Farm-scale carbon and nitrogen fluxes in pastoral dairy production systems using different nitrogen fertilizer regimes

The nitrogen (N) fertilizer application rate (kg ha/1 year) in pastoral dairy systems affects the flow of N through the soil, plant and animal pools of the system. With better understanding of the magnitude of these pools and their fluxes, dairy systems could be managed to improve N use efficiency, therefore reducing losses to the environment. A study with three levels of N fertilizer, 0 (N0), 150 (N150) and 300 (N300) kg N ha−1 year−1, was conducted in the Canterbury region of New Zealand from 1 June 2017 till 31 May 2018. Farm measurements, e.g. pasture and milk production, were used to calibrate three different farm-scale models, DairyNZ's Whole Farm Model, DairyMod, and Overseer®. The models were used to extrapolate periodic farm measurements to predictions of carbon (C) and N pools and fluxes on an annual basis. Pasture and milk production per hectare increased from N0 to N300 by 70% and 58%, respectively. There was a concomitant increase in farm-gate N surplus (input-output) of 43%, resulting in predicted increases in N leaching and greenhouse gas emissions of 72% and 67%, respectively. By increasing N fertilizer from 0 to 300 kg N ha−1 year−1, 53% more feed N flowed through the dairy herd with surplus N deposited as urinary N increasing by 49%. Plant uptake and soil immobilization increased by 58% and 343%, respectively, but not enough to avoid substantial increases in leaching and emission losses. Carbon flux through the soil system increased through increased litter and faecal deposition, but with very little C sequestration because of accelerated microbial respiration rates.