Dinitrogen (N2) and nitrous oxide (N2O) fluxes from grazed pasture soil after cattle urine deposition : A thesis submitted in partial fulfilment of the requirements for the Degree of Doctor of Philosophy at Lincoln University

Abstract

Cattle, grazing pastures, deposit urine onto the soil at high nitrogen (N) rates that exceed the pasture's immediate N demands, increasing the risk of N loss. Nitrous oxide (N2O), a potent greenhouse gas, and dinitrogen (N2) are lost from the cattle urine patches. There is limited information on the in situ loss of N2 from grazed-pasture systems, which is needed for understanding pasture soil N dynamics and balances. The aim of this PhD project was to improve our understanding of the N2O and N2 fluxes from urine-affected pasture soils. There were two field experiments and one laboratory experiment conducted in this PhD project. Different urinary-N rates (400 kg N ha-1 and 800 kg N ha-1), soil types (Te Kowhai clay loam and Horotiu silt loam) and antecedent soil moisture effects on regulating N2O and N2 fluxes from the urine-affected pasture soils were investigated using the 15N flux method. At the end of the project, the N2O and N2 fluxes measured from the field studies were used to validate the performance of denitrification in a process-based model (APSIM) to identify the potential gaps in the model processes that require further improvement to predict N-cycling in grazed pasture. In the first field experiment (Expt. 1), the higher urinary-N rate (800 kg N ha-1) increased the N2O fluxes but did not necessarily produce higher N2 fluxes compared to the lower rate (400 kg N ha-1). It is speculated that the higher urine-N deposited on soil potentially resulted in higher N loss via ammonia volatilisation and nitrate leaching. In the laboratory incubation experiment (Expt. 2), the drier antecedent soil moisture did increase soil N2O emissions, after rewetting with a NO3- solution (to mimic the peak NO3- concentration evolved from urine deposition), due to the “Birch effect” but there was no significant effect on N2 emissions. The antecedent dry soil condition might have delayed the recovery of N2O reductase and therefore increased the N2O/(N2O+N2) product ratio but this effect seemed to only last for 2-3 days. Results from the second field experiment (Expt. 3), showed that soil type plays an important role in regulating N-cycling and in turn affects the gaseous fluxes after urine deposition. The poorly drained Te Kowhai soil emitted more N2O compared to the well-drained Horotiu soil after urine application at the same N rate (800 kg N ha-1) under identical climatic conditions. However, there was no difference in denitrification-derived N2 fluxes between the two soils. The emissions period was aligned with the time when soil NO3- in the 15 - 30 cm depth was > 20 mg N kg-1. This further suggests that N2 fluxes from the field might be mainly produced at deeper soil depths and regulated by the soil factors in the deeper layers. In both field studies, codenitrification was detected but with varied contributions to total N2 productions. In the first field experiment (Expt. 1), codenitrification contributed 1.5% to 2.1% of total N2 production in the Te Kowhai soil after urine deposition while the contributions were 0.89% in the Horotiu soil and 12% in the Te Kowhai soil in the second field study (Expt. 3). Denitrification was found to be the predominant N2 production pathway in pasture soil after urine deposition in this study. However, the role of codenitrification in N2 production after urine deposition needs to be further investigated. The N2O/(N2O+N2) product ratio observed in the field experiments in this PhD project did not correlate with any measured soil factors. This is attributed to the complexity of multiple N transformations happening simultaneously after urine deposition. This indicates that the N2O/(N2O+N2) product ratio might not be a good indicator to predict N2O and N2 fluxes from the field, especially after urine deposition. The poor performance in the prediction of denitrification-related gaseous emissions (N2O and N2) in APSIM using the field flux datasets, despite changing model parameters, suggests that more field N2 fluxes under different conditions should be obtained in the future for the development of the N-cycling process-based model. This study highlights the importance of a comprehensive understanding of the whole N cycle when predicting soil denitrification losses (N2O and N2) from such systems.