Microbial residues are an important component of soil stable carbon (C) pools. However, their response to N-fixing tree species is still unclear. The present study investigated the effects of alder (Alnus sibirica) on microbial residues (as indicated by amino sugars) in the rhizosphere and non-rhizosphere soils of a larch (Larix kaempferi) plantation in eastern Liaoning Province, China. The contents of individual amino sugars significantly increased in the larch rhizosphere and non-rhizosphere soils in the mixed plantation relative to the pure plantation, indicating that alder benefits microbial residue accumulation. Specifically, the glucosamine (GluN) contents were 99.5% (P<0.01) and 154% (P<0.01) higher in the larch rhizosphere and non-rhizosphere soils in the mixed plantation than in the pure plantation, respectively, while the muramic acid (MurN) contents in the rhizosphere and non-rhizosphere soils were 66.1% (P<0.01) and 132.3% (P<0.01) higher in the mixed plantation than in the pure plantation, respectively. We found that alder had no effect on the GluN/MurN ratio, but significantly enriched the amino sugar proportion of soil organic C. Redundancy analysis revealed that total nitrogen was the dominant factor driving changes in amino sugars. This study indicates that alder would significantly stimulate the microbial fixation of C and the stability of soil organic C, which means that it plays a vital role in managing larch plantations.