PARENT SESSION
Oral Session 3: Biogeochemistry I: Litter and Decomposition.
Presiding: J Dilustro
Monday, August 2, 8:00 AM to 11:30 AM, Meeting Room A 107.

Nutrient controls over litter decomposition in a lowland tropical rainforest.

Townsend, Alan^*,, Cleveland, Cory,

ABSTRACT- Lowland tropical rainforests exist across substantial ranges in soil pools of several key nutrients, including nitrogen (N) and phosphorus (P). Yet, the influence of varying N and P levels on decomposition rates remains poorly understood. Here, we report results from a litter decomposition experiment conducted in 80 long-term factorial fertilization plots (control, N, P, NP) located in lowland, highly diverse primary forests on neighboring P-rich alluvial Mollisols and P-poor Oxisols on the Osa Peninsula of southwest Costa Rica. The sites share a similar climate and several tree species, and receive approximately 5m of precipitation per year. More than 1100 litter bags were constructed with leaves of Brosimum utile, a relatively common canopy tree found on both soil types. Every experimental plot received two litter bag strings: one containing leaves from trees growing on the Oxisol, and the other containing Mollisol leaves. Bags were placed in the plots in early April 2003, just after the transition from the short dry season to the extended rainy season. Subsequent litter decomposition was extremely rapid; mass loss averaged nearly 40% after only 2 months, 60% after 5 months, and virtually all material was absent by 9 months. In addition, a sizable fraction of the leaf mass was readily soluble carbon. Prior work suggested that decomposition on the Oxisols was likely to be P-limited, yet despite substantial differences in soil P availability both between the soil types and the experimental plots, no effects of nutrient additions or soil type were seen on rates of litter mass loss. However, when soluble carbon leached from the litter was added to the soil surface, subsequent respiration responses were significantly greater in P-amended (vs. control) soils on the Oxisols. These results suggest that nutrient controls over decomposition are markedly different between the litter and surface soil layers of the forest floor, and that despite the lack of a nutrient effect on litter mass loss, for older, P-poor tropical soils, the ultimate fate of litter carbon may still be strongly controlled by gradients in P availability.

Key words: litter, tropical, decomposition, phosphorus