Soils are the largest terrestrial carbon sink on Earth and increasing soil organic carbon (SOC) content is a means of sequestering atmospheric CO2. Limited work has been done to examine the effect of long-term conservation agriculture management practices on the stability of SOC in prairie soils.

The objective of this study was to assess the nature and permanence of sequestered SOC in contrasting Saskatchewan soils after 21 years of conservation management practices. Ninety fields were sampled in 1996 and again at the same locations in 2018. The fields represent a diverse collection of soil types from within the five soil zones of the province.

Comparisons were made between the 1996 and 2018 soils (0-10 cm) in SOC concentration, along with various labile and dynamic SOC fractions: water-extractable (WEOC), light fraction (LF-C), microbial biomass (MB-C), and respirable CO2-C during a six-week incubation. Spectroscopy (isotope mass and FTIR) was also used to determine the chemical nature of the SOC.

Regardless of soil zone, soils with the smallest initial SOC content accumulated more SOC (ranging from 4.7-9.8 Mg C/ha or 220-454 kg C/ha/year). There was a minor increase in WEOC content (3%), no change in LF-C content, and a substantial increase in MB-C content (41%). Except for greater CO2-C emissions from the 2018 Black soils, there was no difference in CO2-C emissions or percentage of SOC respired between the 2018 and 1996 soils, which suggests similar or greater permanence of the sequestered SOC.

Our results suggest that after 21 years of conservation management practices, more of the SOC is present in an active, dynamic fraction that contributes to soil health and nutrient cycling.