Increased soil release of greenhouse gases shrinks terrestrial carbon uptake enhancement under warming

Warming can accelerate the decomposition of soil organic matter and stimulate the release of soil greenhouse gases (GHGs), but to what extent soil release of methane (CH₄) and nitrous oxide (N₂O) may contribute to soil C loss for driving climate change under warming remains unresolved. By synthesizing 1,845 measurements from 164 peer‐reviewed publications, we show that around 1.5°C (1.16–2.01°C) of experimental warming significantly stimulates soil respiration by 12.9%, N₂O emissions by 35.2%, CH₄ emissions by 23.4% from rice paddies, and by 37.5% from natural wetlands. Rising temperature increases CH₄ uptake of upland soils by 13.8%. Warming‐enhanced emission of soil CH₄ and N₂O corresponds to an overall source strength of 1.19, 1.84, and 3.12 Pg CO₂‐equivalent/year under 1°C, 1.5°C, and 2°C warming scenarios, respectively, interacting with soil C loss of 1.60 Pg CO₂/year in terms of contribution to climate change. The warming‐induced rise in soil CH₄ and N₂O emissions (1.84 Pg CO₂‐equivalent/year) could reduce mitigation potential of terrestrial net ecosystem production by 8.3% (NEP, 22.25 Pg CO₂/year) under warming. Soil respiration and CH₄ release are intensified following the mean warming threshold of 1.5°C scenario, as compared to soil CH₄ uptake and N₂O release with a reduced and less positive response, respectively. Soil C loss increases to a larger extent under soil warming than under canopy air warming. Warming‐raised emission of soil GHG increases with the intensity of temperature rise but decreases with the extension of experimental duration. This synthesis takes the lead to quantify the ecosystem C and N cycling in response to warming and advances our capacity to predict terrestrial feedback to climate change under projected warming scenarios.     

Modeling soil CO<Subscript>2</Subscript> emissions from ecosystems

We present a new soil respiration model, describe a formal model testing procedure, and compare our model with five alternative models using an extensive data set of observed soil respiration. Gas flux data from rangeland soils that included a large number of measurements at low temperatures were used to model soil CO₂ emissions as a function of soil temperature and water content. Our arctangent temperature function predicts that Q₁₀ values vary inversely with temperature and that CO₂ fluxes are significant below 0 °C. Independent data representing a broad range of ecosystems and temperature values were used for model testing. The effects of plant phenology, differences in substrate availability among sites, and water limitation were accounted for so that the temperature equations could be fairly evaluated. Four of the six tested models did equally well at simulating the observed soil CO₂ respiration rates. However, the arctangent variable Q₁₀ model agreed closely with observed Q₁₀ values over a wide range of temperatures (r² = 0.94) and was superior to published variable Q₁₀ equations using the Akaike information criterion (AIC). The arctangent temperature equation explained 16–85% of the observed intra-site variability in CO₂ flux rates. Including a water stress factor yielded a stronger correlation than temperature alone only in the dryland soils. The observed change in Q₁₀ with increasing temperature was the same for data sets that included only heterotrophic respiration and data sets that included both heterotrophic and autotrophic respiration.     

