Nitrogen and Phosphorus Availability in the Rhizosphere and Bulk Soil of Coffee: Shade Tree and Management Effects.

Recent volatility in the world coffee market, along with opportunities presented by niche markets for sustainably-grown coffee, have driven the search for ecologically-based, low-input methods of production. This study was performed at CATIE, Turrialba, Costa Rica, at a 12-year-old experimental coffee production site. We address two research questions: 1) how does the incorporation of Erythrina poeppigiana (a nitrogen-fixing shade tree) affect rhizosphere and bulk soil phosphorus (P) availability for Coffea arabica under low external nutrient inputs and 2) how does intensive organic management versus low-input conventional management affect nitrogen (N) and P availability for C. arabica grown under E. poeppigiana? We use microbiological community structure and function as well as nutrient pools to explain differences in P availability. Rhizosphere and bulk soil samples and C. arabica fine roots (<2.0 mm) were collected at a depth of 0-20cm under coffee plants grown in monoculture (n=9, low-input conventional) and under the shade of E. poeppigiana (n=9, low-input conventional; n=9, intensive organic). Nitrate, ammonium and orthophosphate levels, as well as total N and P in rhizosphere and bulk soil were analyzed, as were % N and P of fine root samples. DNA-based techniques were used to identify differences in soil microbial community composition between treatments, in particular with regards to functional groups involved in transformations of N and P. Additionally, arbuscular mycorrhizal fungal composition of root tissue was contrasted between treatments using molecular techniques. Results will be discussed in detail during the poster presentation.