Contribution of aboveground litter,belowground litter,and rhizosphere respiration to total soil CO<Subscript>2</Subscript> efflux in an old growth coniferous forest

In an old growth coniferous forest located in the central Cascade Mountains, Oregon, we added or removed aboveground litter and terminated live root activity by trenching to determine sources of soil respiration. Annual soil efflux from control plots ranged from 727 g C m⁻² year⁻¹ in 2002 to 841 g C m⁻² year⁻¹ in 2003. We used aboveground litter inputs (149.6 g C m⁻² year⁻¹) and differences in soil CO₂ effluxes among treatment plots to calculate contributions to total soil efflux by roots and associated rhizosphere organisms and by heterotrophic decomposition of organic matter derived from aboveground and belowground litter. On average, root and rhizospheric respiration (R_r) contributed 23%, aboveground litter decomposition contributed 19%, and belowground litter decomposition contributed 58% to total soil CO₂ efflux, respectively. These values fall within the range of values reported elsewhere, although our estimate of belowground litter contribution is higher than many published estimates, which we argue is a reflection of the high degree of mycorrhizal association and low nutrient status of this ecosystem. Additionally, we found that measured fluxes from plots with doubled needle litter led to an additional 186 g C m⁻² year⁻¹ beyond that expected based on the amount of additional carbon added; this represents a priming effect of 187%, or a 34% increase in the total carbon flux from the plots. This finding has strong implications for soil C storage, showing that it is inaccurate to assume that increases in net primary productivity will translate simply and directly into additional belowground storage.     

Effects of Manipulated Above- and Belowground Organic Matter Input on Soil Respiration in a Chinese Pine Plantation

Alteration in the amount of soil organic matter input can have profound effect on carbon dynamics in forest soils. The objective of our research was to determine the response in soil respiration to above- and belowground organic matter manipulation in a Chinese pine (Pinus tabulaeformis) plantation. Five organic matter treatments were applied during a 2-year experiment: both litter removal and root trenching (LRRT), only litter removal (LR), control (CK), only root trenching (RT) and litter addition (LA). We found that either aboveground litter removal or root trenching decreased soil respiration. On average, soil respiration rate was significantly decreased in the LRRT treatment, by about 38.93% ± 2.01% compared to the control. Soil respiration rate in the LR treatment was 30.65% ± 1.87% and in the RT treatment 17.65% ± 1.95% lower than in the control. Litter addition significantly increased soil respiration rate by about 25.82% ± 2.44% compared to the control. Soil temperature and soil moisture were the main factors affecting seasonal variation in soil respiration. Up to the 59.7% to 82.9% seasonal variation in soil respiration is explained by integrating soil temperature and soil moisture within each of the various organic matter treatments. The temperature sensitivity parameter, Q₁₀, was higher in the RT (2.72) and LA (3.19) treatments relative to the control (2.51), but lower in the LRRT (1.52) and LR treatments (1.36). Our data suggest that manipulation of soil organic matter input can not only alter soil CO₂ efflux, but also have profound effect on the temperature sensitivity of organic carbon decomposition in a temperate pine forest.     

Separating the effects of moisture and temperature on soil CO2 efflux in a coniferous forest in the Sierra Nevada mountains

Separating the effects of soil temperature and moisture on soil CO₂ efflux is critical to modeling and understanding the belowground carbon dynamics of forest ecosystems. We developed two analytical procedures to separate the effects of soil temperature and moisture, based on continuous measurements of the CO₂ efflux, temperature and moisture of the soil at a ponderosa pine plantation in the Sierra Nevada Mountains in California, from May 1998 to August 1999. We found that the combined effects of temperature and moisture on the seasonal variation of soil CO₂ efflux could be effectively separated and represented with the product of a temperature term and a moisture term. The relationship between soil CO₂ efflux and temperature could be well described using a power function. This relationship was modified by soil moisture which affects only the coefficient, but not the exponent, of the power function. We also found that when soil moisture was held constant, the temperature effect explained 82% of the temporal variation in CO₂ efflux of the soil. Similarly, when temperature was held constant, the moisture effects explained 84% of the variation. Temperature and moisture together explained 89% of the total temporal variations in soil CO₂ efflux. A multiplicative formulation with power functions representing both temperature and moisture dependences was recommended for modeling soil CO₂ efflux. This formulation can be used to model the seasonal trend of soil CO₂ efflux of the forest based on temperature and moisture, two key variables influenced by climate change and management practices.     