Thermal adaptation of heterotrophic soil respiration in laboratory microcosms

Respiration of heterotrophic microorganisms decomposing soil organic carbon releases carbon dioxide from soils to the atmosphere. In the short term, soil microbial respiration is strongly dependent on temperature. In the long term, the response of heterotrophic soil respiration to temperature is uncertain. However, following established evolutionary trade‐offs, mass‐specific respiration (R_mass) rates of heterotrophic soil microbes should decrease in response to sustained increases in temperature (and vice‐versa). Using a laboratory microcosm approach, we tested the potential for the R_mass of the microbial biomass in six different soils to adapt to three, experimentally imposed, thermal regimes (constant 10, 20 or 30 °C). To determine R_mass rates of the heterotrophic soil microbial biomass across the temperature range of the imposed thermal regimes, we periodically assayed soil subsamples using similar approaches to those used in plant, animal and microbial thermal adaptation studies. As would be expected given trade‐offs between maximum catalytic rates and the stability of the binding structure of enzymes, after 77 days of incubation R_mass rates across the range of assay temperatures were greatest for the 10 °C experimentally incubated soils and lowest for the 30 °C soils, with the 20 °C incubated soils intermediate. The relative magnitude of the difference in R_mass rates between the different incubation temperature treatments was unaffected by assay temperature, suggesting that maximum activities and not Q₁₀ were the characteristics involved in thermal adaptation. The time taken for changes in R_mass to manifest (77 days) suggests they likely resulted from population or species shifts during the experimental incubations; we discuss alternate mechanistic explanations for those results we observed. A future research priority is to evaluate the role that thermal adaptation plays in regulating heterotrophic respiration rates from field soils in response to changing temperature, whether seasonally or through climate change.     

Microbial physiology and soil CO2 efflux after 9 years of soil warming in a temperate forest – no indications for thermal adaptations

Thermal adaptations of soil microorganisms could mitigate or facilitate global warming effects on soil organic matter (SOM) decomposition and soil CO₂ efflux. We incubated soil from warmed and control subplots of a forest soil warming experiment to assess whether 9 years of soil warming affected the rates and the temperature sensitivity of the soil CO₂ efflux, extracellular enzyme activities, microbial efficiency, and gross N mineralization. Mineral soil (0–10 cm depth) was incubated at temperatures ranging from 3 to 23 °C. No adaptations to long‐term warming were observed regarding the heterotrophic soil CO₂ efflux (R₁₀ warmed: 2.31 ± 0.15 μmol m^?2 s^?1, control: 2.34 ± 0.29 μmol m^?2 s^?1; Q₁₀ warmed: 2.45 ± 0.06, control: 2.45 ± 0.04). Potential enzyme activities increased with incubation temperature, but the temperature sensitivity of the enzymes did not differ between the warmed and the control soils. The ratio of C : N acquiring enzyme activities was significantly higher in the warmed soil. Microbial biomass‐specific respiration rates increased with incubation temperature, but the rates and the temperature sensitivity (Q₁₀ warmed: 2.54 ± 0.23, control 2.75 ± 0.17) did not differ between warmed and control soils. Microbial substrate use efficiency (SUE) declined with increasing incubation temperature in both, warmed and control, soils. SUE and its temperature sensitivity (Q₁₀ warmed: 0.84 ± 0.03, control: 0.88 ± 0.01) did not differ between warmed and control soils either. Gross N mineralization was invariant to incubation temperature and was not affected by long‐term soil warming. Our results indicate that thermal adaptations of the microbial decomposer community are unlikely to occur in C‐rich calcareous temperate forest soils.     

施肥方式对紫色土土壤异养呼吸的影响

采用静态暗箱-气相色谱法于2010年12月至2011年10月对不同施肥方式下的紫色土土壤呼吸进行了研究,以揭示施肥方式对紫色土异养呼吸的影响。结果表明:施肥可对土壤异养呼吸产生激发效应。施肥后第5天出现峰值,猪厩肥处理的异养呼吸峰值为2356.8 mg CO2m-2h-1,显著高于秸秆配施氮磷钾(970.1 mgCO2m-2h-1)和常规氮磷钾处理(406.8 mgCO2m-2h-1)(P0.01);小麦季常规氮磷钾、猪厩肥和秸秆配施氮磷钾处理的平均土壤异养呼吸速率为212.9、285.8和305.8mgCO2m-2h-1,CO2排放量为255.1、342.3和369.5 gC/m2,玉米季为408.2、642.8和446.4 mgCO2m-2h-1,CO2排放量为344.7、542.8和376.9 gC/m2,玉米季土壤异养呼吸平均速率及CO2排放量均高于小麦季。全年平均土壤异养呼吸速率分别为310.6、446.3和377.4 mg CO2m-2h-1,CO2排放总量分别为599.8、885.1和746.4 gC/m2。猪厩肥对土壤异养呼吸速率和CO2排放量的影响最大,秸秆配施氮磷钾肥次之,氮磷钾肥最小,说明有机物料的投入是紫色土土壤异养呼吸速率的主要调控措施,低碳氮比的有机物料能促进土壤异养呼吸和CO2的排放。猪厩肥和秸秆配施氮磷钾肥处理相应地表和地下5 cm温度的Q10值分别为2.64、1.88和2.77、1.99,表明低碳氮比的有机物料还能增加土壤异养呼吸Q10值,使土壤异养呼吸速率对温度的敏感性加强。     

