Nitrogen Enrichment Alters Plant Root, Soil Microbial Structure, Diversity, and Function in Mountain Forests of North China

Nitrogen (N) enrichment significantly impacts temperate forest ecosystems, but we lack a comprehensive understanding of the responses of root morphological characteristics, soil microbial communities, and soil multifunctionality concurrently to varying degrees of N enrichment, particularly when exceeding a threefold localized N input in temperate forests. Therefore, we selected four forest communities in China’s temperate forests and experimented with localized N addition to the dominant tree species in each forest community through the root bag method (three N addition treatments were set up: N1, fourfold soil total N; N2, sixfold soil total N; and CK, control). The results showed that (1) N enrichment treatments significantly improved soil multifunctionality and modified root morphological characteristics, leading to increases in RD (root diameter) and RTD (root tissue density) but decreases in SRL (specific root length) and SRA (specific root area). (2) N enrichment treatments also substantially changed microbial community composition and functional taxa. The relative abundance of eutrophic bacteria increased, while that of oligotrophic bacteria and saprotrophic fungi decreased. (3) The microbial α-diversity index decreased, and the microbial co-occurrence networks became less complex and more vulnerable under N enrichment treatments. (4) Soil multifunctionality and the microbial alpha diversity index had a substantial negative correlation. (5) NH4+-N and NO3−-N contents were the key factors affecting microbial dominance phyla, as well as the bacterial Shannon index and the fungal Chao1 index. (6) In addition, soil properties (except NH4+-N and NO3−-N), soil enzyme activities, root morphological characteristics, and the microbial Chao1 index were significantly different among tree species. In summary, N enrichment significantly alters root morphological characteristics and improves soil multifunctionality. Concurrently, it reduced microbial α-diversity, increased the abundance of eutrophic bacteria, and decreased saprophytic fungi, leading to a less complex and more vulnerable microbial community. This study provided important data and insights for a comprehensive study of the repertoire of responses to nitrogen enrichment in temperate forest ecosystems.