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 proposed to replace tie use of the moldboard plow to reduce soil C losses. The effects of no-tillage and plow tillage with and without N fertilization (0 or 90 kg N ha(-1)) on C inputs and outputs of a Typic Argiudoll 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 (Triticum aestivum L.)-soybean [Glycine max (L.) Merr.] rotation. Microbial biomass and the Fate of organic C mineralization during laboratory incubations were also determined. Nitrogen fertilization lad no significant influence on C emission in the field, soil organic matter level or microbial biomass and activity. The annual C budget was negative under 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 fraction (density = 1.6-2.0 g mL(-1)) of the 0- to 20-cm soil layer was 30% higher (P less than or equal to 0.05) under no-tillage. Light (density < 1.6 g mL(-1)) and heavy (density > 2.0 g mL(-1)) organic fractions, total C, and microbial biomass and activity in the 0- to 20-cm laver were not different in no-filled and plowed soils, despite different patterns of distribution with depth. The rates of organic C mineralization during laboratory incubations were the same for both tillage systems. We propose that, after an accumulation phase, soil under no-tillage loses higher amounts of CO2-C than under plowing. The use of no-tillage would not significantly affect soil organic matter pools of the region nl situations with low erosion losses.