Roland Bol

δ13C values of soil organic carbon and their use in documenting vegetation change in a subtropical savanna ecosystem

Abstract Plants with C 3 , C 4 , and CAM photosynthesis have unique δ 13 C values which are not altered significantly during decomposition and soil organic matter formation. Consequently, δ 13 C values of soil organic carbon reflect the relative contribution of plant species with C 3 , C 4 , and CAM photosynthetic pathways to community net primary productivity, and have been utilized to document vegetation change, to quantify soil organic matter turnover, and to refine our understanding of earth–atmosphere–biosphere interactions. Here, we review the basis of this methodology, and illustrate its use as a tool for studying grass–woody plant dynamics in a savanna ecosystem. In the Rio Grande Plains of southern Texas, C 4 grasslands and savannas have been largely replaced by C 3 subtropical thorn woodlands dominated by Prosopis glandulosa . We used δ 13 C values of soil organic matter, above- and belowground plant biomass, and litter in conjunction with radiocarbon dating and dendrochronology to test the hypotheses that: (1) C 3 Prosopis groves in uplands and C 3 Prosopis woodlands in low-lying drainages have been long-term components of the landscape; and (2) Prosopis woodlands of low-lying drainages have expanded up-slope since Anglo-European settlement. Current organic matter inputs were not in isotopic equilibrium with soil organic carbon in any of the patch types sampled. In upland grasslands, δ 13 C values of vegetation (−20‰) were lower than those of soil organic matter (−17‰), suggesting increased C 3 forb abundance in response to long-term, heavy grazing (herbaceous retrogression). In wooded landscape elements, δ 13 C values of current organic matter inputs were characteristic of C 3 plants (−28 to −25‰), while those of the associated soil organic matter were typically −20 to −15‰. These δ 13 C values indicate that woodlands, groves, and shrub clusters dominated almost exclusively by C 3 plants now occupy sites once dominated by C 4 grasses. A particularly strong memory of the C 4 grasslands that once occupied these sites was recorded in the δ 13 C values of organic carbon associated with fine and coarse clay fractions (−18 to −14‰), probably a consequence of the slow organic carbon turnover rates in those soil fractions. When δ 13 C values of soil organic carbon were evaluated in conjunction with radiocarbon measurements of that same carbon, it appeared that herbaceous retrogression and a shift from C 4 grassland to C 3 woodland occurred recently, probably within the last 50–100 years. Demographic characteristics of the dominant tree species corroborated the δ 13 C and 14 C evidence, and indicated widespread establishment of P. glandulosa and associated shrubs over the past 100 years. Together, these data provide direct, spatially explicit evidence that vegetation change has occurred recently across the entire landscape at this site. Environmental conditions where C 3 , C 4 , and CAM plants coexist (e.g., dry, alkaline soils) generally do not favor the preservation of pollen and phytoliths, and these same areas usually lack historical records of vegetation change. Consequently, vegetation dynamics have been difficult to quantify in grasslands, savannas, and woodlands. However, our results demonstrate clearly that δ 13 C values of soil organic matter afford a direct and powerful technique for reconstructing vegetation change in these areas.

Molecular dynamics of organic matter in a cultivated soil.

The dynamics of soil organic carbon are included in global carbon (C) cycle scenarios using different, but generally arbitrary defined, kinetic pools. To improve global C models, better relationships between the chemical structure of soil organic matter (SOM) and its kinetic pools are needed. To assess the molecular residence time of SOM and the relation with plant inputs, pyrolysis–GC/MS–C–IRMS was performed on maize plants and on two samples from the same soil that had undergone a vegetation change from the C3 plant wheat to the C4 plant maize. This vegetation change has added naturally 13C-enriched material to the soil. Most pyrolysis products from the maize were derived from polysaccharides and lignins, and were not detected in soils. However, polysaccharide-derived products were also major pyrolysis products in soils, N-containing or unspecific pyrolysis products were also detected. The residence times (based on 13C natural labelling) revealed a continuum of values, that was independent of chemical structure, with only two pyrolysis products presenting a relatively long residence time (ca. 100 years). An unexpected long life-time for N-containing (∼49 years) and polysaccharide-derived (∼54 years) pyrolysis products was found. Our results suggest that mainly recycling of carbon in carbohydrates and N-containing materials in addition to physical and chemical protection is responsible for SOM stabilization in the slow carbon pool.

