Indonesia lost more tropical forest than all of Brazil in 2012, mainly driven by the rubber, oil palm, and timber industries. Nonetheless, the effects of converting forest to oil palm and rubber plantations on soil organic carbon (SOC) stocks remain unclear. We analyzed SOC losses after lowland rainforest conversion to oil palm, intensive rubber, and extensive rubber plantations in Jambi Province on Sumatra Island. The focus was on two processes: (1) erosion and (2) decomposition of soil organic matter. Carbon contents in the Ah horizon under oil palm and rubber plantations were strongly reduced up to 70% and 62%, respectively. The decrease was lower under extensive rubber plantations (41%). On average, converting forest to plantations led to a loss of 10 Mg C ha À1 after about 15 years of conversion.The C content in the subsoil was similar under the forest and the plantations. We therefore assumed that a shift to higher d 13C values in plantation subsoil corresponds to the losses from the upper soil layer by erosion. Erosion was estimated by comparing the d 13 C profiles in the soils under forest and under plantations. The estimated erosion was the strongest in oil palm (35 AE 8 cm) and rubber (33 AE 10 cm) plantations. The 13 C enrichment of SOC used as a proxy of its turnover indicates a decrease of SOC decomposition rate in the Ah horizon under oil palm plantations after forest conversion. Nonetheless, based on the lack of C input from litter, we expect further losses of SOC in oil palm plantations, which are a less sustainable land use compared to rubber plantations. We conclude that d 13 C depth profiles may be a powerful tool to disentangle soil erosion and SOC mineralization after the conversion of natural ecosystems conversion to intensive plantations when soils show gradual increase of d 13C values with depth.

Abstract. Expansion of palm oil and rubber production, for which global demand is increasing, causes rapid deforestation in Sumatra, Indonesia, and is expected to continue in the next decades. Our study aimed to (1) quantify changes in soil CO 2 and CH 4 fluxes with land-use change and (2) determine their controlling factors. In Jambi Province, Sumatra, we selected two landscapes on heavily weathered soils that differ mainly in texture: loam and clay Acrisol soils. In each landscape, we investigated the reference land-use types (forest and secondary forest with regenerating rubber) and the converted land-use types (rubber, 7-17 years old, and oil palm plantations, 9-16 years old). We measured soil CO 2 and CH 4 fluxes monthly from December 2012 to December 2013. Annual soil CO 2 fluxes from the reference land-use types were correlated with soil fertility: low extractable phosphorus (P) coincided with high annual CO 2 fluxes from the loam Acrisol soil that had lower fertility than the clay Acrisol soil (P < 0.05). Soil CO 2 fluxes from the oil palm (107.2 to 115.7 mg C m −2 h −1 ) decreased compared to the other land-use types (between 178.7 and 195.9 mg C m −2 h −1 ; P < 0.01). Across land-use types, annual CO 2 fluxes were positively correlated with soil organic carbon (C) and negatively correlated with 15 N signatures, extractable P and base saturation. This suggests that the reduced soil CO 2 fluxes from oil palm were the result of strongly decomposed soil organic matter and reduced soil C stocks due to reduced litter input as well as being due to a possible reduction in C allocation to roots due to improved soil fertility from liming and P fertilization in these plantations. Soil CH 4 uptake in the reference landuse types was negatively correlated with net nitrogen (N) mineralization and soil mineral N, suggesting N limitation of CH 4 uptake, and positively correlated with exchangeable aluminum (Al), indicating a decrease in methanotrophic activity at high Al saturation. Reduction in soil CH 4 uptake in the converted land-use types (ranging from −3.0 to −14.9 µg C m −2 h −1 ) compared to the reference land-use types (ranging from −20.8 to −40.3 µg C m −2 h −1 ; P < 0.01) was due to a decrease in soil N availability in the converted land-use types. Our study shows for the first time that differences in soil fertility control the soil-atmosphere exchange of CO 2 and CH 4 in a tropical landscape, a mechanism that we were able to detect by conducting this study on the landscape scale.

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Abstract. Expansion of palm oil and rubber production, for which global demand is increasing, causes rapid deforestation in Sumatra, Indonesia and is expected to continue in the next decades. Our study aimed to (1) quantify changes in soil CO2 and CH4 fluxes with land-use change, and (2) determine their controlling factors. In Jambi Province, Sumatra, we selected two landscapes on heavily weathered soils that differ mainly in texture: loam and clay Acrisol soils. At each landscape, we investigated the reference land uses: forest and secondary forest with regenerating rubber, and the converted land uses: rubber (7–17 years old) and oil palm plantations (9–16 years old). We measured soil CO2 and CH4 fluxes monthly from December 2012 to December 2013. Annual soil CO2 fluxes from the reference land uses were correlated with soil fertility: low extractable phosphorus (P) coincided with high annual CO2 fluxes from the loam Acrisol soil that had lower fertility than the clay Acrisol soil (P < 0.05). Soil CO2 fluxes from the oil palm decreased compared to the other land uses (P < 0.01). Across land uses, annual CO2 fluxes were positively correlated with soil organic carbon (C) and negatively correlated with 15N signatures, extractable P and base saturation. This suggests that the reduced soil CO2 fluxes from oil palm was a result of strongly decomposed soil organic matter due to reduced litter input, and possible reduction in C allocation to roots due to improved soil fertility from liming and P fertilization in these plantations. Soil CH4 uptake in the reference land uses was negatively correlated with net nitrogen (N) mineralization and soil mineral N, suggesting N limitation of CH4 uptake, and positively correlated with exchangeable aluminum (Al), indicating decrease in methanotrophic activity at high Al saturation. Reduction in soil CH4 uptake in the converted land uses compared to the reference land uses (P < 0.01) was due to decrease in soil N availability in the converted land uses. Our study shows for the first time that differences in soil fertility control soil–atmosphere exchange of CO2 and CH4 in a tropical landscape, a mechanism that we were able to detect by conducting this study at the landscape scale.

No abstract