Response of soil microbial activity to temperature,moisture, and litter leaching on a wetland transect during seasonal refilling

In nutrient impoverished landscapes in southwest Australia, terrestrial litter appears to be important in phosphorus (P) turnover and in the gradual accumulation of P in wetland systems. Little is known about the fate of P leached from litter during the wet season and the associated effects of soil microclimate on microbial activity. The effects of temperature, moisture, and litter leaching on soil microbial activity were studied on a transect across a seasonal wetland in southwestern Australia, after the onset of the wet season. Heterotrophic respiration (CO₂ efflux) was higher in the dried lakebed and riparian areas than in upland soils, and higher during the day than at night. There were significant variations in CO₂ efflux with time of sampling, largely caused by the effect of temperature. The addition of litter leachate significantly increased CO₂ efflux, more significantly in soils from upland sites, which had lower moisture and nutrient contents. There was a difference in response of microbial respiration between upland soils and wetland sediments to litter leachate and wetter, warmer conditions. In general, the litter leachate enhanced heterotrophic microbial respiration, and more significantly at warmer conditions (31 °C). The relative fungal to bacterial ratio was 2.9 – 3.2 for surface litter and 0.7–1.0 for soils, suggesting a fungal dominance in heterotrophic respiration of surface litter, but increased bacterial dominance in soils, especially in exposed sediments in the lakebed.     

Allocation of carbon in a mature eucalypt forest and some effects of soil phosphorus availability

Pools and annual fluxes of carbon (C) were estimated for a mature Eucalyptus pauciflora (snowgum) forest with and without phosphorus (P) fertilizer addition to determine the effect of soil P availability on allocation of C in the stand. Aboveground biomass was estimated from allometric equations relating stem and branch diameters of individual trees to their biomass. Biomass production was calculated from annual increments in tree diameters and measurements of litterfall. Maintenance and construction respiration were calculated for each component using equations given by Ryan (1991a). Total belowground C flux was estimated from measurements of annual soil CO₂ efflux less the C content of annual litterfall (assuming forest floor and soil C were at approximate steady state for the year that soil CO₂ efflux was measured). The total C content of the standing biomass of the unfertilized stand was 138 t ha^-1, with approximately 80% aboveground and 20% belowground. Forest floor C was 8.5 t ha^-1. Soil C content (0–1 m) was 369 t ha^-1 representing 70% of the total C pool in the ecosystem. Total gross annual C flux aboveground (biomass increment plus litterfall plus respiration) was 11.9 t ha^-1 and gross flux belowground (coarse root increment plus fine root production plus root respiration) was 5.1 t ha^-1. Total annual soil efflux was 7.1 t ha^-1, of which 2.5 t ha^-1 (35%) was contributed by litter decomposition.The short-term effect of changing the availability of P compared with C on allocation to aboveground versus belowground processes was estimated by comparing fertilized and unfertilized stands during the year after treatment. In the P-fertilized stand annual wood biomass increment increased by 30%, there was no evidence of change in canopy biomass, and belowground C allocation decreased by 19% relative to the unfertilized stand. Total annual C flux was 16.97 and 16.75 t ha^-1 yr^-1 and the ratio of below- to aboveground C allocation was 0.43 and 0.35 in the unfertilized and P-fertilized stands, respectively. Therefore, the major response of the forest stand to increased soil P availability appeared to be a shift in C allocation; with little change in total productivity. These results emphasise that both growth rate and allocation need to be estimated to predict changes in fluxes and storage of C in forests that may occur in response to disturbance or climate change.     

