Seasonal changes in the contribution of root respiration to total soil respiration in a cool-temperate deciduous forest

A trenching method was used to determine the contribution of root respiration to soil respiration. Soil respiration rates in a trenched plot (R _trench) and in a control plot (R _control) were measured from May 2000 to September 2001 by using an open-flow gas exchange system with an infrared gas analyser. The decomposition rate of dead roots (R _D) was estimated by using a root-bag method to correct the soil respiration measured from the trenched plots for the additional decaying root biomass. The soil respiration rates in the control plot increased from May (240–320 mg CO₂ m^–2 h^–1) to August (840–1150 mg CO₂ m^–2 h^–1) and then decreased during autumn (200–650 mg CO₂ m^–2 h^–1). The soil respiration rates in the trenched plot showed a similar pattern of seasonal change, but the rates were lower than in the control plot except during the 2 months following the trenching. Root respiration rate (R _r) and heterotrophic respiration rate (R _h) were estimated from R _control, R _trench, and R _D. We estimated that the contribution of R _r to total soil respiration in the growing season ranged from 27 to 71%. There was a significant relationship between R _h and soil temperature, whereas R _r had no significant correlation with soil temperature. The results suggest that the factors controlling the seasonal change of respiration differ between the two components of soil respiration, R _r and R _h.     

Earthworm Invasion,Fine-root Distributions,and Soil Respiration in North Temperate Forests

The efflux of carbon from soils is a critical link between terrestrial ecosystems and the atmosphere. Current concerns about rising atmospheric carbon dioxide (CO₂) concentrations highlight the need to better understand the dynamics of total soil respiration (TSR, sum of root and heterotroph respiration) in changing environments. We investigated the effects of exotic earthworm invasion on TSR, fine-root distributions, and aboveground litterfall flux in two sugar maple-dominated forests in two locations in New York State, USA. The Arnot Forest in central New York was harvested in the late 19th century and has no history of cultivation. Tompkins Farm in eastern New York regenerated following abandonment from cultivation approximately 75 years ago. Arnot had 20% higher total soil CO₂ efflux (880 g C m^–2year^–1) than Tompkins (715 g C m^–2year^–1). The presence of earthworms had no influence on TSR at either location. However, fine-root (< 1 mm diameter) biomass in earthworm plots (350 g/m²) was significantly lower than in worm-free reference plots (440 g/m²) at Arnot. Fine-root nitrogen (N) concentrations were not influenced by earthworms, and total fine-root N content was significantly reduced in the presence of earthworms at Arnot. Our results indicate that the presence of exotic earthworms is not presently affecting net C emission from soil in these forests. They also suggest a change in root function in earthworm plots that is not associated with higher fine-root N concentration, but that increases efficiency of nutrient uptake and also may enhance the belowground supply of C for heterotroph metabolism.     

Carbon balance of rewetted and drained peat soils used for biomass production: a mesocosm study

Rewetting of drained peatlands has been recommended to reduce CO₂ emissions and to restore the carbon sink function of peatlands. Recently, the combination of rewetting and biomass production (paludiculture) has gained interest as a possible land use option in peatlands for obtaining such benefits of lower CO₂ emissions without losing agricultural land. This study quantified the carbon balance (CO₂, CH₄ and harvested biomass C) of rewetted and drained peat soils under intensively managed reed canary grass (RCG) cultivation. Mesocosms were maintained at five different groundwater levels (GWLs), that is 0, 10, 20 cm below the soil surface, representing rewetted peat soils, and 30 and 40 cm below the soil surface, representing drained peat soils. Net ecosystem exchange (NEE) of CO₂ and CH₄ emissions was measured during the growing period of RCG (May to September) using transparent and opaque closed chamber methods. The average dry biomass yield was significantly lower from rewetted peat soils (12 Mg ha^?1) than drained peat soils (15 Mg ha^?1). Also, CO₂ fluxes of gross primary production (GPP) and ecosystem respiration (ER) from rewetted peat soils were significantly lower than from drained peat soils, but net uptake of CO₂ was higher from rewetted peat soils. Cumulative CH₄ emissions were negligible (0.01 g CH₄ m^?2) from drained peat soils but were significantly higher (4.9 g CH₄ m^?2) from rewetted peat soils during measurement period (01 May–15 September 2013). The extrapolated annual C balance was 0.03 and 0.68 kg C m^?2 from rewetted and drained peat soils, respectively, indicating that rewetting and paludiculture can reduce the loss of carbon from peatlands.     

Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes

Temperate forests of North America are thought to besignificant sinks of atmospheric CO₂. Wedeveloped a below-ground carbon (C) budget forwell-drained soils in Harvard Forest Massachusetts, anecosystem that is storing C. Measurements of carbonand radiocarbon (¹⁴C) inventory were used todetermine the turnover time and maximum rate ofCO₂ production from heterotrophic respiration ofthree fractions of soil organic matter (SOM):recognizable litter fragments (L), humified lowdensity material (H), and high density ormineral-associated organic matter (M). Turnover timesin all fractions increased with soil depth and were2–5 years for recognizable leaf litter, 5–10 years forroot litter, 40–100+ years for low density humifiedmaterial and >100 years for carbon associated withminerals. These turnover times represent the timecarbon resides in the plant + soil system, and mayunderestimate actual decomposition rates if carbonresides for several years in living root, plant orwoody material.Soil respiration was partitioned into two componentsusing ¹⁴C: recent photosynthate which ismetabolized by roots and microorganisms within a yearof initial fixation (Recent-C), and C that is respiredduring microbial decomposition of SOM that resides inthe soil for several years or longer (Reservoir-C).For the whole soil, we calculate that decomposition ofReservoir-C contributes approximately 41% of thetotal annual soil respiration. Of this 41%,recognizable leaf or root detritus accounts for 80%of the flux, and 20% is from the more humifiedfractions that dominate the soil carbon stocks.Measurements of CO₂ and ¹⁴CO₂ in thesoil atmosphere and in total soil respiration werecombined with surface CO₂ fluxes and a soil gasdiffusion model to determine the flux and isotopicsignature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cmof the soil (O + A + Ap horizons). The average residencetime of Reservoir-C in the plant + soil system is8±1 years and the average age of carbon in totalsoil respiration (Recent-C + Reservoir-C) is 4±1years.The O and A horizons have accumulated 4.4 kgC m^–2above the plow layer since abandonment by settlers inthe late-1800's. C pools contributing the most to soilrespiration have short enough turnover times that theyare likely in steady state. However, most C is storedas humified organic matter within both the O and Ahorizons and has turnover times from 40 to 100+ yearsrespectively. These reservoirs continue to accumulatecarbon at a combined rate of 10–30 gC m^{minus 2}yr^–1. This rate of accumulation is only 5–15% of the total ecosystem C sink measured in this stand using eddy covariance methods.     

Spatial–temporal variation in soil respiration in an oak–grass savanna ecosystem in California and its partitioning into autotrophic and heterotrophic components

The spatial upscaling of soil respiration from field measurements to ecosystem levels will be biased without studying its spatial variation. We took advantage of the unique spatial gradients of an oak–grass savanna ecosystem in California, with widely spaced oak trees overlying a grass layer, to study the spatial variation in soil respiration and to use these natural gradients to partition soil respiration according to its autotrophic and heterotrophic components. We measured soil respiration along a 42.5 m transect between two oak trees in 2001 and 2002, and found that soil respiration under tree canopies decreased with distance from its base. In the open area, tree roots have no influence on soil respiration. Seasonally, soil respiration increased in spring until late April, and decreased in summer following the decrease in soil moisture content, despite the further increase in soil temperature. Soil respiration significantly increased following the rain events in autumn. During the grass growing season between November and mid-May, the average of CO₂ efflux under trees was 2.29 μmol m⁻² s⁻¹, while CO₂ efflux from the open area was 1.40 μmol m⁻² s⁻¹. We deduced that oak root respiration averaged as 0.89 μmol m⁻² s⁻¹, accounting for 39% of total soil respiration (oak root + grass root + microbes). During the dry season between mid-May and October, the average of CO₂ efflux under trees was 0.87 μmol m⁻² s⁻¹, while CO₂ efflux from the open areas was 0.51 μmol m⁻² s⁻¹. Oak root respiration was 0.36 μmol m⁻² s⁻¹, accounting for 41% of total soil respiration (oak root + microbes). The seasonal pattern of soil CO₂ efflux under trees and in open areas was simulated by a bi-variable model driven by soil temperature and moisture. The diurnal pattern was influenced by tree physiology as well. Based on the spatial gradient of soil respiration, spatial analysis of crown closure and the simulation model, we spatially and temporally upscaled chamber measurements to the ecosystem scale. We estimated that the cumulative soil respiration in 2002 was 394 gC m⁻² year⁻¹ in the open area and 616 gC m⁻² year⁻¹ under trees with a site-average of 488 gC m⁻² year⁻¹.     

