Conversion of degraded forests to oil palm plantations in the Peruvian Amazonia: Shifts in soil and ecosystem-level greenhouse gas fluxes

Abstract

Oil palm (OP) expansion and associated forest clearance can significantly impact greenhouse gas (GHG) fluxes. We investigated carbon stocks and soil GHG (N₂O, CO₂, CH₄) fluxes in a degraded forest and an adjacent 17-year-old OP plantation in Peruvian Amazonia. The plantation comprised three nitrogen (N) fertilizer treatments: 0 (OPN0), 84 (OPN1), 168 (OPN2) kg N ha⁻¹ y⁻¹. Carbon stocks were inventoried across all pools. GHG and environmental parameters were monitored monthly for 11 months and (bi)daily when fertilizing the OP, with measurements taken both near and far from trees/palms. Ecosystem-scale CO₂equivalent losses from the conversion were computed by balancing carbon stock losses against N₂O emission changes. N₂O emissions (kg N ha⁻¹ y⁻¹) in the forest (6.7 ± 1.2) where litterfall N inputs were large (213 kg N ha⁻¹ y⁻¹) were 11, 5, and 3 times the emissions in OPN0 (0.6 ± 0.2), OPN1 (1.4 ± 0.2), and OPN2 (2.3 ± 0.3). In the plantation, 1 % of the N fertilizer applied was released as N₂O. Across ecosystems, N inputs primarily controlled N₂O emissions. Soil respiration (Mg C ha⁻¹ y⁻¹) was 1.4 times higher in the forest (9.1 ± 0.6) than in the plantation (7.3 ± 1, 5.5 ± 0.5, 6.5 ± 0.3 in OPN0, OPN1, OPN2). The forest was a soil CH₄ (kg C ha⁻¹ y⁻¹) sink (-1.5 ± 0.3) while all OP treatments were sources (0.2 ± 0.3, 0.7 ± 0.5, 0.2 ± 0.4 in OPN0, OPN1, OPN2). Ecosystem carbon stock losses from forest-to-OP conversion were substantial (196.8 ± 44.0 Mg CO₂ ha⁻¹ 15 y⁻¹) and partially offset (14–20 %) by decreased N₂O emissions. Complementary studies for this transition are needed to improve global GHG assessments.