring are currently well aligned,” Sorensen says. “With
warmer winters and reduced snowpack, that timing could become decoupled.”
Earlier snowmelt and thinner snowpacks may reduce winter
microbial activity or shift it earlier in the year, increasing the risk that
nitrogen is lost to streams, lakes, and the atmosphere before plants can use
it. At Sorensen’s Colorado field site, snowmelt now occurs roughly three weeks
earlier than it did 50 years ago, and low-to-no snow winters are expected to
become increasingly common across the Western United States.
“It is important to emphasize that these stark changes in
our environment are likely to happen within many of our lifetimes, not in some
distant time in the future,” he says. “This could adversely affect forest
health in both the Western and Eastern United States; for example, increasing
the frequency of forest fires out West or increasing outbreaks of tree
pathogens in Eastern forests. We may have better options to manage adverse
outcomes if we have a better understanding of overwinter microbial processes.”
The research was made possible by Sorensen’s collaboration
among scientists with expertise in microbial ecology, biogeochemistry,
genomics, and metabolomics; he says his co-authors’ unique expertise made the
project truly a “team-science effort.”
Looking ahead, he is eager to explore new questions raised
by the work, including whether antifreeze compounds produced by microbes
beneath snowpacks influence methane cycling during snowmelt, a connection
previously observed in marine systems but not yet documented in soils.
Ultimately, Sorensen hopes the research encourages people to see snowy ecosystems in a new light. “The next time you’re cross-country skiing or snowshoeing through a forest,” he says, “pause to appreciate that there’s a robust, active microbial community living—and likely thriving—beneath the snow.”
