Nitrous oxide emissions from grazed grasslands: interactions between the N cycle and climate change - a New Zealand case study

Nitrous oxide (N2O) is the third most important greenhouse gas globally. In grazed livestock systems, excreta deposited by grazing animals are the largest source of N2O. Understanding the effect of climate change on N cycling within the plant-soil-animal continuum is critical for future-proofing N2O mitigation strategies for these systems. We describe the impacts of elevated CO2 (eCO(2)), elevated temperatures (eTemp) and elevated or reduced rainfall (eRain or rRain, respectively) on: first, the rates of N cycling processes that produce N2O and second, the amount of urinary N supply as affected by pasture growth and composition, and the N content of the pasture. We use climate change predictions for New Zealand as a case-study example to assess climate change implications for mitigating N2O emissions from grazed grasslands. Climate change effects on soil N processes will most likely result in an increase in nitrification, NH3 volatilisation and denitrification. The effects on mineralisation and N leaching are relatively uncertain. However, overall N2O emissions are likely to increase in grazed grasslands in New Zealand as a result of primarily eTemp and eRain effects on N cycling process rates. The effects of eCO(2) on net primary production, legume growth and biological N fixation are likely to increase the amount of urine N deposited to pastures in the short term, thus also increasing N2O emissions. Although we are more certain about increased N2O emissions due to the effects on N cycling processes, we are less certain about climate change effects on the size of the urinary N source. Development of future-proof N2O mitigation options will require a detailed understanding of the interactions between CO2 and N fertilisation, C/N ratios and microbial activity and integration of component studies at a field scale to fully understand farm systems' responses to climate change.