Bernd Zeller

Decomposition of 15N-labelled beech litter and fate of nitrogen derived from litter in a beech forest.

Abstract The decomposition and the fate of 15N- labelled beech litter was monitored in a beech forest (Vosges mountains, France) over 3 years. Circular plots around beech trees were isolated from neighbouring tree roots by soil trenching. After removal of the litter layer, 15N-labelled litter was distributed on the soil. Samples [labelled litter, soil (0–15 cm depths], fine roots, mycorrhizal root tips, leaves) were collected during the subsequent vegetation periods and analysed for total N and 15N concentration. Mass loss of the 15N-labelled litter was estimated using mass loss data from a litterbag experiment set up at the field site. An initial and rapid release of soluble N from the decomposing litter was balanced by the incorporation of exogenous N into the litter. Fungal N accounted for approximately 35% of the N incorporation. Over 2 years, litter N was continuously released and rates of N and mass loss were equivalent, while litter N was preferentially lost during the 3rd year. Released 15N accumulated essentially at the soil surface. 15N from the decomposing litter was rapidly (i.e. in 6 months) detected in roots and beech leaves and its level increased regularly and linearly over the course of the labelling experiment. After 3 years, about 2% of the original litter N had accumulated in the trees. 15N budgets indicated that soluble N was the main source for soil microbial biomass. Nitrogen accumulated in storage compounds was the main source of leaf N, while soil organic N was the main source of mycorrhizal N. Use of 15N-labelled beech litter as decomposing substrate allowed assessment of the fate of litter N in the soil and tree N pools in a beech forest on different time scales.

Tracing of recently assimilated carbon in respiration at high temporal resolution in the field with a tuneable diode laser absorption spectrometer after in situ 13CO2 pulse labelling of 20-year-old beech trees

The study of the fate of assimilated carbon in respiratory fluxes in the field is needed to resolve the residence and transfer times of carbon in the atmosphere-plant-soil system in forest ecosystems, but it requires high frequency measurements of the isotopic composition of evolved CO2. We developed a closed transparent chamber to label the whole crown of a tree and a labelling system capable of delivering a 3-h pulse of 99% 13CO2 in the field. The isotopic compositions of trunk and soil CO2 effluxes were recorded continuously on two labelled and one control trees by a tuneable diode laser absorption spectrometer during a 2-month chase period following the late summer labelling. The lag times for trunk CO2 effluxes are consistent with a phloem sap velocity of about 1 m h(-1). The isotopic composition (delta13C) of CO2 efflux from the trunk was maximal 2-3 days after labelling and declined thereafter following two exponential decays with a half-life of 2-8 days for the first and a half-life of 15-16 days for the second. The isotopic composition of the soil CO2 efflux was maximal 3-4 days after labelling and the decline was also well fitted with a sum of two exponential functions with a half-life of 3-5 days for the first exponential and a half-life of 16-18 days for the second. The amount of label recovered in CO2 efflux was around 10-15% of the assimilated 13CO2 for soil and 5-13% for trunks. As labelling occurred late in the growing season, substantial allocation to storage is expected.

NanoSIMS Study of Organic Matter Associated with Soil Aggregates: Advantages, Limitations, and Combination with STXM

Direct observations of processes occurring at the mineral-organic interface are increasingly seen as relevant for the cycling of both natural soil organic matter and organic contaminants in soils and sediments. Advanced analytical tools with the capability to visualize and characterize organic matter at the submicrometer scale, such as Nano Secondary Ion Mass Spectrometry (NanoSIMS) and Scanning Transmission X-ray Microscopy (STXM) coupled to Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS), may be combined to locate and characterize mineral-associated organic matter. Taking advantage of samples collected from a decadal (15)N litter labeling experiment in a temperate forest, we demonstrate the potential of NanoSIMS to image intact soil particles and to detect spots of isotopic enrichment even at low levels of (15)N application. We show how microsites of isotopic enrichment detected by NanoSIMS can be speciated by STXM-NEXAFS performed on the same particle. Finally, by showing how (15)N enrichment at one microsite could be linked to the presence of microbial metabolites, we emphasize the potential of this combined imaging and spectroscopic approach to link microenvironment with geochemical process and/or location with ecological function.

