Plant silicon isotopic signature might reflect soil weathering degree

Plants fractionate Si isotopes which provides a useful Si tracer in the Si soil-plant cycle. This study reports plant Si content and Si-isotopic signatures in mature banana plants grown on soils with different weathering degree, but all developed from basaltic pyroclasts in the Mungo area, Cameroon. The δ³⁰Si compositions were determined in various plant parts and soil surface horizons by MC-ICP-MS in dry plasma mode with external Mg doping to a precision of ± 0.15‰ (± 2σ_SD). The Si-isotopic compositions in banana plants grown on weathered clayey soils (+0.54 ± 0.15‰) are heavier than on weakly developed soils rich in fresh ash and pumice (+0.02 ± 0.15‰). The corresponding bulk soils display lower δ³⁰Si value in weathered soil (?1.41‰) than in poorly developed soil (?0.41‰). We suggest that the dissolved Si source for the plant, governed firstly by dissolution of easily weatherable minerals, was isotopically enriched in heavy isotopes through clay formation over long periods. At seasonal to annual time scale, this source is influenced by a combination of following processes: Si adsorption of light isotopes onto Fe oxides, plant Si uptake and recycling in surface horizons. This would provide an isotopically heavier Si source in the more weathered soil since the Fe oxides content increases with weathering. Plant Si-isotopic signature might thus reflect the soil weathering degree. This study further suggests that in addition to weathering processes, rivers isotopic signatures likely depend on the fate of phytoliths in the soil-plant-river system.     

Long-term changes of the δ15N natural abundance of plants and soil in a temperate grassland

Tracing back the N use efficiency of long-term fertilizer trials is important for future management recommendations. Here we tested the changes in natural N-isotope composition as an indicator for N- management within a long-term fertilization lysimeter experiment in a low mountain range pasture ecosystem at Rengen (Eifel Mountains), Germany. Cattle slurry (δ¹⁵N?=?8.9?±?0.5‰) and mineral fertilizers (calcium ammonium nitrate; δ¹⁵N?=??1.0?±?0.2‰) were applied at a rate between 0 and 480 kg N ha^?1?yr^?1 throughout 20 years from 1985 onwards. In 2006, samples were taken from different grass species, coarse and fine particulate soil organic matter, bulk soil and leachates. Total soil N content hardly changed during fertilization experiment. As also N leaching has been small within the stagnant water regime, most N was lost through the gaseous phase beside plant uptake and cutting. Unlike N uptake by plants, the process of N volatilization resulted in strong discrimination against the ¹⁵N isotope. As a consequence, the δ¹⁵N values of top soil samples increased from 1.8?±?0.4‰ to 6.0?±?0.4‰ and that of the plants from ?1.2?±?1.3‰ to 4.8?±?1.2‰ with increasing N fertilizer rate. Samples receiving organic fertilizer were most enriched in δ¹⁵N. The results suggest that parts of the fertilizer N signal was preserved in soils and even discovered in soil organic matter pools with slow N turnover. However, a ¹⁵N/¹⁴N isotope fractionation of up to 1.5‰ added to the δ¹⁵N values recovered in soils and plants, rendering the increase in δ¹⁵N value a powerful indicator to long-term inefficient N usage and past N management in the terrestrial environment.     

Environmental and biological controls on methyl halide emissions from southern California coastal salt marshes

Methyl bromide (CH₃Br) and methyl chloride(CH₃Cl) emission rates from southernCalifornia coastal salt marshes show largespatial and temporal variabilities that arestrongly linked to biological and environmentalfactors. Here we discuss biogeochemical linesof evidence pointing to vegetation as theprimary source of CH₃Br and CH₃Clemissions from salt marshes. Sediments andmacroalgae do not appear to be major producersof these compounds, based on observations thatthe highest fluxes are not inhibited by soilinundation; their emissions are not correlatedwith those of certain gases produced in soils;and emissions from mudflat- andmacroalgae-dominated sites are relativelysmall. In contrast, the seasonal and spatialvariabilities of methyl halide fluxes in thesesalt marshes are consistent with the productionof these compounds by vascular plants, althoughthe possibility of production by microflora orfungi associated with the salt marsh vegetationis not ruled out. Flux chamber measurements ofemission rates are largely correlated to theoverall plant biomass enclosed in the chamber,but appear also to be highly dependent on thepredominant plant species. Emission ratesfollow a diurnal trend similar to the trends ofambient air temperature and photosyntheticallyactive radiation, but not surface soiltemperature. Diurnal variabilities in thecarbon isotope compositions of CH₃Cl andCH₃Br and their relative ratios ofemissions are consistent with simultaneouslycompeting mechanisms of uptake andproduction.     

