Nutrient returns from pasture litterfall during grazing

Abstract

Soil respiration results in carbon dioxide (CO₂) emissions from pasture ecosystems. This is a function of root respiration, rhizosphere respiration, oxidation of soil organic matter (SOM) and litter decomposition; the latter two make a major contribution. Quantification of litterfall during grazing has not been performed in dairy pastures. In this study, close observation of grazing animal behaviour revealed that a fraction of the animal harvested herbage is not ingested and falls on to the pasture soil surface. To quantify this litter, the pasture soil surface was vacuumed before and after each grazing event. Litterfall yields were 72.4 ± 31.2, 53.0 ± 24.4 and 19.4 ± 17.6 kg DM ha⁻¹ (± SD, n = 150) for total litter, post-grazing-fresh (POGF) and post-grazing-senesced (POGS) litters, respectively. Extrapolating this data on an annual basis indicated that 253 and 92 kg C ha⁻¹ y⁻¹ could be applied to the pasture soil as POGF and POGS. Corresponding values for direct CO₂ emissions were calculated to be 81 and 30 kg CO₂-C ha⁻¹ y⁻¹ when using an emission factor (EF) of 32% generated from a laboratory study using clover (Trifolium repens L.) and ryegrass (Lolium perenne L.). Contributions of nitrous oxide (N₂O), another greenhouse gas, were also investigated, and on a CO₂-equivalents basis accounted for 32-76% of the greenhouse gas emissions. The study shows that a small but important fraction of pasture litter contributes to carbon and nitrogen cycling in pastures.