Abstract :
[en] Carbon dioxide (CO2) exchanges between crops and the atmosphere are influenced by both climatic and management
drivers. In this study, a site at the Lonzée Terrestrial Observatory (candidate ICOS site) in Belgium that
had been managed for more than 70 years using conventional farming practices, was monitored over three
complete rotation cycles (sugar beet/winter wheat/seed potato/winter wheat) from 2004 to 2016. Continuous
eddy-covariance measurements and regular biomass samplings were performed in order to obtain the daily and
seasonal Net Ecosystem Exchange (NEE), Gross Primary Productivity (GPP), Total Ecosystem Respiration (TER),
Net Primary Productivity (NPP) and Net Biome Production (NBP). Meteorological data and crop management
practices were also recorded.
Over the 12 crop seasons, total NEE varied both between and within crop types. Crop type influenced net
carbon (C) sequestration, with the seed potato crop exhibiting the smallest C fluxes. Despite differences in CO2
assimilation response to weather variables and in cumulated TER/GPP dynamics, the sugar beet, winter wheat
and maize crops had similar seasonal NEE values. The NEE inter-annual variability, both between and within
crop types, was explained mainly by the length of the active vegetation period, as well as the cumulated photosynthetic
photon flux density and average air temperature during this period.
The establishment of the C budget over the 12 years of study showed that NEE was a downward (negative)
flux (−4.40 ± 0.05 kg C m−2), but NBP was an upward (positive) flux (0.99 ± 0.22 kg C m−2). That is, as
soon as C exportation at harvest and C importation (manure, slimes) were included in the budget, the site
behaved as a C source. The intercrop periods contributed significantly to the total C budget, and the C fluxes
generated during these periods were positively related to crop residue amount and temperature. The largest
uncertainties about the crop C budget were those relating to biomass measurements. Carrying out a soil C
inventory would help to validate the NBP-based estimate of soil C loss.
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