Estimation of autotrophic and heterotrophic components of soil respiration by trenching is sensitive to corrections for root decomposition and changes in soil water content

This study aims to assess the effects of corrections for disturbances such as an increased amount of dead roots and an increase in volumetric soil water content on the calculation of soil CO₂ efflux partitioning. Soil CO₂ efflux, soil temperature and superficial soil water content were monitored in two young beech sites (H1 and H2) during a trenching experiment. Trenching induced a significant input of dead root mass that participated in soil CO₂ efflux and reduced the soil dissolved organic carbon content, while it increased superficial soil water content within the trenched plot. Annual soil CO₂ efflux in control plots was 528 g C m⁻² year⁻¹ at H1 and 527 g C m⁻² year⁻¹ at H2. The annual soil CO₂ efflux in trenched plots was 353 g C m⁻² year⁻¹ at H1 and 425 g C m⁻² year⁻¹ at H2. By taking into account annual CO₂ efflux from decaying trenched roots, the autotrophic contribution to total soil CO₂ efflux reached 69% at H1 and 54% at H2. The partitioning calculation was highly sensitive to the initial root mass estimated within the trenched plots. Uncertainties in the remaining root mass, the fraction of root C that is incorporated into soil organic matter during root decomposition, and the root decomposition rate constant had a limited impact on the partitioning calculation. Corrections for differences in superficial soil water content had a significant impact on annual respired CO₂ despite a limited effect on partitioning.     

Process-level controls on CO<Subscript>2</Subscript> fluxes from a seasonally snow-covered subalpine meadow soil,Niwot Ridge,Colorado

Fluxes of CO₂ during the snow-covered season contribute to annual carbon budgets, but our understanding of the mechanisms controlling the seasonal pattern and magnitude of carbon emissions in seasonally snow-covered areas is still developing. In a subalpine meadow on Niwot Ridge, Colorado, soil CO₂ fluxes were quantified with the gradient method through the snowpack in winter 2006 and 2007 and with chamber measurements during summer 2007. The CO₂ fluxes of 0.71 μmol m⁻² s⁻¹ in 2006 and 0.86 μmol m⁻² s⁻¹ in 2007 are among the highest reported for snow-covered ecosystems in the literature. These fluxes resulted in 156 and 189 g C m⁻² emitted over the winter, ~30% of the annual soil CO₂ efflux at this site. In general, the CO₂ flux increased during the winter as soil moisture increased. A conceptual model was developed with distinct snow cover zones to describe this as well as the three other reported temporal patterns in CO₂ flux from seasonally snow-covered soils. As snow depth and duration increase, the factor controlling the CO₂ flux shifts from freeze–thaw cycles (zone I) to soil temperature (zone II) to soil moisture (zone III) to carbon availability (zone IV). The temporal pattern in CO₂ flux in each zone changes from periodic pulses of CO₂ during thaw events (zone I), to CO₂ fluxes reaching a minimum when soil temperatures are lowest in mid-winter (zone II), to CO₂ fluxes increasing gradually as soil moisture increases (zone III), to CO₂ fluxes decreasing as available carbon is consumed. This model predicts that interannual variability in snow cover or directional shifts in climate may result in dramatically different seasonal patterns of CO₂ flux from seasonally snow-covered soils.     

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.     

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     

Seasonal dynamics of soil CO<Subscript>2</Subscript> efflux in a conventional tilled wheat field of the Loess Plateau,China

An automated chamber system was employed to measure the soil CO₂ efflux (SCE) in situ for 2 years in a conventional wheat field of the Loess Plateau, China under semi-arid conditions. The annual mean SCE values were 2.44 ± 2.52 μmol m⁻² s⁻¹ in 2006 and 2.37 ± 2.33 μmol m⁻² s⁻¹ in 2007. Distinct seasonality in the SCE was observed, with significant differences occurring among four periods divided by harvesting, tillage and sowing. In the period from tillage to sowing, the mean SCE values were 2.82 and 2.69 times the annual mean values in 2006 and 2007, respectively, and SCE accounted for 39% and 48% of the annual total within 14% and 18% of the days of the years. Although there were significant exponential correlations between the SCE and soil temperature, and significant linear correlations between the SCE and soil moisture for measurements conducted in the periods before tillage and after sowing each year, the SCE from tillage to sowing was significantly beyond the correlation curves. These findings indicated that seasonal variation in the SCE in a conventional field was controlled not only by soil temperature and moisture, but also by tillage practice. The confounding effects of climate and practice on SCE should be considered when developing ways to mitigate soil carbon loss in conventional cropland in semi-arid regions.     

