N 2O and CO 2 emissions from three different tropical forest sites in the wet tropics of Queensland, Australia

Abstract

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 N 2O and CO 2 emissions from soils during the wet and the dry season. At all sites, mean N 2O emission rates were significantly higher during the wet season (Bellenden Ker: 242.0±7.4 μg N 2O–N m −2 h −1, Pin Gin Hill: 140.8±5.1 μg N 2O–N m −2 h −1, Kauri Creek: 80.8±3.3 μg N 2O–N m −2 h −1) as compared to the dry season when N 2O-emissions were markedly lower (<20 μg N 2O–N m −2 h −1) due to limitations in soil moisture. During the wet season, mean N 2O emission rates of the Coastal Lowland sites Bellenden Ker and Pin Gin Hill were approximately twofold higher as compared to N 2O 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 CO 2-emissions were not as pronounced as for N 2O-emissions. Mean CO 2 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 N 2O and CO 2 emissions on changes in soil moisture, whereas changes in soil temperature did not significantly influence the magnitude of in situ N 2O and CO 2 emissions. N 2O 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. CO 2 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 N 2O-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 N 2O source strengths at these sites.