Recent Photosynthate Research Articles

Changing sources of soil respiration with time since fire in a boreal forest

AbstractRadiocarbon signatures (Δ14C) of carbon dioxide (CO2) provide a measure of the age of C being decomposed by microbes or respired by living plants. Over a 2‐year period, we measured Δ14C of soil respiration and soil CO2 in boreal forest sites in Canada, which varied primarily in the amount of time since the last stand‐replacing fire. Comparing bulk respiration Δ14C with Δ14C of CO2 evolved in incubations of heterotrophic (decomposing organic horizons) and autotrophic (root and moss) components allowed us to estimate the relative contributions of O horizon decomposition vs. plant sources. Although soil respiration fluxes did not vary greatly, differences in Δ14C of respired CO2 indicated marked variation in respiration sources in space and time.The 14C signature of respired CO2 respired from O horizon decomposition depended on the age of C substrates. These varied with time since fire, but consistently had Δ14C greater (averaging ∼120‰) than autotrophic respiration. The Δ14C of autotrophically respired CO2 in young stands equaled those expected for recent photosynthetic products (70‰ in 2003, 64‰ in 2004). CO2 respired by black spruce roots in stands >40 years old had Δ14C up to 30‰ higher than recent photosynthates, indicating a significant contribution of C stored at least several years in plants.Decomposition of O horizon organic matter made up 20% or less of soil respiration in the younger (<40 years since fire) stands, increasing to ∼50% in mature stands. This is a minimum for total heterotrophic contribution, since mineral soil CO2 had Δ14C close to or less than those we have assigned to autotrophic respiration. Decomposition of old organic matter in mineral soils clearly contributed to soil respiration in younger stands in 2003, a very dry year, when Δ14C of soil respiration in younger successional stands dropped below those of the atmospheric CO2.

Effect of girdling on soil respiration and root composition in a sweet chestnut forest

Rhizosphere respiration is a significant component of the total carbon dioxide (CO 2) efflux from forest soils. Since the root-derived CO 2 is not part of soil C loss, it is worth estimating this fraction and determining its driving forces. In order to determine the dependency of the root respiration on recent photosynthates in a European chestnut ( Castanea sativa Mill.) stand in Southern Switzerland, we girdled 104 tree stems on an area of 625 m 2 at the end of August. The response of forest girdling on the below-ground C cycle was assessed by measuring soil respiration, soil microbial carbon biomass and root composition during the 37 days of experiment. Recent photosynthates contributed significantly to the soil respiration, which declined by 22 and up to 36% on average after girdling. The difference in soil respiration between girdled and control plots was significant only during a short time period (9–20 days after girdling). A significant decline in soil respiration was observed up to a distance of 4 m from the stems. Fine roots from girdled trees were strongly depleted in starch (−90%), although they were functional as shown by the dehydrogenase test. We assume that a large fraction of the starch loss was most likely respired and contributed to the soil respiration or there was a decline in new starch supply due to reduced carbon allocation to the roots.

Seasonal dynamics and partitioning of isotopic CO 2 exchange in a C 3/C 4 managed ecosystem

Ecosystem-atmosphere fluxes of 12CO 2 and 13CO 2 are needed to better understand the impacts of climate and land use change on ecosystem respiration ( F R), net ecosystem CO 2 exchange ( F N), and canopy-scale photosynthetic discrimination ( Δ). We combined micrometeorological and stable isotope techniques to quantify isotopic fluxes of 12CO 2 and 13CO 2 over a corn–soybean rotation ecosystem in the Upper Midwest United States. Results are reported for a 192-day period during the corn (C 4) phase of the 2003 growing season. The isotopomer flux ratio, d 13CO 2/d 12CO 2, was measured continuously using a tunable diode laser (TDL) and gradient technique to quantify the isotope ratios of F R ( δ R) and F N ( δ N). Prior to leaf emergence δ R was approximately −26‰. It increased rapidly following leaf emergence and reached an average value of −12.5‰ at full canopy. δ R decreased to pre-emergence values following senescence. δ N also showed strong seasonal variation and during the main growing period averaged −11.6‰. δ R and δ N values were used in a modified flux partitioning approach to estimate canopy-scale Δ and the isotope ratio of photosynthetically assimilated CO 2 ( δ P) independent of calculating canopy conductance or assuming leaf-scale discrimination factors. The results showed substantial day-to-day variation in Δ with an average value of 4.0‰. This flux-based estimate of Δ was approximately 6‰ lower than the Keeling mixing model estimate and in better agreement with leaf-level observations. These data were used to help constrain and partition F R into its autotrophic ( F Ra) and heterotrophic ( F Rh) components based on the numerical optimization of a mass balance model. On average F Ra accounted for 44% of growing season F R and reached a maximum of 59% during peak growth. The isotope ratio of F Rh ( δ Rh), was −26‰ prior to leaf emergence, and became increasingly 13C enriched as the canopy developed indicating that recent photosynthate became the dominant substrate for microbial activity. Sensitivity analyses substantiated that F Rh had a major influence on the seasonal pattern of δ R, δ N and the isotopic disequilibrium of the ecosystem. These data and parameter estimates are critical for validating and constraining the parameterization of land surface schemes and inverse models that aim to estimate regional carbon sinks and sources and interpreting changes in the atmospheric signal of δ 13CO 2.

