Impact of continuous sugar beet cropping on soil physicochemical properties and microbial communities

Objectives This study aimed to investigate the effects of sugar beet continuous cropping on the physiochemical properties, microbial community structure, and network complexity of rhizosphere and bulk soils. The goal was to reveal the mechanisms underlying sugar beet continuous cropping obstacles and to provide a theoretical basis for their mitigation.

Methods Rhizosphere and bulk soils from sugar beet fields cultivated for one-year and eight consecutive years were collected and analyzed. Soil pH was measured using a pH meter, and other soil physicochemical properties (e.g., nitrate nitrogen and organic carbon) were determined using a flow analyzer and an element analyzer. High-throughput sequencing was employed to analyze microbial community structure. Co-occurrence network and redundancy analysis (RDA) were conducted to explore relationships between microbial communities and environmental factors.

Results Continuous cropping significantly reduced the Chao1, ACE, and Shannon indices of fungal communities (P<0.05), while the bacterial and pathogenic fungal diversity remained unchanged. In non-continuous cropping soils, rhizosphere fungal diversity indices were significantly higher than those in bulk soils. However, under continuous cropping, bulk soils exhibited significantly higher fungal diversity than rhizosphere soils. The relative abundance of pathogenic fungi such as Fusarium increased significantly in continuous-cropped soils, whereas beneficial fungi like Mortierella declined. LEfSe analysis identified pathogens such as Plectosphaerella and Fusarium as key indicator species in continuous-cropped soils. Co-occurrence network analysis revealed reduced fungal network complexity (fewer nodes and edges) and weakened bacterial-fungal interactions (lower modularity) under continuous cropping. Additionally, nitrate nitrogen (NO₃⁻-N) and total organic carbon (TOC) decreased significantly in continuous-cropped soils. RDA confirmed that pH, total nitrogen (TN), NO₃⁻-N, ammonium nitrogen (NH₄⁺-N), and TOC were critical environmental factors shaping microbial communities.

Conclusions Continuous sugar beet cropping exacerbates soil-borne disease transmission by altering fungal community structure, increasing pathogenic fungi abundance, and destabilizing microbial networks. Concurrently, significant changes in environmental factors critically influence microbial communities, particularly fungi, ultimately leading to continuous cropping obstacles.