Plant diversity effects on soil microorganisms support the singular hypothesis.

The global decline in biodiversity has generated concern over the consequences for ecosystem functioning and services. Although ecosystem functions driven by soil microorganisms such as plant productivity, decomposition, and nutrient cycling are of particular importance, interrelationships between plant diversity and soil microorganisms are poorly understood. We analyzed the response of soil microorganisms to variations in plant species richness (1-60) and plant functional group richness (1-4) in an experimental grassland system over a period of six years. Major abiotic and biotic factors were considered for exploring the mechanisms responsible for diversity effects. Further, microbial growth characteristics were assessed following the addition of macronutrients. Effects of plant diversity on soil microorganisms were most pronounced in the most diverse plant communities though differences only became established after a time lag of four years. Differences in microbial growth characteristics indicate successional changes from a disturbed (zymogeneous) to an established (autochthonous) microbial community four years after establishment of the experiment. Supporting the singular hypothesis for plant diversity, the results suggest that plant species are unique, each contributing to the functioning of the belowground system. The results reinforce the need for long-term biodiversity experiments to fully appreciate consequences of current biodiversity loss for ecosystem functioning.

Isotopic evidences for microbiologically mediated and direct C input to soil compounds from three different leaf litters during their decomposition.

We show the potentiality of coupling together different compound-specific isotopic analyses in a laboratory experiment, where (13)C-depleted leaf litter was incubated on a (13)C-enriched soil. The aim of our study was to identify the soil compounds where the C derived from three different litter species is retained. Three (13)C-depleted leaf litter (Liquidambar styraciflua L., Cercis canadensis L. and Pinus taeda L., delta(13)C(vsPDB) approximately -43 per thousand), differing in their degradability, were incubated on a C4 soil (delta(13)C(vsPDB) approximately -18 per thousand) under laboratory-controlled conditions for 8 months. At harvest, compound-specific isotope analyses were performed on different classes of soil compounds [i.e. phospholipids fatty acids (PLFAs), n-alkanes and soil pyrolysis products]. Linoleic acid (PLFA 18:2omega6,9) was found to be very depleted in (13)C (delta(13)C(vsPDB) approximately from -38 to -42 per thousand) compared to all other PLFAs (delta(13)C(vsPDB) approximately from -14 to -35 per thousand). Because of this, fungi were identified as the first among microbes to use the litter as source of C. Among n-alkanes, long-chain (C27-C31) n-alkanes were the only to have a depleted delta(13)C. This is an indication that not all of the C derived from litter in the soil was transformed by microbes. The depletion in (13)C was also found in different classes of pyrolysis products, suggesting that the litter-derived C is incorporated in less or more chemically stable compounds, even only after 8 months decomposition.

Exportation of dissolved (inorganic and organic) and particulate carbon from mangroves and its implication to the carbon budget in the Indian Sundarbans.

Mangroves are known for exchanging organic and inorganic carbon with estuaries and oceans but studies that have estimated their contribution to the global budget are limited to a few mangrove ecosystems which exclude world's largest the Sundarbans. Here, we worked in the Indian Sundarbans and in the Hooghly river/estuary in May (pre-monsoon) and December (post-monsoon), 2014. Aims were, i) to quantify the riverine export of particulate organic carbon (POC) and dissolved organic and inorganic carbon (DOC, DIC)) of the Hooghly into the Bay of Bengal (BoB), ii) to estimate the C export (DOC, DIC, POC) from the Sundarbans into the BoB by using a simple mixing model, as well as iii) to revise the existing C budget constructed for the mangroves. The riverine exports of POC, DOC and DIC account for 0.07TgCyr-1, 0.34TgCyr-1 and 4.14TgCyr-1, respectively, and were largest during the monsoon period. Results revealed that mangrove plant derived organic matter and its subsequent degradation is the primary source of DIC and DOC in the Hooghly estuary whereas POC is linked to soil erosion. Mangroves are identified as a major source of carbon (POC, DOC, DIC) transported from the Sundarbans into the BoB, with export rates of 0.58 TgCyr-1, 3.03TgCyr-1, and 3.69TgCyr-1 respectively, altogether amounting to 7.3TgCyr-1. This C export from the Indian Sundarbans exceeds the 'missing C' of the previous budget, thus necessitating further research to finally resolve the mangrove C budget. However, these first baseline data on C exports from the world's largest deltaic mangrove improves limited global data inventory and signifies the need of acquiring more data from different mangrove settings to reduce uncertainties.

Correlations between the 13C Content of Primary and Secondary Plant Products in Different Cell Compartments and That in Decomposing Basidiomycetes.

