Soil properties and gross nitrogen dynamics in old growth and secondary forest in four types of tropical forest in Thailand

To understand the changes in soil properties and function due to human-induced disruption of mature tropical forest cover in differing climatic zones and forest types, we investigated soil properties and gross soil N dynamics in the top 5 cm of mineral soil in old-growth and young secondary (<30 years) stands of four contrasting forest types (dry dipterocarp, dry evergreen, mixed deciduous, and moist evergreen) in Thailand. In all four forest types, soil total carbon (C) and nitrogen (N) pools were not different between old-growth and young secondary forests. In contrast, soil N turnover rates, such as the gross and net mineralization and nitrification rates, were different depending on forest type (e.g., gross mineralization rate was faster in old-growth than in secondary moist evergreen forest, but faster in secondary than in old-growth for the other three forest types). Although the response of soil N turnover rate to forest succession was inconsistent among the four forest types, the gross nitrification ratio (gross nitrification rate as a proportion of gross mineralization rate) was smaller in secondary than in old-growth forest in all forest types. This shift in the inorganic N production balance by reducing NO3-N production relative to NH4-N production in Southeast Asia warrants further investigation, including the extent of the phenomenon and its effect on plant productivity and species composition.