ALBERT

Description: Croplands are involved in the exchange of carbon dioxide (CO2) between the atmosphere and the biosphere. Further- more, soil carbon (C) stocks play an important role in soil fertility. It is, thus, of great interest to know whether croplands act as a net source or sink of atmospheric CO2, and if soil C stocks are preserved over long timescales. The FLUXNET site CH-Oe2 in Oensingen, Switzerland has been operational since the end of 2003. This cropland is managed under the Swiss framework of the Proof of Ecological Performance (PEP, a variant of integrated management) with a crop rotation centred on winter wheat, which also includes winter barley, winter rapeseed, peas, potato and intermediate cover crops. In addition to eddy covariance measurements, meteorological and soil measurements were available along with information on C imports and exports from organic fertilisation, sowing and harvesting. This study investigates cropland C budgets over 13 years and assesses whether the PEP regulations resulted in a balanced C budget. Strongest CO2 uptake was observed during cereal seasons. C export through harvest, however, offset the strong uptake of the cereal crops. The largest net CO2 emissions to the atmosphere were observed during pea and cover crop seasons. Net biome production, representing the overall C budget, typically ranged between close to C neutral to C losses of up to 407gCm−2 per season, with peas being the largest source. Overall, the field lost 1674gCm−2 over thirteen years (129gCm−2yr−1), which was confirmed by soil C stock measurements at the beginning and the end of the study period. Although managing the field under the regulations of PEP did not result in an overall C sink, model simulations showed that the use of cover crops reduced the C losses compared to leaving the field bare. The use of solid manure improved the C budget by importing substantial amounts of C into the soil while liquid manure had only a small effect. We thus conclude that additional efforts are needed to bring Swiss management practices closer to the goal of preserving soil C in the long-term.