Soil Invertebrates Disrupt Carbon Flow Through Fungal Networks

Annual carbon flux through soil respiration is ten times greater than fossil fuel combustion, but its component parts are poorly understood because they are the product of complex multitrophic interactions between soil organisms. A major component of carbon flux from plants to soil occurs through networks of symbiotic arbuscular mycorrhizal fungi. Here, using 13CO2 pulse labeling, we show that natural densities of the numerically dominant fungal feeding invertebrate Protaphorura armata (order Collembola) reduces 13C enrichment of mycorrhizosphere respiration by 32%. Our findings emphasize the importance of multitrophic interactions in regulating respiration of recent plant photosynthate from soil.

Jasmonic acid induces rapid changes in carbon transport and partitioning in Populus

Here, we tested whether rapid changes in carbohydrate transport and partitioning to storage organs would be induced by jasmonic acid (JA), a plant-produced signal of herbivore attack known to induce resistance. Carbon-11, introduced as (11)CO(2), was used to track real-time carbohydrate transport and partitioning nondestructively in Populus species before and 12 h after application of JA to a single leaf. Jasmonic acid resulted in more rapid [(11)C]-photosynthate export from both local and systemic leaves, as well as greater partitioning of [(11)C]-photosynthate to the stem and roots. In Populus tremuloides, following JA treatment, leaf starch decreased, but there was no change in photosynthetic rates or leaf soluble sugar concentration, indicating that recent photosynthate was diverted from starch accumulation in the leaf to other plant organs. Increasing the supply of photosynthate to roots and stems may shield resources from folivorous predators, and may also facilitate both storage and nutrient uptake, and ultimately lead to greater tolerance, either by enhancing regrowth capacity or by replacing nutrients consumed by herbivores.

Effect of defoliation on patterns of carbon exudation from Agrostis capillaris

AbstractGrassland ecosystems are important global sinks and sources of atmospheric carbon (C). In this study, we used an in‐situ 13CO2 tracer approach to quantify differences in short‐term C exudation from defoliated and non‐defoliated Agrostis capillaris (L.) plants subjected to natural diurnal light and temperature regimes and rainfall events. Results showed: 1. There was no significant difference in overall carbon exudation from the plants as a result of defoliation; 2. defoliation significantly increased exudation of recent photosynthate (i.e., 13C labeled); 3. there was a distinct and statistically significant diurnal trend in C exudation with root C exudation increasing during the day and early evening and decreasing during the night and early morning. The importance of light/temperature and defoliation as drivers of patterns of root C exudation and the contribution of recent assimilate C to atmosphere‐plant‐soil carbon flow are discussed.

