The full carbon balance of a rewetted cropland fen and a conservation-managed fen

•We measure the full C balance of a rewetted cropland and a semi-natural fen.•For both sites, net ecosystem exchange was the largest component of the C budget.•Fluvial C losses were small at both sites.•The semi-natural fen was a C sink, the rewetted fen a C source.•Higher water tables are needed to reduce C losses in rewetted croplands.