Belowground Respiration Research Articles

Measuring and modelling the isotopic composition of soil respiration: insights from a grassland tracer experiment

Abstract. The carbon isotopic composition (δ13C) of CO2 efflux (δ13Cefflux) from soil is generally interpreted to represent the actual isotopic composition of the respiratory source (δ13CRs). However, soils contain a large CO2 pool in air-filled pores. This pool receives CO2 from belowground respiration and exchanges CO2 with the atmosphere (via diffusion and advection) and the soil liquid phase (via dissolution). Natural or artificial modification of δ13C of atmospheric CO2 (δ13Catm) or δ13CRs causes isotopic disequilibria in the soil-atmosphere system. Such disequilibria generate divergence of δ13Cefflux from δ13CRs (termed "disequilibrium effect"). Here, we use a soil CO2 transport model and data from a 13CO2/12CO2 tracer experiment to quantify the disequilibrium between δ13Cefflux and δ13CRs in ecosystem respiration. The model accounted for diffusion of CO2 in soil air, advection of soil air, dissolution of CO2 in soil water, and belowground and aboveground respiration of both 12CO2 and 13CO2 isotopologues. The tracer data were obtained in a grassland ecosystem exposed to a δ13Catm of −46.9 ‰ during daytime for 2 weeks. Nighttime δ13Cefflux from the ecosystem was estimated with three independent methods: a laboratory-based cuvette system, in-situ steady-state open chambers, and in-situ closed chambers. Earlier work has shown that the δ13Cefflux measurements of the laboratory-based and steady-state systems were consistent, and likely reflected δ13CRs. Conversely, the δ13Cefflux measured using the closed chamber technique differed from these by −11.2 ‰. Most of this disequilibrium effect (9.5 ‰) was predicted by the CO2 transport model. Isotopic disequilibria in the soil-chamber system were introduced by changing δ13Catm in the chamber headspace at the onset of the measurements. When dissolution was excluded, the simulated disequilibrium effect was only 3.6 ‰. Dissolution delayed the isotopic equilibration between soil CO2 and the atmosphere, as the storage capacity for labelled CO2 in water-filled soil pores was 18 times that of soil air. These mechanisms are potentially relevant for many studies of δ13CRs in soils and ecosystems, including FACE experiments and chamber studies in natural conditions. Isotopic disequilibria in the soil-atmosphere system may result from temporal variation in δ13CRs or diurnal changes in the mole fraction and δ13C of atmospheric CO2. Dissolution effects are most important under alkaline conditions.

Measurement of carbon dioxide fluxes in a free-air carbon dioxide enrichment experiment using the closed flux chamber technique

Abstract Carbon dioxide (CO2) fluxes, composing net ecosystem exchange (NEE), ecosystem respiration (ER), and soil respiration (SR) were measured in a temperate heathland exposed to elevated CO2 by the FACE (free-air carbon enrichment) technique, raising the atmospheric CO2 concentration from c. 380 μmol mol−1 to 510 μmol mol−1. All CO2 fluxes were measured by the static chamber methodology. Although the FACE technique enriches the atmosphere with CO2 to a fixed level, the above ground CO2 concentrations may nevertheless locally vary strongly (from about ambient to ∼1000 μmol mol−1). Deployment of static chambers to FACE experiments should therefore be performed with great care in order to ensure reproducible conditions with respect to chamber headspace CO2 concentration. We demonstrate that that the fluxes measured by closed chambers relate linearly to the initial headspace CO2 concentration. When changing the initial headspace CO2 concentration from 380 to 510 μmol mol−1 the net CO2 assimilation expressed by NEE increased instantaneously 1.51 times in control plots and 1.71 times in FACE plots. By contrast, ER in control plots decreased, being 0.87 times that measured at the low CO2 concentration, and the flux also decreased in FACE plots, to 0.79 times that at low concentration. Similar SR in control plots was decreased 0.94 times in control plots and 0.88 times in FACE plots. We found that a useful method to achieve stable and reproducible chamber headspace and soil CO2 concentration prior to commencement of flux measurements was to turn off the FACE system at least 10 min in advance. Within 10 min a new equilibrium was established between the soil and atmosphere, apparently due to CO2 degassing from the top soil. The observed increase in SR in response to increased CO2 persisted for up to 18 h during which measurements should be performed. Soil CO2 concentrations were increased by up to 500 μmol mol−1 by the FACE treatment, substantially more than the 130 μmol mol−1 enrichment achieved in the atmosphere suggesting that the increased SR flux was caused by increased belowground respiration.

