Altering ruminant urine composition to reduce urine patch nitrous oxide emissions

Abstract

Grazed pasture systems are a significant source of nitrous oxide (N2O), a potent greenhouse gas and ozone-depleting substance. In grazed pastures, N2O emissions are driven by ruminant livestock urine nitrogen (N) deposition onto soils. Urine N is typically deposited at a rate that exceeds the pasture plant’s immediate N uptake capacity, therefore excess N may be lost from the system as N2O. Two novel methods for mitigating urine patch N2O emissions were identified and evaluated in this PhD: 1. Altering urine N composition to increase the proportion of urine N excreted as non-urea urine nitrogen compounds (NUNCs). These NUNCs may be less labile forms of N, capable of stimulating plant N uptake, or forms of N that degrade to compounds which inhibit nitrification, a key step in soil N2O production. 2. Pasture species may contain active plant secondary metabolites (PSMs) capable of inhibiting nitrification in soil. After grazing ruminant livestock consume PSMs in their forage diet, the livestock would excrete them in their urine, thereby directly applying a nitrification inhibitor to the urine patch. The PSM, aucubin, in the pasture herb species Plantago lanceolata (plantain) was identified for its potential to inhibit nitrification in the urine patch. In Chapter 3, the potential for varying urine N composition to alter urine patch N2O emissions was evaluated in a laboratory trial and a field trial. The laboratory trial tracked the fates of two 15N-labelled NUNCs in soil and the field trial determined the effect of increasing the proportion of urine N excreted as NUNCs, rather than as urea, on urine patch N2O emissions. In the laboratory trial, the 15N-labelled NUNCs, creatine and hypoxanthine, degraded in soil within 102 hours, and significantly contributed to both the soil N and plant N pools within 48 hours. In the field trial, increasing the proportion of urine N excreted as any of the NUNCs did not alter urine patch N2O emissions or any other measured variable. It was concluded that NUNCs rapidly degrade in soils and contribute to inorganic N pools that are substrates for urine patch N2O emissions, similarly to urea N. Therefore, this proposed urine patch N2O mitigation technique is not viable and should not be further pursued. In Chapter 4, both a plantain leaf extract (PLE) and an aucubin solution (AS) were applied with urine or urea (700 kg N ha-1) under laboratory conditions, to mimic livestock excreting PSMs from plantain. This experiment determined whether plantain contained a PSM that could inhibit nitrification in the urine patch, and whether that PSM was aucubin. Overall average soil N2O flux was significantly lower in the Urea + PLE and Urea + AS treatments than in the Urine treatment. Additionally, soil nitrate (NO3-) concentrations on Day 29 were significantly lower in the Urea + AS, Urine + AS, and Urine + PLE treatments, when compared to the Urine treatment. These soil NO3- and N2O flux results indicated that aucubin inhibited nitrification when applied to soil. A subsequent field trial was performed to evaluate the in situ effects of adding PLE or AS to urine (700 kg N ha-1). Soil N2O emissions were lower in both the Urine + PLE and Urine + AS treatments, when compared to the Urine treatment, but the reduction was only statistically significant in the Urine + AS treatment. However, the lack of significant differences in the soil inorganic N data indicated that nitrification inhibition occurred. This field experiment also evaluated whether urine patch N2O flux or NO3- accumulation were reduced in plantain pasture soil, due to the presence of aucubin released via root exudation, but no marked differences were observed. Chapter 5 re-evaluated the in situ effects of adding aucubin to the urine patch, at a lower urine N application rate (500 kg N ha-1), which has been identified as a more typical urine N loading rate from cows grazing plantain. Statistically significant differences in soil NO3- concentrations and soil surface pH indicated that nitrification inhibition occurred 4-7 days after urine application, when aucubin was added in urine. However, this period of inhibitory activity was not sufficient to produce a significant reduction in N2O emissions over the 35 day experiment. It was concluded that this rate of aucubin application in urine was not sufficient to reduce urine patch N2O emissions. In Chapter 6, three rates of aucubin application (47, 243, and 487 kg ha-1) were added to urine and assessed for their potential to inhibit nitrification and subsequent N2O emissions. These rates represented 10, 50, and 100% of the highest calculated potential aucubin excretion rate from cows grazing 100% plantain pasture. Similar to the results reported in Chapters 4 and 5, there was a period from 7-17 days after urine application in the two higher aucubin treatments, where soil N and soil pH measurements indicated that aucubin inhibited nitrification. However, similar to Chapter 5, this period of activity was not sufficient to reduce overall N2O emissions during the experimental period. It was concluded that further research of aucubin as a nitrification inhibitor was warranted, due to the periods of nitrification inhibition activity observed in the studies in Chapters 4-6. Further research is needed on: (i) the input rates and pathways of aucubin input into pasture soils via livestock urinary excretion, plantain root exudation, and/or decomposition of residual herbage; (ii) the fate of aucubin in soil using isotope tracing methods; and (iii) molecular studies to identify the effect of aucubin on soil nitrifiers.