Contrasting soil respiration in young and old-growth ponderosa pine forests

Three years of fully automated and manual measurements of soil CO₂ efflux, soil moisture and temperature were used to explore the diel, seasonal and inter‐annual patterns of soil efflux in an old‐growth (250‐year‐old, O site) and recently regenerating (14‐year‐old, Y site) ponderosa pine forest in central Oregon. The data were used in conjunction with empirical models to determine which variables could be used to predict soil efflux in forests of contrasting ages and disturbance histories. Both stands experienced similar meteorological conditions with moderately cold wet winters and hot dry summers. Soil CO₂ efflux at both sites showed large inter‐annual variability that could be attributed to soil moisture availability in the deeper soil horizons (O site) and the quantity of summer rainfall (Y site). Seasonal patterns of soil CO₂ efflux at the O site showed a strong positive correlation between diel mean soil CO₂ efflux and soil temperature at 64 cm depth whereas diel mean soil efflux at the Y site declined before maximum soil temperature occurred during summer drought. The use of diel mean soil temperature and soil water potential inferred from predawn foliage water potential measurements could account for 80% of the variance of diel mean soil efflux across 3 years at both sites, however, the functional shape of the soil water potential constraint was site‐specific. Based on the similarity of the decomposition rates of litter and fine roots between sites, but greater productivity and amount of fine litter detritus available for decomposition at the O site, we would expect higher rates of soil CO₂ efflux at the O site. However, annual rates were only higher at the O site in one of the 3 years (597 ± 45 vs. 427 ± 80 g C m^?2). Seasonal patterns of soil efflux at both sites showed influences of soil water limitations that were also reflected in patterns of canopy stomatal conductance, suggesting strong linkages between above and below ground processes.     

Sensitivity of Soil Respiration to Variability in Soil Moisture and Temperature in a Humid Tropical Forest

Precipitation and temperature are important drivers of soil respiration. The role of moisture and temperature are generally explored at seasonal or inter-annual timescales; however, significant variability also occurs on hourly to daily time-scales. We used small (1.54 m²), throughfall exclusion shelters to evaluate the role soil moisture and temperature as temporal controls on soil CO₂ efflux from a humid tropical forest in Puerto Rico. We measured hourly soil CO₂ efflux, temperature and moisture in control and exclusion plots (n = 6) for 6-months. The variance of each time series was analyzed using orthonormal wavelet transformation and Haar-wavelet coherence. We found strong negative coherence between soil moisture and soil respiration in control plots corresponding to a two-day periodicity. Across all plots, there was a significant parabolic relationship between soil moisture and soil CO₂ efflux with peak soil respiration occurring at volumetric soil moisture of approximately 0.375 m³/m³. We additionally found a weak positive coherence between CO₂ and temperature at longer time-scales and a significant positive relationship between soil temperature and CO₂ efflux when the analysis was limited to the control plots. The coherence between CO₂ and both temperature and soil moisture were reduced in exclusion plots. The reduced CO₂ response to temperature in exclusion plots suggests that the positive effect of temperature on CO₂ is constrained by soil moisture availability.     

Estimation of carbon storage, carbon inputs, and soil CO2 efflux of alder plantations on granite soil in central Korea: comparison with Japanese larch plantation

Alder is a typical species used for forest rehabilitation after disturbances because of its N₂-fixing activities through microbes. To investigate forest dynamics of the carbon budget, we determined the aboveground and soil carbon content, carbon input by litterfall to belowground, and soil CO₂ efflux over 2 years in 38-year-old alder plantations in central Korea. The estimated aboveground carbon storage and increment were 47.39 Mg C ha⁻¹ and 2.17 Mg C ha⁻¹ year⁻¹. Carbon storage in the organic layer and in mineral soil in the topsoil to 30 cm depth were, respectively, 3.21 and 66.85 Mg C ha⁻¹. Annual carbon input by leaves and total litter in the study stand were, respectively, 1.78 and 2.68 Mg C ha⁻¹ year⁻¹. The aboveground carbon increment at this stand was similar to the annual carbon inputs by total litterfall. The diurnal pattern of soil CO₂ efflux was significantly different in May, August, and October, typically varying approximately twofold throughout the course of a day. In the seasonally observed pattern, soil CO₂ efflux varied strongly with soil temperature; increasing trends were evident during the early growing season, with sustained high rates from mid May through late October. Soil CO₂ efflux was related exponentially to soil temperature (R ² = 0.85, P < 0.0001), but not to soil water content. The Q ₁₀ value for this plantation was 3.8, and annual soil respiration was estimated at 10.2 Mg C ha⁻¹ year⁻¹. An erratum to this article can be found at     