Isotopic estimates of new carbon inputs into litter and soils in a four-year climate change experiment with Douglas-fir

Because soil is a major reservoir of terrestrial carbon and a potential sink for atmospheric CO2, determining plant inputs to soil carbon is critical for understanding ecosystem carbon dynamics. We present a modified method to quantify the effects of global climate change on plant inputs of carbon to soil based on 13C:12C ratio (δ13C) analyses that accounts for isotopic fractionation between inputs and newly created soil carbon. In a four-year study, the effects of elevated CO2 and temperature were determined for reconstructed Douglas-fir (Pseudotsuga mensiezii (Mirb.) Franco) ecosystems in which native soil of low nitrogen content was used. The δ13C patterns in litter and mineral soil horizons were measured and compared to δ13C patterns in live needles, fine roots, and coarse roots. From regression analyses, we calculated the isotopic enrichment in 13C of newly incorporated soil carbon relative to needle and root carbon at 4‰ and 2‰, respectively. These enrichments must be considered when using shifts in soil δ13C to calculate inputs of plant carbon into the soil, and are probably a major factor in the progressive enrichment in 13C with increasing depth in soil profiles. Relative to the total carbon in each layer, the proportion of new carbon from recent photosynthate in each soil layer was 13–15% in the A horizon, 7–9% in litter layers, and 4% in the B2 and C horizons. New carbon in the A horizon was estimated at 370 g C m−2. Carbon concentrations and new carbon in A horizons were correlated (r 2=0.78, n=12), but with a slope of 0.356, indicating that about 36% of net carbon accumulation in the A horizon was from inputs via roots, root exudates or mycorrhizal fungi and 64% of carbon was derived from surface litter decomposition. Under the nitrogen-limited growth conditions used in this study, neither elevated CO2 nor temperature affected soil carbon sequestration patterns.

Temporal dynamics of carbon partitioning and rhizodeposition in wheat.

The temporal dynamics of partitioning and rhizodeposition of recent photosynthate in wheat (Triticum aestivum) roots were quantified in situ in solution culture. After a 30-min pulse of (14)CO(2) to a single intact leaf, (14)C activities of individual carbon fluxes in the root, including exudation, respiration, and root content, were measured continuously over the next 20 h concurrently with (14)C efflux from the leaf. Immediately after the end of the (14)CO(2) pulse, (14)C activity was detected in the root, the hydroponic solution, and in root respiration. The rate of (14)C exudation from the root was maximal after 2 to 3 h, and declined to one-third of maximum after a further 5 h. Completion of the rapid phase of (14)C efflux from the leaf coincided with peak (14)C exudation rate. Thus, exudation flux is much more rapidly and dynamically coupled to current photosynthesis than has been appreciated. Careful cross-calibration of (14)C counting methods allowed a dynamic (14)C budget to be constructed for the root. Cumulative (14)C exudation after 20 h was around 3% of (14)C fixed in photosynthesis. Partitioning of photosynthate between shoot and root was manipulated by partial defoliation before applying the (14)CO(2) pulse to the remaining intact leaf. Although the rate of photosynthesis was largely unaffected by partial defoliation, the proportion of new photosynthate subsequently partitioned to and exuded from the root was substantially reduced. This clearly indicates that exudation depends more on the rate of carbon import into the root than on the rate of photosynthesis.

Response of the carbon isotopic content of ecosystem, leaf, and soil respiration to meteorological and physiological driving factors in a Pinus ponderosa ecosystem

