Nitrogen and carbon footprints of dairy farm systems in China and New Zealand, as influenced by productivity, feed sources and mitigations

•N and C footprint of milk assessed for Chinese housing and NZ grazing dairy systems.•N loss to water was important in NZ while ammonia loss dominated for China.•N footprint decreased by up to 10% with low-N feeds.•In China, optimal manure management could decrease N footprint by over 30%.•In NZ, using clover N fixation and no N fertilizer could reduce N footprint by 25%.