Soil Organic Carbon Stocks and Soil Respiration in Tropical Secondary Forests in Southern Mexico.

The soil CO₂ efflux is recognized as one of the largest fluxes in the global carbon cycle, and small changes in the magnitude of soil respiration could have a large consequence on the concentration of CO₂ in the atmosphere. In this study, we analyzed the soil organic carbon (SOC) stocks and CO₂ efflux from soil respiration in a tropical secondary forest grown after abandonment of swidden agriculture in Southern Mexico. The study was conducted in a semi-evergreen tropical secondary and primary forests in southern part of Yucatan Peninsula, Mexico.  We collected soil samples (up to 30 cm depth) from 32 carbon monitoring plots and analyzed for physical and chemical soil properties. Soil respiration measurements were carried out by using PP systems EGM-4 (an infra-red gas analyzer) during the year 2013. Analysis of variance, correlation and regression analysis were performed to test difference between secondary forests of different age and primary forests and to see the relationship between soil respiration and soil organic as well as inorganic carbon. We found that there was a gradual increase in carbon stock with forest age in organic horizon with significant difference in humus layer. However, contrary to our hypothesis, SOC in mineral soil horizon did not increase with forest age. We neither found a significant relationship with land use intensity. Similarly, different from our hypothesis, soil CO₂ efflux did not correlate to soil organic carbon, it rather correlated to carbonate concentration in the soil. Linear regression analysis showed that soil respiration was higher in carbonate rich soils. Our results suggest that swidden farming adopted by Mayan farmers in the region was not so harmful to jeopardize soil properties and soil carbon accumulation. However, we should consider the previously reported effects on biomass accumulation before implementing any land-use decisions. Higher CO₂ efflux in carbonate rich soils and stability of SOC with forest age in mineral soil horizon can be explained probably by the faster decomposition but the slower ultimate mixing of organic matter in mineral soils of carbonate origin as reported in a few earlier studies. However, it seeks further investigation in partitioning soil CO₂ efflux from autotrophic, heterotrophic and abiotic sources to better understand the role of carbonate soils in atmospheric CO₂ change.