Living roots magnify the response of soil organic carbon decomposition to temperature in temperate grassland

Increasing atmospheric carbon dioxide (CO₂) concentration is both a strong driver of primary productivity and widely believed to be the principal cause of recent increases in global temperature. Soils are the largest store of the world's terrestrial C. Consequently, many investigations have attempted to mechanistically understand how microbial mineralisation of soil organic carbon (SOC) to CO₂ will be affected by projected increases in temperature. Most have attempted this in the absence of plants as the flux of CO₂ from root and rhizomicrobial respiration in intact plant‐soil systems confounds interpretation of measurements. We compared the effect of a small increase in temperature on respiration from soils without recent plant C with the effect on intact grass swards. We found that for 48 weeks, before acclimation occurred, an experimental 3 °C increase in sward temperature gave rise to a 50% increase in below ground respiration (ca. 0.4 kg C m^?2; Q₁₀ = 3.5), whereas mineralisation of older SOC without plants increased with a Q₁₀ of only 1.7 when subject to increases in ambient soil temperature. Subsequent ¹⁴C dating of respired CO₂ indicated that the presence of plants in swards more than doubled the effect of warming on the rate of mineralisation of SOC with an estimated mean C age of ca. 8 years or older relative to incubated soils without recent plant inputs. These results not only illustrate the formidable complexity of mechanisms controlling C fluxes in soils but also suggest that the dual biological and physical effects of CO₂ on primary productivity and global temperature have the potential to synergistically increase the mineralisation of existing soil C.     

Temperature dependence of greenhouse gas emissions from three hydromorphic soils at different groundwater levels

Wetlands contribute considerably to the global greenhouse gas (GHG) balance. In these ecosystems, groundwater level (GWL) and temperature, two factors likely to be altered by climate change, exert important control over CO₂, CH₄ and N₂O fluxes. However, little is known about the temperature sensitivity (Q₁₀) of the combined GHG emissions from hydromorphic soils and how this Q₁₀ varies with GWL. We performed a greenhouse experiment in which three different (plant‐free) hydromorphic soils from a temperate spruce forest were exposed to two GWLs (an intermediate GWL of ?20 cm and a high GWL of ?5 cm). Net CO₂, CH₄ and N₂O fluxes were measured continuously. Here, we discuss how these fluxes responded to synoptic temperature fluctuations. Across all soils and GWLs, CO₂ emissions responded similarly to temperature and Q₁₀ was close to 2. The Q₁₀ of the CH₄ and N₂O fluxes also was similar across soil types. GWL, on the other hand, significantly affected the Q₁₀ of both CH₄ and N₂O emissions. The Q₁₀ of the net CH₄ fluxes increased from about 1 at GWL = ?20 cm to 3 at GWL = ?5 cm. For the N₂O emissions, Q₁₀ varied around 2 for GWL = ?20 cm and around 4 for GWL = ?5 cm. This substantial GWL‐effect on the Q₁₀ of CH₄ and N₂O emissions was, however, hardly reflected in the Q₁₀ of the total GHG emissions (which varied around 2), because the contribution of these gases was relatively small compared to that of CO₂.     

