The effect of tropical land use on soil carbon dynamics: Does reforestation mitigate greenhouse gas emissions?

Tropical land use change accounts for the second largest source of human-caused greenhouse gas emissions. Soils are the largest terrestrial carbon (C) sink, storing two to three times as much C as the atmosphere and above ground vegetation. The majority of soil C is associated with soil aggregates of different size classes that have distinct potentials for C storage. Here we investigated the effect of different tropical land uses on soil C dynamics. We measured soil respiration and bioavailable C and nitrogen (N) from three soil aggregate classes across three common tropical land uses; agricultural cropping system, a tree plantation (purposely afforested for C sequestration), and a transitional premontane rainforest. We found that land use had a significant effect on mean CO₂ flux (r² = 0.84, p = 0.04), with the highest fluxes occurring at the forest plantation and the lowest fluxes occurring at the premontane rainforest. Bioavailable N also differed by land use (r² = 0.74, p < 0.0001), with the greatest amounts observed in the premontane rainforest and the lowest amounts in the tree plantation, suggesting that soil N may be inversely related to soil respiration. Macroaggregates also accounted for the majority ( 70%) of aggregates in the tree plantation. A greater abundance of macroaggregates, a more biologically active aggregate class, may be linked to the large fluxes observed in the tree plantation. Our results indicate that the tree plantation, planted to offset greenhouse gas emissions, may act as a carbon source instead of a sink. As the terrestrial carbon cycle is dominated by the balance between photosynthesis and respiration, photosynthetic studies are needed to better understand the balance of the tree plantation. Finally, the net carbon uptake of tropical forest plantations could be less than anticipated, which has implication for carbon offset programs.