Objectives Fungi play an important role in rhizosphere soil nutrient cycling. Effects of long-term phosphorus fertilizer application on the soil fungal communities in sweet potato rhizosphere were explored, in order to diminish the potential unfavorable impacts of P application methods in the rhizospheric microecological environment.

Methods A long-term experiment was established in 2011 with sweet potato-wheat rotation system located in Nanjing, Jiangsu Province. In 2020, three treatments were selected for this experiment: no phosphorus fertilizer (NK), chemical fertilizers (NPK), and organic and chemical fertilizers (NPKM). The rhizospheric soils of storage and fibrous roots were collected at the storage root expansion stage of sweet potato. The basic soil chemical properties were determined using standard chemical analytical methods, and the relative abundance, composition and diversity of fungal community were analyzed using Illumina Novaseq High-throughput sequencing technology.

Results 1) Long-term P application altered chemical properties of rhizospheric soils. The organic carbon (SOC), dissolved organic carbon (DOC), available phosphorus (AP), and available potassium (AK) were all significantly increased by P application in both storage and fibrous rhizosphere. The SOC, DOC and AK contents were in order of NPKM>NPK>PK (P<0.05), while AP was in order of NPK>NPKM>NK (P<0.05). Only the AP under NPK was significantly different between the two root types. The pH in fibrous and storage rhizosphere were found to be highest in NPKM, and lowest in NPK treatment. The fibrous rhizosphere pH was 0.81 units higher than that in storage rhizosphere under NPKM treatment. 2) The dominant fungi were Ascomycota (70.2%–77.9%), Basidiomycota (5.9%–8.5%), and Mortierellomycota (1.8%–8.1%), regardless of treatments. The relative abundance of Ascomycota in storage rhizosphere was similar among the three treatments, while that in fibrous rhizosphere was significantly (P<0.05) higher in NK treatment than in NPKM treatment. The relative abundance of Basidiomycota and Mortierella was not significant different between NPK and NPKM treatments, nor between the two root types. While the relative abundance of Basidiomycetes and Mortierella in NK treatment decreased by 70.4% and 62.9% compared with NPK treatment, and decreased by 44.0% and 151% compared with NPKM treatment in storage roots. 3) The specific quantities of fungal OTUs were 122 and 113 in storage rhizosphere under NPKM and NPK treatments, respectively, while in NK treatment, it was only 86. The specific quantities of fungal OTUs were 160 in fibrous rhizosphere under NPKM treatment, and 114 and 127 in NPK and NK treatments, respectively. The observed species and Shannon diversity index were significantly higher in fibrous rhizosphere under NK treatment than those in storage rhizosphere (P<0.05). The observed species, and Shannon diversity index were highest under NPKM treatment. Topological network analysis showed that the relationships of fungal communities in the rhizosphere of storage roots were more complex, but those in fibrous rhizosphere had stronger interactions. Redundancy analysis indicated that the AP, SOC, and pH in rhizosphere soil were significantly (P<0.05) correlated with fungal community.

Conclusions Long-term chemical phosphorous fertilization decreased pH, but significantly increased organic carbon and AP in rhizosphere soil. Combined application of chemical and organic fertilizers alleviated the decrease in soil pH, thus maintained higher abundance and diversity of fungi communities in the rhizospheric soils.