Soil microbes compete effectively with plants for organic-nitrogen inputs to temperate grasslands

Although agricultural grassland soils have inherently high rates of net nitrogen (N) mineralization, they often have soil concentrations of soluble organic N that are comparable to inorganic N. We set out to examine in situ the significance of organic N for plant nutrition in grasslands of differing management intensity and soil fertility. Using in situ dual-labeling techniques (glycine-2-13C-15N) we measured preferential uptake of amino-acid N vs. inorganic N [(15NH4)2SO4] in early and late season in low-productivity Agrostis capillaris–Festuca ovina grassland and in agriculturally improved, high-productivity Lolium perenne-dominated grassland. The dominant soluble-N form differed greatly between grasslands. Inorganic N (especially nitrate N) dominated the soluble N pool of the highly productive improved grassland whereas amino acid N was the dominant soluble N form in the low-productivity unimproved grassland. There was no difference in the amount of 15N taken up by plants from the two N forms in either grassland. However, our data indicate that amino-acid N was taken up directly by plants of both grasslands and that more N was captured in this way by plants of low-productivity grassland where amino acids were the dominant soluble N form in soil. Our data from both grasslands also indicate significant microbial competition for added 15N from both N sources, but especially in the low-productivity grassland where the bulk of 15N added was sequestered by the microbial biomass. A significantly greater amount of added 15N was captured by the microbial biomass in the unimproved than in the improved grassland, and substantially more 15N was detected in the microbial biomass than in plant tissue in the unimproved grassland. On the basis of our findings, we predict that subsequent microbial turnover and release of this N into the plant–soil system is the major pathway for plant N capture in these temperate grasslands. Microbial sequestration of added N might be an important mechanism of N retention in these grasslands, especially in the low-productivity systems where microbial N sink strength is greater and organic matter slowly accumulates.

Preferential uptake of soil nitrogen forms by grassland plant species

In this study, we assessed whether a range of temperate grassland species showed preferential uptake for different chemical forms of N, including inorganic N and a range of amino acids that commonly occur in temperate grassland soil. Preferential uptake of dual-labelled (13C and 15N) glycine, serine, arginine and phenylalanine, as compared to inorganic N, was tested using plants growing in pots with natural field soil. We selected five grass species representing a gradient from fertilised, productive pastures to extensive, low productivity pastures (Lolium perenne, Holcus lanatus, Anthoxanthum odoratum, Deschampsia flexuosa, and Nardus stricta). Our data show that all grass species were able to take up directly a diversity of soil amino acids of varying complexity. Moreover, we present evidence of marked inter-species differences in preferential use of chemical forms of N of varying complexity. L. perenne was relatively more effective at using inorganic N and glycine compared to the most complex amino acid phenylalanine, whereas N. stricta showed a significant preference for serine over inorganic N. Total plant N acquisition, measured as root and shoot concentration of labelled compounds, also revealed pronounced inter-species differences which were related to plant growth rate: plants with higher biomass production were found to take up more inorganic N. Our findings indicate that species-specific differences in direct uptake of different N forms combined with total N acquisition could explain changes in competitive dominance of grass species in grasslands of differing fertility.

Preferences for different nitrogen forms by coexisting plant species and soil microbes.

The growing awareness that plants might use a variety of nitrogen (N) forms, both organic and inorganic, has raised questions about the role of resource partitioning in plant communities. It has been proposed that coexisting plant species might be able to partition a limited N pool, thereby avoiding competition for resources, through the uptake of different chemical forms of N. In this study, we used in situ stable isotope labeling techniques to assess whether coexisting plant species of a temperate grassland (England, UK) display preferences for different chemical forms of N, including inorganic N and a range of amino acids of varying complexity. We also tested whether plants and soil microbes differ in their preference for different N forms, thereby relaxing competition for this limiting resource. We examined preferential uptake of a range of 13C15N-labeled amino acids (glycine, serine, and phenylalanine) and 15N-labeled inorganic N by coexisting grass species and soil microbes in the field. Our data show that while coexisting plant species simultaneously take up a variety of N forms, including inorganic N and amino acids, they all showed a preference for inorganic N over organic N and for simple over the more complex amino acids. Soil microbes outcompeted plants for added N after 50 hours, but in the long-term (33 days) the proportion of added 15N contained in the plant pool increased for all N forms except for phenylalanine, while the proportion in the microbial biomass declined relative to the first harvest. These findings suggest that in the longer-term plants become more effective competitors for added 15N. This might be due to microbial turnover releasing 15N back into the plant-soil system or to the mineralization and subsequent plant uptake of 15N transferred initially to the organic matter pool. We found no evidence that soil microbes preferentially utilize any of the N forms added, despite previous studies showing that microbial preferences for N forms vary over time. Our data suggest that coexisting plants can outcompete microbes for a variety of N forms, but that such plant species show similar preferences for inorganic over organic N.