Soil carbon storage, litterfall and CO2 efflux in fertilized and unfertilized larch (Larix leptolepis) plantations

This study evaluated the effects of forest fertilization on the forest carbon (C) dynamics in a 36-year-old larch (Larix leptolepis) plantation in Korea. Above- and below-ground C storage, litterfall, root decomposition and soil CO₂ efflux rates after fertilization were measured for 2 years. Fertilizers were applied to the forest floor at rates of 112 kg N ha⁻¹ year⁻¹, 75 kg P ha⁻¹ year⁻¹ and 37 kg K ha⁻¹ year⁻¹ for 2 years (May 2002, 2003). There was no significant difference in the above-ground C storage between fertilized (41.20 Mg C ha⁻¹) and unfertilized (42.25 Mg C ha⁻¹) plots, and the C increment was similar between the fertilized (1.65 Mg C ha⁻¹ year⁻¹) and unfertilized (1.52 Mg C ha⁻¹ year⁻¹) plots. There was no significant difference in the soil C storage between the fertilized and unfertilized plots at each soil depth (0–15, 15–30 and 30–50 cm). The organic C inputs due to litterfall ranged from 1.57 Mg C ha⁻¹ year⁻¹ for fertilized to 1.68 Mg C ha⁻¹ year⁻¹ for unfertilized plots. There was no significant difference in the needle litter decomposition rates between the fertilized and unfertilized plots, while the decomposition of roots with 1–2 mm diameters increased significantly with the fertilization relative to the unfertilized plots. The mean annual soil CO₂ efflux rates for the 2 years were similar between the fertilized (0.38 g CO₂ m⁻² h⁻¹) and unfertilized (0.40 g CO₂ m⁻² h⁻¹) plots, which corresponded with the similar fluctuation in the organic carbon (litterfall, needle and root decomposition) and soil environmental parameters (soil temperature and soil water content). These results indicate that little effect on the C dynamics of the larch plantation could be attributed to the 2-year short-term fertilization trials and/or the soil fertility in the mature coniferous plantation used in this study.     

Changes in soil carbon flux and carbon stock over a rotation of poplar plantations in northwest China

Forest soil is a major component of terrestrial ecosystems for carbon sequestration and plays an important role in the global carbon cycle. Soil carbon flux and soil carbon pools were investigated in a poplar plantation chronosequence over a rotation in northwest China. Based on continuous field observation in 2007, the results showed that mean soil CO₂ efflux rate was 5.54, 4.81, and 3.93 μmol CO₂ m⁻² s⁻¹ for stands of 2-, 8-, and 15-year-old, respectively, during the growing season. Significant differences in soil respiration of three age classes were mainly because soil temperature, carbon allocation, and fine root growth changed greatly with stand age. Multiple regression analysis suggested that soil temperature and fine root biomass in the upper layer could explain 78–85% of the variation in soil respiration. Mineral soil C stock at 0–40 cm depth was 55.77, 55.09, and 58.14 t ha⁻¹ in the 2-, 8-, and 15-year-old stands, respectively. The average rate of soil C sequestration was 0.13 t ha⁻¹ year⁻¹ following afforestation on former crop lands. Although the plantations had similar management practices and soil types since their establishment, many biotic and abiotic factors such as root biomass and turnover rate, soil condition of the plantations had undergone marked changes at different development stages, which could result in the remarkable differences in soil carbon flux and storage over a rotation. Our results highlight the importance of the development stage within a rotation of poplar plantation in assessment of soil carbon budget.     

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.     

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.     

Annual sediment respiration in estuarine sandy intertidal flats in the Seto Inland Sea, Japan

To quantify organic matter mineralization at estuarine intertidal flats, we measured in situ sediment respiration rates using an infrared gas analyzer in estuarine sandy intertidal flats located in the northwestern Seto Inland Sea, Japan. In situ sediment respiration rates showed spatial and seasonal variations, and the mean of the rates is 38.8 mg CO₂-C m⁻² h⁻¹ in summer. In situ sediment respiration rates changed significantly with sediment temperature at the study sites (r ² = 0.70, p < 0.05), although we did not detect any significant correlations between the rates and sediment characteristics. We prepared a model for estimating the annual sediment respiration based on the in situ sediment respiration rates and their temperature coefficient (Q ₁₀ = 1.8). The annual sediment respiration was estimated to be 92 g CO₂-C m⁻² year⁻¹. The total amount of organic carbon mineralization for the entire estuarine intertidal flats through sediment respiration (43 t C year⁻¹) is equivalent to approximately 25% of the annual organic carbon load supplied from the river basin of the estuary.     

