Increasing soil organic carbon (SOC) content is vital to reducing carbon dioxide from the atmosphere, and enhancing soil and environmental health. The objective of this study was to gain a better understanding of how nitrogen (N) and C within whole soils and physical fractions of soil organic matter (SOM) in the Brown Chernozem soils in the Prairie Soil Carbon Balance Project (PSCB) responded to 21 years of conservation management practices, and to compare their response to a long-term field experiment in the same soil zone.

Soils from the PSCB were sampled in 1996 and 2018 at the 0-10, 10-20 and 20-30 cm layers, while soils from the long-term field experiment were sampled in 1998 and 2016 at the 0-7.5, 7.5-15 and 15-30 cm depths.

Soils were fractionated into particulate organic matter N and C (sand-size fraction; POMN and POMC) and mineral-associated organic matter (silt + clay fraction; MAOMN and MAOMC). For the PSCB, soil total N (STN) and SOC increased by 18% and 13%, respectively, and was more pronounced in fine- than coarse-textured soils. Changes in SOM in the PSCB soils were more pronounced in the MAOM fraction, which increased in the 0-10, 10-20, 0-20 and 0-30 cm depths.

The highest gain in SOC in the long-term field experiment was much lower than that of the PSCB, while loss of STN was observed in the long-term field experiment versus a gain in the PSCB.

Overall, MAOM contained the majority of N and C in both studies, but MAOMN and MAOMC decreased in the long-term field experiment compared to increases in the PSCB sites. These results suggest that the conservation practices likely enhanced the formation and stability of MAOM in the PSCB soils.