Balance between soil N2O emissions and above-ground CO2 uptake in coffee monocultures and agroforestry plantations in Costa Rica

Soil N2O emissions and C storage were measured in coffee monocultures and coffee agroforestry systems (AFS) with N2-fixing shade trees, under two agricultural managements. Two of the coffee plantations (monoculture and AFS with Inga densiflora) were conventionally managed (250 kg mineral N ha-1 y-1); the two others (monoculture and AFS with Erythrina poeppigiana) were organically managed (150 kg organic N ha-1 y-1). Measurements of soil N2O emissions, litterfall, litter and biomass were achieved when the plantations were 7 8 years old (conventional management) and 6 years old (organic management). Soil N2O emissions over the life time of the plantations (32 years) were simulated as a function of N inputs through fertilization and litterfall. Over the same period, CO2uptake by aboveground biomass was calculated by simulating the growth of the coffee plants and shade trees. The annual rate of the litterfall was assumed to be proportional to the above-ground biomass and the standing stock of litter was calculated as a function of litter inputs and decay rate. For both types of agricultural management, measured annual N2O emissions were higher in the AFS (5.8±0.3 and 3.7±0.5 kg N2O -N ha-1 year-1, in the conventional and organic managements, respectively) than in the monocultures (4.3 0.1 and 1.8±0.3 kg N2O -N ha-1 year-1, in the respective managements), coinciding with higher N input through litterfall. The balance between cumulated N2O emissions and CO2uptake over the life time of the plantations was negative in the monocultures (-25.5 and -8.2 Mg CO2-eq ha-1 32 year-1 in the conventional and organic managements, respectively) but positive in the AFS with a greater contribution to climate change mitigation of the organic management (37.3 Mg CO2-eq ha-1 32year-1) than of the conventional one (14.9 Mg CO2-eq ha-1 32year-1). (Texte intégral