Effects of nitrogen deposition and litter layer management on soil CO2, N2O, and CH4 emissions in a subtropical pine forestland

Microbial mechanisms for CO2 and CH4 emissions in Robinia pseudoacacia forests along a North-South transect in the Loess Plateau

Forest soil microbes play a crucial role in regulating atmospheric-soil carbon fluxes. Environmental heterogeneity across forest types and regions may lead to differences in soil CO2 and CH4 emissions. However, the microbial mechanisms underlying these emission variations are currently unclear. In this study, we measured CO2 and CH4 emissions of Robinia pseudoacacia forests along a north-south transect in the Loess Plateau. Using metagenomic sequencing, we investigated the structural and functional profiles of soil carbon cycling microbial communities. Results indicated that the forest CO2 emissions of Robinia pseudoacacia was significantly higher in the north region than in the south region, while the CH4 emission was oppositely. This is mainly attributed to changes in gene abundance driven by soil pH and moisture in participating carbon degradation and methane oxidation processes across different forest regions. The gene differences in carbon fixation processes between regions primarily stem from the Calvin cycle, where the abundance of rbcL, rbcS, and prkB genes dominates microbial carbon fixation in forest soils. Random forest models revealed key genes involved in predicting forest soil CO2 emissions, including SGA1 and amyA for starch decomposition, TYR for lignin decomposition, chitinase for chitin decomposition, and pectinesterase for pectin decomposition. Microbial functional characterization revealed that interregional differences in CH4 emissions during methane metabolism may originate from methane oxidation processes, and the associated gene abundances (glyA, ppc, and pmoB) were key genes for predicting CH4 emissions from forest soils. Our results provide new insights into the microbial mechanisms of CO2 and CH4 emissions from forest soils, which will be crucial for accurate prediction of the forest soil carbon cycle in the future.

Effects of litter and root inputs on soil CH4 uptake rates and associated microbial abundances in natural temperature subalpine forests

Climate change altered the quantities of aboveground plant litter and root inputs, but the effects on soil CH4 uptake rates and underlying mechanisms remain unclear. To investigate these factors, a three-year detritus input and removal treatment (DIRT) study including six treatments (namely, CK, control; NL, litter removal; DL, double litter; NR, root exclusion; NRNL, root exclusion plus litter removal; and NRDL, root exclusion plus double litter) was conducted in broadleaf and coniferous forest subalpine forest ecosystems. The results showed that both the subalpine forest soils acted as sink for atmospheric CH4 across all treatments, while the broadleaf forest had consistently higher CH4 uptake rates than the coniferous forest. Based on the annual mean values, root exclusion (NR, NRNL and NRDL) significantly decreased soil CH4 uptake rates by 35.9 %, 31.0 % and 43.4 % in the broadleaf forest and 36.7 %, 31.9 % and 40.6 % in the coniferous forest compared with CK treatments, respectively. Meanwhile, the mean soil CH4 uptake rates were significantly reduced by 23.6 % and 17.3 % in the broadleaf forest and the coniferous forest under the DL treatments, respectively; nevertheless, the NL treatment significantly increased soil CH4 uptake rates by 19.68 % and 14.4 %, respectively. The results clearly demonstrated that root exclusion exerted a greater influence on soil CH4 uptake rates than plant litter manipulations. Correlation and redundancy analysis (RDA) revealed that the separation of root exclusion treatments from aboveground plant litter manipulations was based on higher soil water content, NH4+-N and NO3--N concentrations, and lower DOC (dissolved organic carbon) concentrations and methanotroph pmoA gene abundance. The results suggest that future alterations in aboveground plant litter and root input, particularly a reduction in root input, can exert a stronger influence on regulating soil CH4 uptake than aboveground litter manipulations in subalpine forests with cold and humid climatic conditions in response to future climate scenarios.

Enhanced CO2 uptake is marginally offset by altered fluxes of non-CO2 greenhouse gases in global forests and grasslands under N deposition

Despite the increasing impact of atmospheric nitrogen (N) deposition on terrestrial greenhouse gas (GHG) budget, through driving both the net atmospheric CO2 exchange and the emission or uptake of non-CO2 GHGs (CH4 and N2 O), few studies have assessed the climatic impact of forests and grasslands under N deposition globally based on different bottom-up approaches. Here, we quantify the effects of N deposition on biomass C increment, soil organic C (SOC), CH4 and N2 O fluxes and, ultimately, the net ecosystem GHG balance of forests and grasslands using a global comprehensive dataset. We showed that N addition significantly increased plant C uptake (net primary production) in forests and grasslands, to a larger extent for the aboveground C (aboveground net primary production), whereas it only caused a small or insignificant enhancement of SOC pool in both upland systems. Nitrogen addition had no significant effect on soil heterotrophic respiration (RH ) in both forests and grasslands, while a significant N-induced increase in soil CO2 fluxes (RS , soil respiration) was observed in grasslands. Nitrogen addition significantly stimulated soil N2 O fluxes in forests (76%), to a larger extent in grasslands (87%), but showed a consistent trend to decrease soil uptake of CH4 , suggesting a declined sink capacity of forests and grasslands for atmospheric CH4 under N enrichment. Overall, the net GHG balance estimated by the net ecosystem production-based method (forest, 1.28 Pg CO2 -eq year-1 vs. grassland, 0.58 Pg CO2 -eq year-1 ) was greater than those estimated using the SOC-based method (forest, 0.32 Pg CO2 -eq year-1 vs. grassland, 0.18 Pg CO2 -eq year-1 ) caused by N addition. Our findings revealed that the enhanced soil C sequestration by N addition in global forests and grasslands could be only marginally offset (1.5%-4.8%) by the combined effects of its stimulation of N2 O emissions together with the reduced soil uptake of CH4 .

