Soil delta15N patterns in old-growth forests of southern Chile as integrator for N-cycling

Old-growth forests of southern Chile represent an important reserve of temperate (rain) forests in the world. Wetter and colder forest ecosystems appear to be more efficient in conserving and recycling N such that mostly non-plant available N species are lost, which could be indicated by more depleted delta15N values of the soil and plants. Hydrological N loss from the old-growth forests in southern Chile occurs mainly via dissolved organic nitrogen and not via dissolved inorganic N. Forest disturbances (e.g. fire, clear-cutting or enhanced N deposition) cause (abrupt) changes in ecosystem N-cycling processes. In this study, we hypothesized that delta15N signatures of soil profiles under old-growth forests could be used as an integrator for ecosystem N-cycling, and changes of these delta15N profiles could be valuable to assess ecosystem resilience towards disturbances. Six old-growth forests were selected in the phytogeographical region of the Valdivian rain forest in southern Chile. One of the sites has been partly burned in February 2002. First, we observed that ecosystems with higher mean annual precipitation and lower mean annual temperature were relatively more depleted in 15N. Secondly, we found that a forest fire caused a 100-fold increase of the nitrate export and induced an enrichment of the soil delta15N signal in the upper 20 cm.     

Characteristics of suspended matter in the River Sava watershed, Slovenia

A combination of C/N ratios, delta(13)C and delta(15)N values in suspended matter was used to examine the seasonal (late summer 2004 and spring 2005) relationship with hydrological characteristics of the River Sava watershed in Slovenia. The values of C/N ratios range from 1.2 to 19.1, delta(13)C values range from-29.2 to-23.0 per thousand and delta(15)N values from 0.5 to 16.7 per thousand and indicate that the samples are a mixture of two end members: modern soils and plant litter. A simple mixing model was used to indicate that soil organic carbon prevails over plant litter and contributes more than 50% of suspended material. The calculated annual particulate organic carbon flux is estimated as 5.2x10(10) g C/year, the annual particulate nitrogen flux 8.5x10(9) g N/year and the total suspended solid flux is estimated to be 1.3x10(12) g/year. Anthropogenic impact was detected only in a tributary stream of the River Sava, which is located in an agriculture-industrial area and is reflected in higher delta(15)N values in suspended matter and high nitrate concentrations in the late summer season.     

Potential for assessing long-term dynamics in soil nitrogen availability from variations in delta15N of tree rings

Numerous researchers have used the isotopic signatures of C, H, and O in tree rings to provide a long-term record of changes in the physiological status, climate, or water-source use of trees. The frequently limiting element N is also found in tree rings, and variation in its isotopic signature may provide insight into long-term changes in soil N availability of a site. However, research has suggested that N is readily translocated among tree ring of different years; such infidelity between the isotopic compositions of the N taken up from the soil and the N contained in the ring of that growth year would obscure the long-term N isotopic record. We used a 15-year 15N-tracer study to assess the degree of N translocation among tree rings in ponderosa pine (Pinus ponderosa) trees growing in a young, mixed-conifer plantation. We also measured delta13C and delta15N values in unlabeled trees to assess the degree of their covariance in wood tissue, and to explore the potential for a biological linkage between them. We found that the maximum delta15N values in rings from the labeled trees occurred in the ring formed one-year after the 15N was applied to the roots. The delta15N value of rings from labeled trees declined exponentially and bidirectionally from this maximum peak, toward younger and older rings. The unlabeled trees showed considerable interannual variation in the delta15N values of their rings (up to 3 and 5 per thousand), but these values correlated poorly between trees over time and differed by as much as 6 per thousand. Removal of extractives from the wood reduced their delta15N value, but the change was fairly small and consistent among unlabeled trees. The delta13C and delta15N values of tree rings were correlated over time in only one of the unlabeled trees. Across all trees, both delta13C values of tree rings and annual stem wood production were well correlated with annual precipitation, suggesting that soil water balance is an important environmental factor controlling both net C gain and transpirational water loss at this site. Our results suggest that interannual translocation of N among tree rings is substantial, but may be predictable enough to remove this source of variation from the tree-ring record, potentially allowing the assessment of long-term changes in soil N availability of a site.     