Causes of variation in mineral soil C content and turnover in differently managed beech dominated forests

Background and aims

Forest soils are important carbon stores and considered as net CO₂ sinks over decadal to centennial time scales. Intensive forest management is thought to reduce the carbon sequestration potential of forest soils. Here we study the effects of decades of forest management (as unmanaged forest, forest under selection cutting, forest under age class management) on the turnover of mineral associated soil organic matter (MOM) in German beech (Fagus sylvatica L.) dominated forests.

Methods

Radiocarbon contents were determined by accelerator mass spectrometry (AMS) in 79 Ah horizon MOM fractions of Cambisols (n?=?13), Luvisols (n?=?51) and Stagnosols (n?=?15). Mean residence times (MRTs) for soil organic carbon (SOC) were estimated with a 2-pool model using the litter input derived from a forest inventory.

Results

MOM fractions from Ah horizons contained 64?±?8.8 % of the bulk SOC. The radiocarbon content of MOM fractions in Ah horizons, expressed as Δ¹⁴C, ranged between ?2.8?‰ and 114?‰ for the three soil groups. Almost all samples contained a detectable proportion of ‘bomb’ carbon fixed from the atmosphere since 1963. Under the assumption that depending on the soil texture between 19 % and 24 % of the SOC from the labile pool is transferred to the stable SOC pool, the corresponding MRTs ranged between 72 and 723 years, with a median of 164 years.

Conclusions

Our results indicate that the MOM fraction of Ah horizons from beech forests contained a high proportion of young carbon, but we did not find a significant decadal effect of forest management on the radiocarbon signature and related turnover times. Instead, both variables were controlled by clay contents and associated SOC concentrations (p?<?0.01). This underlines the importance of pedogenic properties for SOC turnover in the MOM fraction.     

Ectomycorrhizal fungi: A new source of atmospheric methyl halides?

Incomplete source budgets for methyl halides – compounds that release inorganic chlorine and bromine radicals which, in turn, catalyze atmospheric ozone depletion – limit our ability to predict the fate of the stratospheric ozone layer. We report here the first measured emissions of methyl chloride, methyl bromide, and methyl iodide from ectomycorrhizal fungi. We grew nine fungal isolates on growth media containing halide concentrations similar to those found in soils and plant tissues. The observed range of emissions was 0.003–65 μg methyl chloride, 0.001–3 μg methyl bromide, and 0.02–12 μg methyl iodide g^?1 dry weight fungi day^?1. Species varied in production rates of methyl chloride vs. methyl bromide vs. methyl iodide. Cenococcum geophilum, a widespread ectomycorrhizal fungus, was further tested to investigate the effects of halide substrate concentration in growth media. Emissions from this species increased linearly with increasing concentrations of both bromide and iodide. In addition, a subset of four fungi was studied with two media concentrations each of chloride, bromide, and iodide (0.2 or 20 mm ). These fungi had similar responses to halide concentration, despite 1000‐fold differences in baseline emission rates between isolates. Finally, high chloride concentrations (20 mm ) in media did not appear to inhibit emissions of methyl bromide or methyl iodide. Overall, ectomycorrhizal fungi might be an important source of methyl halides to the atmosphere, and substrate concentrations and community composition may influence production levels in ecosystems.     

Pigment production and isotopic fractionations in continuous culture: okenone producing purple sulfur bacteria Part II

