Short-term effect of urea on CH4 flux under the oil palm (Elaeis guineensis) on tropical peatland in Sarawak, Malaysia

Methane flux was measured monthly from August 2002 to July 2003 at an oil palm plantation on tropical peatland in Sarawak, Malaysia, using a closed chamber technique. Urea was applied twice, once in November 2002 and once in May 2003. The monthly CH, flux ranged from -32.78 to 4.17 mu g C m(-2) h(-1). Urea applications increased CH4 emissions in the month of application and emissions remained slightly higher a month later before the effect disappeared in the third month after application (i.e. back to CH, uptake). This effect was the result of increased soil NH4+ content that was not immediately absorbed by the oil palm following urea application, which reduced the oxidation of CH4, resulting in its enhanced emission. By using the Cate-Nelson linear-plateau model, the critical soil NH4+ content causing CH4 emissions in the oil palm ecosystem was 42.75 mg kg(-1) soil. However, the inhibitory effect of NH4+ on the oxidation of CH4 was mitigated by low rainfall and the pyrophosphate solubility index (PSI), where the former might increase oxidation of CH4 and the latter was a reflection of the low soluble substrate for methane production. Thus, the splitting and timing of urea applications are important not only to optimize oil palm yield, but also to reduce soil NH4+ content to minimize CH4 emissions and, therefore, its potential negative impact on the environment.