On-farm comparison of soil carbon storage and associated organic matter composition of Chernozemic profiles in southern Alberta, Canada

Non-Peer Reviewed

Grassland soils are of outstanding importance and provide various ecosystem services that regulate, support, and underpin Canadian agriculture. Prairie ecosystems, however, are currently facing rapid changes in climate/land use that are likely to be accelerated in the coming decades. These alterations are expected to affect their potential to sequester soil organic carbon (SOC), the largest terrestrial C sink. Here, we characterized soil organic matter (SOM) composition along a gradient of land use intensity in a semi-arid climate of southern Alberta, Canada. Soil profiles (0-75 cm) were collected from irrigated croplands (CRP), rotationally grazed (RGZ), and adaptive multi-paddock (AMP) systems and surveyed using elemental and solid-state cross-polarization with magic angle spinning (CPMAS) 13C nuclear magnetic resonance (NMR) analyses. Results indicated that SOC stocks averaged 64.6, 89.1, and 93.4 Mg C ha-1 in CRP, RGZ, and AMP, respectively, with C/N ratios decreasing in the order AMP > RGZ > CRP. SOM roughly consisted of 38% O-alkyl-C, 26% alkyl-C, 22% aromatic-C, and 14% carboxyl-C. SOM aromaticity, measured by the aromaticity index, decreased with depth (P < 0.05) and was greater for AMP (26.2%) than RGZ (24.9%) or CRP (24.7%). By contrast, the alkyl-C/O-alkyl-C ratio, a proxy of SOM decomposability, increased with depth (P < 0.05) and was higher for CRP and RGZ (0.70) than AMP (0.67). Overall, SOM became enriched of plant-derived constituents and was depleted by more decomposed, humified substances in AMP relative to CRP or RGZ. SOM composition is strongly affected by land use intensity, and such changes are important drivers controlling SOC accretion in grassland soils.