施肥对油茶园土壤呼吸和异养呼吸及其温度敏感性的影响

油茶是中国南方重要的木本食用油料树种,研究施肥对油茶园土壤呼吸及其温度敏感性的影响,对于估算中国南方典型种植园林温室气体排放及其对气候变化的响应具有重要意义。设置对照(CK)、施肥(OF)、断根(CK-T)和断根施肥(OF-T)4个处理,采用静态箱-气相色谱法,通过多年观测,分析探讨施肥对油茶园土壤呼吸和异养呼吸及其温度敏感性的影响。结果表明:(1)施肥对油茶园土壤呼吸和异养呼吸无显著影响。研究期间,各处理(OF、CK、OF-T、CK-T)土壤CO_2通量依次为(77.91±2.59)、(73.71±0.97)、(66.82±1.02)mg C m~(-2)h~(-1)和(66.84±3.94)mg C m~(-2)h~(-1);(2)各处理土壤呼吸温度敏感性(Q_(10))表现为OF-T(1.96±0.01)CK-T(1.79±0.03)OF(1.77±0.01)CK(1.75±0.03),其中,OF-T处理下Q_(10)显著高于其他3个处理,即施肥显著增加了断根处理土壤呼吸Q_(10);(3)施肥显著增加了土壤表层NH_4~+-N和NO_3~--N含量,Q_(10)与土壤表层NH_4~+-N和NO_3~--N含量表现出显著的正相关关系。     

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.     

Linking Belowground Carbon Allocation to Anaerobic CH4 and CO2 Production in a Forested Peatland,New York State

Heterotrophic soil microorganisms rely on carbon (C) allocated belowground in plant production, but belowground C allocation (BCA) by plants is a poorly quantified part of ecosystem C cycling, especially, in peat soil. We applied a C balance approach to quantify BCA in a mixed conifer-red maple (Acer rubrum) forest on deep peat soil. Direct measurements of CH₄ and CO₂ fluxes across the soil surface (soil respiration), production of fine and small plant roots, and aboveground litterfall were used to estimate respiration by roots, by mycorrhizae and by free-living soil microorganisms. Measurements occurred in two consecutive years. Soil respiration rates averaged 1.2 bm μmol m^{? 2} s^{? 1} for CO₂ and 0.58 nmol m^{? 2} s^{? 1} for CH₄ (371 to 403 g C m^{? 2} year^{? 1}). Carbon in aboveground litter (144 g C m^{? 2} year^{? 1}) was 84% greater than C in root production (78 g C m^{? 2} year^{? 1}). Complementary in vitro assays located high rates of anaerobic microbial activity, including methanogenesis, in a dense layer of roots overlying the peat soil and in large-sized fragments within the peat matrix. Large-sized fragments were decomposing roots and aboveground leaf and twig litter, indicating that relatively fresh plant production supported most of the anaerobic microbial activity. Respiration by free-living soil microorganisms in deep peat accounted for, at most, 29 to 38 g C m^{? 2} year^{? 1}. These data emphasize the close coupling between plant production, ecosystem-level C cycling and soil microbial ecology, which BCA can help reveal.     