13C and 15N distributions in three spodic dystric cambisols under beech and spruce

The study of natural isotopic abundance signatures is useful to gain further insights in the processes resulting in depthwise changes in the composition of soil organic matter (SOM). Objectives were to describe the delta 13C and delta 15N abundances of SOM with depth in soils from a 153-year old beech (B1), a 119-year old spruce (F1) and a 61-year old spruce (F2) stand at Solling, north-west Germany, and to study, how podzolisation affects the isotopic abundances of 13C and 15N in the SOM. The degree of podzolisation decreased in the order F1 > B1 > F2. At the surface of the humus layer of all three sites, delta 13C values are approximately 1 to 4/1000 higher than in the leaves and needles, probably mainly due to the discrimination of 13C by microbial decomposition. 13C abundances in the organic layers of B1 and F2 increased only slightly from -27.6/1000 PDB (B1, L) to -27.2/1000 PDB (B1, Oh) and from -26.3/1000 PDB (F2, L) to -25.9/1000 PDB (F2, Oh), suggesting that biotic activity resulted in mixing of organic matter. At F1, however, 13C abundance increased from -27.5/1000 PDB (L) to -26.0/1000 PDB (Oh) which reflects the lack of mixing by animals. In the upper 2-4 cm of the mineral soil, i.e., in the eluvial horizons Aeh, 13C values showed a minimum at the spruce sites which was presumably related to a translocation of 13C enriched fulvic acids. Depthwise changes in delta 15N values were not related to podzolisation processes. At all three sites, a 13N enrichment with depth occurred in the mineral soil which is the result of the discrimination of 15N by microbial decomposition.     

Tree species of the Central Amazon and soil moisture alter stable isotope composition of nitrogen and oxygen in nitrous oxide evolved from soil

The use of stable isotopes of N and O in N2O has been proposed as a way to better constrain the global budget of atmospheric N2O and to better understand the relative contributions of the main microbial processes (nitrification and denitrification) responsible for N2O formation in soil. This study compared the isotopic composition of N2O emitted from soils under different tree species in the Brazilian Amazon. We also compared the effect of tree species with that of soil moisture, as we expected the latter to be the main factor regulating the proportion of nitrifier- and denitrifier-derived N2O and, consequently, isotopic signatures of N2O. Tree species significantly affected delta15N in nitrous oxide. However, there was no evidence that the observed variation in delta15N in N2O was determined by varying proportions of nitrifier- vs. denitrifier-derived N2O. We submit that the large variation in delta15N-N2O is the result of competition between denitrifying and immobilizing microorganisms for NO3(-). In addition to altering delta15N-N2O, tree species affected net rates of N2O emission from soil in laboratory incubations. These results suggest that tree species contribute to the large isotopic variation in N2O observed in a range tropical forest soils. We found that soil water affects both 15N and 18O in N2O, with wetter soils leading to more depleted N2O in both 15N and 18O. This is likely caused by a shift in biological processes for 15N and possible direct exchange of 18O between H2O and N2O.     

Short-term changes in delta(13)C and delta(15)N signatures of water discharged from grazed grasslands

The composition of dissolved organic matter (DOM) in a soil is the product of a variety of soil processes. Changes in the composition of DOM in water discharged from soil should, therefore, give an important insight into modifications in these soil processes. We hypothesise that these processes in soils, under different grassland management regimes, would be affected to different extents by the short-term disturbance of a storm event and that evidence of this could be detected in delta(13)C and delta(15)N signatures in drainage and surface runoff waters. During a storm event we collected discharge waters from 1 ha grassland lysimeters, with or without artificial drainage, which received contrasting fertiliser inputs, and delta(13)C and delta(15)N signatures were determined. Changes in (13)C enrichment during the storm event were clearly identifiable, as were differences between plots for (13)C and (15)N, illustrating that this technique has potential to be a useful tool for identifying and investigating short- and long-term changes in soil organic matter dynamics. Copyright 1999 John Wiley & Sons, Ltd.     