Response of soil CO2 efflux to water manipulation in a tallgrass prairie ecosystem

Although CO₂ efflux plays a critical role in carbon exchange between the biosphere and atmosphere, our understanding of its regulation by soil moisture is rather limited. This study was designed to examine the relationship between soil CO₂ efflux and soil moisture in a natural ecosystem by taking advantage of the historically long drought period from 29 July to 21 September 2000 in the southern Central Great Plain, USA. At the end of August when soil moisture content at the top 50 mm was reduced to less than 50 g kg^–1 gravimetrically, we applied 8 levels of water treatments (simulated to rainfall of 0, 10, 25, 50, 100, 150, 200, and 300 mm) with three replicates to 24 plots in a Tallgrass Prairie ecosystem in Central Oklahoma, USA. In order to quantify root-free soil CO₂ efflux, we applied the same 8 levels of water treatments to 24 500-mm soil columns using soil from field adjacent to the experimental plots. We characterized dynamic patterns of soil moisture and soil CO₂ efflux over the experimental period of 21 days. Both soil moisture content and CO₂ efflux showed dramatic increases immediately after the water addition, followed by a gradual decline. The time courses in response to water treatments are well described by Y=Y₀+ate^–bt, where Y is either soil moisture or CO₂ efflux, t is time, Y ₀, a, and b are coefficients. Among the 8 water treatments, the maximal soil CO₂ efflux rate occurred at the 50 mm water level in the field and 100 mm in the root-free soil 1 day after the treatment. The maximal soil CO₂ efflux gradually shifted to higher water levels as the experiment continued. We found the relationship between soil CO₂ efflux and soil moisture using the data from the 21-day experiment was highly scattered, suggesting complex mechanisms determining soil CO₂ efflux by soil moisture.     

Carbon dioxide efflux and pCO2 in soils of threeQuercus ilex montane forests

Soil CO₂ efflux and pCO₂ in the soil atmosphere were measured during one year at three montane sites of Mediterranean sclerophyllous forests in NE Spain. Two sites were located in the upper and lower slopes of a small catchment in the Prades mountains (mean precipitation 550 mm year^–1), and a third site was located on a lower slope in the Montseny mountains (mean precipitation 900 mm year^–1). The three sites were similar in bedrock and vegetation, but differed in soil characteristics and water availability. Seasonal variation of CO₂ efflux and soil pCO₂ were affected by soil temperature and, to a lesser extent, by soil moisture. Annual mean soil CO₂ efflux (considered as soil respiration) was similar at Montseny and at the comparably located site at Prades (83 ± 18 S.E. vs. 75 ± 9 mg CO₂ m^–2 hour^–1 , respectively), and was highest at the Prades upper slope site (122 ± 22 mg C0₂ m^–2 hour^–1 ). Despite those relatively similar CO₂ effluxes, mean soil pCO₂ was much higher at both Prades sites than at Montseny. Soil pCO₂ always increased with depth at Prades while maxima pCO₂ at Montseny were often at 20–30 cm depth. A model based on gas diffusion theory was able to explain why soil pCO₂ was much higher at Prades than at Montseny, and to reproduce the shape of the vertical profile of pCO₂ at the Prades soils. Nevertheless, the model failed to simulate the soil pCO₂ maximum found at 20–30 cm depth at the Montseny site. Model simulations using a time-variable CO₂ production rate suggested that pCO₂ maxima at intermediate depth could be the result of a transient situation instead of an equilibrium one.     

Elevated CO2 and temperature impacts on different components of soil CO2 efflux in Douglas-fir terracosms

