The Mechanisms and Potentially Positive Effects of Seven Years of Delayed and Wetter Wet Seasons on Nitrous Oxide Fluxes in a Tropical Monsoon Forest

Tropical forest soils contribute to global warming and ozone depletion due to large nitrous oxide (N2O) emissions. However, it is unknown whether the soil N2O fluxes will change under ongoing precipitation regime changes. In this study, two typical precipitation regimes were simulated in a tropical monsoon forest for seven years: delayed wet season (DW) and wetter wet season (WW). Although we did not find significant effects of the precipitation changes on soil N2O fluxes annually or seasonally, significant increases in soil N2O emission were observed in treatment months for DW and WW. Importantly, the underlying mechanisms for their effects were different. For the DW treatment, the increase in soil N2O emission was attributed to the increase in abundance of nitrifying bacteria (amoA), which provide nitrate for N2O production, and to the changed microbial structure of denitrifying bacteria (nosZ), which reduce N2O consumption. For the WW treatment, the stimulation of soil N2O emission was attributed to the relatively large increase in N2O production via an increased abundance of denitrifying bacteria (nirS) compared with the increased N2O consumption via increased abundance of N2O reducing bacteria (nosZ). If the projected precipitation regimes of delayed or wetter wet seasons last for more than two months, the N2O emitted from tropical forest soils has the potential to significantly increase due to specific microbial nitrogen transformation processes. This increase implies that the contribution of tropical forests to global warming and ozone depletion would increase, depending on precipitation regimes.