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Litter comprises a major nutrient source when decomposed via soil microbes and functions as subtract that limits gas exchange between soil and atmosphere, thereby restricting methane (CH4) uptake in soils. However, the impact and inherent mechanism of litter and its decomposition on CH4 uptake in soils remains unknown in forest. Therefore, to declare the mechanisms of litter input and decomposition effect on the soil CH4 flux in forest, this study performed a litter-removal experiment in a tropical rainforest, and investigated the effects of litter input and decomposition on the CH4 flux among forest ecosystems through a literature review. Cumulative annual CH4 flux was -3.30 kg CH4-C ha -1 y -1 . The litter layer decreased annual accumulated CH4 uptake by 8% which greater in the rainy season than the dry season in the tropical rainforest. Litter decomposition and the input of carbon and nitrogen in litter biomass reduced CH4 uptake significantly and the difference in CH4 flux between treatment with litter and without litter was negatively associated with N derived from litter input. Based on the literature review about litter effect on soil CH4 around world forests, the effect of litter dynamics on CH4 uptake was regulated by litter-derived nitrogen input and the amount soil inorganic nitrogen content. Our results suggest that nitrogen input via litter decomposition, which increased with temperature, caused a decline in CH4 uptake by forest soils, which could weaken the contribution of the forest in mitigating global warming., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Crown Copyright © 2022. Published by Elsevier B.V. All rights reserved.)

Annual gross primary productivity (AGPP) of terrestrial ecosystems is the largest carbon flux component in ecosystems; however, it's unclear whether photosynthetic capacity or phenology dominates interannual variation of AGPP, and a better understanding of this could contribute to estimation of carbon sinks and their interactions with climate change. In this study, observed GPP data of 494 site-years from 39 eddy covariance sites in Northern Hemisphere were used to investigate mechanisms of interannual variation of AGPP. This study first decomposed AGPP into three seasonal dynamic attribute parameters (growing season length (CUP), maximum daily GPP (GPP max ), and the ratio of mean daily GPP to GPP max (α GPP )), and then decomposed AGPP into mean leaf area index (LAI m ) and annual photosynthetic capacity per leaf area (AGPP lm ). Furthermore, GPP max was decomposed into leaf area index of DOY max (the day when GPP max appeared) (LAI max ) and photosynthesis per leaf area of DOY max (GPP lmax ). Relative contributions of parameters to AGPP and GPP max were then calculated. Finally, environmental variables of DOY max were extracted to analyze factors influencing interannual variation of GPP lmax . Trends of AGPP in 39 ecosystems varied from -65.23 to 53.05 g C m -2 yr -2 , with the mean value of 6.32 g C m -2 yr -2 . Photosynthetic capacity (GPP max and AGPP lm ), not CUP or LAI, was the main factor dominating interannual variation of AGPP. GPP lmax determined the interannual variation of GPP max , and temperature, water, and radiation conditions of DOY max affected the interannual variation of GPP lmax . This study used the cascade relationship of "environmental variables-GPP lmax -GPP max -AGPP" to explain the mechanism of interannual variation of AGPP, which can provide new ideas for the AGPP estimation based on seasonal dynamic of GPP., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2022 Elsevier B.V. All rights reserved.)

Forest soils in the warm-humid tropics significantly contribute to the regional greenhouse gas (GHG) budgets. However, spatial heterogeneity of GHG fluxes is often overlooked. Here, we present a study of N 2 O and CH 4 fluxes over 1.5 years, along a topographic gradient in a rainforest catchment in Xishuangbanna, SW China. From the upper hillslope to the foot of the hillslope, and further to the flat groundwater discharge zone, we observed a decrease of N 2 O emission associated with an increase of soil water-filled-pore-space (WFPS), which we tentatively attribute to more complete denitrification to N 2 at larger WFPS. In the well-drained soils on the hillslope, denitrification at anaerobic microsites or under transient water-saturation was the potential N 2 O source. Negative CH 4 fluxes across the catchment indicated a net soil CH 4 sink. As the oxidation of atmospheric CH 4 is diffusion-limited, soil CH 4 consumption rates were negatively related to WFPS, reflecting the topographic control. Our observations also suggest that during dry seasons N 2 O emission was significantly dampened (<10 μg N 2 O-N m -2  h -1 ) and CH 4 uptake was strongly enhanced (83 μg CH 4 -C m -2  h -1 ) relative to wet seasons (17 μg N 2 O-N m -2  h -1 and 56 μg CH 4 -C m -2  h -1 ). In a post-drought period, several rain episodes induced exceptionally high N 2 O emissions (450 μg N 2 O-N m -2  h -1 ) in the groundwater discharge zone, likely driven by flushing of labile organic carbon accumulated during drought. Considering the global warming potential associated with both GHGs, we found that N 2 O emissions largely offset the C sink contributed by CH 4 uptake in soils (more significant in the groundwater discharge zone). Our study illustrates important topographic controls on N 2 O and CH 4 fluxes in forest soils. With projected climate change in the tropics, weather extremes may interact with these controls in regulating forest GHG fluxes, which should be accounted for in future studies., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2021 Elsevier B.V. All rights reserved.)

Forest ecosystems play an increasingly important role in the global carbon cycle. However, knowledge on carbon exchanges, their spatio-temporal patterns, and the extent of the key controls that affect carbon fluxes is lacking. In this study, we employed 29-site-years of eddy covariance data to observe the state, spatio-temporal variations and climate sensitivity of carbon fluxes (gross primary productivity (GPP), ecosystem respiration (R eco ), and net ecosystem carbon exchange (NEE)) in four representative forest ecosystems in Yunnan. We found that 1) all four forest ecosystems were carbon sinks (the average NEE was -3.40tCha -1 yr -1 ); 2) contrasting seasonality of the NEE among the ecosystems with a carbon sink mainly during the wet season in the Yuanjiang savanna ecosystem (YJ) but during the dry season in the Xishuangbanna tropical rainforest ecosystem (XSBN), besides an equivalent NEE uptake was observed during the wet/dry season in the Ailaoshan subtropical evergreen broad-leaved forest ecosystem (ALS) and Lijiang subalpine coniferous forest ecosystem (LJ); 3) as the GPP increased, the net ecosystem production (NEP) first increased and then decreased when the GPP>17.5tCha -1 yr -1 ; 4) the precipitation determines the carbon sinks in the savanna ecosystem (e.g., YJ), while temperature did so in the tropical forest ecosystem (e.g., XSBN); 5) overall, under the circumstances of warming and decreased precipitation, the carbon sink might decrease in the YJ but maybe increase in the ALS and LJ, while future strength of the sink in the XSBN is somewhat uncertain. However, based on the redundancy analysis, the temperature and precipitation combined together explained 39.7%, 32.2%, 25.3%, and 29.6% of the variations in the NEE in the YJ, XSBN, ALS and LJ, respectively, which indicates that considerable changes in the NEE could not be explained by variations in the temperature and precipitation. Therefore, the effects of other factors (e.g., CO 2 concentration, N/P deposition, aerosol and other variables) on the NEE still require extensive research and need to be considered seriously in carbon-cycle-models., (Copyright © 2017. Published by Elsevier B.V.)