Activity of surface-casting earthworms in a calcareous grassland under elevated atmospheric CO2

Earthworms make up the dominant fraction of the biomass of soil animals in most temperate grasslands and have important effects on the structure and function of these ecosystems. We hypothesized that the effects of elevated atmospheric CO₂ on soil moisture and plant biomass production would increase earthworm activity, expressed as surface cast production. Using a screen-aided CO₂ control facility (open top and open bottom rings), eight 1.2-m² grassland plots in Switzerland have been maintained since March 1994 at ambient CO₂ concentrations (350 μl CO₂ l⁻¹) and eight at elevated CO₂ (610 μl CO₂ l⁻¹). Cumulative earthworm surface cast production measured 40 times over 1 year (April 1995–April 1996) in plots treated with elevated CO₂ (2206 g dry mass m⁻² year⁻¹) was 35% greater (P<0.05) than that measured in plant communities maintained at ambient CO₂ (1633 g dry mass m⁻² year⁻¹). At these rates of surface cast production, worms would require about 100 years to egest the equivalent of the amount of soil now found in the Ah horizon (top 15 cm) under current ambient CO₂ concentrations, and 75 years under elevated CO₂. Elevated atmospheric CO₂ had no influence on the seasonality of earthworm activity. Cumulative surface cast production measured over the 7-week period immediately following the 6-week summer dry period in 1995 (no surface casting) was positively correlated (P<0.05) with the mean soil water content calculated over this dry and subsequent wetter period, when viewed across all treatments. However, no correlations were observed with soil temperature or with annual aboveground plant biomass productivity. No CO₂-related differences were observed in total nitrogen (N_tot) and organic carbon (C_org) concentration of surface casts, although concentrations of both elements varied seasonally. The CO₂-induced increase in earthworm surface casting activity corresponded to a 30% increase of the amount of N_tot (8.9 mg N m⁻² vs. 6.9 mg N m⁻²) and C_org (126 mg C m⁻² vs. 94 mg C m⁻²) egested by the worms in one year. Thus, our results demonstrate an important indirect stimulatory effect of elevated atmospheric CO₂ on earthworm activity which may have profound effects on ecosystem function and plant community structure in the long term. Received: 3 November 1996 / Accepted: 11 January 1997     

Six-year Stable Annual Uptake of Carbon Dioxide in Intensively Managed Humid Temperate Grassland

Variability and future alterations in regional and global climate patterns may exert a strong control on the carbon dioxide (CO₂) exchange of grassland ecosystems. We used 6 years of eddy-covariance measurements to evaluate the impacts of seasonal and inter-annual variations in environmental conditions on the net ecosystem CO₂ exchange (NEE), gross ecosystem production (GEP), and ecosystem respiration (ER) of an intensively managed grassland in the humid temperate climate of southern Ireland. In all the years of the study period, considerable uptake of atmospheric CO₂ occurred in this grassland with a narrow range in the annual NEE from −245 to −284 g C m⁻² y⁻¹, with the exception of 2008 in which the NEE reached −352 g C m⁻² y⁻¹. None of the measured environmental variables (air temperature (Ta), soil moisture, photosynthetically active radiation, vapor pressure deficit (VPD), precipitation (PPT), and so on) correlated with NEE on a seasonal or annual scale because of the equal responses from the component fluxes GEP and ER to variances in these variables. Pronounced reduction of summer PPT in two out of the six studied years correlated with decreases in both GEP and ER, but not with NEE. Thus, the stable annual NEE was primarily achieved through a strong coupling of ER and GEP on seasonal and annual scales. Limited inter-annual variations in Ta (±0.5°C) and generally sufficient soil moisture availability may have further favored a stable annual NEE. Monthly ecosystem carbon use efficiency (CUE; as the ratio of NEE:GEP) during the main growing season (April 1–September 30) was negatively correlated with temperature and VPD, but positively correlated with soil moisture, whereas the annual CUE correlated negatively with annual NEE. Thus, although drier and warmer summers may mildly reduce the uptake potential, the annual uptake of atmospheric CO₂, in this intensively managed grassland, may be expected to continue even under predicted future climatic changes in the humid temperate climate region.     