暖温带麻栎林凋落物调节土壤碳排放通量对降雨脉冲的响应

凋落物既是森林生态系统养分循环的重要构件,又是森林土壤环境和功能的关键调节因子。降雨脉冲导致的土壤碳排放变异是陆地生态系统碳汇能力评价的不确定性来源之一。凋落物在调节土壤碳排放对降雨脉冲的响应中的作用仍缺乏科学的评价。通过在暖温带栎类落叶阔叶林中设置不同凋落物处理(对照、去除凋落物和加倍凋落物)和降雨模拟实验以阐明凋落物数量变化对土壤呼吸脉冲的影响。结果表明：模拟降雨脉冲之前,不同凋落物处理下的土壤呼吸存在显著差异;与对照相比,加倍凋落物导致土壤呼吸速率显著增加57.6%,然而,去除凋落物则对土壤呼吸无显著影响。模拟降雨后52小时内,对照、去除凋落物和加倍凋落物样方的土壤累积碳排放量分别为251.69 gC/m~2,250.93 gC/m~2和409.01 gC/m~2,加倍凋落物处理下的土壤碳排放量显著高于对照和去除凋落物处理;然而,去除凋落物与对照之间无显著差异。此外,不同凋落物处理下土壤呼吸的脉冲持续时间存在显著差异;加倍凋落物显著提高降雨后土壤呼吸脉冲的持续时间,分别比对照和去除凋落物高出262%和158%。多元逐步回归分析表明,土壤总碳排放通量和土壤呼吸的脉冲持续时间与土壤理...     

Fluxes of N2O,CH4 and CO2 on afforested boreal agricultural soils

After drainage of natural boreal peatlands, the decomposition of organic matter increases and peat soil may turn into a net source of CO₂ and N₂O, whereas CH₄ emission is known to decrease. Afforestation is a potential mitigation strategy to reduce greenhouse gas emission from organic agricultural soils. A static chamber technique was used to evaluate the fluxes of CH₄, N₂O and CO₂ from three boreal organic agricultural soils in western Finland, afforested 1, 6 or 23 years before this study. The mean emissions of CH₄ and N₂O during the growing seasons did not correlate with the age of the tree stand. All sites were sources of N₂O. The highest daily N₂O emission during the growing season, measured in the oldest site, was as high as 29 mg N₂O m^–2d^–1. In general, organic agricultural soils are sinks for methane. Here, the oldest site acted as a small sink for methane, whereas the two youngest afforested organic soils were sources for methane with maximum emission rates (up to 154 mg m^–2d^–1) similar to those reported for minerogenous natural peatlands. Soil respiration rates decreased with the age of the forest. The high soil respiration in the younger sites, probably resulted from the high biomass production of herbs, could create soil anaerobiosis and increase methane production. Our results show that afforestation of agricultural peat soils does not abruptly terminate the N₂O emissions during the first two decades, and afforestation can even enhance methane emission for a few years. The carbon accumulation in the developing tree stand can partly compensate the carbon loss from soil.     

Seasonal CO<Subscript>2</Subscript>-exchange variations of temperate semi-desert grassland in Hungary

CO₂ exchange components of a temperate semi-desert sand grassland ecosystem in Hungary were measured 21 times in 2000–2001 using a closed IRGA system. Stand CO₂ uptake and release, soil respiration rate (R _s), and micrometeorological values were determined with two types of closed system chambers to investigate the daily courses of gas exchange. The maximum CO₂ uptake and release were –3.240 and 1.903 mol m^–2 s^–1, respectively, indicating a relatively low carbon sequestration potential. The maximum and the minimum R _s were 1.470 and 0.226 mol(CO₂) m^–2 s^–1, respectively. Water shortage was probably more effective in decreasing photosynthetic rates than R _s, indicating water supply as the primary driving variable for the sink-source relations in this ecosystem type.     

Effects of experimental drought on soil respiration and radiocarbon efflux from a temperate forest soil