Relative carbon isotope ratio ([delta]13C values) of primary and secondary products from different compartments of annual plants, pine needles, wood, and decomposing Basidiomycetes have been determined. An enrichment in 13C was found for storage tissues of annual plants, because of the high level of the primary storage products sucrose and starch; however, the enrichment was even greater in leaf starch. All of these compounds had the same relative 13C enrichment in positions 3 and 4 of glucose. Secondary products in conifer needles (lignin, lipids) were depleted in 13C by 1 to 2 [per mille (thousand) sign] relative to carbohydrates from the same origin. Air pollution caused a small decrease in [delta]13C values; however, the relative content of plant products, especially of the soluble polar compounds, was also affected. Decomposing fungi showed a global accumulation of 13C by 4[per mille (thousand) sign] relative to their substrates in wood. Their chitin was enriched by 2[per mille (thousand) sign] relative to the cellulose of the wood. Hence, Basidiomycetes preferentially metabolize "light" molecules, whereas "heavy" molecules are preferentially polymerized. Our results are discussed on the basis of a kinetic isotope effect on the fructose-1,6-bisphosphate aldolase reaction and of metabolic branching on the level of the triose phosphates with varying substrate fluxes.

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.

Do stable isotopes reflect the food web development in regenerating ecosystems?

We evaluated the use of delta15N- and delta13C-values to monitor the development of food web complexity and biodiversity in a regenerating ecosystem. Therefore a model food chain was established feeding cultivated woodlice (Porcellio dilatatus) on a cellulolytic fungus (Chaetomium globosum) grown on cellulose paper. Two diets of different quality (C:N ratios of 54 vs. 200) with different delta15N- (1.3% vs. 3.1%) but identical delta13C-values caused low and high dietary stress in animals of treatment A and B, respectively. After an incubation time of 7 weeks amount, elemental and isotopic composition of collected faeces and exuviae as well as woodlice and remaining food were determined. The increase of delta15N-values of woodlice relative to the diet was 5.7% and 2.5% in treatments A and B, respectively, whereas delta13C-shifts were 1.0% and 1.6%, showing a reverse relationship. Modelling of elemental and isotopic mass balances indicated that faeces recycling explains the unexpected high 15N-enrichments. Moreover, 13C-enrichments were positively correlated to the degree of starvation. Considering the effects of starvation and recycling of faeces, stable isotopes represent a useful tool to elucidate trophic interactions in regenerating food webs.

An isotopic method for testing the influence of leaf litter quality on carbon fluxes during decomposition.

During microbial breakdown of leaf litter a fraction of the C lost by the litter is not released to the atmosphere as CO(2) but remains in the soil as microbial byproducts. The amount of this fraction and the factors influencing its size are not yet clearly known. We performed a laboratory experiment to quantify the flow of C from decaying litter into the soil, by means of stable C isotopes, and tested its dependence on litter chemical properties. Three sets of (13)C-depleted leaf litter (Liquidambar styraciflua L., Cercis canadensis L. and Pinus taeda L.) were incubated in the laboratory in jars containing (13)C-enriched soil (i.e. formed C4 vegetation). Four jars containing soil only were used as a control. Litter chemical properties were measured using thermogravimetry (Tg) and pyrolysis-gas chromatography/mass spectrometry-combustion interface-isotope ratio mass spectrometry (Py-GC/MS-C-IRMS). The respiration rates and the delta(13)C of the respired CO(2) were measured at regular intervals. After 8 months of incubation, soils incubated with both L. styraciflua and C. canadensis showed a significant change in delta(13)C (delta(13)C(final) = -20.2 +/- 0.4 per thousand and -19.5 +/- 0.5 per thousand, respectively) with respect to the initial value (delta(13)C(initial) = -17.7 +/- 0.3 per thousand); the same did not hold for soil incubated with P. taeda (delta(13)C(final:)-18.1 +/- 0.5 per thousand). The percentages of litter-derived C in soil over the total C loss were not statistically different from one litter species to another. This suggests that there is no dependence of the percentage of C input into the soil (over the total C loss) on litter quality and that the fractional loss of leaf litter C is dependent only on the microbial assimilation efficiency. The percentage of litter-derived C in soil was estimated to be 13 +/- 3% of total C loss.

Carbon isotope effects on the fructose-1,6-bisphosphate aldolase reaction, origin for non-statistical 13C distributions in carbohydrates.

The kinetic and equilibrium isotope effects on the fructose-1, 6-bisphosphate aldolase reaction have been determined using the rabbit muscle enzyme. The natural 13C abundance for both atoms participating in the bond splitting were measured in position C-1 of dihydroxyacetone phosphate and glyceraldehyde 3-P after irreversible conversion to glycerol-3-P and 3-phosphoglycerate, respectively, and chemical degradation. The carbon isotope effects were determined comparing the 13C content of the corresponding positions after partial and complete turnover, and after complete equilibration of the reactants. 13(Vmax/Km) on C-3 was 1.016 +/- 0.007 and 0.997 +/- 0.009 on position C-4, and the equilibrium isotope effects K12/K13 on these positions were 1.0036 +/- 0.0002 and 1.0049 +/- 0.0001. The observed kinetic isotope effect on C-3 is discussed to originate from the formation of the enamine, which comes to equilibrium before the rate determining release of glyceraldehyde 3-P from the ternary complex. The equilibrium isotope effect is seen as the reason for an earlier-found relative 13C enrichment in position C-3 and C-4 of glucose and for varying enrichments in 13C of carbohydrates from different compartments of cells. The kinetic isotope effect is suggested to cause 13C discriminations in the C-3 pool in context with the hexose formation in competition with other dihydroxyacetone phosphate turnover reactions.