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The dynamics of forest recovery after windthrows (i.e., broken or uprooted trees by wind) are poorly understood in tropical forests. The Northwestern Amazon (NWA) is characterized by a higher occurrence of windthrows, greater rainfall, and higher annual tree mortality rates (~2%) than the Central Amazon (CA). We combined forest inventory data from three sites in the Iquitos region of Peru, with recovery periods spanning 2, 12, and 22 years following windthrow events. Study sites and sampling areas were selected by assessing the windthrow severity using remote sensing. At each site, we recorded all trees with a diameter at breast height (DBH) ≥ 10 cm along transects, capturing the range of windthrow severity from old-growth to highly disturbed (mortality > 60%) forest. Across all damage classes, tree density and basal area recovered to >90% of the old-growth values after 20 years. Aboveground biomass (AGB) in old-growth forest was 380 (±156) Mg ha−1 . In extremely disturbed areas, AGB was still reduced to 163 (±68) Mg ha−1 after 2 years and 323 (± 139) Mg ha−1 after 12 years. This recovery rate is ~50% faster than that reported for Central Amazon forests. The faster recovery of forest structure in our study region may be a function of its higher productivity and adaptability to more frequent and severe windthrows. These varying rates of recovery highlight the importance of extreme wind and rainfall on shaping gradients of forest structure in the Amazon, and the different vulnerabilities of these forests to natural disturbances whose severity and frequency are being altered by climate change.

High descending winds generated by convective storms are a frequent and a major source of tree mortality disturbance events in the Amazon, affecting forest structure and diversity across a variety of scales, and more frequently observed in western and central portions of the basin. Soil texture in the Central Amazon also varies significantly with elevation along a topographic gradient, with decreasing clay content on plateaus, slopes and valleys respectively. In this study we investigated the critical turning moments (Mcrit - rotational force at the moment of tree failure, an indicator of tree stability or wind resistance) of 60 trees, ranging from 19.0 to 41.1 cm in diameter at breast height (DBH) and located in different topographic positions, and for different species, using a cable-winch load-cell system. Our approach used torque as a measure of tree failure to the point of snapping or uprooting. This approach provides a better understanding of the mechanical forces required to topple trees in tropical forests, and will inform models of wind throw disturbance. Across the topographic positions, size controlled variation in Mcrit was quantified for cardeiro (Scleronema mincranthum (Ducke) Ducke), mata-matá (Eschweilera spp.), and a random selection of trees from 19 other species. Our analysis of Mcrit revealed that tree resistance to failure increased with size (DBH and ABG) and differed among species. No effects of topography or failure mode were found for the species either separately or pooled. For the random species, total variance in Mcrit explained by tree size metrics increased from an R2 of 0.49 for DBH alone, to 0.68 when both DBH and stem fresh wood density (SWD) were included in a multiple regression model. This mechanistic approach allows the comparison of tree vulnerability induced by wind damage across ecosystems, and facilitates the use of forest structural information in ecosystem models that include variable resistance of trees to mortality inducing factors. Our results indicate that observed topographic differences in windthrow vulnerability are likely due to elevational differences in wind velocities, rather than by differences in soil-related factors that might effect Mcrit.

Old-growth forests are subject to substantial changes in structure and species composition due to the intensification of human activities, gradual climate change and extreme weather events. Trees store ca. 90 % of the total aboveground biomass (AGB) in tropical forests and precise tree biomass estimation models are crucial for management and conservation. In the central Amazon, predicting AGB at large spatial scales is a challenging task due to the heterogeneity of successional stages, high tree species diversity and inherent variations in tree allometry and architecture. We parameterized generic AGB estimation models applicable across species and a wide range of structural and compositional variation related to species sorting into height layers as well as frequent natural disturbances. We used 727 trees (diameter at breast height ≥ 5 cm) from 101 genera and at least 135 species harvested in a contiguous forest near Manaus, Brazil. Sampling from this data set we assembled six scenarios designed to span existing gradients in floristic composition and size distribution in order to select models that best predict AGB at the landscape level across successional gradients. We found that good individual tree model fits do not necessarily translate into reliable predictions of AGB at the landscape level. When predicting AGB (dry mass) over scenarios using our different models and an available pantropical model, we observed systematic biases ranging from -31 % (pantropical) to +39 %, with root-mean-square error (RMSE) values of up to 130 Mg ha-1 (pantropical). Our first and second best models had both low mean biases (0.8 and 3.9 %, respectively) and RMSE (9.4 and 18.6 Mg ha-1) when applied over scenarios. Predicting biomass correctly at the landscape level in hyperdiverse and structurally complex tropical forests, especially allowing good performance at the margins of data availability for model construction/calibration, requires the inclusion of predictors that express inherent variations in species architecture. The model of interest should comprise the floristic composition and size-distribution variability of the target forest, implying that even generic global or pantropical biomass estimation models can lead to strong biases. Reliable biomass assessments for the Amazon basin (i.e., secondary forests) still depend on the collection of allometric data at the local/regional scale and forest inventories including species-specific attributes, which are often unavailable or estimated imprecisely in most regions.

To quantify the contribution of autotrophic microorganisms to organic matter (OM) formation in soils, we investigated natural CO2 vents (mofettes) situated in a wetland in northwest Bohemia (Czech Republic). Mofette soils had higher soil organic matter (SOM) concentrations than reference soils due to restricted decomposition under high CO2 levels. We used radiocarbon ("14C) and stable carbon (13C) isotope ratios to characterize SOM and its sources in two mofettes and compared it with respective reference soils, which were not influenced by geogenic CO2. The geogenic CO2 emitted at these sites is free of radiocarbon and enriched in 13C compared to atmospheric CO2. Together, these isotopic signals allow us to distinguish C fixed by plants from C fixed by autotrophic microorganisms using their differences in 13C discrimination. We can then estimate that up to 27 % of soil organic matter in the 0-10 cm layer of these soils was derived from microbially assimilated CO2. Isotope values of bulk SOM were shifted towards more positive 13C and more negative "14C values in mofettes compared to reference soils, suggesting that geogenic CO2 emitted from the soil atmosphere is incorporated into SOM. To distinguish whether geogenic CO2 was fixed by plants or by CO2 assimilating microorganisms, we first used the proportional differences in radiocarbon and 13C values to indicate the magnitude of discrimination of the stable isotopes in living plants. Deviation from this relationship was taken to indicate the presence of microbial CO2 fixation, as microbial discrimination should differ from that of plants. 13CO2-labelling experiments confirmed high activity of CO2 assimilating microbes in the top 10 cm, where 13C values of SOM were shifted up to 2 ‰ towards more negative values. Uptake rates of microbial CO2 fixation ranged up to 1.59 ± 0.16 1/4g gdw−1 dg'1. We inferred that the negative 13C shift was caused by the activity of autotrophic microorganisms using the Calvin-Benson-Bassham (CBB) cycle, as indicated from quantification of cbbL/cbbM marker genes encoding for RubisCO by quantitative polymerase chain reaction (qPCR) and by acetogenic and methanogenic microorganisms, shown present in the mofettes by previous studies. Combined "14C and 13C isotope mass balances indicated that microbially derived carbon accounted for 8-27 % of bulk SOM in this soil layer. The findings imply that autotrophic microorganisms can recycle significant amounts of carbon in wetland soils and might contribute to observed radiocarbon reservoir effects influencing "14C signatures in peat deposits.

