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
Abstract
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
