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  • 1
  • 2
    Language: English
    In: Soil Biology and Biochemistry, 2010, Vol.42(3), pp.435-444
    Description: By measuring the isotopic signature of soil respiration, we seek to learn the isotopic composition of the carbon respired in the soil ( C ) so that we may draw inferences about ecosystem processes. Requisite to this goal is the need to understand how C is affected by both contributions of multiple carbon sources to respiration and fractionation due to soil gas transport. In this study, we measured potential isotopic sources to determine their contributions to C and we performed a series of experiments to investigate the impact of soil gas transport on C estimates. The objectives of these experiments were to: i) compare estimates of C derived from aboveground and belowground techniques, ii) evaluate the roles of diffusion and advection in a forest soil on the estimates of C , and iii) determine the contribution of new and old carbon sources to C for a Douglas-fir stand in the Pacific Northwest during our measurement period. We found a maximum difference of −2.36‰ between estimates of C based on aboveground vs. belowground measurements; the aboveground estimate was enriched relative to the belowground estimate. Soil gas transport during the experiment was primarily by diffusion and the average belowground estimate of C was enriched by 3.8–4.0‰ with respect to the source estimates from steady-state transport models. The affect of natural fluctuations in advective soil gas transport was little to non-existent; however, an advection–diffusion model was more accurate than a model based solely on diffusion in predicting the isotopic samples near the soil surface. Thus, estimates made from belowground gas samples will improve with an increase in samples near the soil surface. We measured a −1‰ difference in C as a result of an experiment where advection was induced, a value which may represent an upper limit in fractionation due to advective gas transport in forest ecosystems. We found that aboveground measurements of C may be particularly susceptible to atmospheric incursion, which may produce estimates that are enriched in C. The partitioning results attributed 69–98% of soil respiration to a source with a highly depleted isotopic signature similar to that of water-soluble carbon from foliage measured at our site.
    Keywords: Soil Respiration ; Carbon Isotope ; Advection ; Diffusion ; Steady-State ; Partitioning ; Douglas-Fir Forest ; Agriculture ; Chemistry
    ISSN: 0038-0717
    E-ISSN: 1879-3428
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  • 3
    Language: English
    In: Oecologia, 2010, Vol.163(1), pp.227-234
    Description: Patterns in the isotopic signal (stable C isotope composition; δ 13 C) of respiration (δ 13 C R ) have led to important gains in understanding the C metabolism of many systems. Contained within δ 13 C R is a record of the C source mineralized, the metabolic pathway of C and the environmental conditions during which respiration occurred. Because gas samples used for analysis of δ 13 C R contain a mixture of CO 2 from respiration and from the atmosphere, two-component mixing models are used to identify δ 13 C R . Measurement of ecosystem δ 13 C R , using canopy airspace gas samples, was one of the first applications of mixing models in ecosystem ecology, and thus recommendations and guidelines are based primarily on findings from these studies. However, as mixing models are applied to other experimental conditions these approaches may not be appropriate. For example, the range in [CO 2 ] obtained in gas samples from canopy air is generally less than 100 μmol mol −1 , whereas in studies of respiration from soil, foliage or tree stems, the range can span as much as 10,000 μmol mol −1 and greater. Does this larger range in [CO 2 ] influence the precision and accuracy of δ 13 C R estimates derived from mixing models? Does the outcome from using different regression approaches and mixing models vary depending on the range of [CO 2 ]? Our research addressed these questions using a simulation approach. We found that it is important to distinguish between large (〉1,000 μmol mol −1 ) and small (〈100 μmol mol −1 ) ranges of CO 2 when applying a mixing model (Keeling plot or Miller–Tans) and regression approach (ordinary least squares or geometric mean regression) combination to isotopic data. The combination of geometric mean regression and the Miller–Tans mixing model provided the most accurate and precise estimate of δ 13 C R when the range of CO 2 is ≥1,000 μmol mol −1 .
    Keywords: Carbon ; Respiration ; Soil ; Regression ; Isotope
    ISSN: 0029-8549
    E-ISSN: 1432-1939
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  • 4
    Language: English
    In: PloS one, 2015, Vol.10(4), pp.e0122539
    Description: Soil microbial communities play an important role in forest ecosystem functioning, but how climate change will affect the community composition and consequently bacterial functions is poorly understood. We assessed the effects of reduced precipitation with the aim of simulating realistic future drought conditions for one growing season on the bacterial community and its relation to soil properties and forest management. We manipulated precipitation in beech and conifer forest plots managed at different levels of intensity in three different regions across Germany. The precipitation reduction decreased soil water content across the growing season by between 2 to 8% depending on plot and region. T-RFLP analysis and pyrosequencing of the 16S rRNA gene were used to study the total soil bacterial community and its active members after six months of precipitation reduction. The effect of reduced precipitation on the total bacterial community structure was negligible while significant effects could be observed for the active bacteria. However, the effect was secondary to the stronger influence of specific soil characteristics across the three regions and management selection of overstorey tree species and their respective understorey vegetation. The impact of reduced precipitation differed between the studied plots; however, we could not determine the particular parameters being able to modify the response of the active bacterial community among plots. We conclude that the moderate drought induced by the precipitation manipulation treatment started to affect the active but not the total bacterial community, which points to an adequate resistance of the soil microbial system over one growing season.