Understanding the controls over ecosystem‐respired δ13C (δ13CR) is important for applications of isotope‐based models of the global carbon budget as well as for understanding ecosystem‐level variation in isotopic discrimination (Δ). Discrimination may be strongly dependent on synoptic‐scale variation in environmental drivers that control canopy‐scale stomatal conductance (Gc) and photosynthesis, such as atmospheric vapor pressure deficit (vpd) photosynthetically active radiation (PAR) and air temperature (Tair). These potential relationships are complicated, however, due to time lags between the period of carbon assimilation and ecosystem respiration, which may extend up to several days, and may vary with tissue (i.e., leaves versus belowground tissues). Our objective was to determine if relationships exist over a short‐term period (2 weeks) between meteorological and physiological driving factors and δ13CR and its components, soil‐respired δ13C (δ13CR‐soil) and foliage‐respired δ13C (δ13CR‐foliage). We tested for these hypothesized relationships in a 250‐year‐old ponderosa pine forest in central Oregon, United States. A cold front passed through the region 3 days prior to our first sample night, resulting in precipitation (total rainfall 14.6 mm), low vpd (minimum daylight average of 0.36 kPa) and near‐freeze temperature (minimum air temperature of 0.18°C ± 0.3°C), followed by a warming trend with relatively high vpd (maximum daylight average of 3.19 kPa). Over this 2‐week period Gc was negatively correlated with vpd (P < 0.01) while net ecosystem CO2 exchange (NEE) was positively correlated with vpd (P < 0.01), consistent with a vpd limitation to conductance and net CO2 uptake. Consistent with a stomatal influence over Δ, a negative correlation was observed between δ13CR and Gc measured 2 days prior (i.e., a 2‐day time lag, P = 0.04); however, δ13CR was not correlated with other measured variables. Also consistent with a stomatal influence over discrimination, δ13CR‐soil was negatively correlated with Gc (P < 0.01) and positively correlated with vpd and PAR measured one to 3 days prior (P = 0.01 and 0.04, respectively). In contrast, δ13CR‐foliage was not correlated with vpd or Gc, but was negatively correlated with minimum air temperature measured 5 days previously (P < 0.01) supporting the idea that cold air temperatures cause isotopic enrichment of respired CO2. The significant driving parameters differed for δ13CR‐foliage and δ13CR‐soil potentially due to different controls over the isotopic content of tissue‐specific respiratory fluxes, such as differing carbon transport times from the site of assimilation to the respiring tissue or different reliance on recent versus old photosynthate. Consistent with Gc control over photosynthesis and Δ, both δ13CR‐soil and δ13CR‐foliage became enriched as net CO2 uptake decreased (more positive NEE, P < 0.01 for both). The δ13C value of Pinus ponderosa foliage (−27.1‰, whole‐tissue) was 0.5 to 3.0‰ more negative than any observed respiratory signature, supporting the contention that foliage δ13C can be a poor proxy for the isotopic content of respiratory fluxes. The strong meteorological controls over Gc and NEE were associated with similar variation in δ13CR‐soil but only minor variation in δ13CR, leading us to conclude that δ13CR is not controlled solely by either canopy and belowground processes, but rather by their time‐dependent interaction.

Direct measurements of sieve element hydrostatic pressure reveal strong regulation after pathway blockage

According to the Münch hypothesis, solution flow through the phloem is driven by a hydrostatic pressure gradient. At the source, a high hydrostatic pressure is generated in the collection phloem by active loading of solutes, which causes a concomitant passive flow of water, generating a high turgor pressure. At the sink, solute unloading from the phloem keeps the turgor pressure low, generating a source-to-sink hydrostatic pressure gradient. Localised changes in loading and unloading of solutes along the length of the transport phloem can compensate for small, short-term changes in phloem loading at the source, and thus, maintain phloem flow to the sink tissue. We tested directly the hydrostatic pressure regulation of the sieve tube by relating changes in sieve tube hydrostatic pressure to changes in solute flow through the sieve tube. A sudden phloem blockage was induced (by localised chilling of a 1-cm length of stem tissue) while sieve-tube-sap osmotic pressure, sucrose concentration, hydrostatic pressure and flow of recent photosynthate were observed in vivo both upstream and downstream of the block. The results are discussed in relation to the Münch hypothesis of solution flow, sieve tube hydrostatic pressure regulation and the mechanism behind the cold-block phenomenon.

Tree root and soil heterotrophic respiration as revealed by girdling of boreal Scots pine forest: extending observations beyond the first year

ABSTRACTLimitations in available techniques to separate autotrophic (root) and soil heterotrophic respiration have hampered the understanding of forest C cycling. The former is here defined as respiration by roots, their associated mycorrhizal fungi and other micro‐organisms in the rhizosphere directly dependent on labile C compounds leaked from roots. In order to separate the autotrophic and heterotrophic components of soil respiration, all Scots pine trees in 900 m2 plots were girdled to instantaneously terminate the supply of current photosynthates from the tree canopy to roots. Högberg et al. (Nature 411, 789–792, 2001) reported that autotrophic activity contributed up to 56% of total soil respiration during the first summer of this experiment. They also found that mobilization of stored starch (and likely also sugars) in roots after girdling caused an increased apparent heterotrophic respiration on girdled plots. Herein a transient increase in the δ13C of soil CO2 efflux after girdling, thought to be due to decomposition of 13C‐enriched ectomycorrhizal mycelium and root starch and sugar reserves, is reported. In the second year after girdling, when starch reserves of girdled tree roots were exhausted, calculated root respiration increased up to 65% of total soil CO2 efflux. It is suggested that this estimate of its contribution to soil respiration is more precise than the previous based on one year of observation. Heterotrophic respiration declined in response to a 20‐day‐long 6 °C decline in soil temperature during the second summer, whereas root respiration did not decline. This did not support the idea that root respiration should be more sensitive to variations in soil temperature. It is suggested that above‐ground photosynthetic activity and allocation patterns of recent photosynthates to roots should be considered in models of responses of forest C balances to global climate change.

