Intensive cattle grazing affects pasture litter-fall: an unrecognized nitrous oxide source

The rationale for this study came from observing grazing dairy cattle dropping freshly harvested plant material onto the soil surface, hereafter called litter-fall. The Intergovernmental Panel on Climate Change (IPCC) guidelines include NO emissions during pasture renewal but do not consider NO emissions that may result from litter-fall. The objectives of this study were to determine litter-fall rates and to assess indicative NO emission factors (EFs) for the dominant pasture species (perennial ryegrass [ L.] and white clover [ L.]). Herbage was vacuumed from intensively managed dairy pastures before and after 30 different grazing events when cows (84 cows ha) grazed for 24 h according to a rotational system; the interval between grazing events ranged from 21 to 30 d. A laboratory incubation study was performed to assess potential EF values for the pasture species at two soil moisture contents. Finely ground pasture material was incubated under controlled laboratory conditions with soil, and the NO emissions were measured until rates returned to control levels. On average, pre- and postgrazing dry matter yields per grazing event were 2516 ± 636 and 1167 ± 265 kg DM ha (±SD), respectively. Pregrazing litter was absent, whereas postgrazing fresh and senesced litter-fall rates were 53 ± 24 and 19 ± 18 kg DM ha, respectively. Annually, the rotational grazing system resulted in 12 grazing events where fresh litter-fall equaed to 16 kg N ha yr to the soil. Emission factors in the laboratory experiment indicated that the EF for perennial ryegrass and white clover ranged from 0.7 to 3.1%. If such EF values should also occur under field conditions, then we estimate that litter-fall induces an NO emission rate of 0.3 kg NO ha yr. Litter-fall as a source of NO in grazed pastures requires further assessment.