André Mariotti

Experimental determination of nitrogen kinetic isotope fractionation: Some principles; illustration for the denitrification and nitrification processes

SummaryA few principles relative to the presentation and use of nitrogen stable isotopic data are briefly reviewed. Some classical relationships between the isotope composition of a substrate undergoing a single-step unidirectional reaction, are introduced.They are illustrated through controlled experiments on denitrification in a soil, and through nitrification by pure cultures ofNitrosomonas europaea. In the latter case, the isotope fractionation is calculated from the isotopic composition of the residual substrate, then of the product and the result is shown to be statistically the same for the two procedures.The isotopic enrichment factor for denitrification is −29.4±2.4‰ at 20°C, and −24.6±0.9‰ at 30°C; for nitrification this factor is −34.7±2.5‰ under the experimental conditions employed.

The priming effect of organic matter: a question of microbial competition?

It is generally accepted that the low quality of soil carbon limits the amount of energy available for soil microorganisms, and in turn the rate of soil carbon mineralization. The priming effect, i.e. the increase in soil organic matter (SOM) decomposition rate after fresh organic matter input to soil, is often supposed to result from a global increase in microbial activity due to the higher availability of energy released from the decomposition of fresh organic matter. Work to date, however, suggests that supply of available energy induces no effect on SOM mineralization. The mechanisms of the priming effect are much more complex than commonly believed. The objective of this review was to build a conceptual model of the priming effect based on the contradictory results available in the literature adopting the concept of nutritional competition. After fresh organic matter input to soils, many specialized microorganisms grow quickly and only decompose the fresh organic matter. We postulated that the priming effect results from the competition for energy and nutrient acquisition between the microorganisms specialized in the decomposition of fresh organic matter and those feeding on polymerised SOM.

Natural 13C abundance as a tracer for studies of soil organic matter dynamics

A method for measuring the long- and medium-term turnover of soil organic matter is described. Its principle is based on the variations of 13C natural isotope abundance induced by the repeated cultivations of a plant with a high 13C/12C ratio (C4 photosynthetic pathway) on a soil which has never carried any such plant. The 13C/12C ratio in soil organic matter being about equal to the 13C/12C ratio of plant materials from which it is derived, changing the 13C content of the organic inputs to the soil (by altering vegetation from C3 type into C4 type) is equivalent to a true labelling in situ of the organic matter. Two cases of continuous corn cultivation (Zea mays: δ13C = −12%.) on soils whose initial organic matter average δ13C is −26%. were studied. The quantity of organic carbon originating from corn (that is the quantity which had turned-over since the beginning of continuous cultivation) was estimated using the 13C natural abundance data. After 13 yr, 22% of total organic carbon had turned-over, in the system studied. Particle size fractions coarser than 50μm on the one hand, and finer than 2μm on the other. contained the youngest organic matters. The turnover rate of silt-sized fractions was slower

15N isotope biogeochemistry and natural denitrification process in groundwater: Application to the chalk aquifer of northern France

The use of 15N natural isotope tracing in an aquifer contained within chalk rocks in northern France indicates that, under certain hydrogeological conditions, major denitrification occurs. At the boundary where the aquifer becomes confined, the nitrate concentrations decrease in the direction of groundwater flow accompanied by an exponential increase in 15N (expressed in δ15N) of the residual nitrate. This is characteristic of kinetic isotope effects, which accompany the reduction of the nitrate ion during denitrification. Hydrogeochemical and bacteriological observations confirm this process. Natural isotope tracing also permits this process to be distinguished from local dilution with nitrate-free surface water, which would entail a major drop in nitrate values without 15N isotopic enrichment. A model is proposed to explain the relatively small observed magnitude of the isotopic fractionation effect.

Oxygen isotope analyses of co-existing carbonate and phosphate in biogenic apatite: a way to monitor diagenetic alteration of bone phosphate?

Abstract The oxygen isotopic composition of phosphate (δ 18 Op) and structural carbonate (δ 18 Oc) of hydroxylapatite was determined in 31 bone and tooth samples of modern mammals from different countries. These two variables are highly correlate (r 2 = 0.98) and the calculated best fit of linear regression is very similar to the equation calculated from the phosphate and carbonate palaeotemperature equations [1,2]. According to previous measurements [3–6] on fossils of different ages from different areas it seems quite improbable to find isotopically altered skeletal remains showing a good correlation between δ 18 Op and δ 18 Oc, as is the case with modern samples. It therefore seems possible, at least in some cases, to use these measurements for monitoring fossil bone and tooth diagenetic alteration. When a set of points lie on the equilibrium line or close to it, the δ 18 O values could be considered close to the original values. In contrast, when the points lie to the left or to the right of this line this probably means that the values are diagenetically modified, due to interaction with meteoric water or 18 O-enriched water, respectively.