Okenone is a carotenoid pigment unique to certain members of Chromatiaceae, the dominant family of purple sulfur bacteria (PSB) found in euxinic photic zones. Diagenetic alteration of okenone produces okenane, the only recognized molecular fossil unique to PSB. The in vivo concentrations of okenone and bacteriochlorophyll a (Bchl a) on a per cell basis were monitored and quantified as a function of light intensity in continuous cultures of the purple sulfur bacterium Marichromatium purpuratum (Mpurp1591). We show that okenone‐producing PSB have constant bacteriochlorophyll to carotenoid ratios in light‐harvesting antenna complexes. The in vivo concentrations of Bchl a, 0.151 ± 0.012 fmol cell^?1, and okenone, 0.103 ± 0.012 fmol cell^?1, were not dependent on average light intensity (10–225 Lux) at both steady and non‐steady states. This observation revealed that in autotrophic continuous cultures of Mpurp1591, there was a constant ratio for okenone to Bchl a of 1:1.5. Okenone was therefore constitutively produced in planktonic cultures of PSB, regardless of light intensity. This confirms the legitimacy of okenone as a signature for autotrophic planktonic PSB and by extrapolation water column euxinia. We measured the δ¹³C, δ¹⁵N, and δ³⁴S bulk biomass values from cells collected daily and determined the isotopic fractionations of Mpurp1591. There was no statistical relationship in the bulk isotope measurements or stable isotope fractionations to light intensity or cell density under steady and non‐steady‐state conditions. The carbon isotope fractionation between okenone and Bchl a with respect to overall bulk biomass (¹³ε_{pigment – biomass}) was 2.2 ± 0.4‰ and ?4.1 ± 0.9‰, respectively. The carbon isotopic fractionation () for the production of pigments in PSB is more variable than previously thought with our reported values for okenone at ?15.5 ± 1.2‰ and ?21.8 ± 1.7‰ for Bchl a.     

Microbiological and isotopic geochemical investigation of Lake Kislo-Sladkoe,a meromictic water body at the Kandalaksha Bay shore (White Sea)

Microbiological, biogeochemical, and isotopic geochemical investigation of Lake Kislo-Sladkoe (Polusolenoe in early publications) at the Kandalaksha Bay shore (White Sea) was carried out in September 2010. Lake Kislo-Sladkoe was formed in the mid-1900s out of a sea gulf due to a coastal heave. At the time of investigation, the surface layer was saturated with oxygen, while near-bottom water contained sulfide (up to 32 mg/L). Total number of microorganisms was high (12.3 × 10⁶ cells/mL on average). Light CO₂ fixation exhibited two pronounced peaks. In the oxic zone, the highest rates of photosynthesis were detected at 1.0 and 2.0 m. The second, more pronounced peak of light CO₂ fixation was associated with activity of anoxygenic phototrophic bacteria in the anoxic layer at the depth of 2.9 m (413 μg C L^?1 day^?1). Green-colored green sulfur bacteria (GSB) predominated in the upper anoxic layer (2.7–2.9 m), their numbers being as high as 1.12 × 10⁴ cells/mL, while brown-colored GSB predominated in the lower horizons. The rates of both sulfate reduction and methanogenesis peaked in the 2.9 m horizon (1690 μg S L^?1 day^?1 and 2.9 μL CH₄ L^?1 day^-1). The isotopic composition of dissolved methane from the near-bottom water layer (δ¹³C (CH₄) = ?87.76‰) was significantly lighter than in the upper horizons (δ¹³C (CH₄) = ?77.95‰). The most isotopically heavy methane (δ¹³C (CH₄) = ?72.61‰) was retrieved from the depth of 2.9 m. The rate of methane oxidation peaked in the same horizon. As a result of these reactions, organic matter (OM) carbon of the 2.9 m horizon became lighter (?36.36‰), while carbonate carbon became heavier (?7.56‰). Thus, our results demonstrated that Lake Kislo-Sladkoe is a stratified meromictic lake with active microbial cycles of carbon and sulfur. Suspended matter in the water column was mostly of autochthonous origin. Anoxygenic photo-synthesis coupled to utilization of reduced sulfur compounds contributed significantly to OM production.     

Spatial distribution of rhizodeposit carbon of maize (Zea mays L.) in soil aggregates assessed by multiple pulse 13C labeling in the field

Background and aims

Rhizosphere effect is controlled by spatial distribution of rhizodeposits, which may be influenced by soil aggregation and soil moisture regime in relation to water uptake by roots. The objectives of this study were to measure soil organic carbon (SOC) concentration and its δ¹³C abundance by aggregate size in the rooted bulk soil and by distance in the root-free soil vertically and horizontally away from roots, and to measure DOC concentration and its δ¹³C abundance in pore water in the rooted bulk soil after a seasonal pulse labelings of ¹³CO₂ to maize (Zea mays L.).

Methods

Pulse labeling was conducted in the field once a week for 11 weeks. Soil cells (50 mm in diameter and 100 mm long) mimicking root-free soils were imbedded vertically and horizontally 25–50 mm away from the main root of a maize crop. The rooted bulk soils were sampled to extract soil pore water at different suctions and to fractionate aggregates by wet sieving. The root-free soil cells were sliced by 1 mm intervals from the root end to 20 mm away. All the sampling was 12 days after the last labeling after the crop was harvested.