Plant growth in boreal forests is commonly limited by a low supply of nitrogen, a condition that may be aggravated by high tree below-ground allocation of carbon to ectomycorrhizal (ECM) fungi and associated microorganisms. These in turn immobilise N and reduce its availability to plants as boreal ecosystems develop. Here, we studied a boreal forest ecosystem chronosequence created by new land rising out of the sea due to iso-static rebound along the coast of northern Sweden. We used height over the ocean to estimate ecosystem age and examined its relationship to soil microbial community structure and the gross turnover of N. The youngest soils develop with meadows by the coast, followed by a zone of N2-fixing alder trees, and primary boreal conifer forest on ground up to 560 years old. The young soils in meadows contained little organic matter and microbial biomass per unit area. Nitrogen was turned over at low rates when expressed per area (m-2), but specific rates (per gram soil carbon (C)) were the highest found along the transect. In the zone with alder, the amounts of soil C and microbial biomass were much higher (bacterial biomass had doubled and fungal biomass quadrupled). Rates of gross N mineralisation (expressed on an area basis) were highest, but the retention of added labelled NH4+ was lowest in this soil as compared to other ages. The alder zone also had the largest extractable pools of inorganic N in soil and highest N % in plant foliage. In the older conifer forest ecosystems the amounts of soil C and N, as well as biomass of both bacteria and fungi increased. Data on organic matter 14C suggested that the largest input of recently fixed plant C occurred in the younger coniferous forest ecosystems. With increasing ecosystem age, the ratio of microbial C to total soil C was constant, whereas the ratio of microbial N to total soil N increased and gross N mineralization declined. Simultaneously, plant foliar N % decreased and the natural abundance of 15N in the soil increased. More specifically, the difference in δ15N between plant foliage and soil increased, which is related to relatively greater retention of 15N relative to 14N by ECM fungi as N is taken up from the soil and some N is transferred to the plant host. In the conifer forest, where these changes were greatest, we found increased fungal biomass in the F- and H-horizons of the mor-layer, in which ECM fungi are known to dominate (the uppermost horizon with litter and moss is dominated by saprotrophic fungi). Hence, we propose that the decreasing availability of N to the plants and the subsequent decline in plant production in ageing boreal forests is linked to high tree belowground C allocation to ECM fungi, a strong microbial sink for available soil N.

Aims: Subsoil organic carbon (OC) tends to be older and is presumed to be more stable than topsoil OC, but the reasons for this are not yet resolved. One hypothesis is that decomposition rates decrease with increasing soil depth. We tested whether decomposition rates of beech fine root litter varied with depth for a range of soils using a litterbag experiment in German beech forest plots. Methods: In three study regions (Schorfheide-Chorin, Hainich-Dün and Schwäbische-Alb), we buried 432 litterbags containing 0.5 g of standardized beech root material (fine roots with a similar chemical composition collected from 2 year old Fagus sylvatica L. saplings, root diameter < 2 mm) at three different soil depths (5, 20 and 35 cm). The decomposition rates as well as the changes in the carbon (C) and nitrogen (N) concentrations of the decomposing fine root litter were determined at a 6 months interval during a 2 years field experiment. Results: The amount of root litter remaining after 2 years of field incubation differed between the study regions (76 ± 2 % in Schorfheide-Chorin, 85 ± 2 % in Schwäbische-Alb, and 88 ± 2 % in Hainich-Dün) but did not vary with soil depth. Conclusions: Our results indicate that the initial fine root decomposition rates are more influenced by regional scale differences in environmental conditions including climate and soil parent material, than by changes in microbial activities with soil depth. Moreover, they suggest that a similar potential to decompose new resources in the form of root litter exists in both surface and deep soils.

The sensitivity of soil organic matter decomposition to global environmental change is a topic of prominent relevance for the global carbon cycle. Decomposition depends on multiple factors that are being altered simultaneously as a result of global environmental change; therefore, it is important to study the sensitivity of the rates of soil organic matter decomposition with respect to multiple and interacting drivers. In this manuscript, we present an analysis of the potential response of decomposition rates to simultaneous changes in temperature and moisture. To address this problem, we first present a theoretical framework to study the sensitivity of soil organic matter decomposition when multiple driving factors change simultaneously. We then apply this framework to models and data at different levels of abstraction: (1) to a mechanistic model that addresses the limitation of enzyme activity by simultaneous effects of temperature and soil water content, the latter controlling substrate supply and oxygen concentration for microbial activity; (2) to different mathematical functions used to represent temperature and moisture effects on decomposition in biogeochemical models. To contrast model predictions at these two levels of organization, we compiled different data sets of observed responses in field and laboratory studies. Then we applied our conceptual framework to: (3) observations of heterotrophic respiration at the ecosystem level; (4) laboratory experiments looking at the response of heterotrophic respiration to independent changes in moisture and temperature; and (5) ecosystem-level experiments manipulating soil temperature and water content simultaneously.

Over timescales of soil development (millennia), the capacity of soils to stabilize soil organic carbon (SOC) is linked to soil development through changes in soil mineralogy and other soil properties. In this study, an extensive dataset of soil profile chemistry and mineralogy is compiled from the Cowlitz River Chronosequence (CRC), WA USA. The CRC soils range in age from 0.25 to 1200kyr, spanning a developmental gradient encompassing clear changes in soil mineralogy, chemistry, and surface area. Comparison of these and other metrics of soil development with SOC properties reveal several relationships that may be diagnostic of the long-term coupling of soil development and C cycling. Specifically, SOC content was significantly correlated with sodium pyrophosphate extractable metals emphasizing the relevance of organo-metal complexes in volcanic soils. The depth distributions of organo-metals and other secondary weathering products, including the kaolin and short-range order (SRO) minerals, support the so-called "binary composition" of volcanic soils. The formation of organo-metal complexes limits the accumulation of secondary minerals in shallow soils, whereas in deep soils with lower SOC content, secondary minerals accumulate. In the CRC soils, secondary minerals formed in deep soils (below 50cm) including smectite, allophane, Fe-oxides and dominated by the kaolin mineral halloysite. The abundance of halloysite was significantly correlated with bulk soil surface area and 14C content (a proxy for the mean age of SOC), implying enhanced stability of C in deep soils. Allophane, an SRO mineral commonly associated with SOC storage, was not correlated with SOC content or 14C values in CRC soils. We propose conceptual framework to describe these observations based on a general understanding of pedogenesis in volcanic soils, where SOC cycling is coupled with soil development through the formation of and fate of organo-metal or other mobile weathering products. This framework highlights interactions between SOC and soil development, which may be applicable to other soils where organic inputs interact with the products of chemical weathering.

