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.