N2O and CO2 emissions from three different tropical forest sites in the wet tropics of Queensland, Australia

Three different tropical rain forest sites in Northeast Queensland, Australia, two in the Coastal Lowlands (Pin Gin Hill and Bellenden Ker) and one on the Atherton Tablelands (Kauri Creek), were investigated for the magnitude of N2O and CO2 emissions from soils during the wet and the dry season. At all sites, mean N2O emission rates were significantly higher during the wet season (Bellenden Ker: 242.0 +/- 7.4 mug N2O-N m(-2) h(-1), Pin Gin Hill: 140.8 +/- 5.1 mug N2O-N m(-2) h(-1), Kauri Creek: 80.8 +/- 3.3 mug N2O-N m(-2) h(-1)) as compared to the dry season when N2O-emissions were markedly lower (<20 μg N2O-N m(-2) h(-1)) due to limitations in soil moisture. During the wet season, mean N2O emission rates of the Coastal Lowland sites Bellenden Ker and Pin Gin Hill were approximately twofold higher as compared to N2O emission rates of the Atherton Tableland site Kauri Creek. These site differences were found to be due to differences in precipitation and soil moisture, the C-to-N ratio of the organic matter, soil pH and temperature. Site and seasonal differences in CO2-emissions were not as pronounced as for N2O-emissions. Mean CO2 emission rates at the different sites were in a range of 92.2 +/- 1.8 up to 137.3 +/- 4.5 mg C m(-2) h(-1). Correlation analysis revealed a strong dependency of N2O and CO2 emissions on changes in soil moisture, whereas changes in soil temperature did not significantly influence the magnitude of in situ N2O and CO2 emissions. N2O emissions were positively correlated to changes in water filled pore space (WFPS) up to a threshold of 50% WFPS at the Bellenden Ker and Kauri Creek sites and up to a threshold of 60% WFPS at the Pin Gin Hill site. CO2 emission rates were positively correlated to changes in WFPS at dry to moderate soil water contents during the dry season, but were negatively correlated to changes in WFPS during the wet season. Measurements of soil air N2O-concentrations at the different sites revealed the following sequence in magnitude: Bellenden Ker > Pin Gin Hill > Kauri Creek, which is the same as found for the N2O source strengths at these sites. (C) 2002 Elsevier Science Ltd. All rights reserved.