We report methane (CH4) concentration and methane oxidation (MO x) rate measurements from the eastern tropical north Pacific (ETNP) water column. This region comprises low-CH4 waters and a depth interval (~200-760 m) of CH4 supersaturation that is located within a regional oxygen minimum zone (OMZ). MO x rate measurements were made in parallel using tracer-based methods with low-level 14C-CH4 (LL 14C) and 3H-CH4 (3H). The two tracers showed similar trends in MO x rate with water depth, but consistent with previous work, the LL 14C rates (range: 0.034-15 × 10-3 nmol CH4 L-1 d-1) were systematically slower than the parallel 3H rates (range: 0.098-4000 × 10-3 nmol CH4 L-1 d-1). Priming and background effects associated with the 3H-CH4 tracer and LL 14C filtering effects are implicated as the cause of the systematic difference. The MO x rates reported here include some of the slowest rates measured in the ocean to date, are the first rates for the ETNP region, and show zones of slow CH4 turnover within the OMZ that may permit CH4 derived from coastal sediments to travel great lateral distances. The MO x rate constants correlate with both CH4 and oxygen concentrations, suggesting that their combined availability regulates MO x rates in the region. Depth-integrated MO x rates provide an upper limit on the magnitude of regional CH4 sources and demonstrate the importance of water column MO x, even at slow rates, as a sink for CH4 that limits the ocean-atmosphere CH4 flux in the ETNP region.

Radiocarbon is an important tracer of the global carbon cycle that helps to understand carbon dynamics in soils. It is useful to estimate rates of organic matter cycling as well as the mean residence or transit time of carbon in soils. We included a set of functions to model the fate of radiocarbon in soil organic matter within the SoilR package for the R environment for computing. Here we present the main system equations and functions to calculate the transfer and release of radiocarbon from different soil organic matter pools. Similarly, we present functions to calculate the mean transit time for different pools and the entire soil system. This new version of SoilR also includes a group of data sets describing the amount of radiocarbon in the atmosphere over time, data necessary to estimate the incorporation of radiocarbon in soils. Also, we present examples on how to obtain parameters of pool-based models from radiocarbon data using inverse parameter estimation. This implementation is general enough so it can also be used to trace the incorporation of radiocarbon in other natural systems that can be represented as linear dynamical systems.

To examine the effects of vineyard soil management on soil C and N content and quality, we studied harrowed and grass-covered vineyards on a soil developed on plio-pleistocene, marine sediments. A soil naturally covered by grasses adjacent to the vineyards served as control. To reach this goal, we assessed (1) the distribution of C and N and their 13C and 15N signatures in different soil organic matter pools, (2) the amount of C and N as live and dead vine fine roots and their 13C, 15N and 14C signatures, and (3) the stocks of C and N forms accumulated at two soil-depth intervals (0-50 and 50-100cm).Independent of the soil management, the vines increased the total organic C and total N content in the deeper soil horizons because of root turnover and rhizodeposition processes. In the upper horizons, a greater organic matter accumulation was fostered by the presence of the grass cover and the absence of tillage. The grass cover favoured the organic C storage mainly in the form of particulate and highly stabilised organic matter (humic acids and humin), and reduced the soil N content by plant uptake, whereas the harrowing produced a greater abundance of fulvic acids, which were mainly ascribed to oxidative processes enhanced by the soil tillage. In both vineyard soils, decaying vine roots represented an important source of organic C and N, especially in the deepest horizons. Indeed, isotope analyses revealed a more intense degradation of the dead vine roots in the deeper soil portion, where they likely constituted the main substrate for soil microorganisms. In the deepest horizons of the grass-covered vineyard, the greater mean residence time of the decaying vine roots and the lower root production were attributed to the easily available energetic substrates supplied by grass root turnover and rhizodeposition, which were preferentially used by microorganisms. This fact fostered a larger C accumulation in the grass-covered than in the harrowed vineyard. © 2014 Elsevier B.V.

Soils of temperate forests store significant amounts of organic matter and are considered to be net sinks of atmospheric CO2. Soil organic carbon (SOC) turnover has been studied using the δ14C values of bulk SOC or different SOC fractions as observational constraints in SOC models. Further, the δ14C values of CO2 that evolved during the incubation of soil and roots have been widely used together with δ14C of total soil respiration to partition soil respiration into heterotrophic respiration (HR) and rhizosphere respiration. However, these data have not been used as joint observational constraints to determine SOC turnover times. Thus, we focus on (1) how different combinations of observational constraints help to narrow estimates of turnover times and other parameters of a simple two-pool model, the Introductory Carbon Balance Model (ICBM); (2) whether relaxing the steady-state assumption in a multiple constraints approach allows the source/sink strength of the soil to be determined while estimating turnover times at the same time. To this end ICBM was adapted to model SOC and SO14C in parallel with litterfall and the δ14C of litterfall as driving variables. The δ14C of the atmosphere with its prominent bomb peak was used as a proxy for the δ14C of litterfall. Data from three spruce-dominated temperate forests in Germany and the USA (Coulissenhieb II, Solling D0 and Howland Tower site) were used to estimate the parameters of ICBM via Bayesian calibration. Key findings are as follows: (1) the joint use of all four observational constraints (SOC stock and its δ14C, HR flux and its δ14C) helped to considerably narrow turnover times of the young pool (primarily by δ14C of HR) and the old pool (primarily by δ14C of SOC). Furthermore, the joint use of all observational constraints made it possible to constrain the humification factor in ICBM, which describes the fraction of the annual outflux from the young pool that enters the old pool. The Bayesian parameter estimation yielded the following turnover times (mean ± standard deviation) for SOC in the young pool: Coulissenhieb II 1.1 ± 0.5 years, Solling D0 5.7 ± 0.8 years and Howland Tower 0.8 ± 0.4 years. Turnover times for the old pool were 377 ± 61 years (Coulissenhieb II), 313 ± 66 years (Solling D0) and 184 ± 42 years (Howland Tower), respectively. (2) At all three sites the multiple constraints approach was not able to determine if the soil has been losing or storing carbon. Nevertheless, the relaxed steady-state assumption hardly introduced any additional uncertainty for the other parameter estimates. Overall the results suggest that using δ14C data from more than one carbon pool or flux helps to better constrain SOC models. © 2014 Author(s).

Background and aims: Fine root decomposition contributes significantly to element cycling in terrestrial ecosystems. However, studies on root decomposition rates and on the factors that potentially influence them are fewer than those on leaf litter decomposition. To study the effects of region and land use intensity on fine root decomposition, we established a large scale study in three German regions with different climate regimes and soil properties. Methods In 150 forest and 150 grassland sites we deployed litterbags (100 μm mesh size) with standardized litter consisting of fine roots from European beech in forests and from a lowland mesophilous hay meadow in grasslands. In the central study region, we compared decomposition rates of this standardized litter with root litter collected on-site to separate the effect of litter quality from environmental factors. Results: Standardized herbaceous roots in grassland soils decomposed on average significantly faster (24 ± 6 % mass loss after 12 months, mean ± SD) than beech roots in forest soils (12 ± 4 %; p < 0.001). Fine root decomposition varied among the three study regions. Land use intensity, in particular N addition, decreased fine root decomposition in grasslands. The initial lignin:N ratio explained 15 % of the variance in grasslands and 11 % in forests. Soil moisture, soil temperature, and C:N ratios of soils together explained 34 % of the variance of the fine root mass loss in grasslands, and 24 % in forests. Conclusions: Grasslands, which have higher fine root biomass and root turnover compared to forests, also have higher rates of root decomposition. Our results further show that at the regional scale fine root decomposition is influenced by environmental variables such as soil moisture, soil temperature and soil nutrient content. Additional variation is explained by root litter quality. © 2014 The Author(s).