Active microbial RNA turnover in a grassland soil estimated using a 13CO 2 spike

Rhizosphere microbes are critical to the initial transfer and transformation of root carbon inputs to the soil but our understanding of the activity of these organisms remains constrained by their limited culturability. In this study we combined isotopic 13C tracer and molecular approaches to measure the incorporation of recently assimilated plant C into soil microbial RNA and DNA pools as a means to determine the turnover of the ‘active’ rhizosphere community. This required the development of a method for the extraction, purification and preparation of small-sample soil DNA and RNA (<5 μg C) for isotope analysis. Soil, plant and respired CO 2 samples were collected from a 13CO 2 pulse-chase experiment at intervals for 20 days post-labelling. The peak of 13C release in soil/root respired CO 2 came between 5 and 48 h after 13CO 2 pulse-labelling and was followed by a secondary peak of soil heterotroph 13C respiration after 136 h. Results showed that both soil DNA and RNA rapidly incorporated recent photosynthate with greatest 13C found in the ‘active’ microbial RNA fraction reflecting higher rates of microbial RNA turnover. The dilution rate of the pulse derived 13C in RNA-C was used to estimate a microbial RNA turnover of approximately 20% day −1 with a 15–20 day residence time for photosynthate derived 13C in the RNA pool. The findings of this work confirm the rapid transfer of photosynthate C inputs through soil microorganisms to the atmosphere as CO 2 and the potential of the biomolecular-isotope tracer approach in soil C research.

Natural abundance carbon isotope composition of isoprene reflects incomplete coupling between isoprene synthesis and photosynthetic carbon flow.

Isoprene emission from leaves is dynamically coupled to photosynthesis through the use of primary and recent photosynthate in the chloroplast. However, natural abundance carbon isotope composition (delta(13)C) measurements in myrtle (Myrtus communis), buckthorn (Rhamnus alaternus), and velvet bean (Mucuna pruriens) showed that only 72% to 91% of the variations in the delta(13)C values of fixed carbon were reflected in the delta(13)C values of concurrently emitted isoprene. The results indicated that 9% to 28% carbon was contributed from alternative, slow turnover, carbon source(s). This contribution increased when photosynthesis was inhibited by CO(2)-free air. The observed variations in the delta(13)C of isoprene under ambient and CO(2)-free air were consistent with contributions to isoprene synthesis in the chloroplast from pyruvate associated with cytosolic Glc metabolism. Irrespective of alternative carbon source(s), isoprene was depleted in (13)C relative to mean photosynthetically fixed carbon by 4 per thousand to 11 per thousand. Variable (13)C discrimination, its increase by partially inhibiting isoprene synthesis with fosmidomicin, and the associated accumulation of pyruvate suggested that the main isotopic discrimination step was the deoxyxylulose-5-phosphate synthase reaction.

Determination of rhizosphere 13C pulse signals in soil thin sections by laser ablation isotope ratio mass spectrometry.

In grassland ecosystems, soil animals act as key soil engineers and architects. The diversity of soil animals is also a regulator of ecosystem carbon flow. However, our understanding of the link between soil animals, carbon fluxes and soil physical organisation remains poor. An integrated approach based on soil micromorphology and laser ablation stable isotope ratio mass spectrometry (LA-IRMS) was developed to provide spatially distributed data of pulse-derived (13)C tracer from roots in the soil environment. This paper describes the development and testing of a LA-IRMS (13)C/(12)C analytical method on soil thin sections as a means to determine the fate of root carbon derived from photosynthesis into soil. Results from this work demonstrated (1) that micro-scale delta(13)C (per thousand) analysis could be made on targeted features located within a soil thin section and (2) that LA-IRMS delta(13)C (per thousand) measurements made on samples obtained from (13)CO(2) pulse labelled plant-soil blocks confirmed the presence of recent photosynthates in the rhizosphere (1 and 4 weeks post-pulse).

