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

Grassland soils are arguably the most important store of terrestrial carbon, accounting for approximately 22% of terrestrial C stocks. Over the years, management of these grasslands, including the conversion of native vegetation to planted pastures, has resulted in considerable C loss. This highlights that the lost C should be recaptured and future losses avoided, which depends on how these planted pastures are managed. The recapture of lost C has been proposed as a mechanism to mitigate GHG emissions since it should increase soil carbon stocks. Ways to do it include improved pasture management with for example rotational grazing and increased sward diversity. The purpose of the study cited was to examine the effect of management practices on C stocks through measurement of the net ecosystem C balance (NECB) across 68 site-years of New Zealand dairy farms.

Management practices tested included year-round rotational grazing, differing pasture sward composition, irrigation, periodic supplemental feed cropping and pasture renewal.

The results showed that the mean NECB was – 12 ± 30 g C per square meter per year (the negative value means C loss), indicating the soil C stocks of these systems 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 per square meter. This reduced C input was partially overcome with the application of effluent or manure at the time of cultivation. Overall, there was no difference in NECB between irrigated (- 26 ± 114 g C per square meter per year) and non-irrigated (- 19 ± 198 g C per square meter per year) 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. The same applies to pasture renewal, but in both cases some or all of the C loss could be recovered during subsequent years.

It was concluded that the clearest trend across all management swards, was that the use of ryegrass/white clover provides the best opportunity to maintain soil C stocks in New Zealand’s temperate climate, while also producing sufficient biomass for dairy cows. However, it is an open question whether this will remain so under a climate that is warming and characterized by more extreme rainfall and drought events.

Comments: The results cannot be extrapolated to a sub-tropical climate and also not to the pasture dairy production areas of South Africa. Apart from climate, the base level of soil carbon in general is much lower at about 2.5 to 5% than in New Zealand which could be up to 7% +, and therefore close to saturation. Thus, the type of research reported here should also be done in South Africa.