Response of Chaetomium sp. to Nitrogen Input and Its Potential Role in Rhizosphere Enrichment of Lycium barbarum

<i>Lycium barbarum</i> L. (goji berry), a traditional Chinese medicinal plant, depends heavily on nitrogen input to maintain productivity. Nitrogen application also profoundly influences rhizosphere microbial dynamics, which are critical for soil health and plant performance. This study aimed to investigate how the rhizosphere fungal community responds to nitrogen input and explore the potential role of beneficial fungi (e.g., <i>Chaetomium</i>) in goji berry rhizosphere enrichment. A field experiment with four nitrogen levels (0, 53.82, 67.62, and 80.73 g·N m^-2·year^-1, designated as N0, N1, N2, and N3) was conducted to analyze the fungal community structure in the rhizosphere of goji berry using ITS rRNA gene amplicon sequencing. The results showed that nitrogen input significantly altered the rhizosphere fungal community composition and diversity. Redundancy analysis (RDA) and Mantel tests indicated that soil electrical conductivity, total phosphorus, available phosphorus, and nitrate nitrogen were key environmental factors driving the fungal communities' shifts. Notably, specific fungal genera, including <i>Chaetomium</i>, <i>Cladosporium</i>, <i>Gibberella</i>, <i>Fusarium</i>, <i>Pyxidiophora</i>, <i>Acaulium</i>, and <i>Lophotrichus</i>, exhibited differential enrichment across nitrogen levels. In particular, <i>Chaetomium</i> was significantly enriched under the conventional nitrogen treatment (N2), a strain of <i>Chaetomium</i> sp. LC101 was successfully isolated from the goji berry rhizosphere, and its functional roles were verified via pot experiments. Inoculation with <i>Chaetomium</i> sp. LC101 significantly promoted goji berry growth, with the most pronounced effects observed under N0 treatments, root fresh weight, root vitality, and leaf chlorophyll content increased by up to 55.10%, 15.69%, and 43.27%, respectively, compared to non-inoculated controls. Additionally, <i>Chaetomium</i> sp. LC101 regulated rhizosphere nitrogen transformation by enhancing urease, nitrite reductase, and polyphenol oxidase activities while inhibiting nitrate reductase activity. These findings demonstrate that <i>Chaetomium</i> responds sensitively to nitrogen input, with enrichment under moderate nitrogen levels, and acts as a beneficial rhizosphere fungus by promoting plant growth and regulating nitrogen cycling. This study provides novel insights for nitrogen management in the goji berry industry, where synergistic regulation via "nitrogen reduction combined with microbial inoculation" can reduce nitrogen loss, improve yield and quality through functional fungi, and contribute to ecological sustainability.