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.