Carbon balance of a three crop succession over two cropland sites in South West France

Long term flux measurements of different crop species are necessary to improve our understanding of management and climate effects on carbon flux variability as well as cropland potential in terrestrial carbon sequestration. The main objectives of this study were to analyse the seasonal dynamics of CO2 fluxes and to establish the effects of climate and cropland management on the annual carbon balance. CO2 fluxes were measured by means of the eddy correlation (EC) method over two cropland sites, Aurade and Lamasquere, in South West France for a succession of three crops: rapeseed, winter wheat and sunflower at Aurade, and triticale, maize and winter wheat at Lamasquere. The net ecosystem exchange (NEE) was partitioned into gross ecosystem production (GEP) and ecosystem respiration (RE) and was integrated over the year to compute net ecosystem production (NEP). Different methodologies tested for NEP computation are discussed and a methodology for estimating NEP uncertainty is presented. NEP values ranged between -369 +/- 33 g C m(-2) y(-1) for winter wheat at Lamasquere in 2007 and 28 +/- 18 g C m(-2) y(-1) for sunflower at Aurade in 2007. These values were in good agreement with NEP values reported in the literature, except for maize which exhibited a low development compared to the literature. NEP was strongly influenced by the length of the net carbon assimilation period and by interannual climate variability. The warm 2007 winter stimulated early growth of winter wheat, causing large differences in GEP, RE and NEE dynamics for winter wheat when compared to 2006. Management had a strong impact on CO2 flux dynamics and on NEP. Ploughing interrupted net assimilation during voluntary re-growth periods, but it had a negligible short term effect when it occurred on bare soil. Re-growth events after harvest appeared to limit carbon loss: at Lamasquere in 2005 re-growth contributed to store up to 50 g C m(-2). Differences in NEE response to climatic variables (VPD, light quality) and vegetation index were addressed and discussed. Net biome production (NBP) was calculated yearly based on NEP and considering carbon input through organic fertilizer and carbon output through harvest. For the three crops, the mean NBP at Aurade indicated a nearly carbon balanced ecosystem, whereas Lamasquere lost about 100 g C m(-2) y(-1); therefore, the ecosystem behaved as a carbon source despite the fact that carbon was imported through organic fertilizer. Carbon exportation through harvest was the main cause of this difference between the two sites, and it was explained by the farm production type. Lamasquere is a cattle breeding farm, exporting most of the aboveground biomass for cattle bedding and feeding, whereas Aurae is a cereal production farm, exporting only seeds. (C) 2009 Elsevier B.V. All rights reserved.