Estimating the component of soil respiration not dependent on living plant roots: Comparison of the indirect y-intercept regression approach and direct bare plot approach

Abstract Distinguishing between root and non-root derived CO 2 efflux is important when determining rates of soil organic matter turnover, however, in practice they remain difficult to separate. Our aim was to evaluate two methods for determining the component of below-ground respiration not dependent on plant roots (i.e., basal soil respiration; R b ). The first approach estimated R b indirectly from the y -intercept of linear regressions between below-ground respiration ( BGR ) and root biomass. The second approach involved direct measurements of soil respiration from bare plots. To compare the contrasting approaches, BGR and crop biomass measurements were collected throughout the year in a range of agricultural systems. We found that both methods were very closely correlated with each other. Values of R b determined by the intercept approach, however, were slightly higher than those determined by measurement of bare plots. Both approaches showed a seasonal trend with estimates of R b lowest in winter months at 0.02 t C ha −1 month −1 for the y -intercept approach and 0.11 t C ha −1 month −1 for the bare plots approach, even after the data had been corrected for the influence of soil temperature. Highest rates of R b occurred from the height to the end of the crop growing season (0.8–1.5 t C ha −1 month −1 ). The annual CO 2 efflux due to R b was estimated to be 8.1 t C ha −1 y −1 from the y -intercept approach and 6.8 t C ha −1 y −1 from bare plots. Annual BGR was 12.1 t C ha −1 y −1 . We conclude that both methods provide similar estimates of R b , however, logistically the bare plots approach is much easier to undertake than the y -intercept approach.

Partitioning pattern of carbon flux in a Kobresia grassland on the Qinghai‐Tibetan Plateau revealed by field 13C pulse‐labeling

AbstractCharacterizing the carbon turnover in terrestrial ecosystems is critical for understanding and predicting carbon dynamics in ecosystems. We used in situ13C pulse labeling to track photosynthetic carbon fluxes from shoot to roots and to soil in a Kobresia humilis meadow on the Qinghai‐Tibet Plateau. We found that about 36.7% of labeled carbon was translocated out from the shoots within the first 24 h after photosynthetic uptake. This is equivalent to 66.1% of total 13C moving out from the shoot during the 32‐day chase period, indicating a rapid and large translocation of newly fixed carbon to belowground parts in these alpine plants. 58.7% of the assimilated 13C was transferred belowground. At the end of the chase phase, 30.9% was retained in living roots, 3.4% in dead roots, 17.2% lost as belowground respiration and 7.3% remained in the soil. In the four carbon pools (i.e., shoots, living roots, dead roots, and soil pools), living roots consistently had the highest proportion of 13C in the plant–soil system during the 32 days. Based on the 13C partitioning pattern and biomass production, we estimate a total of 4930 kg C ha−1 was allocated belowground during the vegetation growth season in this alpine meadow. Of this, roots accumulated 2868 kg C ha−1 and soils accumulated 613 kg C ha−1. This study suggests that carbon storage in belowground carbon pools plays the most important role in carbon cycles in the alpine meadow.

Carbon partitioning in ectomycorrhizal Scots pine seedlings

Abstract The complete carbon budget and the turnover rate of assimilated carbon of ectomycorrhizal Scots pine seedlings growing on natural humus were determined in microcosm conditions. The main aim was to improve understanding of the partitioning of the assimilated carbohydrates within seedlings associated with multiple ectomycorrhizal fungi, and to discover carbon dynamics of the mycorrhizosphere. Plant photosynthesis and below-ground respiration were measured in order to obtain the actual carbon assimilation and respiration rates at the time of measurements. Soon after the photosynthesis and respiration rate measurements the seedlings were pulse-labeled with 14CO2 to follow carbon allocation to different plant, fungal and soil compartments and rhizosphere respiration. Long-term carbon allocation during the entire life span of the seedlings was estimated by measuring plant and mycorrhizal root-tip biomass. The ectomycorrhizal community was analyzed using morphotyping and ITS-sequencing. The 14C label was detected in rhizosphere respiration after 12 h and it peaked between 36 and 60 h after labeling. More than half of the assimilated carbon was allocated below-ground as biomass or respiration and higher mycorrhizal biomass increased the below-ground carbon turnover. The presence of Suillus variegatus affected the plant carbon balance in several ways. When S. variegatus was present, the below-ground respiration increased and this carbon loss was compensated by higher photosynthetic activity. Other fungal species did not differ between each other in their effects on carbon balance. Our findings indicate that some root-associated mycorrhizal fungal symbionts can significantly alter plant CO2 exchange, biomass distribution, and the allocation of recently photosynthesized plant-derived carbon.