Current carbon cycle-climate models predict that future soil carbon storage will be determined by the balance between CO2 fertilization and warming. However, it is uncertain whether greater carbon inputs to soils with elevated CO2 will be sequestered, particularly since warming hastens soil carbon decomposition rates, and may alter the response of soils to new plant inputs. We studied the effects of elevated CO2 and warming on microbial soil carbon decomposition processes using laboratory manipulations of carbon inputs and soil temperature. We incubated soils from the Aspen Free Air CO2 Enrichment experiment, where no accumulation of soil carbon has been observed despite a decade of increased carbon inputs to soils under elevated CO2. We added isotopically-labeled sucrose to these soils in the laboratory to mimic and trace the effects of increased carbon inputs on soil organic carbon decomposition and its temperature sensitivity. Sucrose additions caused a positive priming of soil organic carbon decomposition, demonstrated by increased respiration derived from soil carbon, increased microbial abundance, and a shift in the microbial community towards faster growing microorganisms. Similar patterns were observed for elevated CO2 soils, suggesting that the priming effect was responsible for reductions in soil carbon accumulation at the site. Laboratory warming accelerated the rate of the priming effect, but the magnitude of the priming effect was not different amongst temperatures, suggesting that the priming effect was limited by substrate availability, not soil temperature. No changes in substrate use efficiency were observed with elevated CO2 or warming. The stimulatory effects of warming on the priming effect suggest that increased belowground carbon inputs from CO2 fertilization are not likely to be stored in mineral soils. © 2014 The Authors.

Soils are globally significant sources and sinks of atmospheric CO2. Increasing the resolution of soil carbon turnover estimates is important for predicting the response of soil carbon cycling to environmental change. We show that soil carbon turnover times can be more finely resolved using a dual isotope label like the one provided by elevated CO2 experiments that use fossil CO2. We modeled each soil physical fraction as two pools with different turnover times using the atmospheric 14C bomb spike in combination with the label in 14C and 13C provided by an elevated CO2 experiment in a California annual grassland. In sandstone and serpentine soils, the light fraction carbon was 21-54% fast cycling with 2-9 yr turnover, and 36-79% slow cycling with turnover slower than 100 yr. This validates model treatment of the light fraction as active and intermediate cycling carbon. The dense, mineral-associated fraction also had a very dynamic component, consisting of ∼7% fast-cycling carbon and ∼93% very slow cycling carbon. Similarly, half the microbial biomass carbon in the sandstone soil was more than 5 yr old, and 40% of the carbon respired by microbes had been fixed more than 5 yr ago. Resolving each density fraction into two pools revealed that only a small component of total soil carbon is responsible for most CO2 efflux from these soils. In the sandstone soil, 11% of soil carbon contributes more than 90% of the annual CO2 efflux. The fact that soil physical fractions, designed to isolate organic material of roughly homogeneous physico-chemical state, contain material of dramatically different turnover times is consistent with recent observations of rapid isotope incorporation into seemingly stable fractions and with emerging evidence for hot spots or micro-site variation of decomposition within the soil matrix. Predictions of soil carbon storage using a turnover time estimated with the assumption of a single pool per density fraction would greatly overestimate the near-term response to changes in productivity or decomposition rates. Therefore, these results suggest a slower initial change in soil carbon storage due to environmental change than has been assumed by simpler (one-pool) mass balance calculations.©Author(s) 2013. CC Attribution 3.0 License.

Two independent programs have collected and analyzed atmospheric CO2samples from Point Barrow, Alaska, for radiocarbon content (Δ14C) over the period 2003-2007. In one program, flask collection, stable isotope analysis, and CO2extraction are performed by the Scripps Institution of Oceanography's CO2Program and CO2is graphitized and measured by accelerator mass spectrometry (AMS) at Lawrence Livermore National Laboratory. In the other program, the University of California, Irvine, performs flask collection, sample preparation, and AMS. Over 22 common sample dates spanning 5 yr, differences in measured Δ14C are consistent with the reported uncertainties and there is no significant bias between the programs. © 2013 by the Arizona Board of Regents on behalf of the University of Arizona.

Fine roots are the most dynamic portion of a plant's root system and a major source of soil organic matter. By altering plant species diversity and composition, soil conditions and nutrient availability, and consequently belowground allocation and dynamics of root carbon (C) inputs, land-use and management changes may influence organic C storage in terrestrial ecosystems. In three German regions, we measured fine root radiocarbon (14C) content to estimate the mean time since C in root tissues was fixed from the atmosphere in 54 grassland and forest plots with different management and soil conditions. Although root biomass was on average greater in grasslands 5.1 ± 0.8 g (mean ± SE, Combining double low line 27) than in forests 3.1 ± 0.5 g (n Combining double low line 27) (p/0.05), the mean age of C in fine roots in forests averaged 11.3 ± 1.8 yr and was older and more variable compared to grasslands 1.7 ± 0.4 yr (ip < 0.001). We further found that management affects the mean age of fine root C in temperate grasslands mediated by changes in plant species diversity and composition. Fine root mean C age is positively correlated with plant diversity (Combining double low line 0.65) and with the number of perennial species ( Combining double low line 0.77). Fine root mean C age in grasslands was also affected by study region with averages of 0.7 ± 0.1 yr ( Combining double low line 9) on mostly organic soils in northern Germany and of 1.8 ± 0.3 yr (n Combining double low line 9) and 2.6 ± 0.3 (n/Combining double low line 9) in central and southern Germany (ip/i0.05). This was probably due to differences in soil nutrient contents and soil moisture conditions between study regions, which affected plant species diversity and the presence of perennial species. Our results indicate more long-lived roots or internal redistribution of C in perennial species and suggest linkages between fine root C age and management in grasslands. These findings improve our ability to predict and model belowground C fluxes across broader spatial scales. © 2012 Author(s).

Bio-char, biomass that has been deliberately charred to slow its rate of decomposition, has been proposed as an amendment with the potential to sequester carbon and improve certain soil properties. Slow pyrolysis (temperature ≤500°C) and hydrothermal carbonization (low temperature, high pressure) are two efficient methods to produce bio-char with high yield and are applicable to a broad range of feedstocks. Chars made using slow pyrolysis (PC) and hydrothermal carbonization (HTC) of the same feedstock material (corn, C4) differed in physical appearance, chemical properties and decomposition behavior. We added these HTC and PC chars as amendments to three soils with C3-derived organic matter that differed in clay content, pH, and land use (managed spruce forest, unmanaged deciduous forest and agriculture), and compared their impacts on carbon sequestration and net greenhouse gas (CO2, 13CO2, N2O and CH4) emissions. HTC addition (1% w/w) significantly increased CO2 emissions in all three soils (p

Understanding the variability and the current value of the Dole Effect, which has been used to infer past changes in biospheric productivity, requires accurate information on the isotopic discrimination associated with respiratory oxygen consumption in each of the biosphere components. Respiration in tree stems is an important component of the land carbon cycle. Here we measured, for the first time, the discrimination associated with tree stem oxygen uptake. The measurements included tropical forest trees, which are major contributors to the global fluxes of carbon and oxygen. We found discrimination in the range of 12.6-21.5g‰, indicating both diffusion limitation, resulting in O2 discrimination values below 20g‰, and alternative oxidase respiration, which resulted in discrimination values greater than 20g‰. Discrimination varied seasonally, between and within tree species. Calculations based on these results show that variability in woody plants discrimination can result in significant variations in the global Dole Effect. © 2012 Author(s).

