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Respired CO2 in woody tissues radially diffuses to the atmosphere or it is transported upward with the transpiration stream, making the origin of CO2 in stem CO2 efflux (EA) uncertain, which may confound stem respiration (RS) estimates. An aqueous 13C-enriched solution was infused into stems of Populus tremula L. trees, and real-time measurements of 13C-CO2 and 12C-CO2 in EA were performed via Cavity Ring Down Laser Spectroscopy (CRDS). The contribution of locally respired CO2 (LCO2) and xylem-transported CO2 (TCO2) to EA was estimated from their different isotopic composition. Mean daily values of TCO2/EA ranged from 13% to 38%, evidencing the notable role that xylem CO2 transport plays in the assessment of stem respiration. Mean daily TCO2/EA did not differ between treatments of drought stress and light exclusion of woody tissues, but they showed different TCO2/EA dynamics on a sub-daily time scale. Sub-daily CO2 diffusion patterns were explained by a light-induced axial CO2 gradient ascribed to woody tissue photosynthesis, and the resistance to radial CO2 diffusion determined by bark water content. Here, we demonstrate the outstanding potential of CRDS paired with 13C-CO2 labelling to advance in the understanding of CO2 movement at the plant-atmosphere interface and the respiratory physiology in woody tissues.

Tropical forest soils are an important source and sink of greenhouse gases (GHGs), with tropical montane forests, in particular, having been poorly studied. The understanding of this ecosystem function is of vital importance for future climate change research. In this study, we explored soil fluxes of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) in four tropical forest sites located on the western flanks of the Andes in northern Ecuador. The measurements were carried out during the dry season from August to September 2018 and along an altitudinal gradient from 400 to 3010 m a.s.l. (above sea level). During this short-term campaign, our measurements showed (1) an unusual but marked increase in CO2 emissions at high altitude, possibly linked to changes in soil pH and/or root biomass, (2) a consistent atmospheric CH4 sink over all altitudes with high temporal and spatial variability, and (3) a transition from a net N2O source to sink along the altitudinal gradient. Our results provide arguments and insights for future and more detailed studies on tropical montane forests. Furthermore, they stress the relevance of using altitudinal transects as a biogeochemical open-air laboratory with a steep in situ environmental gradient over a limited spatial distance. Although short-term studies of temporal variations can improve our understanding of the mechanisms behind the production and consumption of soil GHGs, the inclusion of more rigorous sampling for forest management events, forest rotation cycles, soil type, hydrological conditions and drainage status, ground vegetation composition and cover, soil microclimate, and temporal (seasonality) and spatial (topographic positions) variability is needed in order to obtain more reliable estimates of the CO2, CH4, and N2O source/sink strength of tropical montane forests., Biogeosciences, 18 (2), ISSN:1726-4170

The depolymerization of soil organic matter, such as proteins and (oligo-)peptides, into monomers (e.g. amino acids) is currently considered to be the rate-limiting step for nitrogen (N) availability in terrestrial ecosystems. The mineralization of free amino acids (FAAs), liberated by the depolymerization of peptides, is an important fraction of the total mineralization of organic N. Hence, the accurate assessment of peptide depolymerization and FAA mineralization rates is important in order to gain a better process-based understanding of the soil N cycle. In this paper, we present an extended numerical 15N tracing model Ntrace, which incorporates the FAA pool and related N processes in order to provide a more robust and simultaneous quantification of depolymerization and gross mineralization rates of FAAs and soil organic N. We discuss analytical and numerical approaches for two forest soils, suggest improvements of the experimental work for future studies, and conclude that (i) when about half of all depolymerized peptide N is directly mineralized, FAA mineralization can be as important a rate-limiting step for total gross N mineralization as peptide depolymerization rate; (ii) gross FAA mineralization and FAA immobilization rates can be used to develop FAA use efficiency (NUEFAA), which can reveal microbial N or carbon (C) limitation.