Potential carbon release from permafrost soils of Northeastern Siberia

Permafrost soils are an important reservoir of carbon (C) in boreal and arctic ecosystems. Rising global temperature is expected to enhance decomposition of organic matter frozen in permafrost, and may cause positive feedback to warming as CO₂ is released to the atmosphere. Significant amounts of organic matter remain frozen in thick mineral soil (loess) deposits in northeastern Siberia, but the quantity and lability of this deep organic C is poorly known. Soils from four tundra and boreal forest locations in northeastern Siberia that have been continuously frozen since the Pleistocene were incubated at controlled temperatures (5, 10 and 15°C) to determine their potential to release C to the atmosphere when thawed. Across all sites, CO₂ with radiocarbon (¹⁴C) ages ranging between～21 and 24 ka bp was respired when these permafrost soils were thawed. The amount of C released in the first several months was strongly correlated to C concentration in the bulk soil in the different sites, and this correlation remained the same for fluxes up to 1 year later. Fluxes were initially strongly related to temperature with a mean Q₁₀ value of 1.9±0.3 across all sites, and later were unrelated to temperature but still correlated with bulk soil C concentration. Modeled inversions of Δ¹⁴CO₂ values in respiration CO₂ and soil C components revealed mean contribution of 70% and 26% from dissolved organic C to respiration CO₂ in case of two permafrost soils, while organic matter fragments dominated respiration (mean 68%) from a surface mineral soil that served as modern reference sample. Our results suggest that if 10% of the total Siberian permafrost C pool was thawed to a temperature of 5°C, about 1 Pg C will be initially released from labile C pools, followed by respiration of～40 Pg C to the atmosphere over a period of four decades.     

Site and temporal variation of soil respiration in European beech, Norway spruce, and Scots pine forests

Global warming and changes in rainfall amount and distribution may affect soil respiration as a major carbon flux between the biosphere and the atmosphere. The objectives of this study were to investigate the site to site and interannual variation in soil respiration of six temperate forest sites. Soil respiration was measured using closed chambers over 2 years under mature beech, spruce and pine stands at both Solling and Unterlüß, Germany, which have distinct climates and soils. Cumulative annual CO₂ fluxes varied from 4.9 to 5.4 Mg C ha^?1 yr^?1 at Solling with silty soils and from 4.0 to 5.9 Mg C ha^?1 yr^?1 at Unterlüß with sandy soils. With one exception soil respiration rates were not significantly different among the six forest sites (site to site variation) and between the years within the same forest site (interannual variation). Only the respiration rate in the spruce stand at Unterlüß was significant lower than the beech stand at Unterlüß in both years. Soil respiration rates of the sandy sites at Unterlüß were limited by soil moisture during the rather dry and warm summer 1999 while soil respiration at the silty Solling site tended to increase. We found a threshold of ?80 kPa at 10 cm depth below which soil respiration decreased with increasing drought. Subsequent wetting of sandy soils revealed high CO₂ effluxes in the stands at Unterlüß. However, dry periods were infrequent, and our results suggest that temporal variation in soil moisture generally had little effect on annual soil respiration rates. Soil temperature at 5 cm and 10 cm depth explained 83% of the temporal variation in soil respiration using the Arrhenius function. The correlations were weaker using temperature at 0 cm (r² = 0.63) and 2.5 cm depth (r² = 0.81). Mean Q₁₀ values for the range from 5 to 15 °C increased asymptotically with soil depth from 1.87 at 0 cm to 3.46 at 10 cm depth, indicating a large uncertainty in the prediction of the temperature dependency of soil respiration. Comparing the fitted Arrhenius curves for same tree species from Solling and Unterlüß revealed higher soil respiration rates for the stands at Solling than in the respective stands at Unterlüß at the same temperature. A significant positive correlation across all sites between predicted soil respiration rates at 10 °C and total phosphorus content and C‐to‐N ratio of the upper mineral soil indicate a possible effect of nutrients on soil respiration.     