Response of soil respiration to simulated N deposition in a disturbed and a rehabilitated tropical forest in southern China

Responses of soil respiration (CO₂ emission) to simulated N deposition were studied in a disturbed (reforested forest with previous understory and litter harvesting) and a rehabilitated (reforested forest with no understory and litter harvesting) tropical forest in southern China from October 2005 to September 2006. The objectives of the study were to test the following hypotheses: (1) soil respiration is higher in rehabilitated forest than in disturbed forest; (2) soil respiration in both rehabilitated and disturbed tropical forests is stimulated by N additions; and (3) soil respiration is more sensitive to N addition in disturbed forest than in rehabilitated forest due to relatively low soil nutrient status in the former, resulting from different previous human disturbance. Static chamber and gas chromatography techniques were employed to quantify the soil respiration, following different N treatments (Control, no N addition; Low-N, 5 g N m⁻² year⁻¹; Medium-N, 10 g N m⁻² year⁻¹), which had been applied continuously for 26 months before the respiration measurement. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates observed in the hot and wet growing season (April–September) and the lowest rates in winter (December–February) in both rehabilitated and disturbed forests. Soil respiration rates exhibited significant positive exponential relationship with soil temperature and significant positive linear relationship with soil moisture. Soil respiration was also significantly higher in the rehabilitated forest than in the disturbed forest. Annual mean soil respiration rate in the rehabilitated forest was 20% lower in low-N plots (71 ± 4 mg CO₂-C m⁻² h⁻¹) and 10% lower in medium-N plots (80 ± 4 mg CO₂-C m⁻² h⁻¹) than in the control plots (89 ± 5 mg CO₂-C m⁻² h⁻¹), and the differences between the control and low-N or medium-N treatments were statistically significant. In disturbed forest, annual mean soil respiration rate was 5% lower in low-N plots (63 ± 3 mg CO₂-C m⁻² h⁻¹) and 8% lower in medium-N plots (61 ± 3 mg CO₂-C m⁻² h⁻¹) than in the control plots (66 ± 4 mg CO₂-C m⁻² h⁻¹), but the differences among treatments were not significant. The depressed effects of experimental N deposition occurred mostly in the hot and wet growing season. Our results suggest that response of soil respiration to elevated N deposition in the reforested tropical forests may vary depending on the status of human disturbance. Responsible Editor: Hans Lambers.     

Soil‐surface CO2 efflux and its spatial and temporal variations in a young ponderosa pine plantation in northern California

Soil‐surface CO₂ efflux and its spatial and temporal variations were examined in an 8‐y‐old ponderosa pine plantation in the Sierra Nevada Mountains in California from June 1998 to August 1999. Continuous measurements of soil CO₂ efflux, soil temperatures and moisture were conducted on two 20 × 20 m sampling plots. Microbial biomass, fine root biomass, and the physical and chemical properties of the soil were also measured at each of the 18 sampling locations on the plots. It was found that the mean soil CO₂ efflux in the plantation was 4.43 µmol m^?2 s^?1 in the growing season and 3.12 µmol m^?2 s^?1 in the nongrowing season. These values are in the upper part of the range of published soil‐surface CO₂ efflux data. The annual maximum and minimum CO₂ efflux were 5.87 and 1.67 µmol m^?2 s^?1, respectively, with the maximum occurring between the end of May and early June and the minimum in December. The diurnal fluctuation of CO₂ efflux was relatively small (< 20%) with the minimum appearing around 09.00 hours and the maximum around 14.00 hours. Using daytime measurements of soil CO₂ efflux tends to overestimate the daily mean soil CO₂ efflux by 4–6%. The measurements taken between 09.00 and 11.00 hours (local time) seem to better represent the daily mean with a reduced sampling error of 0.9–1.5%. The spatial variation of soil CO₂ efflux among the 18 sampling points was high, with a coefficient of variation of approximately 30%. Most (84%) of the spatial variation was explained by fine root biomass, microbial biomass, and soil physical and chemical properties. Although soil temperature and moisture explained most of the temporal variations (76–95%) of soil CO₂ efflux, the two variables together explained less than 34% of the spatial variation. Microbial biomass, fine root biomass, soil nitrogen content, organic matter content, and magnesium content were significantly and positively correlated with soil CO₂ efflux, whereas bulk density and pH value were negatively correlated with CO₂ efflux. The relationship between soil CO₂ efflux and soil temperature was significantly controlled by soil moisture with a Q₁₀ value of 1.4 when soil moisture was <14% and 1.8 when soil moisture was >14%. Understanding the spatial and temporal variations is essential to accurately assessment of carbon budget at whole ecosystem and landscape scales. Thus, this study bears important implications for the study of large‐scale ecosystem dynamics, particularly in response to climatic variations and management regimes.     