Results and discussion

The δ¹³C abundance before and after the continuous labeling was ?24.20?±?0.05?‰ and ?23.80?±?0.05?‰ in the rooted bulk soil. The labeling caused increases in δ¹³C abundance in all the aggregates in the rooted bulk soil and down to 14 mm away from the roots in both the root-free cells. The δ¹³C abundance was enriched in the >2 mm and 1–2 mm aggregates (?23.17?±?0.12?‰ and ?23.26?±?0.05?‰) though the SOC concentration was not different among the >0.25 mm aggregates, indicating that rhizodeposits or their metabolites were protected and distributed widely in whole soil through soil aggregation. The δ¹³C abundance in pore water (?24.0?±?0.01?‰) was much lower than those soil aggregates and greatest from the >2 μm soil pores though the DOC concentration was greater from the <20 μm soil pores. The δ¹³C abundance was in general greater in the horizontal cell than in the vertical cell. The δ¹³C abundance decreased with the increasing distance to the roots in the vertical cell and peaked at the 5 and 6 mm distance to the roots in the horizontal cell (?23.66?±?0.11?‰ and ?23.5?±?0.10?‰), possibly due to the drier condition unfavorable to microbial decomposition in the horizontal cell. The higher δ¹³C abundance in the horizontal cell than in the vertical cell was accompanied by a lower SOC concentration and a lower C: N ratio within 3 mm away from the roots, suggesting a stronger priming effect due to the longer residence time of rhizodeposits in the horizontal cell than in the vertical cell.

Conclusions

Rhizodeposits or their metabolites were protected during soil aggregation and distributed to 14 mm beyond the rhizosphere in the natural soil-plant system. This extension is of significance in regulating the formation of soil structure and the priming of soil organic matter during the whole life cycle of plants, which needs further study.     

Hydrologic regulation of gross methyl chloride and methyl bromide uptake from Alaskan Arctic tundra

The Arctic tundra has been shown to be a potentially significant regional sink for methyl chloride (CH₃Cl) and methyl bromide (CH₃Br), although prior field studies were spatially and temporally limited, and did not include gross flux measurements. Here we compare net and gross CH₃Cl and CH₃Br fluxes in the northern coastal plain and continental interior. As expected, both regions were net sinks for CH₃Cl and CH₃Br. Gross uptake rates (−793 nmol CH₃Cl m⁻² day⁻¹ and −20.3 nmol CH₃Br m⁻² day⁻¹) were 20–240% greater than net fluxes, suggesting that the Arctic is an even greater sink than previously believed. Hydrology was the principal regulator of methyl halide flux, with an overall trend towards increasing methyl halide uptake with decreasing soil moisture. Water table depth was one of the best predictors of net and gross uptake, with uptake increasing proportionately with water table depth. In drier areas, gross uptake was very high, averaging −1201 nmol CH₃Cl m⁻² day⁻¹ and −34.9 nmol CH₃Br m⁻² day⁻¹; in flooded areas, gross uptake was significantly lower, averaging −61 nmol CH₃Cl m⁻² day⁻¹ and −2.3 nmol CH₃Br m⁻² day⁻¹. Net and gross uptake was greater in the continental interior than in the northern coastal plain, presumably due to drier inland conditions. Within certain microtopographic features (low‐ and high‐centered polygons), uptake rates were positively correlated with soil temperature, indicating that temperature played a secondary role in methyl halide uptake. Incubations suggested that the inverse relationship between water content and methyl halide uptake was the result of mass transfer limitation in saturated soils, rather than because of reduced microbial activity under anaerobic conditions. These findings have potential regional significance, as the Arctic is expected to become warmer and drier due to anthropogenic climate forcing, potentially enhancing the Arctic sink for CH₃Cl and CH₃Br.     

