Background The intercropping may obviously improve the status of soil microbes and nutrients in root areas, which in turn would promote the growth of intercropped crops. The soil microbial biomass carbon (MBC) and microbial biomass nitrogen (MBN) not only play a critical role in the formation and stability of soil aggregates, but also play an important role in the cycling and transformation of soil materials and energy. However, how the intercropping impacts on soil MBC and MBN by affecting soil aggregates is not clear yet. In order to reveal the effect of intercropping on aggregate microbial biomass carbon and nitrogen in a red soil on a sloping field, this paper examined the effects of different planting patterns on the soil community structure and the distribution characteristics of MBC/MBN in red soil sloping farmland, which may provide basis for selecting the rational planting pattern of red soil slope farmland.

Methods Four treatments of bare land, mono-culture soybean, mono-culture maize and intercropping maize-soybean were set up in the runoff plot of sloping farms in the experimental farm of Yunnan Agricultural University. After three years of different planting patterns, soil samples were collected during the corn harvest period. The samples were then analyzed for soil aggregates, MBC and MBN to examine the differences under monoculture and intercropping treatments.

Results 1) Compared with soybean monoculture and maize monoculture, the mean weight diameter of soil aggregates (D_W) in intercropping treatment increased by 2.78% and 10.45%, the geometric mean diameter (D_G) increased by 9.43%, 23.40%, and ≥ 0.25mm water stability the aggregate (R_0.25) content increased by 7.35% and 15.41%, respectively, and the portion of aggregate destruction (D_p) was reduced by 9.61% and 20.66%, respectively. 2) The average values of MBC and MBC of each size aggregate in the intercropping treatment were higher than those of other treatments, and except for the size of < 0.25mm, the differences of MBC in every grade soil group were significant (P < 0.05), and the MBN in different particle size aggregates were significant (P < 0.05) except for the aggregates with particle size ≥ 5.00 mm and < 0.25 mm. The mean MBC/MBN values of the aggregates of each grain size in the intercropping treatment were all higher than monoculture treatment, with no significant differences (P > 0.05). 3) There was coefficient at soil aggregate stability index and the aggregates MBC, MBN, and the correlation was higher in the medium and large particle size aggregates than in the small particle size aggregates.

Conclusions The results showed that intercropping promoted the formation and stability of red soil aggregates on sloping fields, and increased MBC and MBN, which was an ideal sloping land planting mode in the study area.