Effect is of interest, although not significant
November 13, 2007 By Top Crop Manager
The sheer size and sensory impact of a thunderstorm can be overwhelming at
times. Granted, the combined effect of lightning strikes, high winds and torrential
rains are all realities with which growers must contend. But there is an interesting
facet of thunderstorms and lightning strikes, in particular, that can be of
benefit to a plant.
In the intense heat and pressure generated by lightning, the form of nitrogen
in the atmosphere is actually altered, making it easier for some plants to absorb.
According to Dr. John Lauzon, an assistant professor in the Land Resource Science
department at the University of Guelph, Ontario receives about 18kg to 20kg
of N per hectare per year from the atmosphere. "That number is made up
of two components and roughly half of it is related to atmospheric losses of
ammonia, with the other half related to the lightning strikes," says Lauzon.
Nitrogen gas (N2) accounts for about 78 percent of the
atmosphere, and can react with oxygen in conditions of high heat and pressure.
"When it does, it can form nitric oxide (NO) or nitrite (NO2),"
explains Lauzon. "Either one of those can then react with the water in
the atmosphere to form nitric acid (HNO3). Even if the
nitrite hits the soil before it forms nitric acid, the NO2
would quickly be converted to nitrate (NO3) in the soil."
An uncontrollable process
The total amount of nitrate being formed and delivered is not large, by
any means, adds Lauzon. "If you get 10 percent more storms, you go from
10kg of N per hectare to 11kg," he says, noting there might be a small
effect on a rural region to the lee of a large city.
The interesting facet to the process is the overall impact of lightning and
its utilization by plants. "It's making some N available to the plant that
wouldn't have been available before," says Lauzon. "Nitrogen gas in
the atmosphere is only available to the rhizobia in legume plants, so the process
is making it available to all plants, but it's a relatively small amount."
A fascinating twist
Dr. Terry Gillespie finds the whole situation intriguing and points out that
lightning has a complementary function, as well. A professor emeritus with the
Land Resource Science department at the University of Guelph, and a member of
the Agrometeorology Group, Gillespie notes the positive impacts of ozone being
created by lightning. The heat of lightning combines nitrogen and oxygen to
form nitric oxide (NO) or nitrite (NO2), as Lauzon states.
Ultraviolet light takes the NO2 apart and releases an
atom of oxygen (O) which combines with a molecule of oxygen (O2)
and makes ozone (O3).
"We need oxides of nitrogen, like nitric oxide, and Mother Nature makes
those with lightning, forest fires, volcanoes, anything with high temperatures,"
says Gillespie. "We like to have some ozone in the air because it's a good
cleanser, and it's only when we put excessive amounts of nitrogen oxides into
the air, as we do in the city, that ozone gets out of hand."
Ozone levels in the 20 to 30 parts per billion range are best for cleansing,
but once they reach more than 80ppb, they become an air quality issue.
Like Lauzon, Gillespie questions the degree to which thunderstorms can add
to the nitrogen availability in soil. "In addition to the delivery of nitric
acid, there is a gaseous path, so lightning is producing oxides of nitrogen,
and these in gaseous form are taken up in small amounts through the stomata
in the plant leaves," says Gillespie.
A matter of fact
In research conducted in the late 1990s at Texas A&M University, researchers
attempted to quantify the impact of the heat island effect on large urban
centres. The urban heat island (UHI) effect refers to the impact of higher
temperatures from increased traffic and urban development in metropolitan
areas. Temperature variations in these areas can range from two to six
degrees C higher than surrounding rural areas during the summer months.
The Texas A&M research found, among other things, that the city of Houston
experienced 40 percent more lightning strikes as a result of UHI.
One of the contentions about UHI is its impact on global warming. Opponents
of the phenomenon insist weather data gathered at metropolitan airports
is incorrect because of UHI, thus confounding readings that are used in
global warming models.