Research Objective
To calculate a residue-induced nitrous oxide (N2O) emission factor for the major pulse crops and wheat grown in Saskatchewan.
Objective 4To determine the total contribution (mass balance) of residue-derived N and C into SOM pools and into the biomass growth of a subsequent wheat crop in rotation with the pulses.
To calculate a residue-induced nitrous oxide (N2O) emission factor for the major pulse crops and wheat grown in Saskatchewan; to provide a side-by-side comparison of biological nitrogen fixation (BNF) and net N2O and CO2 budgets from the major pulse crops (and a wheat control) grown in Saskatchewan; to determine the partitioning of nitrogen (N) and carbon (C) in above- and below-ground residues from different pulses to soil organic matter (SOM) pools; to determine the total contribution (mass balance) of residue-derived N and C into SOM pools and into the biomass growth of a subsequent wheat crop in rotation with the pulses.
Wheat grown after the pulses yielded 5% to 25% higher than wheat after wheat. The greatest increases (15– 25%) occurred on lentil and pea residues. Nitrogen (N) content of wheat seed was 5% and 10% greater following lentil and pea. In general, 4% to 6% of above-ground-N and 6% to 12% of below-ground-N in pulse residues was exported with wheat seed. 20% to 30% of urea-N was recovered in wheat.
Overall, pulse residues contributed 6.5 to 16.0 kg N ha-1 to wheat. Nitrous oxide fluxes during the wheat year were greater when the previous crop was a pulse vs wheat. Emissions over the four-year experiment were highest from fertilized wheat, lentil and chickpea residues and lowest from pea and faba bean residues. Below-ground residues contributed 21% of N2O-N vs 12% from above-ground residues and 22% from soil N. The most N2O from pulse residues was released during spring thaw – indicating that considerable N mineralization occurred after harvest. On average, more CO2 was emitted during the pulse phase (5.46 Mg CO2-C ha 1) than the wheat phase (4.50 Mg CO2-C ha-1). Above-ground residues generally resulted in 2-3 times more CO2 than below-ground residues. In short, growing a pulse crop before wheat provided a yield benefit to wheat which was partially due to the N provided from residue decomposition. Pea and lentil residues underwent more extensive decomposition post-harvest than chickpea and faba bean residues and contributed more to wheat yield. However, it appears that this enhanced decomposition also means more N2O was produced, especially with the lentil below-ground residue.