CO2 emissions following cultivation of a temperate permanent pasture

It is well known that frequent cultivation of cropped soils leads to increased soil respiration and loss of soil carbon (C). In contrast, little is known about the impact of occasional cultivation of permanent grasslands on soil C and CO2 dynamics. Occasional cultivation of pastures is common if a pasture is part of an arable-ley rotation, or as part of pasture renewal. Here we report on the CO2 balance following three cultivation events of temperate permanent pasture in New Zealand. For two experiments, one during a drought during late summer/autumn 2008 and one under moist soil conditions in spring 2008, CO2 losses following cultivation were measured using the closed chamber technique. During the spring 2008 experiment, two soils with different clay mineralogy and drainage were studied. During a third cultivation event in autumn 2010 CO2 exchange was measured using eddy covariance. Measured short-term respiratory losses following cultivation across the three experiments ranged from 151 to 329 g C m(-2) over 39 to 43 days. Rates of CO2 loss measured during non-drought conditions were generally higher than those previously reported from studies in Europe and North America, presumably because of generally high soil temperatures, non-limiting moisture conditions and high organic carbon availability at our study site. The 'net impact of cultivation' (taking into account both direct respiratory losses of CO2 and the lack of photosynthetic carbon input following cultivation) across the three experiments ranged between 77 and 406 g C m(-2) over 39-43 days. Both direct CO2 respiratory losses and the net impact of cultivation appeared highly dependent on soil moisture status, with lowest losses measured during a severe drought and highest losses measured in spring when ample moisture was present. Rates of respiratory CO2 losses did not decrease over the duration of our experiments (39-43 days). Our results suggest that when aiming to reduce C losses resulting from cultivation of permanent grassland, it is preferable to cultivate when conditions for soil microbial activity and photosynthesis are sub-optimal; for our study site this meant in autumn instead of spring because of lower soil moisture availability. We also recommend minimising the duration of the period between spraying the old sward and establishment of the new sward or crop. (C) 2013 Elsevier B.V. All rights reserved.