Coordinated nitrogen and sulfur management in S-deficient soils

Experiments have shown that long-term management practices such as crop rotation, tillage, and fertilization schemes affect soil nutrient cycling and crop response to fertilizer. This study consists of measuring N₂O emissions from a long-term crop rotation and nutrient management study that has been conducted since 1929 at the University of Alberta Breton Plots Research Site. Flux measurements of N₂O have been taken for three growing seasons (2013, 2014, 2015) by taking weekly N₂O measurements on the following treatments: control, manure, NPKS, PKS, and NPK treatments within a 2-year, wheat-fallow rotation (wheat and fallow phases), and a 5-year, wheat-oats-barley-hay-hay rotation (wheat phase).

Emissions of N₂O may be derived from both non-fertilizer, and fertilizer N sources. The cumulative N₂O emissions from the 5-year, wheat-oats-barley-hay-hay (WOBHH) rotation were 2.5 times greater than the wheat-fallow rotation and this is attributed to the greater biologically-fixed N inputs during the hay phases of the WOBHH rotation. Management practices that inhibit the nitrification process (i.e. application of nitrification inhibitors) or increase the amount/activity of nitrous oxide (N₂O) reductase (the enzyme used in the conversion of N₂O to N₂) will be more successful at reducing N₂O emissions than management practices that increase crop N uptake. Despite having higher cumulative N₂O-N fluxes, the treatments with long-term, balanced fertilizer applications (NPKS) also had the highest grain yields and lowest yield-normalized N₂O emissions within each rotation. Higher N₂O emissions associated with higher N inputs are offset by increased crop yields. The 2013 and 2014 growing seasons had very similar cumulative N₂O fluxes, and these were greater than the much drier 2015 growing season. Emissions in the first two weeks following fertilizer application and seeding were much higher in 2013. These higher early season emissions were associated with near-saturated soil conditions and were likely a result of anaerobic denitrification. Soil moisture conditions at the time of fertilizer application should be considered. There is a positive relationship between soil total N in the top 6 in. of soil and cumulative N₂O-N emissions. The apparent relationship between soil N and N₂O-N observed here suggests that GHG offsets associated with soil C sequestration should be coupled with BMPs that reduce N₂O-N emissions to be most effective. Further, this suggests that in-field variations in soil C and N levels may result in in-field variations in N₂O emissions, suggesting that variable N management (i.e., precision agriculture) could be used to reduce N₂O emissions at larger scales. A final report for this project will be prepared in early 2016.