We measured the 14C and 13C signatures of CO2 respired from surface and deep soils released through multiple dry/rewetting cycles in laboratory incubations. The C respired from surface soils included components fixed before and after the 1960s. However, that respired from deep soils was derived from organic matter with a mean turnover time estimated in the range of 650-850 years. This reinforces previous research suggesting that a substantial amount of deep soil C is chemically labile but physically inaccessible to microorganisms, but also suggests that substantial amounts of that C may not be so strongly bound to minerals as to be effectively inert, raising the question of why it hasn't already been metabolized. It also demonstrates the contribution of C fixed before the 1960s to CO2 metabolized in the surface soils at this site. © 2011 Elsevier Ltd.

Rooting depth affects soil profiles of water uptake and carbon inputs. Here we explore the importance of deep roots in a mature tropical forest of eastern Amazonia, where a throughfall exclusion experiment was conducted to test the resilience of the forest to experimentally induced drought. We hypothesized that soil water depletion occurred below the depth previously measured by sensors in 11-m-deep soil pits and that only a small root biomass is necessary to affect water uptake and the isotopic signature of soil CO2. A noninvasive electrical profiling method demonstrated greater depletion of soil water in the 11-18 m depth increment in the exclusion plot compared with the control plot by the end of the 3rd year of the experiment. A fine root biomass of only 0.1 g/cm3 measured at 3-6 m was sufficient for soil water drawdown and for imparting an isotopic signature of modern soil 14CO2 in both plots. A soil 13CO2 profile indicated drought stress in the exclusion plot. Fine root inputs of organic C to deep soils are small with respect to the carbon dynamics of the forest, but the deep rooting habit clearly affects the ecosystem water balance and profiles of soil CO2. © 2011 by the Society of American Foresters.

A snow addition experiment in moist acidic tussock tundra at Toolik Lake, Alaska, increased winter snow depths 2-3 m, and resulted in a doubling of the summer active layer depth. We used radiocarbon (Delta(14)C) to (1) determine the age of C respired in the deep soils under control and deepened active layer conditions (deep snow drifts), and (2) to determine the impact of increased snow and permafrost thawing on surface CO(2) efflux by partitioning respiration into autotrophic and heterotrophic components. Delta(14)C signatures of surface respiration were higher in the deep snow areas, reflecting a decrease in the proportion of autotrophic respiration. The radiocarbon age of soil pore CO(2) sampled near the maximum mid-July thaw depth was approximately 1,000 years in deep snow treatment plots (45-55 cm thaw depth), while CO(2) from the ambient snow areas was approximately 100 years old (30-cm thaw depth). Heterotrophic respiration Delta(14)C signatures from incubations were similar between the two snow depths for the organic horizon and were extremely variable in the mineral horizon, resulting in no significant differences between treatments in either month. Radiocarbon ages of heterotrophically respired C ranged from

Previous work has demonstrated that the sealed tube Zn reduction method for converting CO2 to graphite for AMS 14C measurements produces targets that can be measured with high precision and low background for samples of about 1 mg C down to approximately 0.1 mg C at the Keck Carbon Cycle AMS facility at the University of California, Irvine (KCCAMS). Now a modified method has been developed to prepare small-mass samples ranging from 0.015 to 0.1 mg C. In this modified method, the volume of the sealed reactor tube is reduced to ∼1.9 cm3, and the amounts of Zn and TiH2 reagents are reduced proportionally. The amount of Fe catalyst used remains the same to ensure a long lasting current in the AMS. Small-mass samples prepared by this method generally yield 12C+1 currents of about 0.5 μA per 1 μg C. An in situ simultaneous AMS δ13C measurement allows for correction of both graphitization and machine-induced isotopic fractionation, and is a prerequisite for high precision and accurate measurements using the Zn reducing method. Corrections for modern-carbon and dead-carbon background components are applied to samples based on small-mass samples of a 14C free material and of a modern standard covering the sample size range. It was discovered during additional investigation into lowering the modern-carbon background component that baking assembled reactor tubes at 300 °C for 1 h prior to use resulted in significantly lower modern-carbon background values. The accuracy and precision of small-mass samples prepared by this method are size dependent, but is usually ±10-15‰ for the smallest samples (0.015-0.02 mg C), based on duplicate measurements of primary and secondary standards. © 2009 Elsevier B.V. All rights reserved.

Accurate quantification of different soil organic carbon (SOC) fractions is needed to understand their relative importance in the global C cycle. Among the chemical methods of SOC fractionation, oxidative degradation is considered more promising because of its ability to mimic the natural microbial oxidative processes in soil. This study focuses on detailed understanding of changes in structural chemistry and isotopic signatures of SOC upon different oxidative treatments for assessing the ability of these chemicals to selectively isolate a refractory fraction of SOC. Replicated sampling (to ~1 m depth) of pedons classified as Typic Fragiudalf was conducted under four land uses (woodlot, grassland, no-till and conventional-till continuous corn [Zea mays L.]) at Wooster, OH. Soil samples (

The role of DOC for the build-up of soil organic carbon pools is still not well known, but it is thought to play a role in the transport of carbon to a greater depth where it becomes more stable. The aim of this study was to elucidate within-year dynamics of carbon transport from litter to the O (Oe and Oa) and A horizons. Mesocosms with constructed soil profiles were used to study dynamics of C transport from 14C-enriched (about 1000‰) leaf litter to the Oe/Oa and A horizons as well as the mineralization of leaf litter. The mesocosms were placed in the field for 17 months during which time fluxes and 14C content of DOC and CO2 were measured. Changes in 14C in leaf litter and bulk soil C pools were also recorded. Significant simultaneous release and immobilization of DOC occurring in both the O and A horizons was hypothesized. Contrary to our hypothesis, DOC released from the labeled Oi horizon was not retained within the Oe/Oa layer. DOC originating in the unlabeled Oe/Oa layer was also released for transport. Extensive retention of DOC occurred in the A horizon. DOC leaching from A horizon consisted of a mix of DOC from different sources, with a main fraction originating in the A horizon and a smaller fraction leached from the overlaying horizons. The C and 14C budget for the litter layer also indicated a surprisingly large amount of carbon with ambient Δ14C-signature to be respired from this layer. Data for this site also suggested significant contributions from throughfall to dissolved organic carbon (DOC) transport into and respiration from the litter layer. The results from this study showed that DOC retention was low in the O horizon and therefore not important for the O horizon carbon budget. In the A horizon DOC retention was extensive, but annual DOC input was small compared to C stocks and therefore not important for changes in soil C on an annual timescale. © 2008 Elsevier B.V.

