Mitigation potential and trade-offs for nitrous oxide emissions and carbon balances of irrigated mixed-species and ryegrass-clover pastures

Abstract

Grazed grasslands are an important source of N2O emissions. Grasslands also store significant amounts of carbon (C), and net changes to these amounts contribute to their net greenhouse gas budgets. We investigated whether net N2O emissions from irrigated pasture grazed by dairy cows can be reduced by planting more diverse species, compared with conventional ryegrass-clover pasture, and whether there are co-benefits for greenhouse gas reduction by net C gains in the ecosystem, or trade-offs through net C losses. Fluxes of CO2 and N2O were measured by eddy covariance, near the boundary of adjacent five-species mixed pasture (MIX) and ryegrass-white clover pasture (RyWC). A split-footprint approach was applied to separate the CO2 and N2O exchange for the two pastures. The CO2 fluxes were gap-filled with marginal distribution sampling (MDS), and the N2O fluxes with k-nearest-neighbour (kNN) regression. To enable calculation of the net ecosystem C balances (NECB) of the two pastures, non-CO2 carbon imports and exports were also determined. Annual N2O emissions from the MIX pasture were 0.14 g N m−2 yr−1 (mean of two years), compared with 0.23 g N m−2 yr−1 from the RyWC pasture. The N2O emissions accounted for 0.34% and 0.61% of the nitrogen inputs to the MIX and RyWC pasture, respectively. Combining all C gains and losses, both pastures recorded net ecosystem C gains in the first year, of 44 and 282 g C m−2 for MIX and RyWC, respectively. In the second year, both recorded losses: −207 g C m−2 for MIX and −86 g C m−2 for RyWC. Thus, in both years, the net C balance gave the RyWC pasture a considerable advantage over the MIX pasture in the net greenhouse gas balance, and this advantage was one magnitude greater than the difference in N2O emissions, when expressed in CO2-equivalent units.