The impact of forest conversion to oil palm plantation on the internal nitrogen cycle of tropical lowland soils

This thesis seeks to quantify the effect of land use change from tropical forest to oil palm plantation on nitrogen biogeochemical cycling in Sabah, Malaysia (Borneo). Nitrogen cycling process rates and indices were examined across four forests and six oil palm plantations during the inter-monsoon and end of wet season in 2010 and 2012 respectively. Firstly, the study establishes a baseline to assess the impact of land use change along a chronosequence of forest succession. Results indicate that forests follow a trajectory of nitrogen recovery and increased “openness” to nitrogen cycling through secondary forest development. Secondly, the spatial and temporal variation of nitrogen cycling within oil palm plantations is assessed. Results show that plantation management practices result in spatial variability in soil nitrogen. Examining process rates revealed an increasing trend of N\(_2\)O emission and decreasing trends of soil organic matter content as plantations matured. However, season and soil type also affected denitrification and N\(_2\)O emission. Finally, a replicated comparison of process rates in forests and plantations on riparian and terra firme soils revealed that plantation establishment significantly altered rates of nitrogen cycling and resulted in greater emissions of N\(_2\)O from \(terra\) \(firme\) plantations