Although numerous studies indicate that increasing atmospheric CO₂ or temperature stimulate soil CO₂ efflux, few data are available on the responses of three major components of soil respiration [i.e. rhizosphere respiration (root and root exudates), litter decomposition, and oxidation of soil organic matter] to different CO₂ and temperature conditions. In this study, we applied a dual stable isotope approach to investigate the impact of elevated CO₂ and elevated temperature on these components of soil CO₂ efflux in Douglas-fir terracosms. We measured both soil CO₂ efflux rates and the ¹³C and ¹⁸O isotopic compositions of soil CO₂ efflux in 12 sun-lit and environmentally controlled terracosms with 4-year-old Douglas fir seedlings and reconstructed forest soils under two CO₂ concentrations (ambient and 200 ppmv above ambient) and two air temperature regimes (ambient and 4 °C above ambient). The stable isotope data were used to estimate the relative contributions of different components to the overall soil CO₂ efflux. In most cases, litter decomposition was the dominant component of soil CO₂ efflux in this system, followed by rhizosphere respiration and soil organic matter oxidation. Both elevated atmospheric CO₂ concentration and elevated temperature stimulated rhizosphere respiration and litter decomposition. The oxidation of soil organic matter was stimulated only by increasing temperature. Release of newly fixed carbon as root respiration was the most responsive to elevated CO₂, while soil organic matter decomposition was most responsive to increasing temperature. Although some assumptions associated with this new method need to be further validated, application of this dual-isotope approach can provide new insights into the responses of soil carbon dynamics in forest ecosystems to future climate changes.     

Annual soil respiration in broadleaf forests of northern Wisconsin: influence of moisture and site biological,chemical, and physical characteristics

Soil temperature and moisture influence soil respiration at a range of temporal and spatial scales. Although soil temperature and moisture may be seasonally correlated, intra and inter-annual variations in soil moisture do occur. There are few direct observations of the influence of local variation in species composition or other stand/site characteristics on seasonal and annual variations in soil moisture, and on cumulative annual soil carbon release. Soil climate and soil respiration from twelve sites in five different forest types were monitored over a 2-year period (1998–1999). Also measured were stand age, species composition, basal area, litter inputs, total above-ground wood production, leaf area index, forest floor mass, coarse and fine root mass, forest floor carbon and nitrogen concentration, root carbon and nitrogen concentration, soil carbon and nitrogen concentration, coarse fraction mass and volume, and soil texture. General soil respiration models were developed using soil temperature, daily soil moisture, and various site/soil characteristics. Of the site/soil characteristics, above-ground production, soil texture, roots + forest floor mass, roots + forest floor carbon:nitrogen, and soil carbon:nitrogen were significant predictors of soil respiration when used alone in respiration models; all of these site variables were weakly to moderately correlated with mean site soil moisture. Daily soil climate data were used to estimate the annual release of carbon (C) from soil respiration for the period 1998–1999. Mean annual soil temperature did not differ between the 2 years but mean annual soil moisture was approximately 9% lower in 1998 due to a summer drought. Soil C respired during 1998 ranged from 8.57 to 11.43 Mg C ha⁻¹ yr⁻¹ while the same sites released 10.13 and 13.57 Mg C ha⁻¹ yr⁻¹ in 1999; inter-annual differences of 15.41 and 15.73%, respectively. Among the 12 sites studied, we calculated that the depression of soil respiration linked to the drought caused annual differences of soil respiration from 11.00 to 15.78%. Annual estimates of respired soil C decreased with increasing site mean soil moisture. Similarly, the difference of respired carbon between the drought and the non-drought years generally decreased with increasing site mean soil moisture.     

短期增温和增加降水对内蒙古荒漠草原土壤呼吸的影响

利用红外辐射增温装置模拟短期持续增温和降水增加交互作用对内蒙古荒漠草原土壤呼吸作用的影响, 结果表明: 土壤含水量对月土壤呼吸的影响显著大于土壤温度增加的影响, 生长旺季的月土壤呼吸显著大于生长末季; 土壤温度和水分增加都显著影响日土壤呼吸, 但二者的交互作用对土壤呼吸无显著影响。荒漠草原7‒8月平均土壤呼吸速率为1.35 μmol CO₂·m ^-2·s ^-1, 7月份为2.08 μmol CO₂·m ^-2·s ^-1, 8月份为0.63 μmol CO₂·m ^-2·s ^-1。土壤呼吸与地下各层根系生物量呈幂函数关系, 0‒10 cm土层的根系生物量对土壤呼吸的解释率(79.2%)明显高于10‒20 cm土层的解释率(31.6%)。0-10 cm土层的根系生物量是根系生物量的主体, 根系生物量对土壤呼吸的影响具有层次性。在未来全球变暖和降水格局变化的情景下, 荒漠草原土壤水分含量是影响生物量的主导环境因子, 而根系生物量的差异是造成土壤呼吸异质性的主要生物因素, 土壤含水量可通过影响根系生物量控制土壤呼吸的异质性。     