Topographic controls on black carbon accumulation in Alaskan black spruce forest soils: implications for organic matter dynamics

There is still much uncertainty as to how wildfire affects the accumulation of burn residues (such as black carbon (BC)) in the soil, and the corresponding changes in soil organic carbon (SOC) composition in boreal forests. We investigated SOC and BC composition in black spruce forests on different landscape positions in Alaska, USA. Mean BC stocks in surface mineral soils (0.34 ± 0.09 kg C m^?2) were higher than in organic soils (0.17 ± 0.07 kg C m^?2), as determined at four sites by three different ¹³C Nuclear Magnetic Resonance Spectroscopy-based techniques. Aromatic carbon, protein, BC, and the alkyl:O-alkyl carbon ratio were higher in mineral soil than in organic soil horizons. There was no trend between mineral soil BC stocks and fire frequencies estimated from lake sediment records at four sites, and soil BC was relatively modern (<54–400 years, based on mean Δ¹⁴C ranging from 95.1 to ?54.7‰). A more extensive analysis (90 soil profiles) of mineral soil BC revealed that interactions among landscape position, organic layer depth, and bulk density explained most of the variance in soil BC across sites, with less soil BC occurring in relatively cold forests with deeper organic layers. We suggest that shallower organic layer depths and higher bulk densities found in warmer boreal forests are more favorable for BC production in wildfire, and more BC is integrated with mineral soil than organic horizons. Soil BC content likely reflected more recent burning conditions influenced by topography, and implications of this for SOC composition (e.g., aromaticity and protein content) are discussed.     

Abundance and activity of microorganisms at the water-sediment interface and their effect on the carbon isotopic composition of suspended organic matter and sediments of the Kara Sea

At ten stations of the meridian profile in the eastern Kara Sea from the Yenisei estuary through the shallow shelf and further through the St. Anna trough, total microbial numbers (TMN) determined by direct counting, total activity of the microbial community determined by dark CO₂ assimilation (DCA), and the carbon isotopic composition of organic matter in suspension and upper sediment horizons (δ¹³C, ‰) were investigated. Three horizons were studied in detail: (1) the near-bottom water layer (20–30 cm above the sediment); (2) the uppermost, strongly hydrated sediment horizon, further termed fluffy layer (5–10 mm); and (3) the upper sediment horizon (1–5 cm). Due to a decrease in the amount of isotopically light carbon of terrigenous origin with increasing distance from the Yenisei estuary, the TMN and DCA values decreased, and the δ¹³C changed gradually from ?29.7 to ?23.9‰. At most stations, a noticeable decrease in TMN and DCA values with depth was observed in the water column, while the carbon isotopic composition of suspended organic matter did not change significantly. Considerable changes of all parameters were detected in the interface zone: TMN and DCA increased in the sediments compared to their values in near-bottom water, while the 13C content increased significantly, with δ¹³C of organic matter in the sediments being at some stations 3.5–4.0‰ higher than in the near-bottom water. Due to insufficient illumination in the near-bottom zone, newly formed isotopically heavy organic matter (δ¹³C ～ ?20‰) could not be formed by photosynthesis; active growth of chemoautotrophic microorganisms in this zone is suggested, which may use reduced sulfur, nitrogen, and carbon compounds diffusing from anaerobic sediments. High DCA values for the interface zone samples confirm this hypothesis. Moreover, neutrophilic sulfur-oxidizing bacteria were retrieved from the samples of this zone.     

Long‐term effect of biochar on the stabilization of recent carbon: soils with historical inputs of charcoal

This study was set up to identify the long‐term effect of biochar on soil C sequestration of recent carbon inputs. Arable fields (n = 5) were found in Belgium with charcoal‐enriched black spots (>50 m²; n = 14) dating >150 years ago from historical charcoal production mound kilns. Topsoils from these ‘black spots’ had a higher organic C concentration [3.6 ± 0.9% organic carbon (OC)] than adjacent soils outside these black spots (2.1 ± 0.2% OC). The soils had been cropped with maize for at least 12 years which provided a continuous input of C with a C isotope signature (δ¹³C) ?13.1, distinct from the δ¹³C of soil organic carbon (?27.4 ‰) and charcoal (?25.7 ‰) collected in the surrounding area. The isotope signatures in the soil revealed that maize‐derived C concentration was significantly higher in charcoal‐amended samples (‘black spots’) than in adjacent unamended ones (0.44% vs. 0.31%; P = 0.02). Topsoils were subsequently collected as a gradient across two ‘black spots’ along with corresponding adjacent soils outside these black spots and soil respiration, and physical soil fractionation was conducted. Total soil respiration (130 days) was unaffected by charcoal, but the maize‐derived C respiration per unit maize‐derived OC in soil significantly decreased about half (P < 0.02) with increasing charcoal‐derived C in soil. Maize‐derived C was proportionally present more in protected soil aggregates in the presence of charcoal. The lower specific mineralization and increased C sequestration of recent C with charcoal are attributed to a combination of physical protection, C saturation of microbial communities and, potentially, slightly higher annual primary production. Overall, this study provides evidence of the capacity of biochar to enhance C sequestration in soils through reduced C turnover on the long term.     

