Do Cover Crops Increase or Decrease Nitrous Oxide Emissions in Agroecosystems? A Meta-Analysis.

There are many well researched ecological benefits to incorporating cover crops into crop rotations, such as their potential to decrease soil erosion, reduce nitrate leaching and increase soil organic matter.  Some of these benefits impact other elements of the agroecosystem given the coupling of nutrient cycles, notably C and N.  Some field research does point toward an increase in nitrous oxide emissions where cover crops are present.  This work, however, is limited and comes from sites utilizing a range of cover crops generally measured on short time scales (2 years or less).  Thus, a meta-analysis of this research can provide a more robust investigation into the influence of cover crops on nitrous oxide emissions.  To date, 25 peer reviewed articles resulting in 104 data points were analyzed according to their response ratio, the natural log of the N₂O flux with a cover crop divided by the N₂O flux without a cover crop.  There was a significant interaction between cover crop type (grass or legume) and N fertilization rate.  When no nitrogen was applied, a legume cover crop had a response ratio of 1.26, indicating an average N₂O increase in these systems of 26%.  Grass cover cropped systems had an average response ratio of 1 at zero N rates, indicating a net zero N₂O change (p=0.01 between these groups).   Field studies that measured during the two months following cover crop kill and incorporation had the highest mean response ratios as compared to other parts of the cropping season.  The effect of soil pH is also significant, indicating that an increase in pH leads to an increase in response ratio.  Cover crops have the potential to increase or decrease nitrous oxide emissions, depending upon the N fertilization level, soil pH, period of measurement and type of cover crop (grass or legume).