Soil Carbon Pools under Conventional and No‐Tillage Systems in the Argentine Rolling Pampa

The Rolling Pampa is the most important cropping region of Argentina, and its soils are subjected to degradation. No‐tillage has been proposed to replace the 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 yr under a corn ( Zea mays L.)‐wheat ( Triticum aestivum L.)‐soybean [ Glycine max (L.) Merr.] rotation. Microbial biomass and the rate of organic C mineralization during laboratory incubations were also determined. Nitrogen fertilization had 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 ≤ 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 layer were not different in no‐tilled 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 CO 2 ‐C than under plowing. The use of no‐tillage would not significantly affect soil organic matter pools of the region in situations with low erosion losses.