Changes in carbon stock and greenhouse gas balance in a coffee (Coffea arabica) monoculture versus an agroforestry system with Inga densiflora, in Costa Rica

Agroforestry represents an opportunity to reduce CO2 concentrations in the atmosphere by increasing carbon (C) stocks in agricultural lands. Agroforestry practices may also promote mineral N fertilization and the use of N2-fixing legumes that favor the emission of non-CO2 greenhouse gases (GHG) (N2O and CH4). The present study evaluates the net GHG balance in two adjacent coffee plantations, both highly fertilized (250 kg N ha−1 year−1): a monoculture (CM) and a culture shaded by the N2-fixing legume tree species Inga densiflora (CIn). C stocks, soil N2O emissions and CH4 uptakes were measured during the first cycle of both plantations. During a 3-year period (6–9 years after the establishment of the systems), soil C in the upper 10 cm remained constant in the CIn plantation (+0.09 ± 0.58 Mg C ha−1 year−1) and decreased slightly but not significantly in the CM plantation (−0.43 ± 0.53 Mg C ha−1 year−1). Aboveground carbon stocks in the coffee monoculture and the agroforestry system amounted to 9.8 ± 0.4 and 25.2 ± 0.6 Mg C ha−1, respectively, at 7 years after establishment. C storage rate in the phytomass was more than twice as large in the CIn compared to the CM system (4.6 ± 0.1 and 2.0 ± 0.1 Mg C ha−1 year−1, respectively). Annual soil N2O emissions were 1.3 times larger in the CIn than in the CM plantation (5.8 ± 0.5 and 4.3 ± 0.3 kg N-N2O ha−1 year−1, respectively). The net GHG balance at the soil scale calculated from the changes in soil C stocks and N2O emissions, expressed in CO2 equivalent, was negative in both coffee plantations indicating that the soil was a net source of GHG. Nevertheless this balance was in favor of the agroforestry system. The net GHG balance at the plantation scale, which includes additionally C storage in the phytomass, was positive and about 4 times larger in the CIn (14.59 ± 2.20 Mg CO2 eq ha−1 year−1) than in the CM plantation (3.83 ± 1.98 Mg CO2 eq ha−1 year−1). Thus converting the coffee monoculture to the coffee agroforestry plantation shaded by the N2-fixing tree species I. densiflora would increase net atmospheric GHG removals by 10.76 ± 2.96 Mg CO2 eq ha−1 year−1 during the first cycle of 8–9 years.

► The net atmospheric greenhouse gas (GHG) removal from converting a coffee monoculture to a coffee agroforestry plantation shaded by the N2-fixing species Inga densiflora was estimated at 10.76 Mg CO2 eq ha−1 y−1 during the first rotation cycle of 8-9 years.
► N2O emissions resulting mainly from N input largely counterbalanced the potential positive effect of shade trees on C sequestration in soil. The soil negative net GHG balance, indicating a source of GHG to the atmosphere, was yet smaller in the agroforestry system than in the monoculture despite larger soil N2O emissions. C sequestration in the shade tree biomass amply compensated the soil negative balance in the agroforestry system.
► The net balance of GHG at the soil scale which included N2O emissions and changes in soil carbon stock represented a substantial portion of the C sequestration in the phytomass and should therefore be accounted in fertilized coffee plantations.