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Effects of Litter and Throughfall Manipulations on Soil Greenhouse Gas Fluxes in a Subtropical Secondary Forest

Cui, J. 2020. The Chinese University of Hong Kong


Forest soil has an enormous potential in affecting future climate change through soil-atmosphere greenhouse gas (GHG) exchanges. As the largest terrestrial Carbon Dioxide (CO2) efflux, soil respiration (Rs) consists of heterotrophic respiration (Rh) and autotrophic respiration (Ra). Methane (CH4) and Nitrous Oxide (N2O) rank the second and third most important greenhouse gases (GHG) next to CO2 in terms of climate forcing. Altered litter production as a proxy for multiple global changes could trigger ecosystem feedbacks to climate change. Changes in global precipitation regimes have exerted immediate and pronounced impacts on the biogeochemical cycle in the terrestrial ecosystem. However, the responses of soil-atmosphere GHG fluxes to the altered litter or precipitation amount in the previous studies could be divergent, with a limited number of studies to test respiration components (Rh, Ra) and non-CO2 GHGs. The potential mechanisms of the soil-atmosphere interactions remain unclear. In the thesis, field manipulations were carried out to simulate altered litter and precipitation amount over four years in a subtropical secondary forest in Hong Kong. The static chamber measurements were conducted on a weekly to biweekly basis to investigate the temporal variations and the effects of experimental manipulations on soil-atmosphere GHG fluxes. Root exclusion bags were installed to separate the heterotrophic respiration from the total respiration. The soil physiochemical parameters were monitored by the soil core analysis and plant root simulator (PRS) probes. The forest soils at the study site in Tai Po Kau Nature Reserve were mainly the CO2 source, CH4 sink, and N2O source. The Rh/ Rs ratio was 0.52. Significant seasonal patterns were found for all the three GHGs. The emission rates of Rs and its components and N2O were higher in the hot-humid season rather than the cool-dry season, while the CH4 uptake rates were higher in the cool-dry season relative to the hot-humid season. The climate and soil parameters were the major controlling factors of the soil-atmosphere GHG exchanges. Rs responded to the litter positively with positive responses from Ra and Rh. The litter-induced increase in Rs and the priming effect strengthened with experimental duration. Meanwhile, litter duplication could enhance the soil CH4 sink capacity by ~15%. Furthermore, the effect of litter on soil N2O emissions gradually turned from negative to positive during the study period, probably attributable to the distinct annual precipitation and N cycling processes. Overall litter reduction reduced the CO2-equivalent Global warming potential (GWP) of soil GHGs by 14.9%. Whereas litter addition enhanced the GWP by 9.6%, suggesting the positive response to the enhanced litter input associated with the atmospheric CO2 fertilization and global warming might aggravate the consequences of climate change. As for the throughfall manipulation, the rise of Rh could not offset the decline in Ra, leading to an overall decrease of Rs under the experimental drought in the cool-dry season. The soil prevailing moisture and N pools might regulate the response of soil CH4 uptakes to the throughfall, despite the overall non-significant response. The positive response of soil N2O emissions to throughfall acclimated to be negligible at the later stage of study, possibly attributing to the adaptation of microbial communities. Overall throughfall reduction suppressed the GWP by 8.7%, while throughfall addition increased the GWP by a minimal percentage of 0.8%. It is implied that the negative ecosystem response to throughfall reduction might alleviate climate change. Litter and throughfall could also interact with other global change drivers, exerting either synergistic or antagonistic effects on the biogeochemical processes and the biogenic soil GHGs. Therefore, it is proposed that the role of litter and throughfall should be incorporated into the terrestrial C cycle and climate change models for simulation and projection. It is necessary to establish more interactive and long-term manipulation studies for the comprehensive understanding of the ecosystem feedbacks to global changes.

Key Words

greenhouse gas, GHG, CO2, CH4, N2O, soil respiration, heterotrophic respiration, autotrophic respiration, litter, throughfall, subtropical secondary fores