Three-stage phosphorus release model during macrophyte decomposition in macrophyte-dominated eutrophic lake

Phosphorus (P) release during macrophyte decomposition plays a pivotal role in the lake phosphorus cycle, particularly in macrophyte-dominated eutrophic (MDE) lakes. Developing a phosphorus release dynamics model is the key to establishing the complex nutrient cycling model. This study challenges the conventional one-stage phosphorus release model by proposing a three-stage framework, validated through comprehensive in-situ experiments using fresh senescent macrophyte detritus in two hydrologically distinct sub-lakes of a typical MDE lake (Wuliangsu Lake, China). A total of 252 litterbags were systematically arranged to control variables such as detritus type, macrophyte fragment size, vertical positioning of litterbag, and litterbag mesh size. Key findings demonstrate that the three-stage model significantly outperforms the one-stage approach, with determination coefficients (R²) exceeding 0.80 (P < 0.01), compared to R² < 0.67 for the traditional model. The phosphorus release process exhibited distinct stages: Stage 1 (0-4 days): Rapid leaching dominated, with relative release rates (k₁) ranging from 0.240 % d^-1 to 11.510 % d^-1; Stage 2 (4-160/200 days; ice-covered period): Slower microbial-driven decay prevailed, characterized by reduced rates (k₂ = 0.058-0.351 % d^-1); Stage 3 (> 160/200 days; high-temperature phase): Resurgent release (k³ = 0.420-1.941 % d^-1) linked to macroinvertebrate colonization and intensified microbial activity. During the ice thawing process in the second to third transition stage, a sudden surge in phosphorus occurs in plant residues, primarily caused by P adsorption. Based on the data of macroinvertebrate abundance (x_N), quantitative relationships between x_N and k₃ were established (k₃ = k_3-1e^bxN, R² = 0.85), distinguishing microbial (k_3-1) and macroinvertebrate (k_3-2 = k₃-k_3-1) contributions.