Modeling the seasonality of belowground respiration along an elevation gradient in the western Chugach Mountains, Alaska

Belowground respiration is typically the largest flux of carbon from terrestrial ecosystems to the atmosphere, making up >70% of total respiration in boreal forests. Recent work has shown that belowground respiration continues during the snow-covered season in boreal ecosystems, but few studies have made complementary measurements during the snow-free season and it remains uncertain what proportion of annual belowground respiration occurs during winter. Traditional models of the relationship between temperature and respiration assume fixed temperature sensitivity, but it has become clear that the apparent temperature sensitivity of belowground respiration increases as soils approach 0°C. Use of fixed temperature sensitivity to model carbon budgets of northern ecosystems may, therefore, yield misleading results. We measured belowground respiration monthly over 2 years in four ecosystems along an elevation gradient in south-central Alaska. Three models, representing different hypotheses about the relationship between temperature and respiration, were confronted with the data. A logistic model, which allows the temperature sensitivity to vary inversely with temperature, and a variation of the Q10 model, which allows the temperature sensitivity to vary seasonally, performed well at all sites and produced similar estimates of seasonal and annual belowground respiration. The traditional Q10 model performed poorly at all sites and overestimated respiration during the snow-covered season. Annual belowground respiration was generally greater than in ecosystems of interior Alaska, where winters are colder and summers are warmer and drier. Belowground respiration during the snow-covered season made up 6–15% of the annual total—a small, but sensitive, component of annual carbon budgets.

A comparison of tracer methods for quantifying CO2sources in an urban region

The relative contribution of anthropogenic (natural gas and gasoline combustion) and biogenic (aboveground and belowground respiration) CO2sources has previously been quantified with the13C,18O, and14C isotopes of CO2. The unique combination of isotopic signatures of each source allows for top‐down attribution of sources using atmospheric measurements. Other tracers of CO2include carbon monoxide (CO), which is a direct tracer of fossil fuel combustion‐derived CO2as CO and CO2are evolved at specific ratios (RCO/CO2) during combustion depending on fuel source and combustion efficiency. We used the13C,18O, and14C tracers to partition between natural gas, gasoline, and aboveground and belowground respiration during four sampling events in the Los Angeles basin. Additionally, we compared the effectiveness of the independent CO tracer with the14C tracer to distinguish between anthropogenic and biogenic CO2. The three isotope tracer results showed that during the sampling period, fossil fuel combustion was not a dominant source of CO2and aboveground respiration contributed up to approximately 70% of CO2sources during the spring. However, the percent fossil fuel CO2calculated by the CO tracer was not entirely consistent with the fossil fuel CO2calculated by14C, which predicted up to ∼70% of winter CO2from fossil fuel sources. The CO tracer was useful for showing diurnal patterns of CO2sources. However, combustion RCO/CO2values vary significantly, which poses a challenge for accurately identifying CO2sources. Detailed local information about RCO/CO2is required to effectively utilize the CO tracer for quantifying sources of CO2.

Controls on mangrove forest‐atmosphere carbon dioxide exchanges in western Everglades National Park

We report on net ecosystem production (NEP) and key environmental controls on net ecosystem exchange (NEE) of carbon dioxide (CO2) between a mangrove forest and the atmosphere in the coastal Florida Everglades. An eddy covariance system deployed above the canopy was used to determine NEE during January 2004 through August 2005. Maximum daytime NEE ranged from −20 to −25 μmol (CO2) m−2 s−1 between March and May. Respiration (Rd) was highly variable (2.81 ± 2.41 μmol (CO2) m−2 s−1), reaching peak values during the summer wet season. During the winter dry season, forest CO2 assimilation increased with the proportion of diffuse solar irradiance in response to greater radiative transfer in the forest canopy. Surface water salinity and tidal activity were also important controls on NEE. Daily light use efficiency was reduced at high (>34 parts per thousand (ppt)) compared to low (<17 ppt) salinity by 46%. Tidal inundation lowered daytime Rd by ∼0.9 μmol (CO2) m−2 s−1 and nighttime Rd by ∼0.5 μmol (CO2) m−2 s−1. The forest was a sink for atmospheric CO2, with an annual NEP of 1170 ± 127 g C m−2 during 2004. This unusually high NEP was attributed to year‐round productivity and low ecosystem respiration which reached a maximum of only 3 g C m−2 d−1. Tidal export of dissolved inorganic carbon derived from belowground respiration likely lowered the estimates of mangrove forest respiration. These results suggest that carbon balance in mangrove coastal systems will change in response to variable salinity and inundation patterns, possibly resulting from secular sea level rise and climate change.

