Best-practice grazed grassland management, which broadly aims to limit environmental impacts, relies on practices such as rotating grazing areas, maintaining plant cover year- round, irrigation, pasture renewal, periodic cropping of pastures and increasing pasture plant diversity away from conventional ryegrass and white clover. These practices, and others such as planting deeper-rooted pasture species, are also thought beneficial to soil carbon stocks. The effects of these practices are not easy to quantify due to labour-intensive measures and lack of replication between farms.
Scientists from Manaaki Whenua and the University of Waikato recently combined the available data from three dairy farms in Waikato and two in Canterbury over 68 site-years, between 2008 and 2022, to calculate how different management practices have affected soil carbon stocks. On these farms, the net CO₂ exchange of the pasture was continuously measured, and carbon removals and additions associated with grazing, harvesting, effluent and fertiliser application were monitored.
Although the data were limited to five farms and only four main soil types, the results make for interesting, and perhaps unexpected, reading. Soil carbon stocks under grazed pastures were largely steady-state. None of the management practices assessed on these farms showed increases in soil carbon stocks over time, other than when carbon was added in the form of manure or effluent. Some practices, such as periodic feed cropping and pasture renewal, led to net soil carbon loss, although some or all of the carbon could be recovered over subsequent years. Irrigation did not seem to make a difference to soil carbon in Canterbury, while enabling large increases in grass production.
The researchers also found no evidence in their data that pastures with moderately increased diversity (5 species) increased carbon stocks compared with conventional ryegrass/ white clover mixes. It appeared that use of a conventional ryegrass/white clover mix, the most common pasture mix in use in New Zealand over the past century, gave the best opportunity for maintaining soil carbon stocks in New Zealand’s temperate climate, while also producing enough biomass to support dairy cows. However, the studied farms did not use highly diverse species mixes or altered grazing management aiming for higher standing biomass after grazing, which are practices explored by the growing regenerative agriculture movement.
“Research into the carbon effects of these practices is underway” says Dr Johannes Laubach, a senior researcher in greenhouse gases at Manaaki Whenua, “including measurements of the greenhouse gases methane and nitrous oxide, to identify trade-offs or synergy effects of management practices on net greenhouse gas emissions.”