Simulating soil N2O emissions and heterotrophic CO2 respiration in arable systems using FASSET and MoBiLE-DNDC

Modelling of soil emissions of nitrous oxide (N₂O) and carbon dioxide (CO₂) is complicated by complex interactions between processes and factors influencing their production, consumption and transport. In this study N₂O emissions and heterotrophic CO₂ respiration were simulated from soils under winter wheat grown in three different organic and one inorganic fertilizer-based cropping system using two different models, i.e., MoBiLE-DNDC and FASSET. The two models were generally capable of simulating most seasonal trends of measured soil heterotrophic CO₂ respiration and N₂O emissions. Annual soil heterotrophic CO₂ respiration was underestimated by both models in all systems (about 10?C30% by FASSET and 10?C40% by MoBiLE-DNDC). Both models overestimated annual N₂O emissions in all systems (about 10?C580% by FASSET and 20?C50% by MoBiLE-DNDC). In addition, both models had some problems in simulating soil mineral nitrogen, which seemed to originate from deficiencies in simulating degradation of soil organic matter, incorporated residues of catch crops and organic fertilizers. To improve the performance of the models, organic matter decomposition parameters need to be revised.     

Effects of bovine urine, plants and temperature on N2O and CO2 emissions from a sub-tropical soil

Grazing ruminants urinate and deposit N onto pastoral soils at rates up to 1,000 kg ha^?1, with most of this deposited N present as urea. In urine patches, nitrous oxide (N₂O) emissions can increase markedly. Soil derived CO₂ fluxes can also increase due to priming effects.While N₂O fluxes are affected by temperature, no studies have examined the interaction of pasture plants, urine and temperature on N₂O fluxes and the associated CO₂ fluxes. We postulated the response of N₂O emissions to bovine urine application would be affected by plants and temperature. Dairy cattle urine was collected, labelled with ¹⁵N, and applied at 590 kg N ha^?1 to a sub-tropical soil,with and without pasture plants at 11°, 19°, and 23°C. Over the experimental period (28 days), 0.2% (11°C with plants) to 2.2% (23°C with plants) of the applied N was emitted as N₂O. At 11°C, plants had no effect on cumulative N₂O-N fluxes, whereas at 23°C, the presence of plants significantly increased the flux, suggesting plant-derived C supply affected the N₂O producing microbes. In contrast, a significant urine application effect on the cumulative CO₂ flux was not affected by varying temperature from 11?C23°C or by growing plants in the soil. This study has shown that plants and their responses to temperature affect N₂O emissions from ruminant urine deposition. The results have significant implications for forecasting and understanding the effect of elevated soil temperatures on N₂O emissions and CO₂ fluxes from grazed pasture systems.     

Greenhouse gas dynamics in boreal, littoral sediments under raised CO2 and nitrogen supply

SUMMARY 1. The effects of increasing CO₂ and nitrogen loading and of a change in water table and temperature on littoral CH₄, N₂O and CO₂ fluxes were studied in a glasshouse experiment with intact sediment cores including vegetation (mainly sedges), taken from a boreal eutrophic lake in Finland. Sediments with the water table held at a level of 0 or at ?15 cm were incubated in an atmosphere of 360 or 720 p.p.m. CO₂ for 18 weeks. The experiment included fertilisation with NO₃^– and NH₄⁺ (to a total 3 g N m^?2). 2. Changes in the water table and temperature strongly regulated sediment CH₄ and cCO₂ fluxes (community CO₂ release), but did not affect N₂O emissions. Increase in the water table increased CH₄ emissions but reduced cCO₂ release, while increase in temperature increased emissions of both CO₂ and CH₄. 3. The raised CO₂ increased carbon turnover in the sediments, such that cCO₂ release was increased by 16–26%. However, CH₄ fluxes were not significantly affected by raised CO₂, although CH₄ production potential (at 22 °C) of the sediments incubated at high CO₂ was increased. In the boreal region, littoral CH₄ production is more likely to be limited by temperature than by the availability of carbon. Raised CO₂ did not affect N₂O production by denitrification, indicating that this process was not carbon limited. 4. A low availability of NO₃^– did severely limit N₂O production. The NO₃^– addition caused up to a 100‐fold increase in the fluxes of N₂O. The NH₄⁺ addition did not increase N₂O fluxes, indicating low nitrification capacity in the sediments.     

