Effects of Peanut Shells and Mushroom Residues Substitution for Chemical Fertilizers on Soil Biochemical Properties and Microbial Communities in Continuously Cropped Carrot System

: To systematically evaluate the effects of partial substitution of chemical fertilizers with peanut shells and mushroom residues on continuously cropped carrot soil, a two-year field experiment was conducted with four treatments: conventional chemical fertilization (FNP), optimized fertilization (OPT), peanut shell amendment (PS), and mushroom residue amendment (MR). Soil organic carbon (SOC) fractions, six soil enzyme activities, humic substance structure by Fourier transform infrared (FTIR) spectroscopy, microbial biomass carbon and nitrogen (MBC and MBN), and microbial community composition were determined, and redundancy analysis (RDA) was used to identify key driving factors. At harvest, compared with FNP and OPT, PS and MR increased total organic carbon (TOC) by 55.36%-72.85%, labile fractions (water-soluble substances, humic extract, humic acid) increasing by 20.51%-52.55%, while recalcitrant humin accumulated steadily. FTIR revealed that organic amendments reduced humic substance aromatization, increasing the aliphatic-to-aromatic ratio of humic acid by 6.62%-56.76%. PS and MR significantly improved soil enzyme activities by 16.75%-91.97%, with the most pronounced effects on protease and phosphatase; invertase and dehydrogenase activities were highest under MR, whereas β-glucosidase activity was highest under PS. PS and MR increased MBC by 44.42% and 49.71%, and MBN by 79.94% and 85.24%, respectively, relative to FNP. Organic substitution significantly reshaped community composition, enriching Proteobacteria and Ascomycota while suppressing Firmicutes and Basidiomycota, whose relative abundance dropped from 26.1% (FNP) to 2.2% (MR). Beneficial genera such as Pseudomonas , Sphingomonas , Chaetomium , and Trichoderma were enriched under PS and MR. RDA further showed that SOC fractions and six soil enzyme activities were the primary environmental factors shaping microbial community differentiation. In conclusion, substituting chemical fertilizers with peanut shells and mushroom residues effectively improved soil carbon pools, enzyme activities, and microbial community stability in continuously cropped carrot fields, with mushroom residues showing superior effects and providing a scientific basis for green fertilization and sustainable soil management.