Measuring the emission of greenhouse gases from croplands should take into account the crops themselves.
That's the conclusion of a study in the Sept.-Oct. issue of the
Journal of Environmental Quality, which examined the impact of farm
practices such as tillage on the greenhouse gas, nitrous oxide.
Expressing emissions per unit of crop yield rather than on a more
conventional per area basis produced very different results, says the
study's leader, Rod Venterea, research soil scientist with the United
States Department of Agriculture's Agricultural Research Service.
In particular, his team found that total nitrous oxide emissions were
not significantly affected by tillage practices when expressed on an
area basis. When they were calculated per unit yield of grain, however,
emissions were significantly greater under no-tillage compared with
conventional tillage. A byproduct of many agricultural systems, nitrous
oxide is a potent greenhouse gas (GHG) with a heat-trapping potential
more than 300 times that of carbon dioxide.
The findings have important implications for how the greenhouse gases
generated by agriculture are reported, evaluated, and potentially
mitigated. Nitrous oxide emissions were slighter higher under no-till on
a per area basis in the study, Venterea explains, but not high enough
to differ statistically from those under conventional tillage. "But when
we added in the fact that no-tillage also reduced yields, the effect of
tillage did become significant," he says. "The point is that you need
to look at both nitrous oxide emissions and yield together."
While previous studies have shown that practices like fertilizer and
tillage management can affect nitrous oxide emissions, relatively few
have reported the effects of these practices on crop performance at the
same time. In addition, GHG emissions are commonly expressed with
respect to area of field: for example, kilogram nitrous oxide emitted
per hectare. Recent research has suggested that expressing GHG emissions
per unit of yield may be more meaningful, although few studies have
actually done that.
To see how yield-scaled calculations might change the picture on
emissions, USDA-ARS researchers in collaboration with University of
Minnesota colleagues measured the effects of tillage and nitrogen (N)
fertilizer management on nitrous oxide emissions, grain yields, and crop
N uptake over three consecutive growing seasons in Minnesota. The
experiment was conducted in research plots used for corn and soybean
production, which were maintained under either no-till or conventional
tillage for 18 years.
When the scientists calculated nitrous oxide emissions per unit yield
of grain or grain N, they found that emissions under no-tillage were 52
and 66% higher, respectively, than with conventional tillage. In other
words, for this cropping system and climate, Venterea says, no-till
practices would generate substantially more nitrous oxide than would
conventional tillage for the same amount of grain. The effect was due to
lower yields under no-till, combined with slightly greater area-scaled
nitrous oxide emissions.
Reduced yields under continuous no-till management in parts of the
upper Midwest and other regions have been attributed to lower soil
temperatures in spring, which may inhibit plant development. In other
geographic regions, though, no-till can actually increase yields.
"So, for these other regions, expressing GHG emissions on a
yield-basis could reveal benefits to no-till management that otherwise
might not be quantified," Venterea says
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