Pulse‐labeling studies of carbon cycling in Arctic tundra ecosystems: The contribution of photosynthates to methane emission

We investigated a possible mechanism underlying observed correlations between net ecosystem carbon dioxide exchange and methane emission. Using the technique of 14C pulse‐labeling, we traced the movement of carbon fixed by photosynthesis as it moved through wet sedge and moist tussock tundra plant‐soil mesocosms and was emitted as methane to the atmosphere. The 14C tracer provided a definitive way of quantifying the fate of recently fixed carbon. Carbon fixed by photosynthesis was measured as emitted methane from both moist tussock and wet sedge tundra mesocosms within 2 hours after labeling. Integration of time series measurements of methane emission showed that recent photosynthates are an important source of carbon for methane production. Approximately 2% of carbon fixed by photosynthesis at peak growing season was subsequently emitted as methane from moist tussock tundra, and approximately 3% was emitted as methane from wet sedge tundra. Measurements of soil pore water carbon pools demonstrate rapid transfer of 14C from plant carbon to dissolved forms and subsequently to the atmosphere as carbon dioxide or methane.

Using radiocarbon to determine the mycorrhizal status of fungi

Summary Measurements of 13 C in fungal sporocarps are useful in assessing mycorrhizal or saprotrophic status. Because 14 C measurements can indicate the age of fungal carbon (C) and mycorrhizal fungi depend closely on recent photosynthate, 14 C may provide additional insight into possible mycorrhizal status. Sporocarps, needles, and litter from Woods Creek, OR, USA together with archived sporocarps were measured for 14 C content by accelerator mass spectrometry. Known mycorrhizal fungi resembled current‐year needles ( Amanita , Cantharellus and Gomphidius ) or atmospheric CO 2 ( Tuber ) in 14 C and indicated an average age of 0–2 yr for incorporated C, whereas saprotrophic genera ( Pleurocybella , Lepiota and Hypholoma ) were composed of C at least 10 yr old. Of genera tentatively considered mycorrhizal from previous work with 13 C, only Otidia and Sowerbyella appeared mycorrhizal from 14 C measurements, whereas Aleuria , Clavulina , Paurocotylis and Ramaria (sensu lato ) consisted of older carbon and were presumably saprotrophic. 14 C clearly separated known mycorrhizal or saprotrophic fungi and indicated 13 C measurements should be interpreted cautiously on species of unknown status. 14 C results for needles and mycorrhizal fungi suggested that C sources other than atmospheric CO 2 may contribute small amounts of C. Possible sources include storage of carbohydrates and amino acids, organic nitrogen uptake, and incorporation of soil‐respired CO 2 by anaplerotic or photosynthetic pathways.

Transfer of recent photosynthate into mycorrhizal mycelium of an upland grassland: short-term respiratory losses and accumulation of 14C

The movement of carbon from plants into their natural communities of arbuscular-mycorrhizal (AM) fungi was investigated. Mesh-bound cores, which allowed in-growth of AM mycelium to be controlled but excluded roots, were inserted into turf monoliths removed from an upland grassland and were exposed to 14CO 2. Flux of 14C-labelled carbon from plants to hyphae of AM fungi for 70 h post-labelling was measured by (a) trapping CO 2 released from soil cores containing AM hyphae linked to the plants compared to cores from which AM hyphal connections to the plant roots had been severed, and (b) quantification of the total amount of 14C in the cores. Release of 14CO 2 from the cores colonised by active AM mycelium was highest for 0–28 h from the onset of labelling and declined rapidly thereafter. The amount of 14C allocated into mycorrhizal mycelium 0–70 h after labelling accounted for 3.4% of the 14C initially fixed by the plants. The results confirm the rapidity of photosynthate allocation to AM mycelium and demonstrate the importance of the short-term dynamics of C fluxes in undisturbed grasslands.

