CO₂ flux in a wheat-soybean succession in subtropical Brazil: A carbon sink

The subtropical region of Brazil is home to 33% of the soybean [Glycine max (L.) Merr.] growing area and 90% of the wheat (Tritucum aestivum L.) growing area of this country. A soybean-wheat succession with fallow between crops is used in about 11% of the cultivated area. No study has quantified CO₂ fluxes in annual soybean-wheat succession in this region. Hence, this study analyzed the seasonality of CO₂ exchange (net ecosystem exchange [NEE]) in a 2015/2016 wheat-soybean succession in a commercial farm located in Carazinho, Rio Grande do Sul State, Brazil. The eddy covariance method was used to estimate the annual C balance of this system. The NEE was partitioned between gross primary productivity and ecosystem respiration to understand the dynamics of these fluxes during a year of wheat-soybean succession. Considering the net ecosystem balance between photosynthesis and respiration during the growing season, both soybean and wheat absorbed CO₂ from the atmosphere (NEE wheat: -347 ± 4 g C m^-2 ; NEE soybean: -242 ± 3 g C m^-2 ). The fallow periods between growing seasons, however, acted as a source of 156 ± 2 g C m^-2 , reducing the C absorbed by the crops by 27%. For 1 yr, the net biome productivity was -50 g C m^-2 yr^-1 . The results obtained here demonstrate that the wheat-soybean succession was a net C sink under these specific climatic conditions and field management practices and that the long fallow period between crops limited the agroecosystem from becoming a more efficient CO₂ sink.