Objectives The effects of different proportion of green manure replacing chemical fertilizer on soil microbial biomass, and growth, nitrogen (N) absorption, and root morphology of citrus were studied under the same nitrogen level.

Methods One-year-old citrus seedlings were studied in a pot experiment where Dolichos lablab L. and Vigna sinensis Hayata were applied as green manures.There were five N substitution levels of chemical fertilizer (F) with green manure (G) in the experiment: 0 (100%F), 25% (25%G+75%F), 50% (50%G+50%F), 75% (75%G+25%F) and 100% (100%G). The total NPK nutrient input of the treatments were adjusted to equal level. Dry matter, and N content of various organs, citrus root morphology and soil microbial biomass were measured at citrus leafing stage.

Results Compared with 100%F, green manure significantly improved citrus dry matter and N accumulation. The increases in 75%G+25%F and 100% G treatments were as high as 42.71%–82.95% and 38.88%–53.31%, respectively. Green manure treatments increased the soil microbial biomass carbon (MBC) by 5.12%–48.42% and soil microbial N (MBN) by 6.35%–133.67%. Green manure treatments significantly improved root vigor, total root length and root surface area of citrus, with the highest increase in roots with < 1.5 mm diameter. The total length and surface area of roots with < 1.5 mm diameter in the green manure treatments were increased by 88.34%–324.87% and 78.82%–372.91%, respectively, compared with 100%F (P < 0.05). The increase of substitution ratio of Dolichos lablab L. led to the increase in root length and surface area, while the 50% and 100% substitution ratio of Vigna sinensis Hayata led to the highest root length and surface area. In addition, the substitution of Dolichos lablab L. and Vigna sinensis Hayata were 43.95%–47.48% and 40.61%–66.14% higher in root activity than 100%F, respectively, but there was no significant difference between the two green manure types. Green manure directly affected citrus dry matter, and directly or indirectly affected N accumulation by improving the root morphology and activity, and microbial biomass C and N. There was a significant positive correlation between green manure C/N ratio and citrus N accumulation, which directly or indirectly affected citrus dry matter by directly affecting N accumulation, or indirectly changing citrus root morphology, root activity and microbial biomass.

Conclusions Under equal N, P, K input, substitution of chemical fertilizer with Dolichos lablab L. and Vigna sinensis Hayata promoted the growth of soil microorganism, increased citrus root vigor, length and surface area, and promoted the accumulation of dry matter and N of citrus. Without changing the conventional N, P, K fertilizer application rate of citrus, the substitution ratio of 75%–100% of chemical N could promote the citrus production in the study area.