Terrestrial ecosystems are strongly influenced by climate change and soils are key compartments of the globalcarbon (C) cycle in terms of their potential to store or release significant amounts of C. This study is part ofthe interregional IAEA Technical Cooperation Project INT5153 “Assessing the Impact of Climate Change and itsEffects on Soil and Water Resources in Polar and Mountainous Regions” and aims to elucidate driving factors(climatic versus geochemical) of soil organic carbon (SOC) stabilization and greenhouse gas emissions alongaltitudinal transects in different mountain regions.We present novel data from altitudinal transects of four different mountain regions (Zongo, Cordillera Real, Bo-livia; Mount Kilimanjaro, Tanzania; Gongga, Hengduan Mountains, China; Rauris, Hohe Tauern, Austria). Allaltitudinal transects cover a wide range of natural ecosystems under different climatic (MAT, MAP) and soil geo-chemical parameters. Bulk soil samples (four field replicates per ecosystems) were subjected to a combination ofaggregate and particle-size fractionation followed by organic C, total nitrogen (N), stable isotope (13C,15N) andradiocarbon (14C) analyses of all fractions. Bulk soils were further characterized for their texture, geochemistry(Na, K, Ca, Mg, CECpot, Al, Fe, Mn, Si, pH), nutrient status (NH+4, NO−3, Ptot)and incubated for 63 days to assessgreenhouse gas emissions (CO2, CH4, NO, N2O). Moreover, stable C isotope signatures of CO2were determinedto estimate potential sources of soil respiration (using Keeling plots).Cumulative soil CO2emissions (in gCO2-Ckg−1SOC)were highest for the high altitude grassland and forest sitesof Rauris (25.5-25.8) and lowest for the Kilimanjaro forests (4.8-6.9) as well as Bolivian high altitude grasslands(2.5-4.3). Soil CO2emissions were negatively correlated with SOC content (Pearson correlation coefficient rp=-0.35, p=0.002), showing that soils with low SOC contents release the highest amount of CO2per soil C, possiblydue to large fractions of unprotected SOC and thus low SOC stabilization.Particulate organic matter (POM) and sand content were positively correlated with CO2emissions (rp=0.43,p, Geophysical Research Abstracts, 20, ISSN:1029-7006, ISSN:1607-7962

There is currently a lack of necessary measurements and data to fully capture the high degree of heterogeneity inorganic carbon (OC) exported from eroding landscapes. The respective fluxes of modern (biospheric) and aged OCare poorly constrained, and mechanisms controlling OC export have remained elusive. Recently, the age of riverineOC was found to increase with the proportion of human-dominated landscapes within a catchment, potentially dueto aged soil OC that is mobilized by human disturbance and reintroduced into the modern C cycle (Butman et al.,2015). Furthermore, most previous studies have focused on catchment outlets, whereas a greater within-catchmentfocus will undoubtedly provide further direction to pinpoint locations of aged OC export to the river, and to betterconstrain its impact on the C cycle and aquatic biogeochemistry.This study aims (1) to quantify the sources and proportions of modern versus aged OC exported from soils intoriver systems within a catchment and (2) to investigate if theses sources and proportions of OC are dependent onthe different types of land-use within a catchment. Therefore, bulk14C and compound-specific14C analyses ofvegetation markers (long-chain fatty acids, FAs) in soils under different land-use and suspended river sedimentswill be performed and the proportions of modern versus aged OC determined by application of isotope mixingmodels.The study site is located in the mid-hill region of Nepal within a catchment that exhibits different land-uses includ-ing various types of forest and agricultural land-use. Soils were sampled before and sediments during monsoonseasons. A broad data basis is already available for the content and stable C isotopes (δ13C) of bulk OC and FAs.Fatty acids will be collected in sufficient amounts by preparative gas chromatography and will be analyzed for14Cusing accelerator mass spectrometry. Isotope mixing models that use bulk and compound-specific13C and14Cdata will be applied to estimate the relative contributions of modern versus aged OC.The study will generate an extensive14C dataset of bulk OC and specific vegetation markers in soils under differentland-use and suspended river sediments to provide a within-catchment focus on the mechanisms controlling OCexport from terrestrial and aquatic ecosystems. Combination with the available complementary datasets for13Cof bulk OC and specific vegetation markers will reveal the sources and proportions of modern versus aged OCwithin the catchment. This information is needed to develop accurate constraints on fluvial transfer of modernOC, which is of importance to predict OC fluxes quantitatively as a result of anthropogenic change. The studyfurthermore aims to test whether soils under different land-use export different proportions of modern versus agedOC into river systems and thus determines the responds to human disturbance such as changes in soil management.In summary, our study will contribute to a comprehensive understanding of the factors that control the sources,transport pathways and turnover times of terrestrial OC within river systems., Geophysical Research Abstracts, 20, ISSN:1029-7006, ISSN:1607-7962

