Long-term maize and pea intercropping improved subsoil carbon storage while reduced greenhouse gas emissions

Intercropping cereals with legumes is considered a promising option for improving productivity and sustaining soil health. However, the long-term effects of cereal and legume intercropping on soil carbon (C) storage and the physicochemical properties of soil profiles remain elusive. In the present study, an 11-year long-term field experiment was carried out on maize monoculture with zero (control) and conventional 375 kg ha−1 nitrogen (N) application rates (N375), as well as on pea and maize intercropping with zero N (Ps) and a 20% reduction in conventional N (PsN300). The crop yield was obtained each year. The cumulative changes in organic C, bulk density, and other physicochemical properties of the top 20 cm of soil at the beginning and end of the experiment, as well as their spatial changes along the 1 m soil profile at the end, were closely investigated. Greenhouse gas emissions were estimated based on reported emission factors. Pea and maize intercropping under reduced fertilizer N application maintained maize growth and grain yield and increased land use efficiency (land equivalent ratio 1.1) relative to monoculture with farmers’ N inputs. After an 11-year intercropping, soil organic C concentrations increased by 6.7–12.4% in the top 0–20 cm layer and organic C stocks increased by 8.3–17.9%, and bulk density decreased by 6.4–14.2%. Intercropped soils with optimal N application (PsN300) sequestrated 14.8% more organic C in the 0–100 cm soil profile, particularly at depths of 40 cm and below, and the bulk density decreased by 7.0% on average compared with the monoculture with farmers’ N practices. Such intercropping benefits for soil organic C storage and physical properties in the 1-m profile were associated with greater soil microbial biomass C and N and dissolved organic C and N in the profile. Meanwhile, compared to farmers’ N and monoculture, intercropping under reduced N fertilizer application substantially decreased nitrate residues by 30.8% and GHG emissions by 17.8% per hectare or by 15.4% per ton grain yield, which was attributed to less N input, and downregulated enzyme activities involved in soil denitrification. Overall, this study showed that intercropping peas and maize with reduced N inputs is an environmentally sound strategy that improves soil physical quality and enhances deep C storage while reducing greenhouse gas emissions.