Eight years of variations in ecosystem respiration over a residue-incorporated rotation cropland and its controlling factors.

The carbon dioxide emissions from cropland play important roles in the regional carbon budget. In this study, continuous measurements of the ecosystem respiration (RE) were obtained using the eddy covariance technique in a winter wheat-summer maize double cropping agroecosystem mainly between 2004 and 2012 in order to identify the among-year variations in RE and the related factors responsible. The annual RE, estimated by Lloyd and Taylor model, which was the most accurate, was 1866.4 ± 105.75 g C m year and it ranged from 1650.68 g C m year to 1945.57 g C m year during the eight years. The seasonal RE values were 867.98 ± 125.24 g C m year and 890.55 ± 131.34 g C m year for wheat and maize, respectively. Additionally, crop residue carbon ranged from 322.73 g C m year in 2012 and 453.49 g C m year in 2007. Correlation analysis indicated that the interannual variations in wheat and maize RE were correlated with the seasonal mean soil water content (W-W) and maximum leaf area index (W-LAI) of wheat, and seasonal mean air temperature of maize (S-T), respectively. A rest method was attempted to investigate whether these relationships were occasional or inevitable. The rests of RE, i.e. the difference between simulated and observed RE values, were significantly influenced by LAI of wheat and hourly T of maize season but not by hourly W of maize season, indicating that the influence of W-LAI and S-T on RE were inevitable outcomes and that of W-W on wheat RE was occasional. So we suggested that one should not confirm the controlling factors of interannual variations in carbon fluxes just from simple relationships, which may be statistical coincidences and do not correlated with biotical processes.