Spatial Patterns of Soil Surface C Flux in Experimental Canopy Gaps

To explore within-gap spatial patterns of soil surface CO₂ flux, we measured instantaneous soil surface CO₂ flux, soil surface temperature, and soil moisture in north–south transects across canopy gaps and in adjacent contiguous forest from April to November 2010 in a second-growth northern hardwood forest in Wisconsin, USA. Throughout the growing season, soil surface CO₂ flux was higher in the northern 1/3 and northern edge of gaps compared to the central and southern portions. These patterns were driven primarily by within-gap variation in soil temperature, which was itself driven by within-gap patterns of insolation. Most locations in the northern 1/3 and northern edge of gaps had significantly higher modeled total growing season C flux (mean 725 g C m⁻²) compared to the contiguous forest (mean 706 g C m⁻²), whereas C flux in the central and southern portions of gaps (mean 555 g C m⁻²) was significantly lower than both the contiguous forest and the northern portions of gaps.     

Soil respiration of forest ecosystems in Japan and global implications

Within terrestrial ecosystems, soil respiration is one of the largest carbon flux components. We discuss the factors controlling soil respiration, while focusing on research conducted at the Takayama Experimental Site. Soil respiration was affected by soil temperature, soil moisture, rainfall events, typhoons, and root respiration. We consider the temporal and spatial variability of soil respiration at the Takayama Experimental Site and review the variability of annual soil respiration in Japanese forests. In the 26 compiled studies, the values of annual soil respiration ranged from 203 to 1,290 g C m⁻² year⁻¹, with a mean value of 669 g C m⁻² year⁻¹ (SD=264, CV=40). We note the need for more studies and data synthesis for the accurate prediction of soil respiration and soil carbon dynamics in Japanese forests. Finally, several methods for measuring soil respiration rates are compared and the implications of soil respiration rates for global climate change are discussed.     

Whole season net community production in the Weddell Sea

Depletions of total CO₂, nitrate, phosphate and silicate in the surface layer were calculated for cruise ANT XXII/3 with FS Polarstern in March 2005 for estimating the annual net community production. East-west across the Weddell Sea the variation of all depletions is similar, but this holds to a lesser extent for silicate. Depletions in March 2005 are 2–3 times larger than those in January 1993 for the same transect. Very low N:P and C:P depletion ratios seem to point to dominance of diatoms, in the central Weddell Sea more than in the margin. Estimates of annual net community productions are about 1.8 and 3.5 mol C m⁻² year⁻¹ for the interior Weddell Sea and a near-margin region, respectively. The region does not comply with the classical view of a marginal ice zone with high productivity. Net community production is similar to annual export production, implying that remineralization in the ensuing winter be minor.     

CO2 and N-fertilization effects on fine-root length, production, and mortality: a 4-year ponderosa pine study

