Steinbach, Haydée Sara (Univ. of Buenos Aires, Argentina, Av. San Martin 4453, Buenos Aires, C1417DSE, Argentina; Phone: 05411-45248080; Fax: 05411-45248076; Email: steinbac@agro.uba.ar)

Changes in Soil Organic Carbon Contents and Nitrous Oxide Emissions after Introduction of No Till in Pampean Agroecosystems

H. S. Steinbach *, R. Alvarez

During the last decade no till use has widespread in the Argentine Pampas. Around 50 % of agricultural lands are cropped under no till nowadays. Many experiments have been performed to evaluate no till effects on soil organic carbon (SOC) stocks or N₂O emissions but results have not been integrated. Consequently, knowledge about the effect of no till on SOC cannot be generalized to non-experimental sites. Our objective was to generate equations suitable for predicting no till effects on SOC and denitrification in the Pampas and to estimate the potential of no till to sequester carbon and mitigate the global warming effect. A review of published results from tillage experiments conducted in the Pampas during the last 20 years was performed. On an equivalent mass basis 42 paired data from comparisons of no till vs. plow till (mould board plow or disk plow), 18 paired data from comparison of no till vs. reduced till (chisel plow or harrow disk), and 20 paired data from comparisons of reduced till vs. plow till were obtained. Twenty-six denitrification data were also obtained. Data were analyzed by paired t test and linear regression methods. Under no till SOC increased 2.76 t ha^-1 (P = 0.01) in relation to tilled systems, but no differences were detected between plow and reduced till. Climate, soil texture or rotation did not affect the magnitude of carbon sequestration under no till. A linear regression model could be used for predicting SOC under no till, using SOC under tillage (plow or reduced) as independent variable (R² = 0.94, P less than 0.01). The model predicted a relative higher increase of SOC in areas of low organic matter level. Consequently, the impact of introducing no till would be higher in the semiarid portion of the Pampas than in the humid part. SOC increases, due to the introduction of no till, ranged from 15 % in the west semiarid portion of the Pampas to 5 % in the humid west. The conversion of the whole pampean cropping area (26 Mha) to no till will produce a carbon sequestration of 74 Mt C. This carbon sequestration is equivalent to the double of the annual carbon emissions from fossil fuel consumption of the country. The sequestration of carbon in soils under no till would be finish about 10 years from introduction. For wheat and corn, two of the main crops of the Pampas, N₂O emissions are greater under no till during the initial stages of growth. It was estimated a mean increase of 1 kg N ha^-1 yr^-1 emitted by denitrification under no till for the common double crop wheat/soybean-corn-soybean rotation. The emission of N₂O will overcome the mitigation potential of no till due to carbon sequestration in about 40 years and thereof no till will produce to global warming.