The type of fertilizer used, and the manner in which it is applied, can make or break reduced tillage's ability to control greenhouse gases, Agricultural Research Service (ARS) scientists report.
No-till and reduced tillage are promoted as a way farmers can reduce greenhouse gas concentrations in the atmosphere by storing more carbon in soil. But there has been limited information on how tillage or other farm practices affect soil emissions of greenhouse gases other than carbon dioxide.
A study conducted by ARS soil scientist Rod Venterea on the effects of long-term tillage techniques and fertilizer practices has shown that, if not done with care, reduced tillage practices can increase emissions of more powerful greenhouse gases, particularly nitrous oxide. At 300 times the strength of carbon dioxide, nitrous oxide can easily offset the benefit of carbon dioxide reduction. Venterea works at the ARS Soil and Water Management Unit in St. Paul, Minn.
Farm fields are the biggest source of nitrous oxide emissions in the United States, with up to one-third of the agricultural emissions coming from farms in the north central region of the country.
Venterea and colleagues have shown that farmers using no-till should inject nitrogen fertilizer more than 4 inches below the soil surface, beneath the layer of soil that is most conducive to nitrous oxide production.
In field tests, Venterea and his colleagues compared the nitrous oxide emissions from three different tillage systems in combination with anhydrous ammonia, urea nitrogen fertilizer pellets, or liquid urea ammonium nitrate.
Anhydrous ammonia caused about double the losses of nitrous oxide than the other two fertilizers. But combining no-till with anhydrous ammonia injected 6 to 8 inches deep emitted the least nitrous oxide of the three tillage-anhydrous ammonia combinations tested.
In contrast, spreading urea nitrogen fertilizer pellets on a field's surface caused higher nitrous oxide emissions under no-till compared to more intense tillage. Tillage had no effect on emissions when liquid urea ammonium nitrate was applied to the surface.
Venterea conducted the studies in southeastern Minnesota from 2003 to 2004, using soil chambers to capture nitrous oxide emissions.
The research was published in the Journal of Environmental Quality.
ARS is the U.S. Department of Agriculture's chief scientific research agency.