Three-stage carbon release model during macrophyte decomposition

Carbon release during macrophyte decomposing is critical for the carbon cycle in the aquatic ecosystems. Previous mechanic researches have demonstrated the different release rates of nutrients in different stages, e.g., the fast-leaching stage (in initial 4–10 days), the microbe conditioning stage with slow decay rate (from winter to spring with low temperature). Nevertheless, models for predicting these processes are still not available and the conventional used model inherently assumes a constant relative release rate. Here, we conducted a one-year in-situ decomposition experiment using the conventional (net) litterbag technique. The credibility of experimental data is increased by using fresh debris to avoid alternation of material traits caused by the conventional drying pretreatment, which is originally used in experiments for terrestrial debris. Our results demonstrated that the carbon release process showed an obvious three-stage pattern, which can be universally depicted by a piecewise three-stage exponential decaying model (r2 > 0.9). More importantly, contributions of the three key factor (i.e., leaching, microbe and macroinvertebrate) can be explicitly reflected by the corresponding relative release rate (ki, i denote i th decomposition stage), which variate greatly between different stages, with k1/k2 > 11.8 and k3/k2 > 2.63. More importantly, k3 can be further conveniently decomposed into microbe-induced k3-1 and macroinvertebrate-induced k3-2, correspondingly. Also, k3-2 showed an exponential increase with the number of macroinvertebrate (r2 > 0.95). The contributions of the microbe and macroinvertebrate can then be depicted by these two parameters. In conclusion, our model greatly increases the model realism and generality in simulating the carbon release process; Also, the explicit physical meaning of model parameters make the model mechanism clearer. This not only created the chance to further develop a mechanism-based model, but also facilitate the mechanistic study of the impacts of environmental factors (e.g., pH, dissolved oxygen) on k1, k2, k3-1 and k3-2 concerning different kinds of macrophyte, in which the parameters k1, k2, k3-1 and k3-2 acts as important ecological indicators in depicting the relative release rates caused by specific driving forces. These ecological indicators facilitate the future mechanical researches for the impacts of environmental factors on the carbon release.