    Keywords: Droughts ; Soil Microbiology ; Bacteria -- Drug Effects ; Microbial Consortia -- Drug Effects ; RNA, Ribosomal, 16s -- Genetics ; Water -- Pharmacology
    E-ISSN: 1932-6203
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  • 5
    In: Ecosphere, May 2016, Vol.7(5), pp.n/a-n/a
    Description: Crop fields are cultivated across continuities of soil, topography, and local climate that drive biological processes and nutrient cycling at the landscape scale; yet land management and agricultural research are often performed at the field scale, potentially neglecting the context of the surrounding...
    Keywords: Agricultural Landscape ; Isoscape ; Land Management ; Land‐Use Change And Impacts ; Spatial Visualization ; Stable Isotopes
    ISSN: 2150-8925
    E-ISSN: 2150-8925
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  • 6
    In: Ecohydrology, March 2018, Vol.11(2), pp.n/a-n/a
    Description: Kettle holes are glacially created ponds that form within landscape depressions and are numerous across young moraine landscapes. Kettle hole water budgets are based primarily on winter precipitation, and therefore, undergo pronounced short‐term changes in water level fluctuations. Little is known about kettle hole sediment biogeochemistry in NE Germany, especially with regards to hydroperiod. Our objective for this study was to link the abiotic influences demarked by the evaporative isotopic signal from kettle hole water and solute chemistry to sediment organic matter turnover imprinted in the sediment δC and δN isotopic values. From the 20 kettle holes we sampled, 19 of these completely dried out, but on different dates. This dynamic was partially explained by longitudinal and elevational changes over the catchment area illustrating regional controls of kettle hole water balance. At the scale of an individual kettle hole, we estimated evaporation explained up to 38% of water volume loss. The changes in water levels were weakly related to differences in surface sediment elemental N and C concentrations between kettle hole edge and centre positions. These dynamics were primarily driven by redox conditions, Ca, and several nutrient concentrations (dissolved organic carbon, total dissolved nitrogen, total dissolved P, and ammonium) in the water column. Although we did not detect differences in the surface sediment δC and δN values, the δN signature in relation to the C:N ratio highlights the advanced decomposition state of surface sediment OM in temporarily water filled kettle holes.
    Keywords: Evaporation ; Hydrologic Connectivity ; Hydroperiod ; Kettle Holes ; Sediment Biogeochemistry ; Stable Isotopes
    ISSN: 1936-0584
    E-ISSN: 1936-0592
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  • 7
    Language: English
    In: Scientific reports, 19 July 2018, Vol.8(1), pp.10904
    Description: Southwest Siberia encompasses the forest-steppe and sub-taiga climatic zones and has historically been utilized for agriculture. Coinciding with predicted changes in climate for the region is the pressure of agricultural development; however, a characterization of the soil water and carbon dynamics is lacking. We assessed current soil water properties and soil organic carbon turnover in forests and grasslands for two sites that span the forest steppe and sub-taiga bioclimatic zones. Soil evaporation was 0.62 ± 0.17 mm d (mean ± standard error) in grasslands and 0.45 ± 0.08 mm d in the forests of the forest-steppe site. Evaporation at the sub-taiga site was 1.80 ± 1.70 mm d in grasslands and 0.96 ± 0.05 mm d in forest plots. Evaporation was significantly greater at the sub-taiga site than the forest-steppe site. The density of fine roots explained the soil water isotopic patterns between vegetation types and sites. We found soil organic matter turnover to be three times faster in the sub-taiga site than in the forest-steppe site. Our results show that while climate factors, in particular snow levels, between the two sites are drivers for water and carbon cycles, site level hydrology, soil characteristics, and vegetation directly interact to influence the water and carbon dynamics.
