Litter richness decreases decomposition rates in a tropical wetland

Background/Question/Methods

Biodiversity effects on ecosystem function are of interest for several reasons, including recent global changes in landscape-level biological diversity and the often complex, non-additive nature of these effects.  Recently, effects of biodiversity on decomposition have received attention in stream and terrestrial environments. For instance, in terrestrial environments, diversity effects on litter decomposition include increased decomposition rates of recalcitrant litter in the presence of labile litter types, a pattern actively facilitated by nutrient transfer through fungal hyphae, but active nutrient transport is reduced in streams due to disturbance from flow. Few studies, however, have examined biodiversity effects on decomposition in wetlands, where mechanisms that differentiate stream and terrestrial environments (i.e., importance of nutrient transfer) may be intermediate.

In this study, we aimed to investigate litter richness and species composition effects on decomposition dynamics by examining litter decay in mixtures of five emergent macrophytes (Typha domingensis [TD], Eichhornia crassipes [EC], Neptunia prostrata [NP], Thalia geniculata [TG], and Nymphaea amazonum [NA]) commonly found in the tropical wetland at Palo Verde National Park, Costa Rica.  Litter from one to five species was secured using zip-ties in replicated litter bundles of 21 richness combinations (N = 5 replicates/treatment).  Six grams of litter was equally divided among included species in each bundle, and bundles were attached to each of five replicate PVC posts within the wetland.  After initial placement, bundles were collected on days 7, 13 and 73, dried, following which, mass loss, changes in litter chemistry (i.e., C:N, lignins), and associated macroinvertebrate communities were determined.  

Results/Conclusions

Species-specific decomposition rates differed (NA>TG>NP>EC>TD) and variation in decomposition rates decreased with increasing litter richness.  Additionally, a non-additive decrease of litter mass loss in mixed-species bundles was detected by correlating observed and expected percent mass remaining and comparing the calculated correlation coefficient to a null 1:1 relationship (P < 0.001).  C:N ratios and lignin content were correlated with species-specific decomposition rate, and may drive variation in rates of litter mass loss. Further, a statistically significant litter richness X macrophyte species interaction was found for C:N ratio and lignin content (C:N ratio: P < 0.001; lignin: P < 0.001), suggesting that nutrient transfer and breakdown of recalcitrant organic matter types (i.e., lignin) are dependent on litter richness.  These results suggest that litter decomposition in wetlands, like streams, is mediated by litter chemical quality, but, like terrestrial environments, litter richness is also an important driver of decomposition rates.