13C and 15N isotopic fractionation in trees, soils and fungi in a natural forest stand and a Norway spruce plantation

Abstract15N and 13C natural abundances of foliage, branches, trunks, litter, soil, fungal sporophores, mycorrhizas and mycelium were determined in two forest stands, a natural forest and a Norway spruce plantation, to obtain some insights into the role of the functional diversity of saprotrophic and ectomycorrhizal fungi in carbon and nitrogen cycles. Almost all saprotrophic fungi sporophores were enriched in 13C relative to their substrate. In contrast, they exhibited no or very little shift of δ15N. Judging from the amount of C discrimination, ectomycorrhizal fungi seem to acquire carbon from their host or from dead organic matter. Some ectomycorrhizal species seem able to acquire nitrogen from dead organic matter and could be able to transfer it to their host without nitrogen fractionation, while others supply their host with 15N-depleted nitrogen. Moreover ectomycorrhizal species displayed a significant N fractionation during sporophore differentiation, while saprotrophic fungi did not.RésuméLes abondances naturelles du 15N et du 13C de la masse foliaire, des branches, des troncs, de la litière, du sol, des carpophores, des mycorhizes et du mycélium, ont été déterminées dans deux peuplements forestiers, une forêt naturelle et une plantation d’épicéas, afin d’obtenir quelques précisions sur le rôle de la diversité fonctionnelle des champignons saprophytes et ectomycorhiziens dans le cycle du carbone et de l’azote. Presque tous les champignons saprophytes présentent un enrichissement en 13C relativement à leur substrat. Par contre, ils ne présentent pas ou ne présentent que très peu de modifications du δ15N. En fonction de leur taux de discrimination du carbone, les champignons ectomycorhiziens semblent pouvoir acquérir du carbone à la fois à partir de leur hôte et de la matière organique morte. Quelques espèces semblent capables d’acquérir de l’azote organique du sol et de le transférer sans fractionnement à leur hôte alors que d’autres fournissent leur hôte en azote appauvri en 15N. De plus, les espèces ectomycorhiziennes présentent un fractionnement significatif de l’azote pendant la différentiation des carpophores, alors que les champignons saprophytes n’en présentent pas.

Convergence of soil nitrogen isotopes across global climate gradients

Quantifying global patterns of terrestrial nitrogen (N) cycling is central to predicting future patterns of primary productivity, carbon sequestration, nutrient fluxes to aquatic systems, and climate forcing. With limited direct measures of soil N cycling at the global scale, syntheses of the 15N:14N ratio of soil organic matter across climate gradients provide key insights into understanding global patterns of N cycling. In synthesizing data from over 6000 soil samples, we show strong global relationships among soil N isotopes, mean annual temperature (MAT), mean annual precipitation (MAP), and the concentrations of organic carbon and clay in soil. In both hot ecosystems and dry ecosystems, soil organic matter was more enriched in 15N than in corresponding cold ecosystems or wet ecosystems. Below a MAT of 9.8°C, soil δ15N was invariant with MAT. At the global scale, soil organic C concentrations also declined with increasing MAT and decreasing MAP. After standardizing for variation among mineral soils in soil C and clay concentrations, soil δ15N showed no consistent trends across global climate and latitudinal gradients. Our analyses could place new constraints on interpretations of patterns of ecosystem N cycling and global budgets of gaseous N loss.

