Effects of dairy farming management practices on carbon balances in New Zealand’s grazed grasslands: Synthesis from 68 site-years

Globally, the soils underlying managed grasslands are an important reservoir of carbon (C), with the management of the land strongly influencing whether the soils gain, lose or maintain C. Further, improved grassland management, such as rotational grazing and increased sward diversity, are widely promoted to increase soil C sequestration despite limited evidence. Using data from New Zealand’s grazed dairy grasslands, we examined the effect of management practices on C stocks through measurement of the net ecosystem C balance (NECB) across 68 site-years. Management practices tested included year-round rotational grazing, differing pasture sward composition, irrigation, periodic supplemental feed cropping and pasture renewal. Excluding years where supplemental feed cropping occurred, the mean (±95% confidence interval) NECB was −12 ± 30 g C m−2 y−1 (65 site-years; the negative value represents C loss), indicating the soil C stocks of these ecosystems were likely near steady-state. The processes of pasture renewal and the transition between pasture and crops resulted in reduced C inputs from photosynthesis of between −232 and −94 g C m−2. This reduced C input was partially overcome with the application of effluent or manure at the time of cultivation where applied. There was no difference in NECB between irrigated (−26 ± 114 g C m−2 y−1) and non-irrigated (−19 ± 198 g C m−2 y−1) pastures. In general, conventional ryegrass/white clover pastures were more beneficial for C stocks than alternative pasture species and mixes. Periodic cropping for supplemental feed resulted in C losses, with the magnitude depending on crop type and management, although data were limited to three site-years.