We conducted a 4-year study of juvenile Pinus ponderosa fine root (≤2 mm) responses to atmospheric CO₂ and N-fertilization. Seedlings were grown in open-top chambers at three CO₂ levels (ambient, ambient+175 μmol/mol, ambient+350 μmol/mol) and three N-fertilization levels (0, 10, 20 g m⁻² year⁻¹). Length and width of individual roots were measured from minirhizotron video images bimonthly over 4 years starting when the seedlings were 1.5 years old. Neither CO₂ nor N-fertilization treatments affected the seasonal patterns of root production or mortality. Yearly values of fine-root length standing crop (m m⁻²), production (m m⁻² year⁻¹), and mortality (m m⁻² year⁻¹) were consistently higher in elevated CO₂ treatments throughout the study, except for mortality in the first year; however, the only statistically significant CO₂ effects were in the fine-root length standing crop (m m⁻²) in the second and third years, and production and mortality (m m⁻² year⁻¹) in the third year. Higher mortality (m m⁻² year⁻¹) in elevated CO₂ was due to greater standing crop rather than shorter life span, as fine roots lived longer in elevated CO₂. No significant N effects were noted for annual cumulative production, cumulative mortality, or mean standing crop. N availability did not significantly affect responses of fine-root standing crop, production, or mortality to elevated CO₂. Multi-year studies at all life stages of trees are important to characterize belowground responses to factors such as atmospheric CO₂ and N-fertilization. This study showed the potential for juvenile ponderosa pine to increase fine-root C pools and C fluxes through root mortality in response to elevated CO₂.     

Fluxes of greenhouse gases from Andosols under coffee in monoculture or shaded by <Emphasis Type="Italic">Inga densiflora</Emphasis> in Costa Rica

The objective of this study was to evaluate the effect of N fertilization and the presence of N₂ fixing leguminous trees on soil fluxes of greenhouse gases. For a one year period, we measured soil fluxes of nitrous oxide (N₂O), carbon dioxide (CO₂) and methane (CH₄), related soil parameters (temperature, water-filled pore space, mineral nitrogen content, N mineralization potential) and litterfall in two highly fertilized (250 kg N ha⁻¹ year⁻¹) coffee cultivation: a monoculture (CM) and a culture shaded by the N₂ fixing legume species Inga densiflora (CIn). Nitrogen fertilizer addition significantly influenced N₂O emissions with 84% of the annual N₂O emitted during the post fertilization periods, and temporarily increased soil respiration and decreased CH₄ uptakes. The higher annual N₂O emissions from the shaded plantation (5.8 ± 0.3 kg N ha⁻¹ year⁻¹) when compared to that from the monoculture (4.3 ± 0.1 kg N ha⁻¹ year⁻¹) was related to the higher N input through litterfall (246 ± 16 kg N ha⁻¹ year⁻¹) and higher potential soil N mineralization rate (3.7 ± 0.2 mg N kg⁻¹ d.w. d⁻¹) in the shaded cultivation when compared to the monoculture (153 ± 6.8 kg N ha⁻¹ year⁻¹ and 2.2 ± 0.2 mg N kg⁻¹ d.w. d⁻¹). This confirms that the presence of N₂ fixing shade trees can increase N₂O emissions. Annual CO₂ and CH₄ fluxes of both systems were similar (8.4 ± 2.6 and 7.5 ± 2.3 t C-CO₂ ha⁻¹ year⁻¹, −1.1 ± 1.5 and 3.3 ± 1.1 kg C-CH₄ ha⁻¹ year⁻¹, respectively in the CIn and CM plantations) but, unexpectedly increased during the dry season.     

Spatial and temporal patterns of soil respiration over the Japanese Archipelago: a model intercomparison study

We used terrestrial ecosystem models to estimate spatial and temporal variability in and uncertainty of estimated soil carbon dioxide (CO₂) efflux, or soil respiration, over the Japanese Archipelago. We compared five carbon-cycle models to assess inter-model variability: Biome-BGC, CASA, LPJ, SEIB, and VISIT. These models differ in approaches to soil carbon dynamics, root respiration estimation, and relationships between decomposition and environmental factors. We simulated the carbon budget of natural ecosystems over the archipelago for 2001–2006 at 1-day time steps and 2-min (latitude and longitude) spatial resolution. The models were calibrated using measured flux data to accurately represent net ecosystem CO₂ exchange. Each model successfully reproduced seasonal changes and latitudinal gradients in soil respiration. The five-model average of estimated total soil respiration of Japanese ecosystems was 295 Tg C year⁻¹, with individual model estimates ranging from 210 to 396 Tg C year⁻¹ (1 Tg = 10¹² g). The differences between modeled estimates were more evident in summer and in warmer years, implying that they were mainly attributable to differences in modeling the temperature dependence of soil respiration. There was a large discrepancy between models in the estimated contribution of roots to total soil respiration, ranging from 3.9 to 48.4%. Although model calibration reduced the uncertainty of flux estimates, substantial uncertainties still remained in estimates of underground processes from these terrestrial carbon-cycle models.