Plant derived nitrous oxide emissions from intensively grazed dairy pastures

The Intergovernmental Panel on Climate Change (IPCC) includes above- and below-ground residues of all non-N and N-fixing crops in its definition of crop residues. Residues from pastures and from perennial forage crops are only accounted for during pasture renewal. The IPCC also confirms that the nitrogen (N) contained in crop residues in arable systems can contribute significantly to N cycling and be a significant source of nitrous oxide (N₂O) emissions. Despite the fact that 70% of the world’s agricultural area and 90% of New Zealand’s total farm area are pastoral systems, the current IPCC methodology does not consider the potential contribution of pasture residues outside of the renewal period with respect to N₂O emissions. Nitrous oxide is an obligate intermediate in the denitrification process and a by-product of nitrification. These microbial processes cause N₂O to be released from soil into the troposphere. Rates of N₂O emission and microbial pathways for production are dependent, amongst other factors, on soil water content and inorganic N in the soil. Therefore, the questions posed here were: Do pasture residues (collectively called ‘litter’) occur in significant quantities during grazing? And what is the role of herbage embodied-N with respect to N2O emissions? The overall objective of the research was to quantify the contribution of such plant-derived N₂O emissions in intensively grazed dairy pastures to New Zealand’s agricultural greenhouse gas emissions inventory. Experiment 1 (Chapter 4), was a field survey performed at Lincoln University Dairy Farm (LUDF), to quantify grazing-induced litter-fall i.e. the fraction of freshly harvested but un-ingested litter dropped by dairy cattle while grazing. Each paddock at the LUDF was grazed 12 times annually. This research showed, for the first time, that the rate of fresh litter fall equated to 53 ± 24 kg DM ha–1 per grazing event in an intensively grazed dairy pasture and was equal to 4% of the apparent dry matter consumption of the dairy cattle. Annually, fresh and senesced litter equated to N application rates of 15.9 kg N ha–1 y–1 and 3.5 kg N ha–1 y–1, respectively. The aforementioned quantities of litter-fall formed the rationale for further experiments. Experiment 2 (Chapter 5), a field study conducted in two parts (A and B), examined the effect of simulated animal treading on herbage decomposition and its implications on N₂O emissions. Presence or absence of herbage did not affect the N₂O emissions with N₂O emissions increasing regardless of the herbage presence. Soil NO3– levels declined due to treading, presumably due to induced anaerobic conditions and denitrification. The results were confirmed using a 15N technique (part B) which showed that a major fraction of the N₂O emitted under herbage-trodden pasture originated from the soil inorganic N pool. However, the 15N enrichment of the inorganic N pool also showed that the size of the soil inorganic-N pool was diluted due to N being released from either the herbage or the soil organic matter pools as a consequence of treading. Experiment 3 (Chapter 6) investigated the effect of incorporating litter of the dominant New Zealand pasture species (clover and ryegrass) and a pasture supplement (maize) with soil, at two soil water contents (54 and 86% water-filled pore space (WFPS)), incubated at 20oC. At field capacity (86% WFPS), the emission factor (EF) of N₂O equated to 2–3% of the litter-N with no differences due to litter species, while at 54% WFPS, the EF was significantly less with 1.7% > 0.7% = 0.5% for clover, ryegrass and maize, respectively. The decomposition rates were also similar at 86% WFPS. The differences in N₂O emissions were attributed to the biochemical properties of the species’ litter, especially cellulose concentrations and their differing C: N ratios. To further investigate the role of biochemical composition, specifically the C: N ratio of the plant litter to contribute to N₂O emissions, increasing amounts of cellulose were mixed with a constant mass of clover litter and incorporated into a pastoral soil (Experiment 4; Chapter 7). Increasing the C: N ratio via cellulose addition enhanced N₂O emissions, indicating that the incorporated cellulose acted as a labile C source favouring denitrification. Higher N₂O emissions from the highest C: N ratio treatments showed that the biochemical availability of C played a critical role in litter-derived N₂O emissions. Therefore higher emissions observed from the clover litter incorporated in Experiment 3 were most likely due to the labile forms of C embodied in the clover leaf tissues and not just attributable to the amount of N in the litter. In Experiment 5 (Chapter 8), 15N-labelled ryegrass was placed on the surface of a pastoral soil in litterbags at the rate of 213 kg N ha–1 (simulating litter-fall) and N₂O and CO₂ emissions were measured. This current study is the first to report soil N dynamics and N₂O emissions using 15N-labelled pasture litter placed in situ. Approximately 70% of the N₂O originated from the litter when surface-applied. Emissions of N₂O likely resulted from ammonification followed by a coupling of nitrification and denitrification during litter decomposition on the soil surface. The litter contributed to both the 15N enrichment of soil NO3– and N₂O emissions which originated from litter-N. The 15N enrichment of the soil NO3– pool showed that litter-N enhanced the soil inorganic N pool, verifying the conclusions drawn in Experiment 2 (part B), where in situ treading of herbage led to an increase in the soil inorganic N pool as evidenced by the decrease in 15N enrichment of the NO3– pool. The EF of the in situ placed litter was 0.9%; similar to the IPCC default EF value of 1%. This suite of experiments has shown that the contribution(s) of herbage-N to N cycling and N2O emissions are significant, yet, not considered within the current IPCC methodology. If the litter-fall data is extrapolated using the various N contents and EFs measured in this thesis, litter-fall accounts for 4.5–10.9% of the total N₂O-N emitted due to dairy cattle. This thesis has also shown that 4% of the total pasture on-offer can be lost as litter-fall resulting in lower dry matter intake (DMI) of dairy cattle. If this is worked through the inventory calculations, the DMI remains unaffected. However, including the litter-fall-derived N₂O emissions in inventory calculations provides a more accurate and refined accounting of the N₂O-N released from grazed pasture N cycling. Before solid recommendations can be made to alter the IPCC inventory methodologies, further data on the effects of different grazing managements, animal and pasture species, and climate are needed.