Stem and soil nitrous oxide fluxes from rainforest and cacao agroforest on highly weathered soils in the Congo Basin

[摘要] Although tree stems act as conduits for greenhouse gases (GHGs) produced in the soil, the magnitudes of tree contributions to total (soil  +  stem) nitrous oxide (N 2 O) emissions from tropical rainforests on heavily weathered soils remain unknown. Moreover, soil GHG fluxes are largely understudied in African rainforests, and the effects of land-use change on these gases are identified as an important research gap in the global GHG budget. In this study, we quantified the changes in stem and soil N 2 O fluxes with forest conversion to cacao agroforestry. Stem and soil N 2 O fluxes were measured monthly for a year (2017–2018) in four replicate plots per land use at three sites across central and southern Cameroon. Tree stems consistently emitted N 2 O throughout the measurement period and were positively correlated with soil N 2 O fluxes. 15 N-isotope tracing from soil mineral N to stem-emitted 15 N 2 O and correlations between temporal patterns of stem N 2 O emissions, soil–air N 2 O concentration, soil N 2 O emissions and vapour pressure deficit suggest that N 2 O emitted by the stems originated predominantly from N 2 O produced in the soil. Forest conversion to extensively managed, mature ( >20  years old) cacao agroforestry had no effect on stem and soil N 2 O fluxes. The annual total N 2 O emissions were 1.55  ±  0.20 kg N ha −1  yr −1 from the forest and 1.15  ±  0.10 kg N ha −1  yr −1 from cacao agroforestry, with tree N 2 O emissions contributing 11 % to 38 % for forests and 8 % to 15 % for cacao agroforestry. These substantial contributions of tree stems to total N 2 O emissions highlight the importance of including tree-mediated fluxes in ecosystem GHG budgets. Taking into account that our study sites' biophysical characteristics represented two-thirds of the humid rainforests in the Congo Basin, we estimated a total N 2 O source strength for this region of 0.18  ±  0.05 Tg N 2 O-N yr −1 .

[学科分类] 大气科学