Soil moisture affects microbial decay of SOM and rhizosphere respiration (RR) in temperate forest soils, but isolating the response of soil respiration (SR) to summer drought and subsequent wetting is difficult because moisture changes are often confounded with temperature variation. We distinguished between temperature and moisture effects by simulation of prolonged soil droughts in a mixed deciduous forest at the Harvard Forest, Massachusetts. Roofs constructed over triplicate 5 × 5 m² plots excluded throughfall water during the summers of 2001 (168 mm) and 2002 (344 mm), while adjacent control plots received ambient throughfall and the same natural temperature regime. In 2003, throughfall was not excluded to assess the response of SR under natural weather conditions after two prolonged summer droughts. Throughfall exclusion significantly decreased mean SR rate by 53 mg C m^?2 h^?1 over 84 days in 2001, and by 68 mg C m^?2 h^?1 over 126 days in 2002, representing 10–30% of annual SR in this forest and 35–75% of annual net ecosystem exchange (NEE) of C. The differences in SR were best explained by differences in gravimetric water content in the Oi horizon (r²=0.69) and the Oe/Oa horizon (r²=0.60). Volumetric water content of the A horizon was not significantly affected by throughfall exclusion. The radiocarbon signature of soil CO₂ efflux and of CO₂ respired during incubations of O horizon, A horizon and living roots allowed partitioning of SR into contributions from young C substrate (including RR) and from decomposition of older SOM. RR (root respiration and microbial respiration of young substrates in the rhizosphere) made up 43–71% of the total C respired in the control plots and 41–80% in the exclusion plots, and tended to increase with drought. An exception to this trend was an interesting increase in CO₂ efflux of radiocarbon‐rich substrates during a period of abundant growth of mushrooms. Our results suggest that prolonged summer droughts decrease primarily heterotrophic respiration in the O horizon, which could cause increases in the storage of soil organic carbon in this forest. However, the C stored during two summers of simulated drought was only partly released as increased respiration during the following summer of natural throughfall. We do not know if this soil C sink during drought is transient or long lasting. In any case, differential decomposition of the O horizon caused by interannual variation of precipitation probably contributes significantly to observed interannual variation of NEE in temperate forests.     

Effects of soil phosphorus availability,temperature and moisture on soil respiration in Eucalyptus pauciflora forest

Rates of soil respiration (CO₂ efflux) were measured for a year in a mature Eucalyptus pauciflora forest in unfertilized and phosphorus-fertilized plots. Soil CO₂ efflux showed a distinct seasonal trend, and average daily rates ranged from 124 to 574 mg CO₂ m^–2 hr^–1. Temperature and moisture are the main variables that cause variation in soil CO₂ efflux; hence their effects were investigated over a year so as to then differentiate the treatment effect of phosphorus (P) nutrition.Soil temperature had the greatest effect on CO₂ efflux and exhibited a highly significant logarithmic relationship (r² = 0.81). Periods of low soil and litter moisture occurred during summer when temperatures were greater than 10 °C, and this resulted in depression of soil CO₂ efflux. During winter, when temperatures were less than 10 °C, soil and litter moisture were consistently high and thus their variation had little effect on soil CO₂ efflux. A multiple regression model including soil temperature, and soil and litter moisture accounted for 97% of the variance in rates of CO₂ efflux, and thus can be used to predict soil CO₂ efflux at this site with high accuracy. Total annual efflux of carbon from soil was estimated to be 7.11 t C ha^–1 yr^–1. The model was used to predict changes in this annual flux if temperature and moisture conditions were altered. The extent to which coefficients of the model differ among sites and forest types requires testing.Increased soil P availability resulted in a large increase in stem growth of trees but a reduction in the rate of soil CO₂ efflux by approximately 8%. This reduction is suggested to be due to lower root activity resulting from reduced allocation of assimilate belowground. Root activity changed when P was added to microsites within plots, and via the whole tree root system at the plot level. These relationships of belowground carbon fluxes with temperature, moisture and nutrient availability provide essential information for understanding and predicting potential changes in forest ecosystems in response to land use management or climate change.     

Winter CO2 flux from soil and snow surfaces in a cool-temperate deciduous forest, Japan

We measured diurnal and wintertime changes in CO₂ fluxes from soil and snow surfaces in a Japanese cool-temperate Quercus/Betula forest between December 1994 and May 1995. To evaluate the relationship between these winter fluxes and temperature, flux measurements were made with the open-flow infrared gas analyzer (IRGA) method rather than with the more commonly used closed chamber method or the snow CO₂ profile method. The open-flow IRGA method proved to be more successful in measurements of winter CO₂ fluxes than the two standard methods. Despite colder air temperatures, soil temperature profiles were greater than 0°C because of the thermal insulation effect of deep snowpack. This reveals that soil temperature is satisfactory for microbial respiration throughout the winter. Unfrozen soils under the snowpack showed neither diurnal nor wintertime trends in CO₂ fluxes or in soil surface temperature, although there was a daily snow surface CO₂ flux of 0.18–0.32 g m^–2. By combining this with other reference data, Japanese cool-temperate forest soils in snowy regions can be estimated to emit < 100 g m^–2 carbon over an entire winter, and this value accounts for < 15% of the annual emission. In the present study, when data for all winter fluxes were taken together, fluxes were most highly correlated with deep soil temperatures rather than the soil surface temperature. Such a high correlation can be attributed to the relatively increased respiration of the deep soil where the temperature was higher than the soil surface temperature. Thus, deeper soil temperature is a better predictor of winter CO₂ fluxes in cold and snowy ecosystems.     