Temporal variation in CO<Subscript>2</Subscript> efflux from soil and snow surfaces in a Japanese cedar (<Emphasis Type="Italic">Cryptomeria japonica</Emphasis>) plantation,central Japan

CO₂ efflux from soil and snow surfaces was measured continuously in a Japanese cedar (Cryptomeria japonica D. Don) forest in central Japan using an open dynamic chamber system. The chamber opens and closes automatically and records measurements based on an open-flow dynamic method. Between May and December, mean soil CO₂ efflux ranged from 1,529 mg CO₂ m⁻² h⁻¹ in September to 255 mg CO₂ m⁻² h⁻¹ in December. The seasonal change in CO₂ efflux from the soil paralleled the seasonal pattern of soil temperature. No marked diurnal trends in soil CO₂ efflux were observed on days without rainfall, whereas significant pulses in soil CO₂ efflux were observed on days with rainfall. In this plantation, soil CO₂ efflux frequently responded to rainfall. Measurements of changes from litter-covered soil to snow-covered surfaces revealed that CO₂ efflux decreased from values of ca. 250 mg CO₂ m⁻² h⁻¹ above soil to less than 33 mg CO₂ m⁻² h⁻¹ above snow. Soil temperature alone explained 66% of the overall variation in soil CO₂ efflux, but explained approximately 85% of the variation when data from two anomalous periods were excluded. Moreover, we found a significant correlation between soil CO₂ efflux and soil moisture (which explained 44% of the overall variation) using a second-order polynomial function. Our results suggest that the seasonality of CO₂ efflux is affected not only by soil temperature and moisture, but also by drying and rewetting cycles and by litterfall pulses.     

Characteristics of soil CO2 efflux variability in an aseasonal tropical rainforest in Borneo Island

Although soil carbon dioxide (CO₂) efflux from tropical forests may play an important role in global carbon (C) balance, our knowledge of the fluctuations and factors controlling soil CO₂ efflux in the Asian tropics is still poor. This study characterizes the temporal and spatial variability in soil CO₂ efflux in relation to temperature/moisture content and estimates annual efflux from the forest floor in an aseasonal intact tropical rainforest in Sarawak, Malaysia. Soil CO₂ efflux varied widely in space; the range of variation averaged 17.4 μmol m⁻² s⁻¹ in total. While most CO₂ flux rates were under 10 μmol m⁻² s⁻¹, exceptionally high fluxes were observed sporadically at several sampling points. Semivariogram analysis revealed little spatial dependence in soil CO₂ efflux. Temperature explained nearly half of the spatial heterogeneity, but the effect varied with time. Seasonal variation in CO₂ efflux had no fixed pattern, but was significantly correlated with soil moisture content. The correlation coefficient with soil moisture content (SMC) at 30 and 60 cm depth was higher than at 10 cm depths. The annual soil CO₂ efflux, estimated from the relationship between CO₂ efflux and SMC at 30 cm depth, was 165 mol m⁻² year⁻¹ (1,986 g C m⁻² year⁻¹). As this area is known to suffer severe drought every 4–5 years caused by the El Nino-Southern Oscillation, the results suggest that an unpredictable dry period might affect soil CO₂ efflux, leading an annual variation in soil C balance.     