Diet, physiology and ecology of fossil mammals as inferred from stable carbon and nitrogen isotope biogeochemistry: implications for Pleistocene bears

Low β15N values of Wurmian cave bear (Ursus spelaeus) bone collagen indicate a strictly vegetarian diet, and negative δ13C values suggest a forested habitat. Tooth collagen δ15N values are about 2%0 higher than bone collagen within one individual, similar to that found in the modern black bear (Ursus americanus). These data suggest an influence of a 15N-enriched milk diet during the synthesis of tooth collagen, which is partly formed before weaning. Thus, tooth collagen δ15N values are not reliable for adult diet reconstruction. Tooth collagen δ13C values are around 1%0 lower than bone collagen, suggesting a 13C-depleted milk diet. A similar pattern of variation is seen in the average δ13C and δ15N values of several individuals from one locality. Enamel carbonate hydroxylapatite δ13C values are low in cave bears (around −14%0) when compared to carnivores (around −12%0) and herbivores (−10%0). This is probably due to lipid use while hibernating during winter. A similar pattern of enamel carbonate hydroxylapatite δ13C values differences is found between Deningers bear (Ursus deningeri), carnivores and herbivores in 200,000 to 600,000 year old caves, suggesting a similar physiology for both fossil bear species.

Isotopic biogeochemistry ( 13 C, 18 O) of mammalian enamel from African Pleistocene hominid sites

The carbon and oxygen isotope composition of carbonate in enamel hydroxylapatite can provide information on photosynthetic pathways of plants at the base of food webs, and on hydrological conditions. Retrieval ofpalaeoenvironmental information from isotopic composition of vertebrate fossils is complicated by potential diagenetic overprinting. In this study, alteration has been assessed by examining the extent to which expected biological carbon and oxygen isotope patterns are disrupted in fossils of species whose diets can be independently predicted by other criteria. The biological patterns used are 1) the differences in carbon isotope composition between grazers and browsers, and 2) the differences in oxygen isotope composition between hippopotamus and terrestrial herbivores. Results obtained on enamel samples from Tighenif (Algeria, =700,000 yr), Melka-Kunture (Ethiopia, 0.7-1.5 myr), and Anabo Koma (Djibouti, -1.6 myr) suggest that in vivo carbon and oxygen isotope compositions are preserved in most cases. Moreover, in all three regions, modern patterns of C3 versus C4 grass dominance were present within the Pleistocene.

Natural isotopic composition of nitrogen as a tracer of origin for suspended organic matter in the Scheldt estuary

Abstract The natural isotopic composition of suspended particulate organic nitrogen was determined in the Southern Bight of the North Sea and in the Scheldt estuary. These data show that δ 15 N constitutes a convenient tracer of the origin of the suspended matter. In the winter, in the absence of intensive primary production, the suspended organic matter of the Scheldt estuary is a mixture of two components: a continental detrital component characterized by a low δ value of 1.5%. and a marine component with a mean δ value of 8%.. During the phytoplankton flowering period, lasting from early May to October, intensive primary production occurs throughout the estuary giving rise to a third source of organic matter. This material is characterized by high δ values reflecting the isotopic composition of ammonia, the nitrogenous nutrient assimilated by phytoplankton in the estuary. The nitrification process occuring in the mixing area of the Scheldt estuary leads to higher downstream δ values of ammonia (>20%.) which permits the distinction between estuarine from fresh-water phytoplankton. Simple isotopic budget calculations show that, both in the upstream part and in the downstream part, autochthonous phytoplanktonic material contributes a major part of the total suspended matter in the Scheldt estuary during summer.

Isotopic biogeochemistry (13C,15N) of fossil vertebrate collagen: application to the study of a past food web including Neandertal man

Abstract TheC/Nratio and amino acid composition of organic matter extracted from fossil mammal bones from the Paleolithic site at Marillac (Charentes, France) shown that this organic matter comes from collagen.δ 13 Candδ 15 Nvalues of known-diet fossil species demonstrate that these values have been preserved through fossilization processes, and that these fossil mammals can be used as ecological references to determine the Neandertal position in the past food web. Initial Neandertalδ 13 Candδ 15 N values suggest that he was mostly carnivorous.

Isotopic composition of nitrate-nitrogen as a marker of riparian and benthic denitrification at the scale of the whole Seine River system

Nitrogen budgets established for large river systems reveal that up to 60% of the nitrate exported from agricultural soils is eliminated, either when crossing riparian wetlands areas before even reaching surface waters, or within the rivers themselves through benthic denitrification. The study of nitrogen isotope ratios of riverine nitrates could offer an elegant means to assess the extent of denitrification and thus confirm these budgets, as it is known that denitrification results in a natural 15N enrichment of residual nitrates. The results reported here, for the Seine river system (France), demonstrate the feasibility of this isotopic approach at the scale of large watersheds. On the basis of in situ observations carried out in a large storage reservoir in the upstream Seine catchment (Der Lake), where intensive benthic denitrification occurs, as well as on the basis of laboratory experiments of denitrification under controlled conditions, it is shown that the isotopic discrimination associated with benthic denitrification is minimal (ε of NO3-N ranging from −1.5 to −3.6‰), probably because the rate-limiting step of the process consists of nitrate diffusion through the water-sediment interface. Riparian denitrification on the contrary, when it implies nitrate reduction during convective transfer through reducing environements, causes a much more significant isotopic enrichment of 15N of residual nitrate (ε about −18‰). The authors report measurements of nitrogen isotopic composition of nitrate from rivers of various stream orders in the Seine river system under summer conditions. Anomalies in the data with respect to the values expected from the mixture of the various sources of nitrate are here attributed to riparian denitrification. However, the authors show that because of the patchy distribution of actively denitrifying riparian zones within the drainage network, the isotopic signature conferred to residual nitrate in river water intrinsically provides only a minimum estimate of the extent of denitrification.