N deposition, N transformation and N leaching in acid forest soils

Nitrogen deposition, mineralisation, uptake and leaching were measured on a monthly basis in the field during 2 years in six forested stands on acidic soils under mountainous climate. Studies were conducted in three Douglas-fir [Pseudotsuga menziesii (Mirb.) Franco] plantations (D20: 20 year; D40: 40 yr; D60: 60 yr) on abandoned croplands in the Beaujolais Mounts; and two spruce (Picea abies Karst.) plantations (S45: 45 yr; S90: 90 yr) and an old beech (Fagus sylvatica L.) stand (B150: 150 yr) on ancient forest soils in a small catchment in the Vosges Mountains. N deposition in throughfall varied between 7–8 kg ha−1 year−1 (D20, B150, S45) and 15–21 kg ha−1 yr−1 (S90, D40, D60). N in annual litterfall varied between 20–29 kg ha−1 (D40, D60, S90), and 36–43 kg ha−1 (D20, S45, B150). N leaching below root depth varied among stands within a much larger range, between 1–9 kg ha−1 yr−1 (B150, S45, D60) and 28–66 kg ha−1 yr−1 (D40, S90, D20), with no simple relationship with N deposition, or N deposition minus N storage in stand biomass. N mineralisation was between 57–121 kg ha−1 yr−1 (S45, D40, S90) and between 176–209 kg ha−1 yr−1 in (B150, D60 and D20). The amounts of nitrogen annually mineralised and nitrified were positively related. Neither general soil parameters, such as pH, soil type, base saturation and C:N ratio, nor deposition in throughfall or litterfall were simply related to the intensity of mineralisation and/or nitrification. When root uptake was not allowed, nitrate leaching increased by 11 kg ha−1 yr−1 at S45, 36 kg ha−1 yr−1 at S90 and between 69 and 91 kg ha−1 yr−1 at D20, D40, B150 and D60, in relation to the nitrification rates of each plot. From this data set and recent data from the literature, we suggest that: high nitrification and nitrate leaching in Douglas-fir soils was likely related to the former agricultural land use. High nitrification rate but very low nitrate leaching in the old beech soil was related to intense recycling of mineralised N by beech roots. Medium nitrification and nitrate leaching in the old spruce stand was related to the average level of N deposition and to the deposition and declining health of the stand. Very low nitrification and N leaching in the young spruce stand were considered representative of fast growing spruce plantations receiving low N deposition on acidic soils of ancient coniferous forests. Consequently, we suggest that past land use and fine root cycling (which is dependent on to tree species and health) should be taken into account to explain the variability in the relation between N deposition and leaching in forests.

Influence of tree species on gross and net N transformations in forest soils

We compared N fluxes in a 150-year-old Fagus sylvatica coppice and five adjacent 25-year-old plantations of Fagus sylvatica, Picea abies, Quercus petraea, Pinus laricio and Pseudotsuga menziesii. We measured net N mineralization fluxes in the upper mineral horizon (A1, 0–5 cm) for 4 weeks and gross N mineralization fluxes for two days. Gross rates were measured during the 48-h period after addition of 15NH4 and 15NO3. Mineralization was measured by the 15NH4 dilution technique and gross nitrification by 15NO3 production from the addition of 15NH4, and by 15NO3 dilution. Net and gross N mineralization was lower in the soil of the old coppice, than in the plantations, both on a soil weight and organic nitrogen basis. Gross nitrification was also very low. Gross nitrification measured by NO3 dilution was slightly higher than measured by 15NO3 production from the addition of 15NH4. In the plantations, gross and net mineralization and nitrification from pool dilution were lowest in the spruce stand and highest in the beech and Corsican pine stands. We concluded that: (1) the low net mineralization in the soil of the old coppice was related to low gross rate of mineralization rather than to the concurrent effect of microbial immobilisation of mineral N; (2) the absence of nitrate in the old coppice was not related to the low rate of mineralization nor to the absence of nitrifyers, but most probably to the inhibition of nitrifyers in the moder humus; (3) substituting the old coppice by young stands favours nitrifyer communities; and (4) heterotrophic nitrifyers may bypass the ammonification step in these acid soils, but further research is needed to check this process and to characterize the microbial communities.RésuméNous avons mesuré les flux de minéralisation nette d’azote au cours d’une incubation de quatre semaines et les flux bruts d’azote au cours d’une incubation de deux jours dans 6 sols prélevés dans une comparaison d’espèces forestières. Nous avons comparé les horizons A1 d’un taillis sous futaie (TSF) de Fagus sylvatica et de cinq plantations adjacentes de 25 ans de Fagus sylvatica, Picea abies, Quercus petraea, Pinus laricio et Pseudotsuga menziesii. Les taux bruts ont été mesurés 48 h après l’addition de 15NH4, et 15NO3. La minéralisation brute a été calculée à partir de la dilution de 15NH4 et la nitrification brute à partir de la dilution de 15NO3 mais aussi de la production de 15NO3 à partir de l’apport de 15NH4. La minéralisation brute et nette est la plus basse dans le TSF, exprimée par gramme de sol ou d’azote organique. La nitrification nette et brute mesurée par enrichissement en 15NO3 est très faible, mais la nitrification brute est sensiblement plus élevée lorsqu’on l’évalue par dilution isotopique du 15NO3. Dans les plantations, la minéralisation et la nitrification brute et nette sont plus faibles sous épicéa et plus élevées sous hêtre et pin Laricio. Nous en concluons que (1) la faible minéralisation d’azote dans le TSF est directement liée à une faible minéralisation brute et non à l’expression d’une immobilisation microbienne de l’azote minéral formé; (2) l’absence de nitrate dans le TSF n’est pas liée à l’absence de nitrifiants mais plutôt à l’inhibition de leur activité sous le moder; (3) la coupe rase du TSF et sa plantation entraîne une levée partielle ou totale de cette inhibition; et (4) l’activité de nitrifiants hétérotrophes sans libération intermédiaire de NH4 est possible dans ces sols acides. Des études plus approfondies devraient permettre de vérifier ce point et d’identifier ces populations.

