The study investigated the effects of planting density and area of floating-bed plants on the removal of nitrogen and phosphorus from water using the floating-bed of Iris pseudacorus L. as the experimental subject. Five planting densities (4, 8, 12, 16, and 20 plants/m2) and one control group were set via a water tank control experiment. Additionally, three planting areas were established, namely S (representing the experimental floating-bed area of 80 cm×80 cm), 50%S, and 25%S. The results showed that the floating-bed of I. pseudacorus significantly reduced the total nitrogen content in the water (p<0.05), but not the total phosphorus content (p>0.05). Moreover, planting density significantly affected the nitrogen and phosphorus concentrations in water (p<0.05). The total nitrogen and phosphorus removal rate was the highest at 16 and 8 plants/m2, respectively. In contrast, both nitrogen and phosphorus removal rates were the lowest at 12 plants/m2. The variation of planting density significantly affected the contribution rates of nitrogen and phosphorus removal by stems, leaves, and roots (p<0.05). The contribution rates in 12 and 16 plants/m2 were higher than those in other low- and high-density groups. The variation of floating-bed planting area also significantly affected the total phosphorus removal rate from water (p<0.05), but not the total nitrogen removal rate from water (p>0.05). An increased planting area was beneficial for enhancing the phosphorus removal, while variations in the planting area significantly affected the contribution rate of roots to nitrogen and phosphorus removal (p<0.05), with the highest values for 50%S. Variations in planting density or area can alter the planting distance and living space of plants, affecting the plant biomass and nutrient distribution, such as nitrogen and phosphorus distributions to varying degrees, and influencing the different growth characteristics of plant stems, leaves, and roots. The ability of plants to absorb and remove nitrogen and phosphorus from water depends not only on the total plant biomass and nutrient reserves, but also the synergy of the absorption through stems, leaves, and roots. Therefore, in the practical application of floating-bed, the proper planting density and area should be selected according to the characteristics of water quality.
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