Emissions of nitrous oxide from the leaves of grasses

Nitrous oxide (N2O) emissions from pastoral agriculture are considered to originate from the soil as a consequence of microbial activity during soil nitrification and denitrification. However, recent studies have identified the plant canopy as a potentially significant source of N2O emissions to the atmosphere. Understanding the extent and mechanisms of plant emissions may provide new mitigation opportunities as current options only target soil microbial processes. We developed an experimental apparatus and protocol to partition N2O emissions between the leaves of grasses and the soil and measured emissions from ten common grass species found in New Zealand pastures. The chamber design enabled us to identify measurable changes in N2O concentration over a period of 1 h and to distinguish a range of emissions from 0.001 to 0.25 mg N2O-N/m(2) leaf area/h. There was a 10-fold variation among species; Holcus lanataus, Lolium perenne and Paspalum dilatatum had the highest leaf N2O emissions and Poa annua the lowest. Grasses do emit N2O from their leaves and the rate that this occurs varies among grass species. The emission does not appear to arise from formation of N2O in plant leaves but more likely reflects transport of N2O from the soil. Differences in emission rates appear to arise from a plant influence on the rate of formation of N2O in the soil rather than the rate of transportation through the plant.