Land-use effects on leaf-litter breakdown in streams in a tropical lowland catchment

The expansion of oil palm and cattle grazing in the tropics continues to alter numerous ecosystem functions. The generated land-use change is potentially impacting stream leaf-litter breakdown, a fundamental process for freshwater ecosystems. To assess the effect of land-use change, we studied breakdown rates of forest (Pachira aquatica, Pouroma aspera, Sloanea ampla, and Hippocratea volubilis) and oil palm (Elaeis guineensis) leaves after a 26-day immersion in streams surrounded by rainforests, grazing lands, or oil palm plantations with and without riparian buffers. In addition, we assessed breakdown drivers by deploying litter bags (248) of two mesh sizes (15 mm and 0.5 mm) allowing or restricting macroinvertebrates’ access (134 coarse-mesh bags and 114 fine-mesh bags). Forest leaf breakdown by microbes (fine-mesh) was lower by 55% in the plantations compared to forests; while microbial oil palm-leaf breakdown was lower by 32% in the unbuffered plantations compared to forests (p < 0.05). Total litter breakdown was lower (p > 0.05) in the plantations but not when these preserved riparian buffers. Litter breakdown was driven primarily by microbes in all land uses except in the buffered plantations, possibly due to increased shredder biomass. These results suggest that oil palm agriculture may decrease microbial stream leaf-litter breakdown, especially in streams with no riparian buffers.