Rutledge, S., Mudge, P.L., Campbell, D.I., Woodward, S.L., Goodrich, J.P., Wall, A.M., Kirschbaum, M.U.F., Schipper, L.A. (2015). Carbon balance of an intensively grazed temperate dairy pasture over four years. Agriculture, Ecosystems and Environment 206: 10-20.

Abstract. We estimated the net ecosystem carbon (C) balance (NECB) of a temperate pasture in the North Island of New Zealand for four years (2008–2011). The pasture was intensively managed with addition of fertiliser and year-round rotational grazing by dairy cows. Climatic conditions and management practices had a large impact on CO₂ exchange, with a severe drought in one year and cultivation in another both causing large short-term (~3 months) net losses of CO₂–C (100–200 g C m^-2). However, CO₂ was regained later in both of these years so that on annual timescales, the site was a CO₂ sink or CO₂ neutral. Management practices such as effluent application and harvesting silage also influenced non-CO₂–C fluxes, and had a large impact on annual NECB. Despite these major environmental or management perturbations, both NEP and NECB were relatively constant on annual timescales. It is likely that this apparent resilience of the CO₂ and C balance to perturbations was at least partly attributable to the relatively warm temperatures, also in winter, providing good growing conditions year-round (in the absence of major perturbations such as moisture stress). In several instances, the farmer’s decisions aimed at maintaining a constant milk yield between years also appeared to contribute to a relatively stable C balance.

Averaged over the full four-year study period, the site was a net sink for both CO₂ (NEP = 165 ± 51 g C m^-2 y-1), and total C (NECB = 61 ± 53 g C m^-2 y^-1) after non-CO₂–C fluxes were accounted for. Annual NEP and NECB values were similar to results collated from other managed temperate grasslands on mineral soils globally, for which average NEP and NECB were 188 ± 44 g C m^-2 y^-1 and 44 ± 33 g C m^-2 y^-1, respectively. In the global dataset, we noted a general trend for increased C sequestration with increasing NEP, suggesting that it may be possible to meet the dual goal of increased pasture production (thus milk, meat and fiber production) and increasing soil C storage in managed temperate grasslands. Identification of management practices that increase C storage while maintaining or enhancing pasture production requires more standardised reporting between NECB studies, and experiments involving side-by-side comparison of treatment and control plots.

Keywords: Net ecosystem exchange; Carbon dioxide; Grassland; Pasture; Grazed agriculture; Eddy covariance; Cultivation; Carbon balance.

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