Organic Matter Dynamics and Carbon Sequestration Rates for a Tillage Chronosequence in a Brazilian Oxisol

Amounts and rates of C sequestration under no‐tillage are not known for a major ecological region of south Brazil. These were assessed in a Brazilian Oxisol under a plow and no‐tillage chronosequence located in Paraná State. The chronosequence consisted of six treatments: (i) native field (NF); (ii) 1‐yr plow conversion of native field to cropland (PNF‐1); (iii) no‐tillage for 10 yr (NT‐10); (iv) no‐tillage for 20 yr (NT‐20); (v) no‐tillage for 22 yr (NT‐22); and (vi) conventional tillage for 22 yr (CT‐22). Soil samples were collected from five depths. No‐tillage, compared with the NF treatment, caused a significant increase in soil organic C (SOC) storage. More than 60% of this increase occurred in the 0‐ to 10‐cm soil layer. There was a decrease in the amount of SOC in the CT‐22 compared with the NF soil treatment and 97% of this loss also occurred in the 0‐ to 10‐cm layer. There was a close relationship between the SOC content and the amount of crop residues input ( R 2 = 0.74, P ≤ 0.05). There were increased SOC concentrations in the finer particle‐size fractions (<20 μm) of no‐tillage surface soil compared with the NF or CT‐22 soils. However, the percentage of SOC derived from crop residues in no‐tillage treatments, as assessed by 13C natural abundance (δ), was generally greater in the coarse (>20 μm) than in the finer (<20 μm) particle‐size fractions. The C sequestration rate for no‐tillage was 80.6 g C m −2 yr −1 for the 0‐ to 20‐cm depth and 99.4 g C m −2 yr −1 for the 0‐ to 40‐cm depth. The no‐tillage C sequestration potential for South Brazil was estimated as 9.37 Tg C yr −1