Background/Question/Methods:

In an agroforest, crops are cultivated under the shade of perennial over-story trees. In many regions, coffee (Caffea arabica) is still cultivated in traditional agroforestry systems despite economic pressure to maximize yields through monoculture farming. In general, nutrient cycles are tighter in agroforestry systems, and the combination of leaf litter and pruning residues from shade trees comprise the primary pathway for nutrient transfer between biomass and soil.  Litter quality, climate regimes, and microbial activity regulate the decomposition of organic material from agroforestry species, and consequently control their potential to supply nutrients and organic matter to the system.  We investigated decomposition over one year in a coffee agroforest located in the Central Valley of Costa Rica. The purpose of this study was to determine the effect of (1) canopy tree species and (2) substrate quality on decomposition rates and patterns of nitrogen (N) and phosphorus (P) release.  

Results/Conclusions:
In general, decomposition was rapid, with 50% of the mass lost after 8 weeks, and 80% of the mass lost over the one-year study period (average k of 3yr^-1).  To address the effect of canopy type, we investigated the decomposition of coffee, poró (Erythrina poeppigiana), and banana (Musa spp.) under a single-species (poró) and a mixed-species (poró and banana) canopy.  Decomposition was rapid under both canopies with 50% of the mass lost after 8 weeks, and roughly 20% of the original mass remaining at the end of the one-year study period.  Litter decomposing under the poró canopy retained a greater proportion of P than litter under the mixed canopy (p=0.008), and we observed nutrient immobilization at the end of the dry season (p<0.0001). To investigate the effect of substrate quality on decomposition, we conducted a common-garden experiment that included a fourth substrate (laurel; Cordia alliodora) under the mixed-species canopy.  Average rates of mass and nutrient loss were comparable to the previous study, and decomposition rates did not differ significantly with substrate. Nevertheless, a smaller proportion of N and P remained in high quality substrates compared to low quality substrates (p=0.0009, p=0.003 respectively).  Agroforesters must manage fertilizer inputs and internal cycling.  Our study suggests that pruning should occur roughly every four months to keep pace with decomposition rates.  In addition, mixed canopies may be favored over single-species canopies, as P mineralization is enhanced in diverse systems.  Finally, we suggest farmers quickly homogenize litter and pruning residues to maintain even nutrient release to the crop.