Effects on soil chemistry of tropical deforestation for agriculture and subsequent reforestation with special reference to changes in carbon and nitrogen

Abstract

While soils can act either as a source or as a sink for atmospheric carbon dioxide and nitrogen, depending on land cover change and management, there is a great need to improve our understanding of the dynamics of soil structure and chemistry following land conversion in tropical regions. In this study, we investigated various soil structural and chemical variables (bulk density, pH, cation exchange capacity, phosphorus, magnesium, potassium), soil carbon (C), soil nitrogen (N), δ13C, and δ15N in relation to conversion of rain forest to tea cultivation and subsequent changes due to agricultural abandonment and reforestation with Caribbean pine. Although Caribbean pine has shown potential for reforestation throughout Asia and Latin America, effects on soil properties in comparison with other land uses have not been quantified. Our study compared: (1) the original mixed-dipterocarp rain forest, (2) tea plantations, (3) Kekilla fernlands and (4) Caribbean pine plantations. Tea plantations show higher bulk density (BD) as an evidence of compaction. Although soil C concentration in tea plantations were lower than other land uses at 0–10cm, when bulk density was used with C as a composite measure, tea plantation showed the highest value for both soil C and N stocks. Disregarding tea plantations, Caribbean pine plantations had the highest C but showed the lowest N stocks at both soil depths. Soil δ13C and δ15N values of all land uses increase with increasing soil depth; but Caribbean pine plantations showed the greatest increase in δ13C at both soil depths (e.g. −27.82±0.38‰ at 0–10cm to −26.50±0.37‰ at 10–20cm). For both Kekilla fernlands and Caribbean pine plantations the relationship between δ13C and δ15N was strongly linear. By comparing the physical and chemical soil properties of these land uses with undisturbed rain forest, we established baseline data to determine influence of land conversion on soil properties. Based on soil pH, CEC and other major nutrients (P, K, Mg), there are strong legacy effects of land use potentially from both fertilization and fire. Our results also showed strong evidence that δ13C and δ15N was both increased with depth under pine reforestation and fernlands, suggesting soils can recuperate with a consistent input of litter and slower decomposition processes. There is some evidence that recruitment of natural regeneration beneath pine can help facilitate faster litter decomposition that can revert soil structure and fertility to a status similar to that of the original forest.