Submitted to: Agronomy for Sustainable Development
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/10/2007
Publication Date: 12/28/2007
Citation: Phillips, R.L. 2007. Denitrification in frozen agricultural soil. A review. Agronomy for Sustainable Development 28:1-7. Interpretive Summary: Up to 27% of the nitrogen (N) in cropping systems is lost to the atmosphere by soil microbes that use plant available N when oxygen is limited, such as when soils are saturated. This process, called denitrification, reduces the amount of fertilizer-N, depletes soil organic carbon, and emits gases associated with global warming. Practitioners of sustainable agronomy aim to improve plant N-use efficiency and reduce emissions of the greenhouse gas nitrous oxide by synchronizing N application and plant nutritional requirements. Denitrification occurs both in summer and winter, but our current understanding is based on data collected during the growing season. This review summarizes knowledge of denitrification during the growing season and points to gaps in the knowledge during the off-season (see figure below). Sustainable management of N in cropping systems (greater N-use efficiency and lower greenhouse gas emissions) could be facilitated by understanding of denitrification in winter.
Technical Abstract: Nitrogen (N) in agricultural fertilizers is denitrified by soil bacteria when oxygen is limited, which effectively removes plant-available N from the soil to the atmosphere. Reported denitrification rates range from 0 to 239 kg N ha-1 yr-1 and may reduce the amount of N available for crop growth by 27%, depending upon conditions. Additionally, one of the gaseous products of microbial denitrification, nitrous oxide (N2O), is a recognized pollutant that contributes to stratospheric ozone destruction and radiative forcing in the troposphere. Practitioners of sustainable agronomy aim to improve plant N-use efficiency and reduce emissions of the greenhouse gas N2O by synchronizing N application and plant nutritional requirements. It is difficult to predict denitrification rates based on our current knowledge, both during and after the growing season, but high rates are consistently reported in irrigated cropping systems following heavy applications of fertilizer-N. Denitrification at sub-zero soil temperatures may represent a significant sink for fertilizer-N in cropping systems, yet factors controlling denitrification are not clearly defined at sub-zero soil temperatures. Here, the three factors required for microbial denitrification (limited O2 availability, electron donors and electron acceptors) are reviewed based on soil research performed both above and below 0° C. Gaps in the knowledge of denitrification rates in cropping systems during the off-season, particularly when soils are frozen, are identified. Research questions are posed which aim to unravel how denitrification might be influenced by sub-zero soil temperatures, compared to soil above 0° C. Sustainable management of N in cropping systems (greater N-use efficiency and lower greenhouse gas emissions) could be facilitated by understanding of denitrification during the off-season in winter.