Nitrogen addition may promote soil organic carbon storage and CO2 emission but reduce dissolved organic carbon in Zoige peatland

With the increase of nitrogen (N) deposition, N input can affect soil C cycling since microbes may trigger a series of activities to balance the supply and demand of nutrients. However, as one of the largest C sinks on earth, the role of extra N addition in affecting peatland soil C and its potential mechanism remains unclear and debated. Therefore, this study chose the largest peatland in China (i.e., Zoige, mostly N-limited) to systematically explore the potential changes of soil C, microbes, and ecoenzymes caused by extra N input at the lab scale incubation. Three different types of soils were collected and incubated with different levels of NH₄NO₃ solution for 45 days. After incubation, N input generally increased soil organic C (SOC) but decreased dissolved organic carbon (DOC) in Zoige peatland soils. Moreover, CO₂ and CH₄ emissions were significantly increased after high N input (equal to 5 mg NH₄NO₃ g^-1 dry soils). Through a series of analyses, it was observed that microbial communities and ecoenzyme activities mainly influenced the changes of different C components. Collectively, this study implied that the increasing N deposition might help C sequestration in N-limited peatland soils; simultaneously, the risk of increased CO₂ and CH₄ by N input in global warming should not be ignored.

Responses of Soil N2O Emission and CH4 Uptake to N Input in Chinese Forests across Climatic Zones: A Meta-Study

Enhanced nitrogen (N) deposition has shown significant impacts on forest greenhouse gas emissions. Previous studies have suggested that Chinese forests may exhibit stronger N2O sources and dampened CH4 sinks under aggravated N saturation. To gain a common understanding of the N effects on forest N2O and CH4 fluxes, many have conducted global-scale meta-analyses. However, such effects have not been quantified particularly for China. Here, we present a meta-study of the N input effects on soil N2O emission and CH4 uptake in Chinese forests across climatic zones. The results suggest that enhanced N inputs significantly increase soil N2O emission (+115.8%) and decrease CH4 uptake (−13.4%). The mean effects were stronger for N2O emission and weaker for CH4 uptake in China compared with other global sites, despite being statistically insignificant. Subtropical forest soils have the highest emission factor (2.5%) and may respond rapidly to N inputs; in relatively N-limited temperate forests, N2O and CH4 fluxes are less sensitive to N inputs. Factors including forest type, N form and rate, as well as soil pH, may also govern the responses of N2O and CH4 fluxes. Our findings pinpoint the important role of Southern Chinese forests in the regional N2O and CH4 budgets.

Contribution of Litter Layer to Greenhouse Gas Fluxes between Atmosphere and Soil Varies with Forest Succession

Surface litter layer strongly influences CO2, N2O, and CH4 fluxes (FCO2, FN2O, and FCH4) between the atmosphere and forest floor through litter decomposition (litter-internal, fL-L) or interactions between litter and mineral soil (litter-induced, fL-S). However, the relative contribution of fL-L or fL-S to these greenhouse gas (GHG) fluxes in forests at different succession stages remain unclear. We conducted a field experiment where surface litter was either removed (LR), left intact (CT), doubled (LD), or exchanged (LE) in a Masson pine forest (PF, early stage of succession) and an evergreen broadleaved forest (BF, climax of succession) at the Dinghushan Nature Reserve in southern China, and studied the responses of FCO2, FN2O, and FCH4 from August 2012 to July 2013. The results showed that both FCO2 and FN2O were increased by LD treatment with a greater increase in BF (41% for FCO2 and 30% for FN2O) and decreased by LR treatment with the greater decrease in PF (−61% for FCO2 and −58% for FN2O). LD treatment decreased FCH4 by 14% in PF and 6% in BF, and LR treatment increased FCH4 by 5% in PF and 18% in BF. fL-S contributed more to FCO2 (36%) and FN2O (45%) than fL-L in PF, whereas contributions of fL-L to FCO2 (41%) and FN2O (30%) were much bigger than fL-S in BF. The greater FCH4 in PF and BF resulted from the contributions of fL-L (−14%) and fL-S (−12%), respectively. Our results indicated that fL-L is the major source of GHG fluxes in BF, whereas fL-S dominates GHG fluxes in PF. The results provide a scientific reference for quantifying the contributions of fL-L and fL-S to GHG fluxes during the subtropical forest succession and should be considered in ecosystem models to predict global warming in the future.

How Can Litter Modify the Fluxes of CO2 and CH4 from Forest Soils? A Mini-Review

Forests contribute strongly to global carbon (C) sequestration and the exchange of greenhouse gases (GHG) between the soil and the atmosphere. Whilst the microbial activity of forest soils is a major determinant of net GHG exchange, this may be modified by the presence of litter through a range of mechanisms. Litter may act as a physical barrier modifying gas exchange, water movement/retention and temperature/irradiance fluctuations; provide a source of nutrients for microbes; enhance any priming effects, and facilitate macro-aggregate formation. Moreover, any effects are influenced by litter quality and regulated by tree species, climatic conditions (rainfall, temperature), and forest management (clear-cutting, fertilization, extensive deforestation). Based on climate change projections, the importance of the litter layer is likely to increase due to an litter increase and changes in quality. Future studies will therefore have to take into account the effects of litter on soil CO2 and CH4 fluxes for various types of forests globally, including the impact of climate change, insect infestation, and shifts in tree species composition, as well as a better understanding of its role in monoterpene production, which requires the integration of microbiological studies conducted on soils in different climatic zones.