Effects of Continuous Cropping on Soil Phosphorus Fractions and Associated Microbial Community Structure in Greenhouse Tomato Cultivation
Long-term continuous cropping is common in greenhouse tomato production and can cause soil degradation, nutrient imbalance, and yield decline; however, its effects on soil phosphorus (P) fractionation and P-cycling microbial communities remain unclear. This study aimed to determine how continuous cropping duration affects soil P fractions, phosphatase activities, phoD- and pqqC-harboring microbial communities, and tomato yield. Soils representing eight continuous cropping cycles (1, 2, 6, 8, 10, 16, 26, and 32 cycles; two cropping cycles per year) were used in a greenhouse pot experiment. Soil chemical attributes, Hedley P fractions, acid and alkaline phosphatase activities, the diversity and composition of phoD- and pqqC-harboring microbial communities, and tomato yield were evaluated. With increasing continuous cropping cycles, total P, available P, and soil organic matter increased by 171.2%, 210.9%, and 59.3%, respectively. Labile P increased during the early and intermediate cropping cycles but declined after 16 cycles, whereas moderately labile and non-labile P fractions accumulated progressively. Acid phosphatase activity increased, alkaline phosphatase activity decreased, and the richness and diversity of phoD- and pqqC-harboring microbial communities generally declined. Tomato yield peaked after six cropping cycles but subsequently decreased, with a 20.25% reduction after 32 cycles compared with that after one cycle. These findings demonstrate that prolonged continuous cropping promotes soil P accumulation but shifts P toward less readily available fractions and reduces the diversity of P-cycling microbial communities, which is associated with decreased tomato productivity.