米槠和杉木人工林土壤呼吸及其组分分析

区分森林土壤呼吸组分是了解生态系统碳循环的重要环节。该文以福建省三明市格氏栲自然保护区米槠(Castanopsis carlesii)人工林和邻近的杉木(Cunninghamia lanceolata)人工林为研究对象, 于2012年8月至2013年7月, 采用LI-8100开路式土壤碳通量系统, 通过挖壕沟方法, 测定了土壤呼吸及异养呼吸的速率, 同时测定了5 cm深处的土壤温度和0-12 cm深处的土壤含水量。利用指数模型和双因素模型, 分析土壤呼吸及其组分与土壤温度和土壤含水量的关系, 同时计算了土壤呼吸各组分在土壤呼吸中所占的比例, 并分析了不同森林类型对土壤呼吸及其组分的影响。结果表明: 米槠人工林和杉木人工林土壤呼吸及其组分的季节变化显著, 均呈单峰型曲线, 与5 cm深处的土壤温度呈极显著正相关关系。土壤温度可以分别解释米槠人工林土壤呼吸、自养呼吸和异养呼吸变化的70.3%、73.4%和58.2%, 可以解释杉木人工林土壤呼吸、自养呼吸和异养呼吸变化的77.9%、65.7%和79.2%。土壤呼吸及其组分与土壤含水量没有相关关系。米槠和杉木人工林自养呼吸的年通量分别为4.00和2.18 t C·hm^-2·a^-1, 占土壤呼吸年通量的32.5%和24.1%; 异养呼吸年通量分别为8.32和6.88 t C·hm^-2·a^-1, 分别占土壤呼吸年通量的67.5%和75.9%, 米槠人工林土壤呼吸及其组分的年通量都大于杉木人工林。     

Grubbing by wild boars (Sus scrofa L.) and its impact on hardwood forest soil carbon dioxide emissions in Switzerland

Interest in soil C storage and release has increased in recent years. In addition to factors such as climate/land-use change, vertebrate animals can have a considerable impact on soil CO₂ emissions. To date, most research has considered herbivores, while the impact of omnivorous animals has rarely been investigated. Our goal was to determine how European wild boars (Sus scrofa L.), large omnivores that consume soil-inhabiting animals and belowground plant parts by grubbing in the soil, affect soil C dynamics. We measured soil respiration (CO₂), temperature, and moisture on paired grubbed and non-grubbed plots in six hardwood forest stands for a 3-year period and sampled fine root and microbial biomass at the beginning and after 2 years of the study. We also measured the percentage of freshly disturbed forest soil within the larger surroundings of each stand and used this information together with hunting statistics and forest cover data to model the total amount of CO₂ released from Swiss forest soils due to grubbing during 1 year. Soil CO₂ emissions were significantly higher on grubbed compared to non-grubbed plots during the study. On average 23.1% more CO₂ was released from these plots, which we associated with potential alterations in CO₂ diffusion rates, incorporation of litter into the mineral soil and higher fine root/microbial biomass. Thus, wild boars considerably increased the small-scale heterogeneity of soil properties. Roughly 1% of Switzerland’s surface area is similar to our sites (boar density/forest cover). Given the range of forest soil disturbance of 27–54% at our sites, the geographic information system model predicted that boar grubbing would lead to the release of an additional 49,731.10–98,454.74 t CO₂ year⁻¹. These values are relatively small compared to total soil emissions estimated for Swiss hardwood forests and suggest that boars will have little effect on large-scale emissions unless their numbers increase and their range expands dramatically.     

Soil CO<Subscript><Emphasis Type="Bold">2</Emphasis></Subscript> efflux and fungal and bacterial biomass in a plantation and a secondary forest in wet tropics in Puerto Rico

We examined the effects of root and litter exclusion on the rate of soil CO₂ efflux and microbial biomass using trenching and tent separation techniques in a secondary forest (SF) and a pine (Pinus caribaea Morelet) plantation in the Luquillo Experimental Forest in Puerto Rico. Soil surface CO₂ efflux was measured using the alkali trap method at 12 randomly-distributed locations in each treatment (control, root exclusion, litter exclusion, and both root and litter exclusion) in the plantation and the SF, respectively. We measured soil CO₂ efflux every two months and collected soil samples at each sampling location in different seasons to determine microbial biomass from August 1996 to July 1997. We found that soil CO₂ efflux was significantly reduced in the litter and root exclusion plots (7-year litter and/or root exclusion) in both the secondary forest and the pine plantation compared with the control. The reduction of soil CO₂ efflux was 35.6% greater in the root exclusion plots than in the litter exclusion plots in the plantation, whereas a reversed pattern was found in the secondary forest. Microbial biomass was also reduced during the litter and root exclusion period. In the root exclusion plots, total fungal biomass averaged 31.4% and 65.2% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 24% and 8.3% lower than the control plots in the plantation and the secondary forest, respectively. In the litter exclusion treatment, total fungal biomass averaged 69.2% and 69.7% lower than the control plots in the plantation and the secondary forest, respectively, while the total bacterial biomass was 48% and 50.1% lower than the control plots in the plantation and the secondary forest, respectively. Soil CO₂ efflux was positively correlated with both fungal and bacterial biomass in both the plantation the secondary forest. The correlation between soil CO₂ efflux and active fungal biomass was significantly higher in the plantation than in the secondary forest. However, the correlation between the soil CO₂ efflux and both the active and total bacterial biomass was significantly higher in the secondary forest than in the plantation in the day season. In addition, we found soil CO₂ efflux was highly related to the strong interactions among root, fungal and bacterial biomass by multiple regression analysis (R² > 0.61, P < 0.05). Our results suggest that carbon input from aboveground litterfall and roots (root litter and exudates) is critical to the soil microbial community and ecosystem carbon cycling in the wet tropical forests.     