Drainage increases CO2 and N2O emissions from tropical peat soils

Tropical peatlands are vital ecosystems that play an important role in global carbon storage and cycles. Current estimates of greenhouse gases from these peatlands are uncertain as emissions vary with environmental conditions. This study provides the first comprehensive analysis of managed and natural tropical peatland GHG fluxes: heterotrophic (i.e. soil respiration without roots), total CO₂ respiration rates, CH₄ and N₂O fluxes. The study documents studies that measure GHG fluxes from the soil (n = 372) from various land uses, groundwater levels and environmental conditions. We found that total soil respiration was larger in managed peat ecosystems (median = 52.3 Mg CO₂ ha^?1 year^?1) than in natural forest (median = 35.9 Mg CO₂ ha^?1 year^?1). Groundwater level had a stronger effect on soil CO₂ emission than land use. Every 100 mm drop of groundwater level caused an increase of 5.1 and 3.7 Mg CO₂ ha^?1 year^?1 for plantation and cropping land use, respectively. Where groundwater is deep (≥0.5 m), heterotrophic respiration constituted 84% of the total emissions. N₂O emissions were significantly larger at deeper groundwater levels, where every drop in 100 mm of groundwater level resulted in an exponential emission increase (exp(0.7) kg N ha^?1 year^?1). Deeper groundwater levels induced high N₂O emissions, which constitute about 15% of total GHG emissions. CH₄ emissions were large where groundwater is shallow; however, they were substantially smaller than other GHG emissions. When compared to temperate and boreal peatland soils, tropical peatlands had, on average, double the CO₂ emissions. Surprisingly, the CO₂ emission rates in tropical peatlands were in the same magnitude as tropical mineral soils. This comprehensive analysis provides a great understanding of the GHG dynamics within tropical peat soils that can be used as a guide for policymakers to create suitable programmes to manage the sustainability of peatlands effectively.     

Drivers of temperature sensitivity of decomposition of soil organic matter along a mountain altitudinal gradient in the Western Carpathians

Mountain forest soils contain an important stock of carbon. Their altitudinal gradient can serve as a model for research on the potential risk of increased emission of carbon dioxide to the atmosphere, in a positive feedback of global warming. Using soil samples collected at three elevations (600, 900, and 1200 m a.s.l.) from five separate slopes of the Carpathian Mountains (Poland), we studied the effects of soil physical, chemical and microbial properties controlling the temperature sensitivity (Q₁₀ values) of organic matter decomposition in forest soils. Data of soil basal respiration rate measured in laboratory conditions at six different temperatures (5, 10, 15, 20, 25 and 30 °C) were fitted to a Gaussian function. The modelled soil respiration rates differed between altitudes at temperature exceeding 15 °C, and the respiration rate of soil from 1200 m a.s.l. was higher than in soils from the two lower elevations. Based on the modelled respiration values, we calculated Q₁₀ values in the low (Q₁₀L, 0–10 °C), medium (Q₁₀M, 10–20 °C) and high (Q₁₀H, 20–30 °C) temperature ranges. The Q₁₀ values did not differ between elevations. Q₁₀L and Q₁₀M were negatively related only with the C:N ratio. Temperature sensitivity of decomposition of soil organic matter was not affected by bacterial activity and functional diversity (assessed using Biolog^® ECO plates), microbial biomass or community structure (inferred from phospholipid fatty acid assays). Our findings support a kinetics-based theory of the higher temperature sensitivity of more chemically recalcitrant soil organic matter, put forward by other authors.     