Heterotrophic microbial communities use ancient carbon following glacial retreat.

When glaciers retreat they expose barren substrates that become colonized by organisms, beginning the process of primary succession. Recent studies reveal that heterotrophic microbial communities occur in newly exposed glacial substrates before autotrophic succession begins. This raises questions about how heterotrophic microbial communities function in the absence of carbon inputs from autotrophs. We measured patterns of soil organic matter development and changes in microbial community composition and carbon use along a 150-year chronosequence of a retreating glacier in the Austrian Alps. We found that soil microbial communities of recently deglaciated terrain differed markedly from those of later successional stages, being of lower biomass and higher abundance of bacteria relative to fungi. Moreover, we found that these initial microbial communities used ancient and recalcitrant carbon as an energy source, along with modern carbon. Only after more than 50 years of organic matter accumulation did the soil microbial community change to one supported primarily by modern carbon, most likely from recent plant production. Our findings suggest the existence of an initial stage of heterotrophic microbial community development that precedes autotrophic community assembly and is sustained, in part, by ancient carbon.

Molecular insight into soil carbon turnover.

Curie-point pyrolysis-gas chromatography coupled on-line to mass spectrometry (Py-GC/MS) and isotope ratio mass spectrometry (Py-GC/IRMS) were used to determine the individual turnover rate of specific carbohydrates, lignin, lipids and N-containing compounds from French arable soils. The analysed soils were cultivated, either continuously with a C3 plant (wheat delta(13)C-value = -25.2 per thousand), or transferred to a C4 plant (maize delta(13)C-value = -11.4 per thousand) cropping 23 years ago. Most pyrolysis products identified were related to carbohydrates (furans), lipids (hydrocarbons and derivatives of benzene), proteins (nitriles and pyrrole) and lignins (phenols). The relative yield of all individual pyrolysis products was similar in the samples from the maize and control wheat soil. The isotopic enrichment between identical pyrolysis products from the two soils varied from 1 to 12 delta (delta) units, indicating that after 23 years of cultivation 7 to 90% of their C was derived from maize. This suggests a slow mean turnover time varying from 9 to 220 years. Based on the differences in isotopic enrichment of chemical structures after vegetation change the pyrolysis products could be divided into three groups: (i) pyrolysis products with a nearly complete C4 signal, e. g. phenol, derived from lignin degradation products, (ii) pyrolysis products with an intermediate isotopic enrichment of 6-8 per thousand, most likely to be a composite of remaining (possibly physically protected) fragments derived from both maize and native wheat, and (iii) pyrolysis products showing only low enrichments in (13)C of 1-3 per thousand. Most of their precursors were found to be proteinaceaous materials. This indicates that proteins or peptides are indeed preserved during decomposition and humification processes occurring in the soil. Our study highlights the potential of Py-GC/MS-C-IRMS to further novel insights into the dynamics of soil organic constituents. Copyright 1999 John Wiley & Sons, Ltd.

