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

External nitrogen (N) input into pastoral dairy systems is an important driver of productivity. Nitrogen fertilizer rate (kg N/ha/year) is one such an input that affects the flow of N through the soil, plant and animal pools of the system, either as mineral N or, because of its close association with carbon (C), as organic matter. The magnitude of the pools and fluxes are mediated by microbial communities, which, if understood better, could be managed to improve efficiency of converting input into output, therefore reducing losses to the environment. A farmlet experiment with three levels of N fertilizer, (0 (N0), 150 (N150) and 300 (N300) kg N/ha/year), was conducted in the Canterbury region of New Zealand from 1 June 2017 till 31 May 2018. Farm measurements of pastures and animals 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 C and N pools and fluxes on an annual basis. As expected, pasture and milk production per hectare increased from N0 to N300 by 70 and 58%, respectively. But withThere 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. The pasture production, milk production, N surplus, N leaching and GHG emissions of the N150 system was intermediate between the N0 and N300 treatments. With increased N fertilizer more feed-N flows flowed through the dairy herd with more surplus N deposited as urinary N, which increases increased the soil mineral N pool. Plant uptake and immobilization increased as well, but not enough to avoid substantial increases in leaching and emission losses. These rResults suggest that the rate of C flux through the soil system is accelerated by adding more N fertilizer, with very little C sequestration happening because of accelerated microbial respiration rates. It is suggested that, apart from limiting N fertilizer input into pastoral dairy systems, microbiome management should prioritize N processing in the rumen where N gets dissociated from organic matter, but also has the potential to be incorporated into microbial biomass with a resultant reduction in environmental losses.