Molecular investigations into a globally important carbon pool: permafrost‐protected carbon in Alaskan soils

The fate of carbon (C) contained within permafrost in boreal forest environments is an important consideration for the current and future carbon cycle as soils warm in northern latitudes. Currently, little is known about the microbiology or chemistry of permafrost soils that may affect its decomposition once soils thaw. We tested the hypothesis that low microbial abundances and activities in permafrost soils limit decomposition rates compared with active layer soils. We examined active layer and permafrost soils near Fairbanks, AK, the Yukon River, and the Arctic Circle. Soils were incubated in the lab under aerobic and anaerobic conditions. Gas fluxes at ?5 and 5 °C were measured to calculate temperature response quotients (Q₁₀). The Q₁₀ was lower in permafrost soils (average 2.7) compared with active layer soils (average 7.5). Soil nutrients, leachable dissolved organic C (DOC) quality and quantity, and nuclear magnetic resonance spectroscopy of the soils revealed that the organic matter within permafrost soils is as labile, or even more so, than surface soils. Microbial abundances (fungi, bacteria, and subgroups: methanogens and Basidiomycetes) and exoenzyme activities involved in decomposition were lower in permafrost soils compared with active layer soils, which, together with the chemical data, supports the reduced Q₁₀ values. CH₄ fluxes were correlated with methanogen abundance and the highest CH₄ production came from active layer soils. These results suggest that permafrost soils have high inherent decomposability, but low microbial abundances and activities reduce the temperature sensitivity of C fluxes. Despite these inherent limitations, however, respiration per unit soil C was higher in permafrost soils compared with active layer soils, suggesting that decomposition and heterotrophic respiration may contribute to a positive feedback to warming of this eco region.     

Soil-atmospheric exchange of CO2, CH4, and N2O in three subtropical forest ecosystems in southern China

The magnitude, temporal, and spatial patterns of soil‐atmospheric greenhouse gas (hereafter referred to as GHG) exchanges in forests near the Tropic of Cancer are still highly uncertain. To contribute towards an improvement of actual estimates, soil‐atmospheric CO₂, CH₄, and N₂O fluxes were measured in three successional subtropical forests at the Dinghushan Nature Reserve (hereafter referred to as DNR) in southern China. Soils in DNR forests behaved as N₂O sources and CH₄ sinks. Annual mean CO₂, N₂O, and CH₄ fluxes (mean±SD) were 7.7±4.6 Mg CO₂‐C ha^?1 yr^?1, 3.2±1.2 kg N₂O‐N ha^?1 yr^?1, and 3.4±0.9 kg CH₄‐C ha^?1 yr^?1, respectively. The climate was warm and wet from April through September 2003 (the hot‐humid season) and became cool and dry from October 2003 through March 2004 (the cool‐dry season). The seasonality of soil CO₂ emission coincided with the seasonal climate pattern, with high CO₂ emission rates in the hot‐humid season and low rates in the cool‐dry season. In contrast, seasonal patterns of CH₄ and N₂O fluxes were not clear, although higher CH₄ uptake rates were often observed in the cool‐dry season and higher N₂O emission rates were often observed in the hot‐humid season. GHG fluxes measured at these three sites showed a clear increasing trend with the progressive succession. If this trend is representative at the regional scale, CO₂ and N₂O emissions and CH₄ uptake in southern China may increase in the future in light of the projected change in forest age structure. Removal of surface litter reduced soil CO₂ effluxes by 17–44% in the three forests but had no significant effect on CH₄ absorption and N₂O emission rates. This suggests that microbial CH₄ uptake and N₂O production was mainly related to the mineral soil rather than in the surface litter layer.     

Respiration of the growing shoots of Scots pine

The respiratory CO₂ exchange and the growth of the annual shoots were followed in Scots pine (Pinus sylvestris L.) trees growing under extreme continental forest-steppe conditions near the lake Baikal. The temperature coefficient of dark respiration (Q₁₀) in growing shoots dropped down from 3.2–4.0 (in the temperature range of 10–20°C) to 1.5–2.0 (in the temperature range of 20–30°C). The changes in averaged daily respiration rates correlated with the changes in shoot growth increments and temperature (with the multiple determination coefficient of 0.94). Growth respiration of the axial shoots during the phenophase reached 80% of the total respiration costs, with the coefficients of growth respiration and maintenance respiration 0.32 and 0.021. In young crown shoots, the average value of CO₂ evolution in the light combined for the whole observation period (years 1976–2004) was about 1 kg/dm², that is 9% of CO₂ evolution from the trunk surface.     

