The efficacy of Plantago lanceolata for mitigating nitrous oxide emissions from cattle urine patches

Urine deposited by grazing animals is the main source of nitrous oxide (N₂O) emissions in New Zealand. Recent studies have suggested that certain pasture plants, for example plantain (Plantago lanceolata), can curb N₂O emissions from livestock systems. This study aimed to i) evaluate the potential of plantain for reducing N₂O emissions from cattle urine patches; ii) determine the effect of including plantain in animal diets on urine-N loading and its influence on N₂O emissions; and, iii) evaluate whether any effects on N₂O emissions reduction could be attributed to a 'urine' or a 'plant' effect. A static chamber method was used to measure N₂O fluxes from urine collected from cows fed a 0, 15, 30 or 45% plantain mixed with "standard" ryegrass/clover (Lolium perenne/Trifolium repens) diet and applied to plots with the corresponding percentage of plantain in the sward. In addition, we measured N₂O emissions from different proportions of plantain in the sward (0, 30, 60 and 100%) that received urine collected from cows fed on ryegrass/clover. The urine N loading rates of animals fed plantain, significantly reduced with increasing proportions of plantain in the diet (r² = 0.987, P < 0.01). There was a trend of lower N₂O emissions with an increasing proportion of plantain in the diet (r² = 0.830, P < 0.08). However, there was no significantly difference in the N₂O emission factors (P > 0.10). Following applications of standard urine, total N₂O emissions and emission factor reduced linearly as the proportion of plantain in the sward increased (r² = 0.969, P < 0.05 and 0.974, P < 0.05, respectively). The results suggest that the efficacy of plantain as a N₂O mitigation option is due to both a reduction in urinary N excretion and a plant effect. The latter could be due to biological nitrification inhibition (BNI) caused by the release of root exudates and/or changes in the soil microclimate.