Controls of nitrogen isotope patterns in soil profiles

To determine the dominant processes controlling nitrogen (N) dynamics in soils and increase insights into soil N cycling from nitrogen isotope (δ¹⁵N) data, patterns of ¹⁵N enrichment in soil profiles were compiled from studies on tropical, temperate, and boreal systems. The maximum ¹⁵N enrichment between litter and deeper soil layers varied strongly with mycorrhizal fungal association, averaging 9.6 ± 0.4‰ in ectomycorrhizal systems and 4.6 ± 0.5‰ in arbuscular mycorrhizal systems. The ¹⁵N enrichment varied little with mean annual temperature, precipitation, or nitrification rates. One main factor controlling ¹⁵N in soil profiles, fractionation against ¹⁵N during N transfer by mycorrhizal fungi to host plants, leads to ¹⁵N-depleted plant litter at the soil surface and ¹⁵N-enriched nitrogen of fungal origin at depth. The preferential preservation of ¹⁵N-enriched compounds during decomposition and stabilization is a second important factor. A third mechanism, N loss during nitrification and denitrification, may account for large ¹⁵N enrichments with depth in less N-limited forests and may account for soil profiles where maximum δ¹⁵N is at intermediate depths. Mixing among soil horizons should also decrease differences among soil horizons. We suggest that dynamic models of isotope distributions within soil profiles that can incorporate multiple processes could provide additional information about the history of nitrogen movements and transformations at a site.     

Trophic fractionation of carbon and nitrogen stable isotopes in Chironomus riparius reared on food of aquatic and terrestrial origin

1. Trophic fractionation was studied in short‐term laboratory feeding experiments with larvae of the deposit‐feeding midge Chironomus riparius. Larvae were fed food of terrestrial (oats, peat) and aquatic origin (Spirulina, Tetraphyll^®). 2. By analysing both whole larvae and isolated gut contents we were able to distinguish between the isotopic signature of recently ingested food and that of assimilated carbon and nitrogen in body tissue. Additionally we studied the effects of microbial conditioning, i.e. the colonisation and growth on food particles of microbes, on the isotopic signal of food resources. 3. Nitrogen fractionation for the different food types ranged from 0.67‰ to 2.68‰ between consumer and diet and showed that isotopic fractionation can be much lower than the value of 3.4‰ that is commonly assumed. 4. Microbial degradation of food particles resulted in an approximate doubling of the δ¹⁵N in 8 days, from 6.24 ± 0.05‰ to 11.36 ± 0.56‰. Values for δ¹³C increased only marginally, from ?20.66 ± 0.11‰ to ?20.34 ± 0.12‰. These results show that microbial conditioning of food may affect dietary isotope signatures (in particular N) and, unless accounted for, could introduce an error in measures of trophic fractionation. Microbial conditioning could well account for some of the variation in fractionation reported in the literature.     

Soil carbon inventories and δ13C along a moisture gradient in Botswana

We present a study of soil organic carbon (SOC) inventories and δ¹³C values for 625 soil cores collected from well‐drained, coarse‐textured soils in eight areas along a 1000 km moisture gradient from Southern Botswana, north into southern Zambia. The spatial distribution of trees and grass in the desert, savannah and woodland ecosystems along the transect control large systematic local variations in both SOC inventories and δ¹³C values. A stratified sampling approach was used to smooth this variability and obtain robust weighted‐mean estimates for both parameters. Weighted SOC inventories in the 0–5 cm interval of the soils range from 7 mg cm^?2 in the driest area (mean annual precipitation, MAP=225 mm) to 41±12 mg cm^?2 in the wettest area (MAP=910 mm). For the 0–30 cm interval, the inventories are 37.8 mg cm^?2 for the driest region and 157±33 mg cm^?2 for the wettest region. SOC inventories at intermediate sites increase as MAP increases to approximately 400–500 mm, but remain approximately constant thereafter. This plateau may be the result of feedbacks between MAP, fuel load and fire frequency. Weighted δ ¹³C values decrease linearly in both the 0–5 and 0–30 cm depth intervals as MAP increases. A value of –17.5±1.0‰ characterizes the driest areas, while a value of ?25±0.7‰ characterizes the wettest area. The decrease in δ ¹³C value with increasing MAP reflects an increasing dominance of C₃ vegetation as MAP increases. SOC in the deeper soil (5–30 cm depth) is, on average, 0.4±0.3‰ enriched in ¹³C relative to SOC in the 0–5 cm interval.     

