Impact of land use change on soil organic carbon stocks and greenhouse gas balance in tropical regions

Introduction

For centuries, Sumatra's lowland forests stored some of the densest terrestrial carbon pools on Earth. By the mid-twentieth century, roughly 75% of the island remained forested [1] . That figure has since fallen below 30%, driven primarily by conversion to oil palm plantations and smallholder agriculture [2] . The carbon released during this transformation accounts for a substantial share of Indonesia's total greenhouse gas emissions, placing land use change at the center of national climate policy debates. Soil organic carbon represents the largest terrestrial carbon reservoir, holding an estimated 1,500 Pg C globally in the top meter [3] . Tropical soils are particularly responsive to disturbance because warm temperatures and high moisture accelerate organic matter decomposition once protective forest cover is removed [4] . Yet the magnitude of SOC loss varies greatly depending on what replaces the forest. Agroforestry, with its multi-layered canopy structure, may retain more soil carbon than monoculture systems [5] , but direct field comparisons across multiple land use types remain scarce for Sumatran conditions. This research measured SOC stocks and estimated net GHG balances across five land use types in Sumatra to quantify carbon consequences of different conversion pathways and identify which productive land uses best preserve soil carbon.

Seasonal variations in carbon fluxes

Soil CO₂ efflux in cropland plots increased 38% during the wet season (October-March) compared to dry season, driven by enhanced microbial activity. Primary forest showed more stable respiration due to canopy buffering. N₂O emissions peaked during wet-dry transitions across all fertilized land uses, matching denitrification patterns under wetting-drying cycles [6] .

Material and Methods

Material

Fieldwork was conducted collaboratively between Kyoto University and partner institutions in Sumatra from April 2020 to March 2023, under approval from Kyoto University Research Ethics Board (Protocol KU-ENV/2020-042, March 8, 2020). Ninety plots (18 per land use type) were established across Jambi, Riau, and South Sumatra provinces. Each plot measured 40 m × 40 m. Soil cores were extracted using a hydraulic sampler to 100 cm depth in 10 cm increments. Gas samples for GHG analysis were collected using static closed chambers and analyzed on a Shimadzu GC-2014 gas chromatograph. SOC was determined by dry combustion on an Elementar vario MACRO cube [7] .

Methods

SOC stocks were calculated from carbon concentration and bulk density measurements at each depth increment, summed to 100 cm. GHG fluxes (CO₂, CH₄, N₂O) were measured monthly from three permanent chamber bases per plot. Net ecosystem GHG balance was estimated by combining soil carbon stock changes (measured over the 3-year period), biomass carbon increments (from allometric equations), and cumulative soil GHG emissions [8] . Land use history was reconstructed from Landsat time-series imagery and farmer interviews. Statistical comparisons used one-way ANOVA followed by Tukey's HSD test at p<0.05 in R v4.3.0.

Soil quality analysis

Baseline characterization included texture (hydrometer), pH (1:2.5), total nitrogen (Kjeldahl), available phosphorus (Bray-1), and CEC (ammonium acetate). Bulk density was from undisturbed cores at 10 cm increments. Microbial biomass carbon was measured by chloroform fumigation at 0-10 and 10-30 cm depths [9] .

Results

Discussion

The 51.1% SOC loss upon forest-to-cropland conversion exceeds the global average of 42% reported by Don et al. [10] , likely reflecting high baseline SOC in Sumatran lowlands and intensive smallholder tillage [11] . Agroforestry matching secondary forest SOC within 15 years is consistent with Nair et al. [12] for humid tropical tree-based systems. Oil palm remaining a net GHG source despite biomass accumulation reflects continued SOC decline and N₂O from fertilization, as Melling et h. [13] reported for Sarawak mineral soils. Optimizing nitrogen management could shift the balance, since N₂O accounted for roughly 34% of oil palm's GHG output.

Limitations

The chronosequence approach assumes current land uses represent conversion trajectories, which may not hold where management intensity changed over time. GHG measurements captured soil-level fluxes only, excluding canopy exchanges. The 3-year observation period may not reflect longer-term stabilization dynamics. Spatial replication across three provinces improved generalizability but introduced confounding variation in soil type and rainfall.

Conclusion

Land use conversion in tropical Sumatra caused SOC losses up to 51.1% when primary forest gave way to annual cropland. Agroforestry emerged as the most carbon-efficient productive alternative, accumulating SOC at 1.14 Mg/ha/yr and functioning as a net carbon sink. Oil palm and cropland were net GHG sources at 2.7 and 3.4 Mg CO₂-eq/ha/yr respectively. Secondary forests regained about 76% of original SOC over 15-25 years. These results support policies incentivizing agroforestry over monoculture expansion in tropical frontier landscapes.

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