A vast body of research has demonstrated the ecological implications of coffee management systems, but less is known about the impacts on nutrient cycling. In shade-grown systems, coffee is grown in the understory of larger species creating a spatial mosaic of leaf litter. Leaf litter from Coffea species is unique in its chemical composition due to caffeine. Caffeine, a secondary defense compound produced to defend against herbivores, may also deter detritovores.
Our research examined decomposition of the two dominant coffee species, Coffea arabica and Coffea robusta (canephora). We hypothesized that C. arabica would decompose faster because of the lower average concentration of caffeine and the weaker physical leaf structure. We also hypothesized that leaves of both species would decompose more quickly in plots where Coffea sp. are grown in comparison to the forested control site.
Tethered line decomposition experiments were conducted at a coffee farm in Chiapas, Mexico. Tethered lines with leaves of both species were placed in three environments: plots where C. arabica is grown, plots where C. robusta is grown and a forest where coffee is not cultivated. Samples were collected weekly for six weeks, dried and weighed. Mass loss and carbon to nitrogen ratios were used to assess decomposition.
Decomposition of C. robusta was slower than C. arabica in plots where C. arabica is grown (p < 0.005), which could be due to higher caffeine levels or structural differences in leaf material. However, there was no difference in decomposition rates between C. arabica and C. robusta in plots where C. robusta is grown or in the control sites (p=0.54, p= 0.78). This suggests there may be some “home-field” advantage, potentially due to differences in decomposer communities between sites. For both species of leaves, decomposition was slowest in control plots, where coffee is not grown, which may be a reflection of the decomposer community or the relative value of Coffea leaves in comparison to other species present. These results demonstrate the importance of species- and location-specific effects on decomposition and have significant implications for nutrient cycling in shade-grown coffee systems.