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.     

Seasonal patterns in soil surface CO<Subscript>2</Subscript> flux under snow cover in 50 and 300 year old subalpine forests

Soil CO₂ flux can contribute as much as 60–80% of total ecosystem respiration in forests. Although considerable research has focused on quantifying this flux during the growing season, comparatively little effort has focused on non-growing season fluxes. We measured soil CO₂ efflux through snow in 50 and ~300 year old subalpine forest stands near Fraser CO. Our objectives were to quantify seasonal patterns in wintertime soil CO₂ flux; determine if differences in soil CO₂ flux between the two forest ages during the growing season persist during winter; and to quantify the sample size necessary to discern treatment differences. Soil CO₂ flux during the 2002–2003 and 2003–2004 snow season averaged 0.31 and 0.35 μmols m⁻² s⁻¹ for the young and old forests respectively; similar to the relative difference observed during summer. There was a significant seasonal pattern of soil CO₂ flux during the winter with fluxes averaging 0.22 μmols m⁻² s⁻¹ in December and January and increasing to an average of 0.61 μmols m⁻² s⁻¹ in May. Within-plot variability for measurements used in calculating flux was low. The coefficients of variation (CV) for CO₂ concentration, snowpack density, and snow depth were 17, 8 and 14%, respectively, yielding a CV for flux measurements within-plot of 29%. A within plot CV of 29% requires 8 sub-samples per plot to estimate the mean flux with a standard error of ±10% of the mean. Variability in CO₂ flux estimates among plots (size = 400 m²) was similar to that within plot and was also low (CV = ~28%). With a CV of 28% among plots, ten plots per treatment would have a 50% probability of detecting a 25% difference in treatment means for α = 0.05.     

Effects of soil frost on soil respiration and its radiocarbon signature in a Norway spruce forest soil

Apart from a general increase of mean annual air temperature, climate models predict a regional increase of the frequency and intensity of soil frost with possibly strong effects on C cycling of soils. In this study, we induced mild soil frost (up to −5 °C in a depth of 5 cm below surface) in a Norway spruce forest soil by removing the natural snow cover in the winter of 2005/2006. Soil frost lasted from January to April 2006 and was detected down to 15 cm depth. Soil frost effectively reduced soil respiration in the snow removal plots in comparison to undisturbed control plots. On an annual basis 6.2 t C ha⁻¹ a⁻¹ were emitted in the control plots compared with 5.1 t C ha⁻¹ a⁻¹ in the snow removal plots. Only 14% of this difference was attributed to reduced soil respiration during the soil frost period itself, whereas 63% of this difference originated from differences during the summer of 2006. Radiocarbon (Δ¹⁴C) signature of CO₂ revealed a considerable reduction of heterotrophic respiration on the snow removal plots, only partly compensated for by a slight increase of rhizosphere respiration. Similar CO₂ concentrations in the uppermost mineral horizons of both treatments indicate that differences between the treatments originated from the organic horizons. Extremely low water contents between June and October of 2006 may have inhibited the recovery of the heterotrophic organisms from the frost period, thereby enhancing the differences between the control and snow removal plots. We conclude that soil frost triggered a change in the composition of the microbial community, leading to an increased sensitivity of heterotrophic respiration to summer drought. A CO₂ pulse during thawing, such as described for arable soils several times throughout the literature, with the potential to partly compensate for reduced soil respiration during soil frost, appears to be lacking for this soil. Our results from this experiment indicate that soil frost reduces C emission from forest soils, whereas mild winters may enhance C losses from forest soils.     