    Keywords: Article;
    E-ISSN: 2045-2322
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  • 8
    In: New Phytologist, October 2013, Vol.200(1), pp.144-157
    Description: The oxygen stable isotope composition of plant organic matter (OM) (particularly of wood and cellulose in the tree ring archive) is valuable in studies of plant–climate interaction, but there is a lack of information on the transfer of the isotope signal from the leaf to heterotrophic tissues. We studied the oxygen isotopic composition and its enrichment above source water of leaf water over diel courses in five tree species covering a broad range of life forms. We tracked the transfer of the isotopic signal to leaf water‐soluble OM and further to phloem‐transported OM. Observed leaf water evaporative enrichment was consistent with values predicted from mechanistic models taking into account nonsteady‐state conditions. While leaf water‐soluble OM showed the expected 18O enrichment in all species, phloem sugars were less enriched than expected from leaf water enrichment in Scots pine (Pinus sylvestris), European larch (Larix decidua) and Alpine ash (Eucalyptus delegatensis). Oxygen atom exchange with nonenriched water during phloem loading and transport, as well as a significant contribution of assimilates from bark photosynthesis, can explain these phloem 18O enrichment patterns. Our results indicate species‐specific uncoupling between the leaf water and the OM oxygen isotope signal, which is important for the interpretation of tree ring data.
    Keywords: Broadleaf ; Conifer ; Diel Course ; Oxygen Atom Exchange ; Phloem Transport
    ISSN: 0028-646X
    E-ISSN: 1469-8137
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  • 9
    In: Global Change Biology, August 2016, Vol.22(8), pp.2861-2874
    Description: Drought duration and intensity are expected to increase with global climate change. How changes in water availability and temperature affect the combined plant–soil–microorganism response remains uncertain. We excavated soil monoliths from a beech ( L.) forest, thus keeping the understory plant–microbe communities intact, imposed an extreme climate event, consisting of drought and/or a single heat‐pulse event, and followed microbial community dynamics over a time period of 28 days. During the treatment, we labeled the canopy with with the goal of (i) determining the strength of plant–microbe carbon linkages under control, drought, heat and heat–drought treatments and (ii) characterizing microbial groups that are tightly linked to the plant–soil carbon continuum based on C‐labeled s. Additionally, we used 16S sequencing of bacteria from the Ah horizon to determine the short‐term changes in the active microbial community. The treatments did not sever within‐plant transport over the experiment, and carbon sinks belowground were still active. Based on the relative distribution of labeled carbon to roots and microbial s, we determined that soil microbes appear to have a stronger carbon sink strength during environmental stress. High‐throughput sequencing of the 16S revealed multiple trajectories in microbial community shifts within the different treatments. Heat in combination with drought had a clear negative effect on microbial diversity and resulted in a distinct shift in the microbial community structure that also corresponded to the lowest level of label found in the s. Hence, the strongest changes in microbial abundances occurred in the heat–drought treatment where plants were most severely affected. Our study suggests that many of the shifts in the microbial communities that we might expect from extreme environmental stress will result from the plant–soil–microbial dynamics rather than from direct effects of drought and heat on soil microbes alone.
    Keywords: 13 Co 2 Pulse Labeling ; 16s Rrna Next‐Generation Sequencing ; Climate Extremes ; Drought ; Forest Understory ; Heat‐Pulse ; Microbial Community Structure ; Plant–Soil–Microbe Carbon Continuum ; Plfa S
    ISSN: 1354-1013
    E-ISSN: 1365-2486
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  • 10
    Language: English
    In: Forest Ecology and Management, 15 June 2014, Vol.322, pp.27-36
    Description: Top dieback on Norway spruce has frequently occurred in stands of southern Norway and it is a serious threat to the productivity and stability of economically important spruce stands. The underlying dieback mechanisms are unclear; often the whole stand is not affected, but only individual trees. Drought stress is hypothesized as a crucial trigger for the onset of symptoms; therefore, we studied the response-effect relationships of water limitation and tree specific traits. We analyzed year ring anatomy, i.e. wood density, as an estimate of drought vulnerability, and carbon and oxygen isotope composition of the year rings as an estimate of leaf physiology. At two sites in SE Norway, we grouped declining and symptomless trees in direct vicinity of each other into pairs for comparison of anatomical and physiological traits. For one site, we observed a distinct lower wood density and higher radial growth of declining trees in comparison with the healthy trees over several years. We identified high vulnerability to cavitation due to lower wood density as a trait of individuals prone to dieback. We observed lower intrinsic water-use efficiency ( ) associated with increased stomatal conductance. The healthy trees had lower stomatal conductance, which most likely prevented water losses during dry periods. Within a population, we observed a trade-off between long-term growth performance under “average” conditions and a different response for “extreme” events. These resource strategies will be important for Norway spruce management, especially for regions facing an increase in the frequency of drought events.
    Keywords: Picea Abies ; Stable Isotope ; Wood Density ; Water-Use Efficiency ; Hydraulic Failure ; Carbon Starvation ; Forestry ; Biology
    ISSN: 0378-1127
    E-ISSN: 1872-7042
    Source: ScienceDirect Journals (Elsevier)
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