Large differences in CO2 emissions from two dairy farms on a drained peatland driven by contrasting respiration rates during seasonal dry conditions

Abstract

Drained peatlands are major sources of CO2 to the atmosphere, yet the effects of land management and hydrological extremes have been little-studied at spatial scales relevant to agricultural enterprises. We measured fluxes of CO2 using the eddy covariance (EC) technique at two adjacent dairy farms on a drained peatland in Aotearoa New Zealand with remaining peat depths 5.5–8 m. One site (SD) had shallow surface drains and mean water table depth (WTD) −657 mm, while the other site (BD) had deep field border drains and mean WTD −838 mm. Net ecosystem CO2 production (NEP) was similar at the two sites when the soils were moist but diverged during late-summer drying, with site BD having 4.56 t C ha−1 greater CO2 emission than site SD over the four-month dry period. Soil drying reduced gross primary production (GPP) at both sites, while ecosystem respiration (ER) was reduced at site SD but not at site BD. The low dry season respiration rates at site SD contributed to near-zero annual NEP, while higher respiration rates at site BD led to annual CO2 loss of −4.95 ± 0.59 t C ha−1 yr−1. Accounting for other imports and exports of carbon, annual net ecosystem carbon balances were −2.23 and −8.47 t C ha−1 yr−1 at sites SD and BD, respectively. It is likely that the contrasting dry season respiration rates resulted from differences in soil physical properties affecting soil moisture vertical redistribution and availability to plants and microbes rather than from the relatively small differences in WTD. These differences could be caused by soil physical disturbances during pasture renewal or paddock recontouring, or time since initial drainage. Therefore, improved soil management might provide practical mitigation against excessive CO2 emissions during dry conditions, including droughts.