Soil carbon balance in a clonal Eucalyptus plantation in Congo: effects of logging on carbon inputs and soil CO2 efflux

Soil CO₂ efflux was measured in clear‐cut and intact plots in order to quantify the impact of harvest on soil respiration in an intensively managed Eucalyptus plantation, and to evaluate the increase in heterotrophic component of soil respiration because of the decomposition of harvest residues. Soil CO₂ effluxes showed a pronounced seasonal trend, which was well related to the pattern of precipitation and soil water content and were always significantly lower in the clear‐cut plots than in the intact plots. On an annual basis, soil respiration represented 1.57 and 0.91 kg_C m^?2 yr^?1 in intact and clear‐cut plots, respectively. During the first year following harvest, residues have lost 0.79 kg_C m^?2 yr^?1. Our estimate of heterotrophic respiration was calculated assuming that it was similar to soil respiration in the clear‐cut area except that the decomposition of residues did not occur, and it was further corrected for differences in soil water content between intact and clear‐cut plots and for the cessation of leaf and fine root turnover in clear cut. Heterotrophic respiration in clear‐cut plots was estimated at 1.18 kg_C m^?2 yr^?1 whereas it was only 0.65 kg_C m^?2 yr^?1 in intact plots (41% of soil respiration). Assumptions and uncertainties with these calculations are discussed.     

Microbial Respiration in Arctic Upland and Peat Soils as a Source of Atmospheric Carbon Dioxide

Knowledge on soil microbial respiration (SMR) rates and thus soil-related CO₂ losses from Arctic soils is vital because of the crucial importance of this ecosystem within the global carbon (C) cycle and climate system. Here, we measured SMR from various habitats during the growing season in Russian subarctic tundra by applying two different approaches: ¹⁴C partitioning approach and root trenching. The variable habitats encompassed peat and mineral soils, bare and vegetated surfaces and included both dry and moist ones. The field experiment was complemented by laboratory studies to measure bioavailability of soil carbon and identify sources of CO₂. Differences in bioavailability of soils, measured in the laboratory as basal soil respiration rates, were generally greater than inter-site differences in SMR rates measured in situ, suggesting secondary constraints at field conditions, such as soil C content. There was a tendency towards lower SMR in vegetated peat plateaus compared to upland mineral tundra (on average 137 vs. 185 g CO₂ m^?2 growing season^?1, respectively), but no significant differences were found. Surprisingly, the bare surfaces (peat circles) with 3500-year-old C at the surface exhibited about the largest SMR among all sites as shown by both methods. This was related to the general development of peat plateaus in the region, and uplifting of deeper peat with high C content to the surface during the genesis of peat circles. This observation is particularly relevant for decomposition of deeper peat in vegetated peat plateaus, where soil material similar to the bare surfaces can be found. The data indicate that the large stocks of C stored in permafrost peatlands are principally available for decomposition despite old age.     

Microbial biomass and nitrogen cycling responses to fertilization and litter removal in young northern hardwood forests

The influence of site fertility on soil microbial biomass and activity is not well understood but is likely to be complex because of interactions with plant responses to nutrient availability. We examined the effects of long-term (8 yr) fertilization and litter removal on forest floor microbial biomass and N and C transformations to test the hypothesis that higher soil resource availability stimulates microbial activity. Microbial biomass and respiration decreased by 20–30 % in response to fertilization. Microbial C averaged 3.8 mg C/g soil in fertilized, 5.8 mg C/g in control, and 5.5 mg C/g in litter removal plots. Microbial respiration was 200 µg CO₂-C g^–1 d^–1 in fertilized plots, compared to 270 µg CO₂-C g^–1 d^–1 in controls. Gross N mineralization and N immobilization did not differ among treatments, despite higher litter nutrient concentrations in fertilized plots and the removal of substantial quantities of C and N in litter removal plots. Net N mineralization was significantly reduced by fertilization. Gross nitrification and NO₃ ^– immobilization both were increased by fertilization. Nitrate thus became a more important part of microbial N cycling in fertilized plots even though NH₄ ⁺ availability was not stimulated by fertilization.Soil microorganisms did not mineralize more C or N in response to fertilization and higher litter quality; instead, results suggest a difference in the physiological status of microbial biomass in fertilized plots that influenced N transformations. Respiration quotients (qCO₂, respiration per unit biomass) were higher in fertilized plots (56 µg CO₂-C mg C^–1 d^–1) than control (48 µg CO₂-C mg C^–1 d ^–1) or litter removal (45 µg CO₂-C mg C^–1 d^–1), corresponding to higher microbial growth efficiency, higher proportions of gross mineralization immobilized, and lower net N mineralization in fertilized plots. While microbial biomass is an important labile nutrient pool, patterns of microbial growth and turnover were distinct from this pool and were more important to microbial function in nitrogen cycling.