SOIL RESPIRATION AND CARBON INPUTS FROM CROPS IN A WHEAT SOYBEAN ROTATION UNDER DIFFERENT TILLAGE SYSTEMS

The production of CO2 in the field and the contribution of carbon from crops to the soil were evaluated for the double crop wheat-soyabean r otation on a typical soil of the Rolling Pampa to assess the effects o f two tillage systems, mouldboard ploughing and shallow discing, on th e soil carbon balance. Microbial biomass and respiration under control led conditions were also determined. No differences in soil microbial biomass contents were detected between tillage systems after two years , but the biological activity of incubated soils and the mineralized f raction of organic carbon were greater (P=0.05) at the 0-5 cm depth in disc tillage. This suggested an increase in the labile fraction of or ganic matter in that layer, though the total carbon content of the soi l did not vary significantly. Soil moisture was not a limiting factor at any time of the year and production of CO2 in the field was regulat ed by temperature (r>0.89, P=0.01). There were no differences between tillage systems in the emission of CO2 to the atmosphere, which was es timated at 11.6 t C/ha/yr. The contribution in dry matter from the cro ps ranged from 15.3 to 17.0 t/ha/yr, and the carbon input was approxim ately 7.0 t/ha/yr. Consequently, the soil lost carbon with the wheat-s oyabean rotation but tillage systems did not affect carbon inputs and losses from the agrosystem.