Methane emissions from soils: synthesis and analysis of a large UK data set

Nearly 5000 chamber measurements of CH₄ flux were collated from 21 sites across the United Kingdom, covering a range of soil and vegetation types, to derive a parsimonious model that explains as much of the variability as possible, with the least input requirements. Mean fluxes ranged from ?0.3 to 27.4 nmol CH₄ m^?2 s^?1, with small emissions or low rates of net uptake in mineral soils (site means of ?0.3 to 0.7 nmol m^?2 s^?1) and much larger emissions from organic soils (site means of ?0.3 to 27.4 nmol m^?2 s^?1). Less than half of the observed variability in instantaneous fluxes could be explained by independent variables measured. The reasons for this include measurement error, stochastic processes and, probably most importantly, poor correspondence between the independent variables measured and the actual variables influencing the processes underlying methane production, transport and oxidation. When temporal variation was accounted for, and the fluxes averaged at larger spatial scales, simple models explained up to ca. 75% of the variance in CH₄ fluxes. Soil carbon, peat depth, soil moisture and pH together provided the best sub‐set of explanatory variables. However, where plant species composition data were available, this provided the highest explanatory power. Linear and nonlinear models generally fitted the data equally well, with the exception that soil moisture required a power transformation. To estimate the impact of changes in peatland water table on CH₄ emissions in the United Kingdom, an emission factor of +0.4 g CH₄ m^?2 yr^?1 per cm increase in water table height was derived from the data.     

Stable carbon isotope ratio of tree leaves, boles and fine litter in a tropical forest in Rondônia, Brazil

Leaves of 208 trees were collected for isotopic analysis together with wood from 36 tree boles and 18 samples of fine litter from a terra-firme forest located at Samuel Ecological Reserve, Rondônia State, in the southwestern Amazon region. The range of δ¹³C values in leaves was from ?28 to ?36‰, with an average (±1 SD) of ?32.1?±?1.5‰, which was more negative than the δ¹³C values of bole samples (?28.4?±?2.0‰) and fine litter (?28.7?±?2.0‰). These values are within the range found for tropical and subtropical forests. Pooling the δ¹³C values for leaf samples from trees of the same height gave averages which were positively correlated with plant height at a highly significant level, with a slope of 0.06 and an intercept of ?33.3‰ and a correlation coefficient r ²=0.70 (P<0.001).     

Nitrogen addition alters mineralization dynamics of 13C‐depleted leaf and twig litter and reduces leaching of older DOC from mineral soil

Recent reviews indicate that N deposition increases soil organic matter (SOM) storage in forests but the undelying processes are poorly understood. Our aim was to quantify the impacts of increased N inputs on soil C fluxes such as C mineralization and leaching of dissolved organic carbon (DOC) from different litter materials and native SOM. We added 5.5 g N m^?2 yr^?1 as NH₄NO₃ over 1 year to two beech forest stands on calcareous soils in the Swiss Jura. We replaced the native litter layer with ¹³C‐depleted twigs and leaves (δ¹³C: ?38.4 and ?40.8‰) in late fall and measured N effects on litter‐ and SOM‐derived C fluxes. Nitrogen addition did not significantly affect annual C losses through mineralization, but altered the temporal dynamics in litter mineralization: increased N inputs stimulated initial mineralization during winter (leaves: +25%; twigs: +22%), but suppressed rates in the subsequent summer. The switch from a positive to a negative response occurred earlier and more strongly for leaves than for twigs (?21% vs. 0%). Nitrogen addition did not influence microbial respiration from the nonlabeled calcareous mineral soil below the litter which contrasts with recent meta‐analysis primarily based on acidic soils. Leaching of DOC from the litter layer was not affected by NH₄NO₃ additions, but DOC fluxes from the mineral soils at 5 and 10 cm depth were significantly reduced by 17%. The ¹³C tracking indicated that litter‐derived C contributed less than 15% of the DOC flux from the mineral soil, with N additions not affecting this fraction. Hence, the suppressed DOC fluxes from the mineral soil at higher N inputs can be attributed to reduced mobilization of nonlitter derived ‘older’ DOC. We relate this decline to an altered solute chemistry by NH₄NO₃ additions, an increased ionic strength and acidification resulting from nitrification, rather than to a change in microbial decomposition.     

