Enclosure Area Shifts Biomass Allocation in Vallisneria spinulosa Yan Growth System During Enclosure Cultivation Process: A Pilot Study

Vallisneria spinulosa Yan (V. spinulosa Yan), as a submerged macrophyte and herbal plant, simultaneously exhibits ecological restoration functions in water bodies and medicinal value. However, it cannot be efficiently planted and cultivated in turbid water bodies. Enclosure cultivation technology can solve this issue in real ecological engineering, and enclosure area is an important parameter. The impact of enclosure area on growth characteristics, water purification, and microbial communities of the V. spinulosa Yan growth system in the enclosure cultivation process was studied, with planting density (50 plants·m−2) and soil volume per pot kept constant. The results showed that enclosure area primarily shifted biomass allocation: aboveground biomass decreased, and belowground biomass increased with increasing enclosure area, with the root-to-shoot ratio rising from 0.449 (A1, 1 × 1 m enclosure area) to 1.013 (A16, 4 × 4 m enclosure area). Total biomass, leaf number, soluble protein, and soluble sugar did not differ significantly among treatments. Leaf length was significantly greatest in A1. Notably, chlorophyll a, chlorophyll b, and total chlorophyll were significantly lowest in the A4 (2 × 2 m enclosure area) treatment, while SOD (superoxide dismutase) activity was highest in A1 and POD (peroxidase) activity was lowest in A1, all surviving global multiple-comparison correction. Nutrient removal loads (TN, nitrate nitrogen, TP, and PO43−-P) were significantly higher in planted treatments than in the unplanted control, but pairwise comparisons among planted treatments showed no significant differences. Furthermore, A1 exhibited significantly higher TP (total phosphorus) and PO43−-P (phosphate) removal loads than the other planted treatments, and its TN (total nitrogen) removal load was significantly higher than that of A9 (3 × 3 m enclosure area). Overall, enclosure area primarily shifted biomass allocation and induced complex physiological responses, without uniformly improving growth or water purification performance. This study can provide valuable references for efficient cultivation of submerged macrophytes and enhancing water purification effects in real ecological engineering.