The effects of elevated atmospheric CO<Subscript>2</Subscript> and nitrogen amendments on subsurface CO<Subscript>2</Subscript> production and concentration dynamics in a maturing pine forest

Profiles of subsurface soil CO₂ concentration, soil temperature, and soil moisture, and throughfall were measured continuously during the years 2005 and 2006 in 16 locations at the free air CO₂ enrichment facility situated within a temperate loblolly pine (Pinus taeda L.) stand. Sampling at these locations followed a 4 by 4 replicated experimental design comprised of two atmospheric CO₂ concentration levels (ambient [CO₂]^a, ambient + 200 ppmv, [CO₂]^e) and two soil nitrogen (N) deposition levels (ambient, ambient + fertilization at 11.2 g_N m⁻² year⁻¹). The combination of these measurements permitted indirect estimation of belowground CO₂ production and flux profiles in the mineral soil. Adjacent to the soil CO₂ profiles, direct (chamber-based) measurements of CO₂ fluxes from the soil–litter complex were simultaneously conducted using the automated carbon efflux system. Based on the measured soil CO₂ profiles, neither [CO₂]^e nor N fertilization had a statistically significant effect on seasonal soil CO₂, CO₂ production, and effluxes from the mineral soil over the study period. Soil moisture and temperature had different effects on CO₂ concentration depending on the depth. Variations in CO₂ were mostly explained by soil temperature at deeper soil layers, while water content was an important driver at the surface (within the first 10 cm), where CO₂ pulses were induced by rainfall events. The soil effluxes were equal to the CO₂ production for most of the time, suggesting that the site reached near steady-state conditions. The fluxes estimated from the CO₂ profiles were highly correlated to the direct measurements when the soil was neither very dry nor very wet. This suggests that a better parameterization of the soil CO₂ diffusivity is required for these soil moisture extremes.     

Impacts of forest management type and season on soil carbon fluxes in Eastern Mau Forest,Kenya

The value of ecosystems functions performed by forests in the climate change era has prompted increasing attention towards assessment of carbon stocks and fluxes in tropical forests. The aim of this study was to understand how forest management approaches and environmental controls impacted on soil CO₂ efflux in a tropical Eastern Mau forest which is one of the blocks of the greater Mau complex in Kenya. Nested experimental design approach was employed where 32 plots were nested into four blocks (disturbed natural, undisturbed natural, plantation and glades). In 10 m² plots, data were collected on soil CO₂ efflux, soil temperature and soil moisture using soda lime methods, direct measurement and proxy techniques, respectively. There was significant forest management type effect (F_3,127 = 3.01, p = 0.033) and seasonality effect (t test = 3.31, df = 1, p < 0.05) on mean soil CO₂ efflux. The recorded mean soil CO₂ efflux levels were as follows: plantation forest (9.219 ± 3.067 g C M^?2 day^?1), undisturbed natural forest (8.665 ± 4.818 g C M^?2 day^?1), glades (8.592 ± 3.253 g C M^?2 day^?1) and disturbed natural forest (7.198 ± 3.457 g C M^?2 day^?1). The study concludes that managing a forest in plantation form is primarily responsible for forest soil CO₂ efflux levels due to aspects such as increased microbial activity and root respiration. However, further studies are required to understand the role and impact of soil CO₂ efflux on the greater forest carbon budget.