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

Abstract

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.

Introduction

The concentration of CO₂ and other greenhouse gases (GHG) in the atmosphere has increased considerably over the last four decades (Rogner et al., 2007). This increase is mainly a result of the burning of fossil fuels and the conversion of tropical forests to agricultural production, which have caused negative changes in the global climate (Rogner et al., 2007). Reduction of GHG concentrations in the atmosphere in order to mitigate climate change can be achieved through two major processes: (1) reducing anthropogenic emissions; and (2) creating and/or enhancing GHG sinks in the biosphere (Albrecht and Kandji, 2003, Oelbermann et al., 2004). By including trees in agricultural production systems, agroforestry offers a potential as biomass energy provider and thus presents interesting opportunities for CO₂ mitigation through the substitution of fossil fuel by wood energy and the protection of existing forests (Verchot et al., 2005). Moreover, agroforestry can increase the amount of carbon (C) stored in agricultural systems while still allowing for the growing of food crops (Montagnini and Nair, 2004). Tree components in agroforestry systems can be significant sinks of atmospheric C (De Miguel Magaña et al., 2004); and organic inputs from litter, pruned biomass and roots decay and exudation of the trees can maintain or increase soil organic content as well as improve soil nutrients and reduce loss through erosion (Montagnini and Nair, 2004, Oelbermann et al., 2006). In contrast to these C stock gains, agroforestry may involve practices that favor the emission of non-CO₂ GHG (N₂O and CH₄) including mineral nitrogen (N) fertilization and the use of N₂-fixing leguminous species to provide N to the system and/or shade for the crop growing underneath the tree canopy (Verchot et al., 2005, Hergoualc’h et al., 2008). Many studies have established that N fertilization in agriculture increases soil N₂O emissions (Stehfest and Bouwman, 2006) and decreases the uptake of atmospheric CH₄ (Hütsch et al., 1993, Chu et al., 2007). N₂-fixing species are suspected to contribute to the increase of soil emissions of N₂O (Rochette and Janzen, 2005, Verchot et al., 2008) and to the reduction of the soil CH₄ sink (Palm et al., 2002), although to date results in the literature are contradictory (Verchot et al., 2008). N₂O and CH₄ have a global warming potential 298 and 25 times larger than CO₂, respectively (Forster et al., 2007), it is therefore important to evaluate in agroforestry systems how the changes in their dynamics compare with increased atmospheric CO₂ uptakes.

Coffee agriculture represents 7.5% of the world's permanent crops (FAO, 2005) and can use large amounts of N fertilizer. Originally grown under permanent shade, coffee cultivated in full sunlight has gradually become a common cultivation system over the last 40 years in many tropical countries worldwide. It was estimated that by 1990 more than half of the coffee-producing area in Latin America was changed to an intensive monoculture system shaded by either one species of shade tree or exposed to direct sunlight. Estimates suggest that 41% of the 2.7 million ha of coffee production lands in Latin America have been converted to unshaded or reduced shade plantations (Gobbi, 2000). In Costa Rica coffee monocultures expanded in the most favorable areas such as the Central Valley, with altitudes of 1000–1300 m and mean temperatures of <22 °C (Samper, 1999). However, in lower lying areas (altitude < 700 m, mean air temperature > 24 °C) poorly suited for monoculture, Coffea arabica is grown under the shade of trees, which moderate the microclimate for the benefit of the crop (van Kanten and Vaast, 2006, Siles et al., 2010). Nowadays coffee monocultures and agroforestry systems represent 40% and 60% of coffee cultivations in Costa Rica, respectively (Vaast and Harmand, 2002). Generally large amounts of mineral N fertilizer (150–250 kg N ha⁻¹ year⁻¹) are applied in both cultivation types and in agroforestry systems, coffee is commonly shaded by regularly pruned N₂-fixing leguminous trees, such as Erythrina spp. and Inga spp., or non-leguminous timber trees, such as Eucalyptus spp. (Harmand et al., 2007a, Harmand et al., 2007b).

This study considers the effect of agroforestry practices in coffee plantations of Costa Rica on C stocks and non-CO₂ GHG, N₂O and CH₄. Two plantations were studied; a coffee monoculture and a coffee agroforestry system shaded by the N₂-fixing tree species Inga densiflora. The two plots were adjacent, planted with a similar coffee density and both received a commonly applied N fertilization rate of 250 kg N ha⁻¹ year⁻¹. The two main questions addressed in this study were whether in these intensively fertilized coffee plantations (1) the soil was a net source or sink of GHG and (2) agroforestry contributed to increase C sequestration and enhance net GHG removal. Our approach consisted of measuring C stocks in the phytomass (trees, coffee plants and litter) and in the soil (0–40 cm) 7 years after plantation establishment. We measured the evolution of soil C stocks in the plantations by comparing the stocks at two different times (6 and 9 years after the establishment of the plantations) and by calculating the annual soil C stock change as the difference in stocks divided by the number of years (3) between the measurements. Soil emissions of N₂O and uptakes of CH₄ were monitored for a 1-year period (Hergoualc’h et al., 2008) when the plantations were 6–7 years old. Finally, the net GHG balance, expressed in CO₂-equivalent, was evaluated both at soil scale and at plantation scale. At soil scale the net balance was calculated from the changes in soil C stock and soil N₂O emissions. At plantation scale it included additionally C storage in the phytomass.