A method was devised for analysing small discrete gas samples (50 mL syringe) by cavity ring-down spectroscopy (CRDS). Measurements were accomplished by inletting 50 mL syringed samples into an isotopic-CO2 CRDS analyser (Picarro G2131-i) between baseline readings of a reference air standard, which produced sharp peaks in the CRDS data feed. A custom software script was developed to manage the measurement process and aggregate sample data in real time. The method was successfully tested with CO2 mole fractions (xCO2) ranging from 20 000 ppm and δ13C–CO2 values from −100 up to +30 000 ‰ in comparison to VPDB (Vienna Pee Dee Belemnite). Throughput was typically 10 samples h−1, with 13 h−1 possible under ideal conditions. The measurement failure rate in routine use was ca. 1 %. Calibration to correct for memory effects was performed with gravimetric gas standards ranging from 0.05 to 2109 ppm xCO2 and δ13C–CO2 levels varying from −27.3 to +21 740 ‰. Repeatability tests demonstrated that method precision for 50 mL samples was ca. 0.05 % in xCO2 and 0.15 ‰ in δ13C–CO2 for CO2 compositions from 300 to 2000 ppm with natural abundance 13C. Long-term method consistency was tested over a 9-month period, with results showing no systematic measurement drift over time. Standardised analysis of discrete gas samples expands the scope of application for isotopic-CO2 CRDS and enhances its potential for replacing conventional isotope ratio measurement techniques. Our method involves minimal set-up costs and can be readily implemented in Picarro G2131-i and G2201-i analysers or tailored for use with other CRDS instruments and trace gases.

1 .pdf copy (1 Pag.) of the abstract published by the Organization. © Author(s) 2016. CC Attribution 3.0 License., In this contribution, key components of the model are evaluated in the context of common experimental designs for sediment fingerprinting studies namely simple, nested and distributed catchment sampling programmes. Illustrative examples using geochemical and compound specific stable isotope datasets are presented and used to discuss best practice with specific attention to (1) the tracer selection process, (2) incorporation of fixed effects relating to sample timeframe and sediment type in the modelling process, (3) deriving and using informative priors in sediment fingerprinting context and (4) transparency of the process and replication of model results by other users.

The role of eroding landscapes in organic carbon stabilization operating as C sinks or sources has been frequently discussed, but the underlying mechanisms are not fully understood. Our analysis aims to clarify the effects of soil redistribution on physical and biogeochemical soil organic carbon (SOC) stabilization mechanisms along a hillslope transect. The observed mineralogical differences seem partly responsible for the effectiveness of geochemical and physical SOC stabilization mechanisms as the mineral environment along the transect is highly variable and dynamic. The abundance of primary and secondary minerals and the weathering status of the investigated soils differ drastically along this transect. Extractable iron and aluminum components are generally abundant in aggregates, but show no strong correlation to SOC, indicating their importance for aggregate stability but not for SOC retention. We further show that pyrophosphate extractable soil components, especially manganese, play a role in stabilizing SOC within non-aggregated mineral fractions. The abundance of microbial residues and measured 14C ages for aggregated and non-aggregated SOC fractions demonstrate the importance of the combined effect of geochemical and physical protection to stabilize SOC after burial at the depositional site. Mineral alteration and the breakdown of aggregates limit the protection of C by minerals and within aggregates temporally. The 14C ages of buried soil indicate that C in aggregated fractions seems to be preserved more efficiently while C in non-aggregated fractions is released, allowing a re-sequestration of younger C with this fraction. Old 14C ages and at the same time high contents of microbial residues in aggregates suggest either that microorganisms feed on old carbon to build up microbial biomass or that these environments consisting of considerable amounts of old C are proper habitats for microorganisms and preserve their residues. Due to continuous soil weathering and, hence, weakening of protection mechanisms, a potential C sink through soil burial is finally temporally limited., Biogeosciences, 12 (5), ISSN:1726-4170