Soil microbial and nutrient properties in the rhizosphere of coffee under agroforestry management

- We determined coffee rhizosphere soil nutrient concentrations and nitrifier populations.
- Soil nitrates accumulate in rhizosphere of shade coffee compared to monoculture coffee.
- Soil pH and AOB abundance positively correlate under shade and monoculture coffee.
- Under monoculture coffee, ammonium:nitrate ratios vary with AOB abundance.

The integration of shade trees in coffee production can positively contribute to increased yields in these cropping systems. However, little work has examined nutrient-microbial dynamics in the rhizosphere of coffee under shade trees. In this study, we compared nutrient concentrations and nitrifier populations in rhizosphere soil of coffee under shade and in monoculture as well as in rhizosphere versus bulk soils. Our study was performed in a low altitude, wet coffee zone of Costa Rica, at a 12-year-old agroforestry experimental farm with coffee (Coffea arabica) grown under full sun and under the commonly used dinitrogen (N2) fixing shade tree, Erythrina poeppigiana. Rhizosphere and bulk soils were collected and analyzed for inorganic N, available P, exchangeable base cations and pH status. Real-time PCR was used to determine the relative abundance of ammonia oxidizing bacteria (AOB). We show a nutrient increase (11-158%), particularly nitrates, in the rhizosphere soil compared to bulk soil of coffee. Moreover, this accumulation was greater in the rhizosphere of coffee associated with E. poeppigiana than under monoculture. AOB abundance was strongly related to soil pH (r = 0.83; P = 0.002) across both treatments, while AOB abundance varied with soil ammonium:nitrate ratios (r = −0.82; P = 0.0420) exclusively under monoculture coffee. Our study, for the first time to our knowledge, illustrates soil nutrient differences and nutrient-microbial relationships under coffee in monoculture and in a N2-fixing shade tree system.