Articles | Volume 13, issue 10
Biogeosciences, 13, 2927–2944, 2016

Special issue: OzFlux: a network for the study of ecosystem carbon and water...

Biogeosciences, 13, 2927–2944, 2016

Research article 18 May 2016

Research article | 18 May 2016

Carbon budgets for an irrigated intensively grazed dairy pasture and an unirrigated winter-grazed pasture

John E. Hunt1, Johannes Laubach1, Matti Barthel1,2, Anitra Fraser1, and Rebecca L. Phillips1 John E. Hunt et al.
  • 1Landcare Research, P.O. Box 69040, Lincoln 7640, New Zealand
  • 2Department of Environmental Systems Science, ETH Zürich, 8092 Zürich, Switzerland

Abstract. Intensification of pastoral agriculture is occurring rapidly across New Zealand, including increasing use of irrigation and fertiliser application in some regions. While this enables greater gross primary production (GPP) and livestock grazing intensity, the consequences for the net ecosystem carbon budget (NECB) of the pastures are poorly known. Here, we determined the NECB over one year for an irrigated, fertilised and rotationally grazed dairy pasture and a neighbouring unirrigated, unfertilised, winter-grazed pasture. Primary terms in the NECB calculation were: net ecosystem production (NEP), biomass carbon removed by grazing cows and carbon (C) input from their excreta. Annual NEP was measured using the eddy-covariance method. Carbon removal was estimated with plate-meter measurements calibrated against biomass collections, pre- and post-grazing. Excreta deposition was calculated from animal feed intake. The intensively managed pasture gained C (NECB  =  103 ± 42 g C m−2 yr−1) but would have been subject to a non-significant C loss if cattle excreta had not been returned to the pasture. The unirrigated pasture was C-neutral (NECB  =  −13 ± 23 g C m−2 yr−1). While annual GPP of the former was almost twice that of the latter (2679 vs. 1372 g C m−2 yr−1), ecosystem respiration differed by only 68 % between the two pastures (2271 vs. 1352 g C m−2 yr−1). The ratio of GPP to the total annual water input of the irrigated pasture was 37 % greater than that of the unirrigated pasture, i.e. the former used the water input more efficiently than the latter to produce biomass. The NECB results agree qualitatively with those from many other eddy-covariance studies of grazed grasslands, but they seem to be at odds with long-term carbon-stock studies of other New Zealand pastures.

Short summary
Intensification of dairying is rapid in New Zealand, with unknown impact on stocks of soil carbon (C). We investigated changes of soil C over one year for an irrigated, fertilised, rotationally-grazed pasture and an unirrigated winter grazed pasture. The irrigated pasture gained more C from net CO2 uptake, excreta deposition and fertiliser application than was lost by grazing. The unirrigated pasture lost a small amount of C. Intensive dairy practices may thus increase soil C in the short term.
Final-revised paper