YSM Issue 93.2

Ecology
FOCUS
Results
To analyze the effect of SOM on growth,
the researchers used a statistical method
called regression to quantify the impact of
each measured variable on plant growth. The
regression models showed that aboveground
plant growth increased as SOM levels
increased until a threshold concentration
of around five percent, after which wheat
biomass began to decline. For soils with
optimum irrigation, this decline started
occurring at around six percent SOM.
Across all SOM concentrations, the
biggest difference in aboveground biomass
was observed between the two experimental
extremes: the pots with optimum fertilizer
and irrigation versus the pots with no
fertilizer and half irrigation. However, this
difference was largest at the lowest one
percent SOM concentration (pots with
optimum treatment produced 3.45 times
more aboveground biomass) and became less
dramatic when SOM levels were at or greater
than give percent (optimum pots produced
1.6 times more biomass). This supports the
hypothesis that SOM contribution can, in
some cases, compensate for plants that are
not receiving any supplemental input and
substitute in for mineral N fertilizer. But this
raises more questions of cost and reward—
will productivity of mineral fertilized soils
always outpace that of soils sustained by
organic matter alone? And what about
the reverse hypothesis: can added mineral
nitrogen fertilizer easily compensate for
lower SOM levels?
Nitrification
Though SOM levels did not seem to
exhibit a strong correlative relationship
with net rates of nitrification, they did have
an impact on net rates of N mineralization,
the process by which organic nitrogen is
converted to plant accessible inorganic
forms. As SOM levels increased, rates of
N mineralization increased. This effect
was greater in fertilized soils compared
to unfertilized soils. However, after SOM
concentrations passed a specific threshold
(around seven percent), pots with optimum
treatment began experiencing decreases in
net rates of nitrification: the plants had less
nitrogen accessible to them at eight percent
SOM as opposed to five percent SOM.
Oldfield hypothesizes that this eventual
decrease in nitrification rate may be related
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to increased microbial biomass that is
correlated with higher SOM concentrations.
These microbes themselves need to draw
upon specific nutrients in the soil, including
nitrogen, phosphorous and sulfur, which
may lead to a competitive environment
for nutrients and oxygen in the soil. In
such an environment, less resources
are available for plant use, which could
explain why productivity began to decline
instead of leveling off at the highest SOM
concentrations. “However, it’s very hard
to get a holistic picture of the forms of
nitrogen. A follow up study would be almost
the exact same experimental setup, just
with different levels of nitrogen fertilizer,”
Oldfield said. This would help determine if
these nutritional elements become limiting
at high levels of SOM.
Final Conclusions
Returning to the original question,
can soil organic matter substitute for
agricultural inputs such as insufficient
fertilization and irrigation? These results,
obtained by the systematic variation of
variables, demonstrate an optimistic
answer: building up SOM levels in soil will
have beneficial impacts on productivity.
Though it may not be a perfect replacement
for N fertilizer, SOM can still help cut back
on costly fertilizer inputs without risking
a lowered yield. “We know through other
research that’s being done right now that
agricultural soils tend to have very low
organic matter concentrations as a result of
tillage and other conventional practices...
You rarely see farm soil that is nine percent
organic matter,” said Oldfield when asked
whether the SOM threshold of five percent
would pose a problem.
Some scientists and agriculturists
continue to argue that though productivity
may increase with higher SOM
concentrations, these benefits will never
outpace or outweigh those brought about
by additional mineral fertilizer. However,
this perspective fails to take into account
the cost and availability of fertilizer. “There
are potential outcomes that don’t directly
translate to yield but are enhancements
in other environmental outcomes that we
do care about. This could be mitigating
agricultural runoff to improve water
quality, improving biological activity of
microbial communities, and enhancing
carbon sequestration,” Oldfield said.
What’s next?
Given that many groups such as the
USDA and policy makers rely on the
general notion that “more is better” when
it pertains to SOM levels in soil, Oldfield
is determined to continue delving into the
nuances and intricacies of organic matter
in soil. She briefly explains how increasing
organic matter could pose drawbacks:
increased SOM concentrations are related
to increases in nitrous oxide emissions, a
very potent greenhouse gas. “I’m interested
in linking [this research] to other outcomes
besides yield,” she says. Her ultimate
research goal is to run this experiment on
a much larger scale and get the “full farm
look,” so she can not only measure crop
growth, but also bigger profitability issues
such as balancing yield against costs and
observing ecosystem outcomes. ■
ART BY ANASTHASIA SHILOV
CINDY KUANG
CINDY KUANG is a first-year prospective Neuroscience major in Timothy Dwight College. In
addition to writing for YSM, she also participates in Danceworks and the Chinese American
Students Association.
for threshold effects of soil organic matter on crop growth. Ecological Applications
Soil Use and
Management
of temperate regions: a review. Soil & Tillage Research
September 2020 Yale Scientific Magazine 17