We assessed the degree to which ectomycorrhizal fungi exploit organic nitrogen in situ. In an Alaskan boreal forest, we identified pairs of sporocarps from five taxa of ectomycorrhizal fungi. We added 13C-labeled alanine to the soil surrounding one sporocarp within each pair; the second served as an unlabeled control. Peak rates of 13C-respiration from alanine were higher in the labeled sporocarp plots than the controls, indicating that the 13C-alanine was detectably respired from the soil. “Reference” plots adjacent to the sporocarps served as an indication of background 13C-respiration rates released by the soil community as a whole. Ectomycorrhizal sporocarps displayed higher 13C-respiration rates than their reference plots. Thus, the sporocarps and associated mycorrhizal mycelium appeared to contribute significantly to the release of alanine-derived 13CO2, confirming the hypothesis that ectomycorrhizal fungi may access soil amino acid pools under natural conditions.

Nutrient availability in the arctic is expected to increase in the next century due to accelerated decomposition associated with warming and, to a lesser extent, increased nitrogen deposition. To explore how changes in nutrient availability affect ecosystem carbon (C) cycling, we used radiocarbon to quantify changes in belowground C dynamics associated with long-term fertilization of graminoid-dominated tussock tundra at Toolik Lake, Alaska. Since 1981, yearly fertilization with nitrogen (N) and phosphorus (P) has resulted in a shift to shrub-dominated vegetation. These combined changes have altered the quantity and quality of litter inputs, the vertical distribution and dynamics of fine roots, and the decomposition rate of soil organic C. The loss of C from the deep organic and mineral soil has more than offset the C accumulation in the litter and upper organic soil horizons. In the litter and upper organic horizons, radiocarbon measurements show that increased inputs resulted in overall C accumulation, despite being offset by increased decomposition in some soil pools. To reconcile radiocarbon observations in the deeper organic and mineral soil layers, where most of the ecosystem C loss occurred, both a decrease in input of new root material and a dramatic increase of decomposition rates in centuries-old soil C pools were required. Therefore, with future increases in nutrient availability, we may expect substantial losses of C which took centuries to accumulate. © 2007 Springer Science+Business Media, LLC.

The sealed tube zinc reduction method for converting CO2 to graphite for AMS 14C measurements was originally developed for rapid production of graphite in biomedical tracer experiments. The method was usually thought to have low precision and a high background. We have modified the zinc reduction method originally outlined in Vogel [J.S. Vogel, Radiocarbon 34 (3) (1992) 344] by carefully controlling the amounts of reagents (zinc, titanium hydride and Co or Fe catalyst) and now routinely obtain a precision of 2-3‰ and a relatively low background of ∼50,000 14C years when analyzing for 14C at the Keck Carbon Cycle AMS facility at UC Irvine. Fractionation of carbon isotopes does occur during graphitization and depends on the graphitization yield, which can be affected by the amounts of reagents used and other conditions. The δ13C of our zinc-reduced graphite is usually lighter by 2-3‰ than the CO2 from which it is made, but this is corrected for in our system by simultaneous measurement of 13C/12C along with 14C/12C by the spectrometer. This method is suitable for 14C enriched samples, as well as natural abundance 14C samples, especially those with modern 14C contents. With improved precision and background, we believe that many disciplines can benefit from this technique because of its low cost and rapid production of graphite. © 2007 Elsevier B.V. All rights reserved.

The major driving factors of soil CO2 production - substrate supply, temperature, and water content - vary vertically within the soil profile, with the greatest temporal variations of these factors usually near the soil surface. Several studies have demonstrated that wetting and drying of the organic horizon contributes to temporal variation in summertime soil CO2 efflux in forests, but this contribution is difficult to quantify. The objectives of this study were to partition CO2 production vertically in a mixed hardwood stand of the Harvard Forest, Massachusetts, USA, and then to use that partitioning to evaluate how the relative contributions of CO2 production by genetic soil horizon vary seasonally and interannually. We measured surface CO2 efflux and vertical soil profiles of CO2 concentration, temperature, water content, and soil physical characteristics. These data were applied to a model of effective diffusivity to estimate CO2 flux at the top of each genetic soil horizon and the production within each horizon. A sensitivity analysis revealed sources of uncertainty when applying a diffusivity model to a rocky soil with large spatial heterogeneity, especially estimates of bulk density and volumetric water content and matching measurements of profiles and surface fluxes. We conservatively estimate that the O horizon contributed 40-48% of the total annual soil CO2 efflux. Although the temperature sensitivity of CO2 production varied across soil horizons, the partitioning of CO2 production by horizon did not improve the overall prediction of surface CO2 effluxes based on temperature functions. However, vertical partitioning revealed that water content covaried with CO2 production only in the O horizon. Large interannual variations in estimates of O horizon CO2 production indicate that this layer could be an important transient interannual source or sink of ecosystem C. © 2006 Blackwell Publishing Ltd.

Soil moisture affects microbial decay of SOM and rhizosphere respiration (RR) in temperate forest soils, but isolating the response of soil respiration (SR) to summer drought and subsequent wetting is difficult because moisture changes are often confounded with temperature variation. We distinguished between temperature and moisture effects by simulation of prolonged soil droughts in a mixed deciduous forest at the Harvard Forest, Massachusetts. Roofs constructed over triplicate 5×5m2 plots excluded throughfall water during the summers of 2001 (168mm) and 2002 (344 mm), while adjacent control plots received ambient throughfall and the same natural temperature regime. In 2003, throughfall was not excluded to assess the response of SR under natural weather conditions after two prolonged summer droughts. Throughfall exclusion significantly decreased mean SR rate by 53mgCm-2h-1 over 84 days in 2001, and by 68mgCm-2h-1 over 126 days in 2002, representing 10-30% of annual SR in this forest and 35-75% of annual net ecosystem exchange (NEE) of C. The differences in SR were best explained by differences in gravimetric water content in the Oi horizon (r250.69) and the Oe/Oa horizon (r250.60). Volumetric water content of the A horizon was not significantly affected by throughfall exclusion. The radiocarbon signature of soil CO2 efflux and of CO2 respired during incubations of O horizon, A horizon and living roots allowed partitioning of SR into contributions from young C substrate (including RR) and from decomposition of older SOM. RR (root respiration and microbial respiration of young substrates in the rhizosphere) made up 43-71% of the total C respired in the control plots and 41-80% in the exclusion plots, and tended to increase with drought. An exception to this trend was an interesting increase in CO2 efflux of radiocarbon-rich substrates during a period of abundant growth of mushrooms. Our results suggest that prolonged summer droughts decrease primarily heterotrophic respiration in the O horizon, which could cause increases in the storage of soil organic carbon in this forest. However, the C stored during two summers of simulated drought was only partly released as increased respiration during the following summer of natural throughfall. We do not know if this soil C sink during drought is transient or long lasting. In any case, differential decomposition of the O horizon caused by interannual variation of precipitation probably contributes significantly to observed interannual variation of NEE in temperate forests. © 2005 Blackwell Publishing Ltd.

