Temporal and spatial effects of crop diversity on soil carbon and nitrogen storage and vertical distribution

Species-diverse plant communities can promote the accumulation of soil organic carbon (SOC) and nitrogen (SON) in both natural and semi-natural ecosystems. It is not clear however, how the accumulation and depth distribution of soil C and N might be affected in species-diverse croplands. Using a 15-year crop diversity field experiment, we ask how SOC and SON accumulation and vertical distribution (0-to-100 cm soil depth) changes in response to agricultural diversity, expressed as crop rotation (temporal effect) and intercropping (spatial effect) of staple crop species including wheat and faba bean (C₃) and maize (C₄). We found that rotation and intercropping across crop combinations altered the depths at which C and N were stored, over the 0–100 cm soil profile, but not the total C and N stored. There was greater SOC storage over the 0–100 cm soil profile in faba bean/maize intercropping than maize-faba bean rotation. Total SOC between 0 and 40 cm soil depth in intercropping increased by 125–164 kg C ha^-1 yr^-1 compared to rotation systems across crop combinations. However, rotations increased SOC at 60–80 cm in depth by 104 kg ha^-1 yr^-1 compared to intercropping. Also, SON stocks in rotation systems, at 60–80 cm, increased by 18 kg ha^-1 yr^-1 compared to intercropping systems. The accumulation and vertical distribution of SOC₃ derived from roots of wheat and faba bean contributed to increase total SOC stocks and SOC vertical distribution. SOC₄ derived from roots of maize did not have the same effect. Crop rotations promoted deeper root-associated SOC₃ accumulation whereas intercropping promoted shallower root-associated SOC₃ accumulation. Our study demonstrates how the type of crop diversification influences soil C and N sequestration via regulating root distribution, root composition and root decomposition. These findings have practical implications for soil C and N management in croplands.