Post-glacial variations in distributions, 13C and 14C contents of aliphatic hydrocarbons and bulk organic matter in three types of British acid upland soils

The post-glacial variations in distributions and 13C contents for individual aliphatic hydrocarbons and bulk organic matter down the profiles of three stratified organic upland soils (peaty gley, podzol and acid brown earth) in the U.K. were studied by a combination of conventional radiocarbon dating and accelerator mass spectrometry (AMS) and gas chromatography (GC), gas chromatography-mass spectrometry (GC-MS) and gas chromatography-isotope ratio mass spectrometry (GC-IRMS) characterisation of bulk and individual compounds. The reduction with depth of the total organic carbon content (TOC) of the soils was accompanied by concentration decreases in aliphatic hydrocarbons, attributed primarily to diagenetic degradation and, secondly, to changes in primary production since the last deglaciation. In contrast, the 13C content of TOC generally increased with depth and age in the soils, which was again paralleled by the 13C content of individual aliphatic hydrocarbons. CO2 contributed by fossil fuel burning can only explain changes of about 1‰, with the additional (2–4‰) 13C enrichment for individual n-alkanes from horizons in the podzol (< 2000 yr BP) and acid brown earth (< 3000 yr BP) being attributed primarily to the contributions of isotopically heavier (relative to plants) n-alkanes by soil micro-organisms. The large 13C enrichment (5–7‰) for TOC, aliphatic hydrocarbon fractions and individual n-alkanes from a peaty gley horizon older than 10,000 yr BP was also attributed to the effects of environmental conditions on isotopic fractionation during photosynthesis. Compared with n-alkanes from higher plants, the hopanoids were enriched on average by 4–5‰ in 13C and showed little variation down the soil cores, suggesting a source of heterotrophic bacteria using carbohydrates/proteins as their major carbon source, and effecting little isotopic fractionation during hopanoid biosynthesis, and/or from soil cyanobacteria using dissolved CO2 in water. The radiocarbon ages of the soil TOCs showed a nearly linear increase with depth, suggesting little bio-disturbance and consistent accumulation of organic C at this site. The 14C ages of the aliphatic hydrocarbon fractions isolated from peaty gley soil horizons (measured by AMS) increased linearly with depth and the age of the lower gleyed horizon was ca. 3000 yr older than that of the bulk soil organic matter. The presence of n-alkanes derived from higher-plant leaf waxes in the oldest horizons of peaty gley soil indicates a 3000 yr earlier development of vegetation since the last glaciation than that estimated simply by the age of TOC.

Quantification of soil carbon inputs under elevated CO2: C3 plants in a C4 soil

The objective of this investigation was to quantify the differences in soil carbon stores after exposure of birch seedlings (Betula pendula Roth.) over one growing season to ambient and elevated carbon dioxide concentrations. One-year-old seedling of birch were transplanted to pots containing ‘C4 soil’ derived from beneath a maize crop, and placed in ambient (350 μL L−1) and elevated (600 μL L−1) plots in a free-air carbon dioxide enrichment (FACE) experiment. After 186 days the plants and soils were destructively sampled, and analysed for differences in root and stem biomass, total plant tissue and soil C contents and δ13C values. The trees showed a significant increase (+50%) in root biomass, but stem and leaf biomasses were not significantly affected by treatment. C isotope analyses of leaves and fine roots showed that the isotopic signal from the ambient and elevated CO2 supply was sufficiently distinct from that of the ‘C4 soil’ to enable quantification of net root C input to the soil under both ambient and elevated CO2. After 186 days, the pots under ambient conditions contained 3.5 g of C as intact root material, and had gained an additional 0.6 g C added to the soil through root exudation/turnover; comparable figures for the pots under elevated CO2 were 5.9 g C and 1.5 g C, respectively. These data confirm the importance of soils as an enhanced sink for C under elevated atmospheric CO2 concentrations. We propose the use of ‘C4 soils’ in elevated CO2 experiments as an important technique for the quantification of root net C inputs under both ambient and elevated CO2 treatments.

Innovative methods in soil phosphorus research: A review

Phosphorus (P) is an indispensable element for all life on Earth and, during the past decade, concerns about the future of its global supply have stimulated much research on soil P and method development. This review provides an overview of advanced state-of-the-art methods currently used in soil P research. These involve bulk and spatially resolved spectroscopic and spectrometric P speciation methods (1 and 2D NMR, IR, Raman, Q-TOF MS/MS, high resolution-MS, NanoSIMS, XRF, XPS, (µ)XAS) as well as methods for assessing soil P reactions (sorption isotherms, quantum-chemical modeling, microbial biomass P, enzymes activity, DGT, 33P isotopic exchange, 18O isotope ratios). Required experimental set-ups and the potentials and limitations of individual methods present a guide for the selection of most suitable methods or combinations.