SOIL CARBON POOLS UNDER CONVENTIONAL AND NO-TILLAGE SYSTEMS IN THE ARGENTINE ROLLING PAMPA

The Rolling Pampa is the mast important cropping region of Argentina, and its soils are subjected to degradation. No-tillage has been propos ed to replace tie use of the moldboard plow to reduce soil C losses. T he effects of no-tillage and plow tillage with and without N fertiliza tion (0 or 90 kg N ha(-1)) on C inputs and outputs of a Typic Argiudol l and on organic C level and density fractions were studied in a field experiment at the end of 15 gr under a corn (Zea mays L.)-wheat (Trit icum aestivum L.)-soybean [Glycine max (L.) Merr.] rotation. Microbial biomass and the Fate of organic C mineralization during laboratory in cubations were also determined. Nitrogen fertilization lad no signific ant influence on C emission in the field, soil organic matter level or microbial biomass and activity. The annual C budget was negative unde r both tillage systems, but no-tillage lost about 0.7 to 1.5 Mg ha(-1) yr(-1) more C than did plow tillage. Carbon in the medium-density fra ction (density = 1.6-2.0 g mL(-1)) of the 0- to 20-cm soil layer was 3 0% higher (P less than or equal to 0.05) under no-tillage. Light (dens ity < 1.6 g mL(-1)) and heavy (density > 2.0 g mL(-1)) organic fractio ns, total C, and microbial biomass and activity in the 0- to 20-cm lav er were not different in no-filled and plowed soils, despite different patterns of distribution with depth. The rates of organic C mineraliz ation during laboratory incubations were the same for both tillage sys tems. We propose that, after an accumulation phase, soil under no-till age loses higher amounts of CO2-C than under plowing. The use of no-ti llage would not significantly affect soil organic matter pools of the region nl situations with low erosion losses.