Decomposition of plant material is an important source of energy and nutrients in freshwater wetlands and understanding factors controlling detrital dynamics in these systems is crucial. Establishment of the invasive, floating-mat plant Eichhornia crassipes in dense beds can lead to altered native plant and animal communities. Further, given that differences between macroinvertebrates and the physicochemical environment associated with E. crassipes and native vegetation (i.e., Hydrocotyle umbellate and Neptunia prostrata) are substantial, decomposition dynamics within mats dominated by E. crassipes relative to mats of native plant species may differ.

To investigate decomposition of E. crassipes [E] and N. prostrata [N], 5-g samples were secured using zip-ties in 1 m² enclosures at Palo Verde National Park, Costa Rica. Experimental enclosures were assigned one of five species composition treatments (N =5 replicates), each varying in the proportion of living individuals of these two plant species (e.g., 16 E:0 N, 8 E:8 N, 0 E:16 N). Decomposition samples were removed 0, 7, 14 and 109 days after initial placement to examine mass loss, tissue chemistry (i.e., C:N, total phenolics, tannins, and lignins), and associated macroinvertebrate communities.
Results/Conclusions

To estimate species-specific decomposition rates by species composition treatment, mass loss data were fitted to two exponential decay models: the single exponential decay model, where the parameter k is estimated,  and the decreasing-coefficient decay model, where two parameters, a and b, are estimated.  Fits of the single exponential decay model were statistically significant (P < 0.05) for all treatments and plant species, although no plant species (P = 0.09) or species composition (P = 0.63) effects on k were detected.  Estimates of both a and b from the decreasing-coefficient decay model where statistically significant for N. prostrata in all treatments.  However, while estimates of a were significant for E. crassipes in all treatments (P < 0.05), estimates of b were significant only in the 8 E:8 N (P = 0.002) and 0 E:16 N (P = 0.03) treatments.  Additionally, no species composition effects were detected for estimates of a (P = 0.72) or b (P = 0.25), but both parameters showed a statistically significant plant species effect (P < 0.01). 

Overall, the decreasing-coefficient decay model better fit decomposing N. prostrata, whereas the single exponential decay model better fit decomposing E. crassipes, suggesting different decomposition patterns for these two species. Changes in decomposition in freshwater wetlands invaded by E. crassipes may influence other ecosystem processes.