Snow distribution, soil temperature and late winter CO2 efflux from soils near the Arctic treeline in northwest Alaska

The Arctic treeline is advancing in many areas and changes in carbon (C) cycling are anticipated. Differences in CO2 exchange between adjacent forest and tundra are not well known and contrasting conclusions have been drawn about the effects of forest advance on ecosystem C stocks. Measurements of CO2 exchange in tundra and adjacent forest showed the forest was a greater C sink during the growing season in northern Canada. There is, however, reason to expect that forests lose more C than tundra during the wintertime, as forests may accumulate and retain more snow. Deeper snow insulates the soil and warmer soils should lead to greater rates of belowground respiration and CO2 efflux. In this study, I tested the hypotheses that forests maintain a deeper snowpack, have warmer soils and lose more C during winter than adjacent tundra near the Arctic treeline in northwest Alaska. Measurements of snow depth, soil temperature and CO2 efflux were made at five forest and two treeline sites in late winter of three consecutive years. Snow depth and soil temperature were greater in forest than treeline sites, particularly in years with higher snowfall. There was a close exponential correlation between soil temperature and CO2 efflux across sites and years. The temperature-efflux model was driven using hourly soil temperatures from all the sites to provide a first approximation of the difference in winter C loss between treeline and forest sites. Results showed that greater wintertime C loss from forests could offset greater summertime C gain.

The significance of phloem transport for the speed with which canopy photosynthesis and belowground respiration are linked

Ecosystem respiration is known to vary following changes in canopy photosynthesis. However, the timing of this coupling is not well understood. Here, we summarize the literature on soil and ecosystem respiration where the speed of transfer of photosynthetic sugars from the plant canopy via the phloem to the roots was determined. Estimates of the transfer speed can be grouped according to whether the study employed isotopic or canopy/soil flux-based techniques. These two groups should provide different estimates of transfer times because transport of sucrose molecules, and pressure-concentration waves, in phloem differ. A steady-state and a dynamic photosynthesis/phloem-transport/soil gas diffusion model were employed to interpret our results. Starch storage and partly soil gas diffusion affected transfer times, but phloem path-length strongly controlled molecule transfer times. Successful modelling required substantially different phloem properties (higher specific conductivity and turgor pressure difference) in tall compared with small plants, which is significant for our understanding of tall trees' physiology. Finally, we compared isotopic and flux-based approaches for the determination of the link between canopy photosynthesis and ecosystem respiration. We conclude that isotopic approaches are not well suited to document whether changes in photosynthesis of tall trees can rapidly affect soil respiration.

Depression of belowground respiration rates at simulated high moose population densities in boreal forests

Large herbivores can affect the carbon cycle in boreal forests by changing productivity and plant species composition, which in turn could ultimately alter litter production, nutrient cycling, and the partitioning between aboveground and belowground allocation of carbon. Here we experimentally tested how moose (Alces alces) at different simulated population densities affected belowground respiration rates (estimated as CO2 flux) in young boreal forest stands situated along a site productivity gradient. At high simulated population density, moose browsing considerably depressed belowground respiration rates (24-56% below that of no-moose controls) except during June, where the difference only was 10%. Moose browsing depressed belowground respiration the most on low-productivity sites. Soil moisture and temperature did not affect respiration rates. Impact of moose on belowground respiration was closely linked to litter production and followed Michaelis-Menten dynamics. The main mechanism by which moose decrease belowground respiration rates is likely their effect on photosynthetic biomass (especially decreased productivity of deciduous trees) and total litter production. An increased productivity of deciduous trees along the site productivity gradient causes an unequal effect of moose along the same gradient. The rapid growth of deciduous trees may offer higher resilience against negative effects of moose browsing on litter production and photosynthate allocation to roots.