Carbon mineralization of Chinese fir (Cunninghamia lanceolata) soils under different temperature and humidity conditions

Burned and unburned mineral soils (0–10 cm) from a 40-year-old Chinese fir (Cunninghamia lanceolata) forest in Nanping, Fujian, China were incubated for 90 days at different temperatures (25 °C and 35 °C) and humidity [25%, 50%, and 75% of water holding capacity (WHC)] conditions. Carbon (C) mineralization of all soils was determined using CO₂ respiration method. The results showed that CO₂ evolution rates of the burned and control soils exhibited similar temporal patterns, and similar responses to temperature and moisture. CO₂ evolution rates for all soil samples decreased with incubation time. At different humidity conditions, average rate of C mineralization and cumulative mineralized C from burned and control soils were significantly higher at 35 °C than at 25 °C. This implied that C mineralization was less sensitive to soil moisture than to temperature. In both soils at 25 °C or 35 °C, the amount of soil evolved CO₂ over the 90 days incubation increased with increasing moisture content from 25% to 75% WHC. A temperature coefficient (Q₁₀) varied with soil moisture contents. The maximum values recorded for Q₁₀ were 1.7 in control soil and 1.6 in burned soil both at 25% WHC. However, there were no significant differences in Q₁₀ values between the control and burned soils over all moisture ranges (P > 0.05). The data of cumulative C–CO₂ released from control and burned soils were fitted to two different kinetic models. The two simultaneous reactions model described mineralization better than the first-order exponential model, which reflected the heterogeneity of substrate quality. Based on these results, it is possible to conclude that temperature and moisture are important in the controls of C mineralization, and the combined effects of these variables need to be considered to understand and predict the response of CO₂ release in subtropical ecosystems to climate change.     

Effects of temperature and oxygenavailability on greenhouse gas and nutrient dynamics in sediment of a eutrophic mid-boreal lake

The effects of oxygen conditions and temperature on dynamics of greenhousegases (CH₄, CO₂, N₂O) and nutrients(NH₄ ⁺, NO₂ ^–+NO₃ ^–, tot-P) were studied in sediment of hyper-eutrophic LakeKevätön, Finland. Undisturbed sediment cores were incubated at 6, 11,16, and 23 °C in a laboratory microcosm using a continuouswater flowtechnique with an oxic or anoxic water flow. The production of CO₂increased with increasing temperature in both oxic (Q₁₀ 3.2 ±0.6) and anoxic (Q₁₀ 2.3 ± 0.4) flows. The release ofCH₄ increased with temperature in anoxic conditions (Q₁₀2.3 ± 0.2), but was negligible with the oxic flow at all temperatures.The release of NH₄ ⁺ increased with temperature with the oxic and anoxic flows(Q₁₀ 2.4 ± 0.1). There was a net production of NO₂ ^–, NO₃ ^– and N₂O with the oxic flow at temperatures below16 °C. The release of phosphorus was greater from the anoxicsediments and increased with temperature with both the anoxic (Q₁₀2.9 ± 0.5) and oxic (Q₁₀ 1.9 ± 0.1) flows. It isprobable that the temperature of boreal lakes and the associated oxygendeficiency will increase as the climate becomes warmer. Our experiments showedthat this change would increase the global warming potential of greenhousegasesreleased from sediments of eutrophic lakes predominately attributable to theincrease in the CH₄ production. Furthermore, warming would alsoaccelerate the eutrophication of lakes by increasing release of phosphorus andmineral nitrogen from sediments, which further enhance CH₄productionin sediments.