Land use and organic carbon content of some subtropical soils

Summary

The assumption that the organic matter content of tropical forest soils is oxidized to atmospheric carbon dioxide when these soils are converted to agricultural use was tested using results of soil surveys in Puerto Rico (1940's, 1960's, and 1980's). Results showed that under intensive agricultural use, soil carbon in the top 18 cm of soil was about 30–37 Mg/ha, regardless of climatic conditions. Reduced intensity of agricultural use resulted in an increase of soil carbon in the order of 0.3–0.5 Mg.ha⁻¹. yr⁻¹ over a 40-yr period. Rates of soil carbon accumulation were inversely related to the sand content of soils. The relation between rates of soil carbon accumulation and climate or soil texture were better defined at higher soil carbon content. Soils under pasture accumulated soil carbon and often contained similar or greater amounts than adjacent mature forest soils (60–150 Mg/ha in the top 25 or 50 cm). Soils in moist climates exhibited greater variations in soil carbon content with changes in land use (both in terms of loss and recovery) than did soils in dry climates. However, in all life zones studied, the recovery of soil carbon after abandonment of agriculture was faster than generally assumed. Low carbon-to-nitrogen ratios suggested that intensively used soils may be stable in their nutrient retention capacity. The observed resiliency of these soils suggested that their role as atmospheric carbon sources has been overestimated, while their potential role as atmospheric carbon sinks has been underestimated.