Dark septate root endophytic fungi increase growth of Scots pine seedlings under elevated CO2 through enhanced nitrogen use efficiency

Although increasing concentrations of atmospheric CO2 are predicted to have substantial impacts on plant growth and functioning of ecosystems, there is insufficient understanding of the responses of belowground processes to such increases. We investigated the effects of different dark septate root endophytic (DSE) fungi on growth and nutrient acquisition by Pinus sylvestris seedlings under conditions of N limitation and at ambient and elevated CO2 (350 or 700 μ1 CO2 l−1). Each seedling was inoculated with one of the following species: Phialocephala fortinii (two strains), Cadophora finlandica, Chloridium paucisporum, Scytalidium vaccinii, Meliniomyces variabilis and M. vraolstadiae. The trial lasted 125 days. During the final 27 days, the seedlings were labeled with 14CO2 and 15NH4+. We measured extraradical hyphal length, internal colonization, plant biomass, 14C allocation, and plant N and 15N content. Under elevated CO2, the biomass of seedlings inoculated with DSE fungi was on average 17% higher than in control seedlings. Simultaneously, below-ground respiration doubled or trebled, and as a consequence carbon use efficiency by the DSE fungi significantly decreased. Shoot N concentration decreased on average by 57% under elevated CO2 and was lowest in seedlings inoculated with S. vaccinii. Carbon gain by the seedlings despite reduced shoot N concentration indicates that DSE fungi increase plant nutrient use efficiency and are therefore more beneficial to the plant under elevated CO2.

Root‐derived CO2 efflux via xylem stream rivals soil CO2 efflux

Respiration consumes a large portion of annual gross primary productivity in forest ecosystems and is dominated by belowground metabolism. Here, we present evidence of a previously unaccounted for internal CO(2) flux of large magnitude from tree roots through stems. If this pattern is shown to persist over time and in other forests, it suggests that belowground respiration has been grossly underestimated. Using an experimental Populus deltoides plantation as a model system, we tested the hypothesis that a substantial portion of the CO(2) released from belowground autotrophic respiration remains within tree root systems and is transported aboveground through the xylem stream rather than diffusing into the soil atmosphere. On a daily basis, the amount of CO(2) that moved upward from the root system into the stem via the xylem stream (0.26 mol CO(2) m(-2) d(-1)) rivalled that which diffused from the soil surface to the atmosphere (0.27 mol CO(2) m(-2) d(-1)). We estimated that twice the amount of CO(2) derived from belowground autotrophic respiration entered the xylem stream as diffused into the soil environment. Our observations indicate that belowground autotrophic respiration consumes substantially more carbohydrates than previously recognized and challenge the paradigm that all root-respired CO(2) diffuses into the soil atmosphere.

The significance of below-ground fractions when considering N and C partitioning within chickpea (Cicer arietinum L.)

The aim of this controlled environment experiment was to quantify the distribution of leaf-fed-15N and canopy fed-13C within nodulating, non-nodulating or N fertilized non-nodulating Cicer arietinum L. and in their surrounding rhizosphere soil, excluding soil + root respiration. Nodulating chickpea partitioned 32% of its total N and 27% of its total recoverable C below-ground, of which only 50% of N and 36% of C were in the clean root fraction. Non-nodulating chickpea allocated equal recoverable C but slightly less N (28%) below-ground but lost less C from plant induced below-ground respiration. The importance of this below-ground partitioning for crop systems C and N balances is highlighted by their large (45% and 33%, for N and C, respectively) contribution to the total plant derived residue (recyclable) fraction. Recovered 15N and 13C were greater (P < 0.05) in the outer-rhizosphere (459 µg 15N and 3.2 mg 13C core−1) than in the inner-rhizosphere soil (detached from roots during freeze-drying; 18 µg 15N and 67 µg 13C core−1) in relation with the relative size of these compartments. This highlights the significance of the outer-rhizosphere soil when estimating C and N budgets and quantifying rhizodeposition, and the benefit of a double (15N, 13C) isotope approach to determine this flow against large background soil C and N pools.