Separating ecosystem and soil respiration into autotrophic and heterotrophic component sources is necessary for understanding how the net ecosystem exchange of carbon (C) will respond to current and future changes in climate and vegetation. Here, we use an isotope mass balance method based on radiocarbon to partition respiration sources in three mature black spruce forest stands in Alaska. Radiocarbon (Δ14C) signatures of respired C reflect the age of substrate C and can be used to differentiate source pools within ecosystems. Recently-fixed C that fuels plant or microbial metabolism has Δ14C values close to that of current atmospheric CO2, while C respired from litter and soil organic matter decomposition will reflect the longer residence time of C in plant and soil C pools. Contrary to our expectations, the Δ14C of C respired by recently excised black spruce roots averaged 14% greater than expected for recently fixed photosynthetic products, indicating that some portion of the C fueling root metabolism was derived from C storage pools with turnover times of at least several years. The Δ14C values of C respired by heterotrophs in laboratory incubations of soil organic matter averaged 60% higher than the contemporary atmosphere Δ14CO2, indicating that the major contributors to decomposition are derived from a combination of sources consistent with a mean residence time of up to a decade. Comparing autotrophic and heterotrophic Δ14C end members with measurements of the Δ14C of total soil respiration, we calculated that 47-63% of soil CO2 emissions were derived from heterotrophic respiration across all three sites. Our limited temporal sampling also observed no significant differences in the partitioning of soil respiration in the early season compared with the late season. Future work is needed to address the reasons for high Δ14C values in root respiration and issues of whether this method fully captures the contribution of rhizosphere respiration. © 2005 Blackwell Publishing Ltd.

Soils play a key role in the global cycling of carbon (C), storing organic C, and releasing CO2 to the atmosphere. Although a large number of studies have focused on the CO2 flux at the soil-air interface, relatively few studies have examined the rates of CO2 production in individual layers of a soil profile. Deeper soil horizons often have high concentrations of CO2 in the soil air, but the sources of this CO2 and the spatiotemporal dynamics of CO2 production throughout the soil profile are poorly understood. We studied CO2 dynamics in six soil profiles arrayed across a grassland hillslope in coastal southern California. Gas probes were installed in each profile and gas samples were collected weekly or biweekly over a three-year period. Using soil air CO2 concentration data and a model based on Fick's law of diffusion, we modeled the rates of CO2 production with soil profile depth. The CO2 diffusion constants were checked for accuracy using measured soil air 222Rn activities. The modeled net CO2 production rates were compared with CO2 fluxes measured at the soil surface. In general, the modeled and measured net CO2 fluxes were very similar although the model consistently underestimated CO2 production rates in the surficial soil horizons when the soils were moist. Profile CO2 production rates were strongly affected by the inter- and intra-annual variability in rainfall; rates were generally 2-10 times higher in the wet season (December to May) than in the dry season (June to November). The El Niño event of 1997-1998, which brought above-average levels of rainfall to the study site, significantly increased CO2 production in both the surface and subsurface soil horizons. Whole profile CO2 production rates were approximately three times higher during the El Niño year than in the following years of near-average rainfall. During the dry season, when the net rates of CO2 flux from the soil profiles are relatively low (4-11 mg C- CO2 m-2 h-1), 20%-50% of the CO2 diffusing out of the profiles appears to originate in the relatively moist soil subsurface (defined here as those horizons below 40 cm in depth). The natural abundance 14C signatures of the CO2 and soil organic C suggest that the subsurface CO2 is derived from the microbial mineralization of recent organic C, possibly dissolved organic C transported to the subsurface horizons during the wet season. © 2005 Springer Science+Business Media, Inc.

We investigated the decomposability of soil organic matter (SOM) along a chronosequence of rainforest sites in Hawaii that form a natural fertility gradient and at two long-term fertilization experiments. To estimate turnover times and pool sizes of organic matter, we used two independent methods: (1) long-term incubations and (2) a three-box soil model constrained by radiocarbon measurements. Turnover times of slow-pool SOM (the intermediate pool between active and passive pools) calculated from incubations ranged from 6 to 20 y in the O horizon and were roughly half as fast in the A horizon. The radiocarbon-based model yielded a similar pattern but slower turnover times. The calculation of the 14C turnover times is sensitive to the lag time between photosynthesis and incorporation of organic C into SOM in a given horizon. By either method, turnover times at the different sites varied two- or threefold in soils with the same climate and vegetation community. Turnover times were fastest at the sites of highest soil fertility and were correlated with litter decay rates and primary productivity. However, experimental fertilization at the two least-fertile sites had only a small and inconsistent effect on turnover, with N slowing turnover and P slightly speeding it at one site. These results support studies of litter decomposition in suggesting that while plant productivity can respond rapidly to nutrient additions, decomposition may respond much more slowly to added nutrients. © 2005 Springer Science+Business Media, Inc.

Understanding how tropical forest carbon balance will respond to global change requires knowledge of individual heterotrophic and autotrophic respiratory sources, together with factors that control respiratory variability. We measured leaf, live wood, and soil respiration, along with additional environmental factors over a 1-yr period in a Central Amazon terra firme forest. Scaling these fluxes to the ecosystem, and combining our data with results from other studies, we estimated an average total ecosystem respiration (R eco) of 7.8 μmol·m-2·s-1. Average estimates (per unit ground area) for leaf, wood, soil, total heterotrophic, and total autotrophic respiration were 2.6, 1.1, 3.2, 5.6, and 2.2 μmol·m-2·s-1, respectively. Comparing autotrophic respiration with net primary production (NPP) estimates indicated that only ∼30% of carbon assimilated in photosynthesis was used to construct new tissues, with the remaining 70% being respired back to the atmosphere as autotrophic respiration. This low ecosystem carbon use efficiency (CUE) differs considerably from the relatively constant CUE of ∼0.5 found for temperate forests. Our Reco estimate was comparable to the above-canopy flux (Fac) from eddy covariance during defined sustained high turbulence conditions (when presumably Fac = Reco) of 8.4 (95% CI = 7.5-9.4). Multiple regression analysis demonstrated that ∼50% of the nighttime variability in Fac was accounted for by friction velocity (u*, a measure of turbulence) variables. After accounting for u* variability, mean Fac varied significantly with seasonal and daily changes in precipitation. A seasonal increase in precipitation resulted in a decrease in Fac, similar to our soil respiration response to moisture. The effect of daily changes in precipitation was complex: precipitation after a dry period resulted in a large increase in Fac, whereas additional precipitation after a rainy period had little effect. This response was similar to that of surface litter (coarse and fine), where respiration is greatly reduced when moisture is limiting, but increases markedly and quickly saturates with an increase in moisture.

Drying and wetting cycles of O horizon in forest soils have not received much attention, partly due to methodological limitations for nondestructive monitoring of the O horizon water content. The objective of this study was to determine the importance of moisture limitations in the O horizon of a temperate forest on summertime soil respiration. We measured soil respiration in three replicated plots in a mixed deciduous forest at Harvard Forest, Massachusetts, weekly from May to October 2001. Direct Current (DC) half-bridge sensors that had been calibrated using destructive samples of the Oi and Oe/ Oa horizons were placed in the Oi and Oe/Oa horizons to record hourly changes of gravimetric water contents. Soil temperature explained 47% of the variation in soil respiration using the Arrhenius equation. The residuals of the temperature model were linearly correlated with gravimetric water content of the Oi horizon (r2 = 0.72, P < 0.0001) and Oe/Oa horizon (r2 = 0.56, P < 0.001), indicating that temporal variation in soil respiration can be partly explained by water content of the O horizon. Additionally, a laboratory study was performed to evaluate drying/wetting cycles of the O horizon at constant temperature. Even small simulated rainfall amounts of 0.5 mm significantly increase CO2 flux from dry O horizon within a few minutes. The duration of CO2 pulses increased with the amount of applied water, lasting from a few hours to days. A strong correlation between CO2 release and water content of the O horizons demonstrates a clear regulatory role of litter water content on decomposition within the O horizons.