A pulse-labeling experiment to determine the contribution of recent plant photosynthates to net methane emission in arctic wet sedge tundra

We conducted a 14C pulse-labeling experiment under field conditions to estimate the contribution of recent photosynthates to methane (CH 4) emission in arctic wet sedge tundra dominated by Carex aquatilis and Eriophorum angustifolium. The average CH 4 emission rate from plant–soil mesocosms in this study was 0.45 g C m −2 d −1. Carbon assimilated by plants via photosynthesis during pulse-labeling turned over rapidly and appeared as emitted CH 4 within 24 h. Integration of flux measurements made over a 2-week period shows that the contribution of recent photosynthates to mid-season CH 4 emission is relatively low. Less than 1% of the 14C-labeled carbon dioxide taken up through photosynthesis was emitted as 14CH 4 during this study.

Biodegradable dissolved organic carbon in forest soil solution and effects of chronic nitrogen deposition

Using a flow-through bioreactor, biodegradable dissolved organic carbon (BDOC) in forest floor solution was determined for a red pine ( Pinus resinosa) plantation and a naturally regenerated mixed hardwood forest (dominant species: Quercus velutina, Q. rubra, Acer rubrum, Fagus grandifolia, Prunus serotina). The forests have received chronic-N fertilization at three different rates (0, 50, 150 kg N ha −1 year −1) since 1988. Both BDOC concentration and %BDOC [% of dissolved organic carbon (DOC)] were significantly higher in the summer (18–20 mg C l −1, ≈30%) than spring or fall for both forest types. The BDOC depletion that was hypothesized to occur with chronic-N application and N saturation was not found in either stand. Instead, for the hardwood stand, BDOC concentration increased with N application, probably due to increased BDOC production in the forest floor with increases in available N. The strong negative relationship between litterfall mass and both BDOC concentration ( r 2=0.36) and %BDOC ( r 2=0.60) for the hardwood stand suggests that the primary source of BDOC in the hardwood forest floor is recent photosynthate allocated to fine roots for their growth, and organic compounds released from them (e.g., exudates and mucilage) rather than the leachate of freshly fallen autumn litter. A significant but weaker positive relationship between BDOC and ambient temperature for both stands suggests that the decomposition of forest floor organic matter is an additional source of BDOC in the forest floor. The production of non-biodegradable DOC (NBDOC) in the forest floor was not sensitive to ambient temperature or the chronic-N treatment. This suggests that an abiotic process, such as chemical equilibration between forest floor and forest floor solution, is responsible for the concentration of non-labile DOC. High %BDOC in throughfall (50–75%) for both stands suggests that throughfall is possibly an additional source of BDOC in the forest floor.

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 be significant sinks of atmospheric CO2. We developed a below-ground carbon (C) budget for well-drained soils in Harvard Forest Massachusetts, an ecosystem that is storing C. Measurements of carbon and radiocarbon ( 14 C) inventory were used to determine the turnover time and maximum rate of CO2 production from heterotrophic respiration of three fractions of soil organic matter (SOM): recognizable litter fragments (L), humified low density material (H), and high density or mineral-associated organic matter (M). Turnover times in all fractions increased with soil depth and were 2-5 years for recognizable leaf litter, 5-10 years for root litter, 40-100+ years for low density humified material and >100 years for carbon associated with minerals. These turnover times represent the time carbon resides in the plant + soil system, and may underestimate actual decomposition rates if carbon resides for several years in living root, plant or woody material. Soil respiration was partitioned into two components using 14 C: recent photosynthate which is metabolized by roots and microorganisms within a year of initial fixation (Recent- C), and C that is respired during microbial decomposition of SOM that resides in the soil for several years or longer (Reservoir-C). For the whole soil, we calculate that decomposition of Reservoir-C contributes approximately 41% of the total annual soil respiration. Of this 41%, recognizable leaf or root detritus accounts for 80% of the flux, and 20% is from the more humified fractions that dominate the soil carbon stocks. Measurements of CO 2 and 14 CO2 in the soil atmosphere and in total soil respiration were combined with surface CO2 fluxes and a soil gas diffusion model to determine the flux and isotopic signature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cm of the soil (O + A + Ap horizons). The average residence time of Reservoir-C in the plant + soil system is 81 years and the average age of carbon in total soil respiration (Recent-C + Reservoir-C) is 41 years. The O and A horizons have accumulated 4.4 kgC m -- 2 above the plow layer since abandon- ment by settlers in the late-1800's. C pools contributing the most to soil respiration have short enough turnover times that they are likely in steady state. However, most C is stored as humified organic matter within both the O and A horizons and has turnover times from 40 to