Regulation of soil surface respiration in a grazed pasture in New Zealand

This study investigated the regulation of soil surface respiration ( R s) in a grazed pasture, in New Zealand. The interactive responses of R s to soil temperature ( T s) and root zone volumetric water content ( θ) were quantified and modelled using soil cores in controlled conditions. Chamber measurements of R s, made at a field site approximately twice a month throughout a growing season, were used to validate the model. A closed gas-exchange system was used in the field to measure ecosystem respiration ( R e) and R s in order to partition the contribution of above- and below-ground respiration. This result was then applied to partition nighttime measurements of R e using eddy covariance into R s and above-ground respiration ( R a). Eddy covariance soil surface respiration ( R sEC) estimates were compared to modelled estimates of R s. The response of R s to T s could be described using an Arrhenius-type function, while the response of R s to normalised θ ( θ n) was described by linear relationship. The model showed that when θ n was >0.9, R s remained constant. In the field, R s, was strongly influenced by T s, which ranged from a minimum of 12.2 °C to a maximum of 20.1 °C throughout the measurement period. During the year, θ was rarely low enough to limit R s. The base value of respiration at 10 °C ( R 10) at the field site was 0.21 mg CO 2 m −2 s −1. The model was able to explain half of the variability observed in the field measurements of R s. Total R e was comprised of 84% R s and 16% R a. For nights with valid eddy covariance data, the estimate of R sEC from measurements of nighttime R e multiplied by the fraction of below-ground to total respiration was 48% less than the estimate from the model. Total growing season R s estimated using the model was 1.94 kg (C) m −2.

Biophysical controls on rhizospheric and heterotrophic components of soil respiration in a boreal black spruce stand

We conducted a root-exclusion experiment in a 125-year-old boreal black spruce (Picea mariana (Mill.) BSP) stand in 2004 to quantify the physical and biological controls on temporal dynamics of the rhizospheric (R(r)) and heterotrophic (R(h)) components of soil respiration (R(s)). Annual R(r), R(h) and estimated moss respiration were 285, 269 and 57 g C m(-2) year(-1), respectively, which accounted for 47, 44 and 9% of R(s) (611 g C m(-2) year(-1)), respectively. A gradual transition from R(h)-dominated (winter, spring and fall) to R(r)-dominated (summer) respiration was observed during the year. Soil thawing in spring and the subsequent increase in soil water content (theta) induced a small and sustained increase in R(h) but had no effect on R(r). During the remainder of the growing season, no effect of theta was observed on either component of R(s). Both components increased exponentially with soil temperature (T(s)) during the growing season, but R(r) showed greater temperature sensitivity than R(h) (Q(10) of 4.0 and 3.0, respectively). Temperature-normalized variations in R(r) were highly correlated with eddy covariance estimates of gross ecosystem photosynthesis, and the correlation was greatest when R(r) was lagged by 24 days. Within diurnal cycles, variations in T(s) were highly coupled to variations in R(h) but were significantly decoupled from R(r). The patterns observed at both time scales strongly suggest that the flow of photosynthates to the rhizosphere is a key driver of belowground respiration processes but that photosynthate supply may control these processes in several ways.

Interspecific variation in photosynthesis and respiration balance of three seagrasses in relation to light availability

Low light levels are a major factor determining the depth distribution of submerged macrophytes. In the Indo-Pacific tropical region, different seagrass species have specific zonation patterns from intertidal to subtidal areas along depth and light gradients. Cymodocea serrulata gen- erally occurs in deeper water than Thalassia hemprichii and C. rotundata, suggesting that C. serru- lata probably has the lowest minimum photon requirement of the 3 species. We compared photosyn- thesis and respiration balances of these 3 tropical seagrasses by examining photosynthesis-irradiance (P-E) curves for leaves and for the whole plant bodies. Sample collection and measurement were conducted on 2 occasions (July and February) on Ishigaki Island, southwestern Japan. Leaf photo- synthetic traits were not concordant with the observed zonation pattern of the species. When respira- tion rates of the root and rhizome were taken into account, T. hemprichii had a significantly higher light compensation point (158.1 µmol photons m -2 s -1 in July, 187.2 in February) than C. rotundata (47.9 in July, 41.0 in February) and C. serrulata (85.1 in July, 40.0 in February). The large amount of belowground respiration in T. hemprichii was a major cause of the variation in the compensation irra- diance. This result is in concordance with the realized distribution pattern of T. hemprichii, which is sparse under low light conditions caused by depth or heavy siltation. Conversely, C. serrulata had less belowground biomass and a smaller respiration demand, enabling the species to maintain a better photosynthetic/respiration balance under lower light. C. rotundata had a lower saturating irra- diance, a lower compensation point and a lower respiration rate than the other species. Thus, C. rotundata has the potential to inhabit the deepest water, although its realized distribution range is shallower than that of C. serrulata, suggesting that other factors limit the depth range of C. rotundata.

