Soil fertility in organically and conventionally-grown coffee: Nutrient availability and mycorrhizas

Background/Question/Methods

Coffee (Coffea arabica) is one of many plant species that form mutualistic associations with arbuscular mycorrhizal fungi (AMF), in which the plants exchange sugars from photosynthesis for enhanced uptake of phosphorus and other poorly mobile soil nutrients and for protection from pathogens via fungal mycelia. While the benefits of traditional shade-grown and organic coffee farming methods have been well-characterized for a variety of animal species, the effects of different coffee farming methods on belowground communities are much less well-understood. In this two-year study, we characterized management effects in twenty-five conventional and organic coffee fields in two regions of Costa Rica, Santa Elena de Monteverde and San Vito de Coto Brus, based on use of herbicides, pesticides, and synthetic fertilizers; amount of shade; richness of shade species; depth of leaf litter; elevation; slope; and coffee cultivars. Soil and coffee root samples were collected in each field in order to determine soil nutrient availability, soil pH, richness of AMF spores, and percent root colonization of coffee by AMF.  

Results/Conclusions

Shade, shade tree diversity, and leaf litter depth were greater in organic than in conventional coffee fields. While availability of macronutrients (N,P,K) was higher in conventional coffee fields, availability of most micronutrients was higher in sites with organic coffee. Number of AMF species varied by coffee field but was similar between conventional and organic coffee. AMF root colonization was greater in organic fields than in conventional fields, but the effect depended on region. Root colonization by AMF was positively correlated and AMF spore richness was negatively correlated with soil copper. Our results suggest that nutrient cycling in conventional and organic coffee fields bears further consideration. Management of biotic factors contributing to soil fertility in coffee agroecosystems has implications both for improving nutritional status of coffee plants and for conserving AMF diversity in the extensive area of the mid-elevational tropics that has been converted to coffee fields.