The Fate of 15N-Labelled Nitrogen Inputs to Coniferous and Broadleaf Forests

Nitrogen in forest soils is mainly composed of organic N compounds originating from litterfall. During leaf senescence of the forest vegetation, N compounds are either allocated to perennial tissues or remain in the leaf litter, mainly as polyphenol-protein condensates. For example, senescent beech leaves are composed of 45% cellulose and hemicellulose, 5 to 10% lignin and 25 to 35% brown polyphenol condensates which contain about 70% of the litter N (Berthelin et al. 1994). Beech litter has a C/N mass ratio of 50–70 and evolves into soil organic matter with a C/N ratio ranging from 10 to 30 depending on the humus type. These organic N compounds in forest soils are highly protected from major N losses due to their high chemical stability and low mobility.

Impact of Douglas-fir and Scots pine seedlings on plagioclase weathering under acidic conditions

The weathering of soil minerals in forest ecosystems increases nutrient availability for the trees. The rate of such weathering and its relative contribution to forest tree nutrition, is a major issue when evaluating present and potential forest stand productivity and sustainability. The current paper examines the weathering rate of plagioclase with and without Douglas-fir or Scots pine seedlings, in a laboratory experiment at pH 3–4 and 25 °C. All nutrients, with the exception of Ca, were supplied in sufficient amounts in a nutrient solution. The objective of the experiment was to evaluate the potential of trees to mobilise Ca from the mineral plagioclase that contained 12% of Ca. Amounts of nutrients supplied in the nutrient solution, amounts accumulated in the living tissue of the seedlings and amounts leached from the experimental vessels, were measured. A weathering balance, accounting for leached + accumulated − supplied amounts, was established. Bio-induced weathering, defined as the weathering increase in the presence of trees, relative to the weathering rate without trees (geochemical weathering; control vessels), under the present experimental conditions, explained on average, 40% of total weathering (biological + geochemical). These conditions appeared more beneficial to Scots pine (higher relative growth rate, higher Ca incorporation) than to Douglas-fir.

A multi-scale approach to determine accurate elemental and isotopic ratios by nano-scale secondary ion mass spectrometry imaging

RATIONALE Nano-scale secondary ion mass spectrometry (NanoSIMS) is still hampered by a lack of appropriate calibration method for the quantification of elemental and isotopic ratios in heterogeneous materials such as soil samples. The potential of (13)C-(15)N-labeled density fractions of soil to calibrate the C/N, (13)C/(12)C and (15)N/(14)N ratios provided by NanoSIMS was evaluated. METHODS The spatial organization of soil particles found at the macro- and micro-scales were compared. The C/N, (13)C/(12)C and (15)N/(14)N ratios measured at the macroscopic scale from different density fractions using an elemental analyzer coupled to an isotope ratio mass spectrometer (EA/IRMS) were compared with the corresponding micro-scale NanoSIMS measurements. When the macro- and micro-scales patterns were similar, macroscopic scale measurements obtained by EA/IRMS and the corresponding NanoSIMS C/N and (15)N/(14)N ratios averaged per fraction were used to obtain correction equations. The correction method using the internal calibration procedure was compared with the traditional one using a single organic standard. RESULTS It was demonstrated that the correction method using an internal calibration procedure was applicable for NanoSIMS images acquired on more than 500 µm(2) per fraction and provided more accurate C/N and (15)N/(14)N ratios than the traditional correction method. CONCLUSIONS As long as the NanoSIMS sampling was representative of the macroscopic properties, the correction method using an internal calibration procedure allowed better quantification of the isotope tracers and characterization of the C/N ratios. This method not only produced qualitative images, but also accurate quantitative parameters from which ecological interpretations can be derived.