Carbon and nitrogen sequestration in soils under different management in the semi-arid Pampa (Argentina)

• No tillage promotes surface TOC and TN accumulation.
• No tillage increases TOC and TN stocks up to 100 cm depth when compared with RT.
• Soil with soybean–maize rotation shows more TOC stock than soil under soybean monoculture.
• Tillage system has a greater impact on soil C stock than crop sequence.

Soil management affects distribution and the stocks of soil organic carbon and total nitrogen. The aim of this study was to evaluate the effect of different crop sequences and tillage systems on the vertical distribution and stocks of soil carbon and nitrogen. We hypothesized that no-tillage promotes surface organic carbon and total nitrogen accumulation, but does not affect the C and N stocks, when compared with reduced tillage. In addition, the incorporation of maize in the crop sequence increases total organic carbon and total nitrogen stocks. Observations were carried out in 2010 in an experiment located in the semiarid Argentine Pampa, on an Entic Haplustoll. A combination of three tillage systems (no tillage, no tillage with cover crop in winter and reduced tillage) and two crop sequences (soybean–maize and soybean monoculture) were assessed. After 15 years of management treatments, soil samples to a depth of 100 cm at seven intervals, were taken and analyzed for bulk density, organic carbon and total nitrogen. Total organic carbon stock up to a depth of 100 cm showed significant differences between soils under different tillage systems (reduced tillage < no tillage = no tillage with cover crop), the last ones having 8% more than the reduced tillage treatment. Soybean–maize had 3% more organic C up to 100 cm depth than the soybean monoculture. Total nitrogen stock was higher under no-till treatments than under reduced tillage, both at 0–50 and 0–100 cm depths. Total organic carbon stratification ratios (0–5 cm/5–10 cm) were around 1.6 under no-till and lower under reduced tillage. The stratification ratio explains less than 40% of soil carbon stock. Tillage system had a greater impact on soil carbon stock than crop sequence.