Across Southeast Asia, tropical forests are being cleared at an alarming rate to make way for commercial plantations. Yet the hidden costs beneath the ground may be far more consequential than previously recognized.

In a study published in Journal of Environmental Management on June 6, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences revealed that replacing rainforests with rubber trees reduces soil organic carbon and alters its chemical composition, potentially compromising long-term climate benefits.

The researchers investigated the impact of tropical rainforest conversion to rubber plantations on soil organic carbon (SOC) cycling. They focused on analyzing the dynamic changes and driving factors of plant- and microbe-derived SOC. Examining a chronosequence of rubber plantations aged nine, 21, and 37 years—all of which were established on land that was once primary tropical rainforest—the researchers found that the transition from rainforest to young rubber plantations triggered a dramatic 43% drop in SOC across the top 40 centimeters of soil.

Using lignin phenols and amino sugars as biomarkers to trace plant- and microbial-derived carbon, respectively, the researchers found that the transition from rainforest to young rubber plantations triggered a dramatic 43% drop in SOC across the top 40 centimeters of soil. This decline, however, was not permanent. As the plantations matured, both biomarker pools showed signs of recovery, likely due to increased leaf litter, greater root inputs, and reduced microbial decomposition of lignin.

Traditionally, plant-derived lignin has been considered the primary stable component of soil carbon. However, the researchers found that microbial residues contributed 5.2 to 9.3 times more to SOC than plant-derived carbon across all sites. Moreover, the rubber plantations consistently exhibited lower glucosamine-to-muramic acid ratios than the rainforest soils, indicating a compositional shift in the microbial residue pool.

According to the authors, this change "may reduce the long-term persistence potential of microbial-derived SOC."

"Our results indicate that, although the quantity of soil carbon can recover with plantation age, its composition and the mechanisms governing its persistence are fundamentally changed," said YUAN Xia, first author of the study. "This is not simply about putting carbon back into the soil—it matters what kind of carbon it is and how long it will remain there."

As rubber plantations matured, both the soil organic carbon and its component biomarkers showed signs of recovery. This was driven by increased litter and root inputs, as well as reduced microbial degradation of lignin. However, even in 37-year-old plantations, the carbon pool composition remained distinct from that of the original rainforest.

"Our study offers critical evidence that sustainable rubber plantation management must go beyond merely maintaining soil carbon stocks," said LIU Wenjie of XTBG. "To truly support climate change mitigation, we need to develop practices that foster the recovery and persistence of the most stable soil carbon components. That means minimizing soil disturbance and enhancing below-ground carbon inputs."