The effect of soil warming on bulk soil vs. rhizosphere respiration

AbstractThere has been considerable debate on whether root/rhizosphere respiration or bulk soil respiration is more sensitive to long‐term temperature changes. We investigated the response of belowground respiration to soil warming by 3 °C above ambient in bare soil plots and plots planted with wheat and maize. Initially, belowground respiration responded more to the soil warming in bare soil plots than in planted plots. However, as the growing season progressed, a greater soil‐warming response developed in the planted plots as the contribution of root/rhizosphere respiration to belowground respiration declined. A negative correlation was observed between the contribution of root/rhizosphere respiration to total belowground respiration and the magnitude of the soil‐warming response indicating that bulk soil respiration is more temperature sensitive than root/rhizosphere respiration. The dependence of root/rhizosphere respiration on substrate provision from photosynthesis is the most probable explanation for the observed lower temperature sensitivity of root/rhizosphere respiration. At harvest in late September, final crop biomass did not differ between the two soil temperature treatments in either the maize or wheat plots. Postharvest, flux measurements during the winter months indicated that the response of belowground respiration to the soil‐warming treatment increased in magnitude (response equated to a Q10 value of 5.7 compared with ∼2.3 during the growing season). However, it appeared that this response was partly caused by a strong indirect effect of soil warming. When measurements were made at a common temperature, belowground respiration remained higher in the warmed subplots suggesting soil warming had maintained a more active microbial community through the winter months. It is proposed that any changes in winter temperatures, resulting from global warming, could alter the sink strength of terrestrial ecosystems considerably.

Partitioning heterotrophic and rhizospheric soil respiration in a mature Douglas-fir (Pseudotsuga menziesii) forest

Total belowground respiration (Rs) was partitioned into heterotrophic (Rh) and rhizospheric (Rr) respiration to determine the amount of CO2 originating from each component in a coastal Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) forest. Rh was measured within cylinders from which roots, hyphae, and associated rhizosphere organisms were excluded by a 0.5 μm nylon mesh and installed 50 cm into the soil. Rs was 12 Mg C·ha–1·year–1 and ranged from 0.71 to 6.57 g C·m–2·day–1 during the 15 month experiment. Rh was 7.8 Mg C·ha–1·year–1, which contributed 65% of Rs, mostly between May and August. Rr was 4.2 Mg C·ha–1·year–1 (35% of Rs) and peaked in spring and fall. Soil temperature described the variability in Rs (p = 0.0004) better than soil moisture (p = 0.6156) and Rh was more closely related to temperature than was Rr. Values of Q10 were 1.7 for Rs and 2.2 for Rh. We also assessed three potential sources of error associated with this root-exclusion technique: respiration from decaying severed roots, stimulated respiration as a result of cylinder installation, and lateral diffusion of CO2 into cylinders. None of these artifacts were found to be significant sources of error in this experiment.

How do climate warming and species richness affect CO2fluxes in experimental grasslands?

This paper presents the results of 2 yr of CO(2) flux measurements on grassland communities of varying species richness, exposed to either the current or a warmer climate. We grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers. Half of these chambers were exposed to ambient air temperatures, while the other half were warmed by 3 degrees C. Equal amounts of water were added to heated and unheated communities, implying drier soils if warming increased evapotranspiration. Three main CO(2) fluxes (gross photosynthesis, above-ground and below-ground respiration) were measured multiple times per year and reconstructed hourly or half-hourly by relating them to their most important environmental driver. While CO(2) outputs through respiration were largely unchanged under warming, CO(2) inputs through photosynthesis were lowered, especially in summer, when heat and drought stress were higher. Above-ground CO(2) fluxes were significantly increased in multispecies communities, as more complementary resource use stimulated productivity. Finally, effects of warming appeared to be smallest in monocultures. This study shows that in a future warmer climate the CO(2) sink capacity of temperate grasslands could decline, and that such adverse effects are not likely to be mitigated by efforts to maintain or increase species richness.