Rock fragments in soil can contain significant amounts of organic carbon. We investigated the nature and dynamics of organic matter in rock fragments in the upper horizons of a forest soil derived from sandstone and compared them with the fine earth fraction (

We investigated the variation of stable (δ13C) soil carbon isotopes in relation to depth in seven of the most important savanna areas to adjacent contiguous forests in the Amazon region. The δ13C of bulk organic matter in all profiles from forested sites increased with soil depth. In forest profiles from Amapá, Alter do Chão, and Roraima, the enrichment was less than 3.5‰ between deeper soil and surface layers, suggesting that C3 plants have remained the dominant vegetation cover. On the other hand, in forest soil profiles from Humaitá and Carolina sites, the δ13C enrichment was greater than 3.5‰, indicating the influence of past C4 vegetation or a mixture of C3/C4 vegetation (woody savanna). The surface δ13C values in the savanna profiles were 5-13‰ greater than the comparable forest profiles, indicating the influence of C4 vegetation. Two kinds of isotopic distribution were observed in deeper layers. The savanna profiles at Alter do Chão, Chapada dos Parecis, and Redenção had relatively constant δ13C values throughout the profile, suggesting minor past changes in the vegetation composition. In profiles at Amapá, Roraima, Humaitá, and Carolina, δ13C values decreased with depth from the surface and converged with comparable forest values, suggesting more woody savanna in the past than exists currently.

Soil organic matter (SOM) formation is one of the least understood steps of the global carbon cycle. An example is uncertainty around the role of plant communities in regulating SOM formation and turnover. Here we took advantage of the highly controlled conditions at the San Dimas lysimeter installation to quantify the influence of oak and pine vegetation on SOM dynamics. SOM turnover rates, estimated using total C and 14C content of litter and physically separable soil fractions, were faster under oak than under pine. In contrast to the rapid turnover for the oak litter (< 2 years), the delay in litter incorporation into the mineral soil under pine was a controlling factor of SOM fluxes. © 2001 Published by Elsevier Science Ltd.

To reconcile observations of decomposition rates, carbon inventories, and net primary production (NPP), we estimated long-term averages for C exchange in boreal forests near Thompson, Manitoba. Soil drainage as defined by water table, moss cover, and permafrost dynamics, is the dominant control on direct fire emissions. In upland forests, an average of about 10-30% of annual NPP was likely consumed by fire over the past 6500 years since these landforms and ecosystems were established. This long-term, average fire emission is much larger than has been accounted for in global C cycle models and may forecast an increase in fire activity for this region. While over decadal to century times these boreal forests may be acting as slight net sinks for C from the atmosphere to land, periods of drought and severe fire activity may result in net sources of C from these systems.

Despite the importance of Arctic and boreal regions in the present carbon cycle, estimates of annual high-latitude carbon fluxes vary in sign and magnitude. Without accurate estimates of current carbon fluxes from Arctic and boreal ecosystems, predicting the response of these systems to global change is daunting. A number of factors control carbon turnover in high-latitude soils, but because they are unique to northern systems, they are mostly ignored by biogeochemical models used to predict the response of these systems to global change. Here, we review those factors. First, many northern systems are dominated by mosses, whose extremely slow decomposition is not predicted by commonly used indices of litter quality. Second, cold temperature, permafrost, waterlogging, and substrate quality interact to stabilize soil organic matter, but the relative importance of these factors, and how they respond to climate change, is unknown. Third, recent evidence suggests that biological activity occuring over winter can contribute significantly to annual soil carbon fluxes. However, the controls over this winter activity remain poorly understood. Finally, processes at the landscape scale, such as fire, permafrost dynamics, and drainage, control regional carbon fluxes, complicating the extrapolation of site-level measurements to regional scales.

Soil organic matter (SOM) is a central contributor to soil quality as it mediates many of the chemical, physical, and biological processes controlling the capacity of a soil to perform successfully. SOM properties (e.g. C/N ratio, macro-organic matter) have been proposed as diagnostic criteria of overall soil fitness, but their use is hampered by a poor understanding of the basic biochemical principles underlying SOM processes. The objective of this project was to determine the influence of scrub oak (Quercus dumosa Nutt.) and Coulter pine (Pinus coulteri B. Don) vegetation on decomposition and SOM formation processes in a lysimeter installation constructed in 1936 in the San Gabriel mountains of Southern California. Soil samples archived during construction of the installation, and A horizons sampled in 1987, were fractionated according to density and mineral particle size to isolate the water floatable (macro-organic matter), fine silt and clay fractions. Carbon turnover rates were determined on all fractions from AMS 14C measurements. Solid state CPMAS TOSS 13C NMR was used to semiquantitatively characterize the chemical structure of organic matter on fresh litter and soil fractions. For the two soils, there was a progressive decrease in O-alkyl C, and an increase in alkyl and carbonyl C from the litter to the floatable, fine silt and clay fractions. These compositional differences were due to the oxidative degradation of the litter material, with preferential decomposition of the cellulose and hemicellulose entities and selective preservation of recalcitrant waxes and resins. In all soil fractions, turnover rates of carbon were longer for the pine than for the oak lysimeter (up to 10 times longer). Also under pine, there was a gradual increase in turnover rate progressing from the floatable to the clay fraction, and differences in turnover rates among fractions may be explained based on differences in carbon chemistry. In contrast, under oak, rapid carbon turnover for all fractions suggested intense biological activity in this soil. © 2000 Published by Elsevier Science B.V.

Understanding terrestrial carbon metabolism is critical because terrestrial ecosystems play a major role in the global carbon cycle. Furthermore, humans have severely disrupted the carbon cycle in ways that will alter the climate system and directly affect terrestrial metabolism. Changes in terrestrial metabolism may well be as important an indicator of global change as the changing temperature signal. Improving our understanding of the carbon cycle at various spatial and temporal scales will require the integration of multiple, complementary and independent methods that are used by different research communities. Tools such as air sampling networks, inverse numerical methods, and satellite data (top-down approaches) allow us to study the strength and location of the global- and continental-scale carbon sources and sinks. Bottom-up studies provide estimates of carbon fluxes at finer spatial scales and examine the mechanisms that control fluxes at the ecosystem, landscape, and regional scales. Bottom-up approaches include comparative and process studies (for example, ecosystem manipulative experiments) that provide the necessary mechanistic information to develop and validate terrestrial biospheric models. An iteration and reiteration of top-down and bottom-up approaches will be necessary to help constrain measurements at various scales. We propose a major international effort to coordinate and lead research programs of global scope of the carbon cycle.