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.

Representativeness of Scholander chamber measurements as a function of time after leaf removal.

The objectives of the study were to determine the maximum tolerable period between leaf removal from a plant and the determination of leaf water potential (Ψ l ), and to evaluate different ways of packing leaves after their removal from a plant (LR). Two experiments (Exp) were conducted in a randomized complete block design with four replicates, using bean, maize, and soybean crops. The predawn leaf water potential (Ψ PD ) (Exp1) and the minimum leaf water potential (Ψ min ) (Exp1 and Exp2) were determined. The maximum tolerable period between LR varied among species, ranging from 80 to 90 min for bean and soybean, and from 40 to 60 min for maize. A box with low interior air temperature and capable of blocking the effect of solar radiation reduced variation in leaf water potential. These results indicate that the determination of the Ψ l in bean, soybean, and maize crops does not need to be carried out immediately after leaf removal.