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Variations in CO2 exchange for dairy farms with year-round rotational grazing on drained peatlands

Campbell, David I., Wall, Aaron M., Nieveen, Joost P., Schipper, Louis A.
Agriculture, ecosystems & environment 2015 v.202 pp. 68-78
autumn, biomass, carbon, carbon dioxide, dairy farming, ecosystems, eddy covariance, farms, land management, net ecosystem exchange, pastures, peat, peat soils, peatlands, photosynthesis, rotational grazing, spring, water stress, water table, New Zealand
It is commonly assumed that agricultural peatlands are net sources of CO2 to the atmosphere because of lowered water tables and intensive land management altering the balance of plant productivity and respiration. Yet actual farm-scale fluxes of CO2 have been infrequently quantified. We measured net ecosystem exchange of CO2 (NEE) using a permanent and a mobile eddy covariance tower installed over dairy farms with year-round rotational grazing on deep peats in New Zealand. The permanent tower was in place for one year and the mobile tower was deployed for periods of 3–4 weeks at three other farms on peat between spring and autumn. At all sites, grazing cycles caused large variations in pasture biomass and consequent daytime NEE and we accounted for these variations using an index of photosynthesising biomass (phytomass index, Lohila et al., 2004) automatically derived from daily CO2 flux measurements. We estimated annual CO2 loss of 190gCm−2yr−1 for the permanent site, which is in broad agreement with other agricultural peatland studies. Including other farm-scale exports of C, overall net ecosystem carbon loss estimated for the permanent site was 294gCm−2yr−1. Accounting for changes in phytomass index, daytime NEE was similar for permanent-mobile site farm pairings, except when there were very large differences in water table depths between farms in autumn. In contrast, night-time respiration losses were almost identical between farms even when water tables were markedly different, suggesting that spatial differences in NEE in these agricultural peatlands are caused by reduced photosynthesis in dry periods, due to plant water stress, rather than increased respiration. Comparisons between permanent and mobile towers appeared a useful approach for determining spatial variability of CO2 fluxes from peat soils. Taken together, our results suggested that the CO2 losses measured at the permanent site were representative of CO2 losses for farmed peats in the Waikato region when the water table was within ∼0.5m of the surface. Where water tables were deeper net CO2 losses would be expected to be greater due to reduced pasture photosynthesis and production. Maintaining higher water tables might achieve dual benefits of increasing pasture productivity and reducing CO2 losses.