Mg and Ca root uptake and vertical transfer in soils assessed by an in situ ecosystem-scale multi-isotopic (26Mg & 44Ca) tracing experiment in a beech stand (Breuil-Chenue, France)

Background and aims

The sustainability of forest ecosystems may be at stake especially in forests on base-poor soils due to reduced nutrient deposition and intensified silvicultural practices. Understanding nutrient availability and cycling is therefore essential to manage forest soil fertility. This study aims to assess in a beech plot Mg and Ca vertical transfer in soil and root uptake using an isotopic tracing experiment.

Methods

A simulated rainfall containing a small amount (960 g?Mg.ha^?1; 530 g Ca.ha^?1) of highly enriched ²⁶Mg and ⁴⁴Ca was sprayed on the forest floor of a 35-yr-old beech plot. The isotopic composition of fine roots and of the soil exchangeable Mg and Ca pool was monitored during 1 year. A pool and flux model (IsoMod) was developed to predict the labeling of the soil and vertical transfer of tracers.

Results

Tracers (⁴⁴Ca and ²⁶Mg) were immediately retained in the thin litter layer. During the following year, Mg and to a lesser extent Ca were progressively released. After 1 year, the exchangeable Mg and Ca pools of the upper mineral layer (0–5 cm) were strongly labeled (~660?‰, representing ~55 % of the tracer input and ~370?‰, ~41 % of the tracer input respectively). A significant proportion (~8 % ²⁶Mg, ~2 % ⁴⁴Ca) of tracer was leached through the soil, below 10 cm. This amount was much larger than what was predicted using a simple mixing model. The Ca and Mg isotopic composition of fine roots at all depths was close or lower than that of exchangeable Ca and Mg respectively.

Conclusions

An in situ ecosystem-scale ²⁶Mg and ⁴⁴Ca isotopic tracing experiment was successfully carried out. Tracers were at first strongly retained in the litter layer, then progressively transferred to soil horizons below. Nutrient cycling of Mg and Ca were proven to be very different. Mg had a higher mobility in the soil than Ca, and nutrient uptake sources were proven to be different.     

Stoichiometry and temperature sensitivity of methanogenesis and CO2 production from saturated polygonal tundra in Barrow,Alaska

Arctic permafrost ecosystems store ~50% of global belowground carbon (C) that is vulnerable to increased microbial degradation with warmer active layer temperatures and thawing of the near surface permafrost. We used anoxic laboratory incubations to estimate anaerobic CO₂ production and methanogenesis in active layer (organic and mineral soil horizons) and permafrost samples from center, ridge and trough positions of water‐saturated low‐centered polygon in Barrow Environmental Observatory, Barrow AK, USA. Methane (CH₄) and CO₂ production rates and concentrations were determined at ?2, +4, or +8 °C for 60 day incubation period. Temporal dynamics of CO₂ production and methanogenesis at ?2 °C showed evidence of fundamentally different mechanisms of substrate limitation and inhibited microbial growth at soil water freezing points compared to warmer temperatures. Nonlinear regression better modeled the initial rates and estimates of Q₁₀ values for CO₂ that showed higher sensitivity in the organic‐rich soils of polygon center and trough than the relatively drier ridge soils. Methanogenesis generally exhibited a lag phase in the mineral soils that was significantly longer at ?2 °C in all horizons. Such discontinuity in CH₄ production between ?2 °C and the elevated temperatures (+4 and +8 °C) indicated the insufficient representation of methanogenesis on the basis of Q₁₀ values estimated from both linear and nonlinear models. Production rates for both CH₄ and CO₂ were substantially higher in organic horizons (20% to 40% wt. C) at all temperatures relative to mineral horizons (<20% wt. C). Permafrost horizon (~12% wt. C) produced ~5‐fold less CO₂ than the active layer and negligible CH₄. High concentrations of initial exchangeable Fe(II) and increasing accumulation rates signified the role of iron as terminal electron acceptors for anaerobic C degradation in the mineral horizons.