Changes in soil carbon under long-term maize in monoculture and legume-based rotation

about 20 Mg C ha ‐1 . The effects of fertilization on soil C were small (~6 Mg C ha ‐1 ), and differences were observed only in the monoculture system. The NMR results suggest that the chemical composition of organic matter was little affected by the nature o f crop residues returned to the soil. The total quantity of maize-derived soil C was different in each system, because the quanti ty of maize residue returned to the soil was different; hence the maize-derived soil C ranged from 23 Mg ha ‐1 in the fertilized and 14 Mg ha ‐1 in the unfertilized monoculture soils (i.e., after 35 maize crops) to 6‐7 Mg ha ‐1 in both the fertilized and unfertilized legume-based rotation soils (i.e., after eight maize crops). The proportion of maize residue C returned to the soil and retaine d as soil organic C (i.e., Mg maize-derived soil C/Mg maize residue) was about 14% for all maize cropping systems. The quantity of C3-C below the plow layer in legume-based rotation was 40% greater than that in monoculture and about the same as that under either continuous grass or forest. The soil organic matter below the plow layer in soil under the legume-based rotation appeare d to be in a more biologically resistant form (i.e., higher aromatic C content) compared with that under monoculture. The retenti on of maize residue C as soil organic matter was four to five times greater below the plow layer than that within the plow layer. We conclude that residue quality plays a key role in increasing the retention of soil C in agroecosystems and that soils under leg umebased rotation tend to be more “preservative” of residue C inputs, particularly from root inputs, than soils under monoculture.