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 rotation on a typical soil of the Rolling Pampa to assess the effects of two tillage systems, mouldboard ploughing and shallow discing, on the soil carbon balance. Microbial biomass and respiration under controlled 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 fraction 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 organic matter in that layer, though the total carbon content of the soil 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 regulated 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 estimated at 11.6 t C/ha/yr. The contribution in dry matter from the crops ranged from 15.3 to 17.0 t/ha/yr, and the carbon input was approximately 7.0 t/ha/yr. Consequently, the soil lost carbon with the wheat-soyabean rotation but tillage systems did not affect carbon inputs and losses from the agrosystem.