Stable isotope ratios and uric acid preservation in termites belonging to three feeding habits in Thailand

Nitrogen and carbon stable isotope ratios and uric acid concentrations in termites sampled from a dry evergreen forest in Thailand, were determined across three kinds of feeding habits. Feeding habits of Microcerotermes crassus, which is an abundant wood-feeder, and Dicuspiditermes makhamensis, a common soil-feeding termite, were confirmed by isotopic signatures. Lichen feeding termites (Hospitalitermes birmanicus, H. bicolor and H. ataramensis) were characterized by low delta15N values, suggesting that they assimilated nitrogen deposited from the atmosphere. There was also a significant difference in uric acid concentrations between termites representing different feeding habits. No significant relationships were found between uric acid concentrations and delta15N or delta13C in Hospitalitermes. However, delta15N values were correlated with C/N ratios in H. birmanicus, except in one colony of H. ataramensis. delta13C values in both species were negatively correlated with C/N ratios.     

Estimating spatial and seasonal phytoplankton delta13C variations in an estuarine mangrove ecosystem

In estuarine ecosystems, large spatial and seasonal variations in delta13C values of primary producers can occur, and knowledge of these variations may be crucial when interpreting stable isotope data of higher trophic levels. Obtaining clean phytoplankton samples for isotope analysis is usually impossible in such systems, and analysis of total suspended matter is not a simple proxy for phytoplankton delta13C variations. Based on a few simple assumptions regarding the C and N content of the two end-members (terrestrial detritus and phytoplankton) and the delta13C of the terrestrial component, we here present a simple model to estimate the phytoplankton delta13C variations using an existing dataset on the delta13C and elemental (C:N) composition of suspended organic matter from an estuarine mangrove ecosystem in southeast India. These variations are related to the monthly rainfall pattern during the sampling period. It is stressed that this method estimates approximate phytoplankton delta13C values, which should not be used in e.g., mixing models. However, we propose that in cases where sufficiently large datasets are available, the described procedure could provide a valuable method to semi-quantitatively estimate the seasonal or spatial variations of the phytoplankton delta13C signal.     

Investigations on the Nitrogen Metabolism of Forest Trees by Mathematical Modelling of Natural Isotope Ratios

Abstract

Based on an experimental investigation of nitrogen concentrations and δ¹⁵N-values in different parts of Picea abies (L.) trees and soil samples and of their dependence on the age of plant parts and on damaging stress, mathematical models which represent main features of nitrogen turnover in the forest tree - soil system and associated isotope variations are developed. These models consider parts of 3 cycles of nitrogen: (1) transport of soluble nitrogen within trees, (2) turnover of protein nitrogen in different plant parts, and (3) the exchange of N between tree and soil via litter and the uptake of mineralized soil N by plant roots. Isotope effects of protein synthesis and mineralization of organic soil N are included. The models give a qualitative explanation of the data and lead to a better understanding of isotopic variations as indicators of forest damage.     

Nutrient cycling responses to fire frequency in the Kruger National Park (South Africa) as indicated by stable isotope analysis

Fires, which are an intrinsic feature of southern African ecosystems, produce biogenic and pyrogenic losses of nitrogen (N) from plants and soils. Because of the long history of fires in these savannas, it was hypothesized that N2 fixation by legumes balances the N losses caused by fires. In this study, the N2 fixation activity of woody legumes was estimated by analyzing foliar delta15N and proportional basal area of N2 fixing species along experimental fire gradients in the Kruger National Park (South Africa). In addition, soil carbon (C) and N pools, foliar phosphorus (P) and gross N mineralization and nitrification rates were measured, to indicate the effects of fires on nutrient stocks and the possible N cycling processes modified by fires. Although observations of increased soil C/N and mineralization rates in frequently burned plots support previous reports of N losses caused by fires, soil %N did not decrease with increasing fire frequency (except in 1 plot), suggesting that N losses are replenished in burned areas. However, relative abundance and N2 fixation of woody legumes decreased with fire frequency in two of the three fire gradients analyzed, suggesting that woody legume N2 fixation is not the mechanism that balances N losses. The relatively constant %N along fire gradients suggests that these ecosystems have other mechanisms to balance the N lost by fires, which could include inputs by atmospheric deposition and N2 fixation by forbs, grasses and soil cyanobacteria. Soil isotopic signatures have been previously used to infer patterns of fire history. However, the lack of a relationship between soil delta15N and fire frequency found in this study indicates that the effects of fires on ecosystem delta15N are unpredictable. Similar soil delta15N along fire gradients may reflect the contrasting effects of increased N gaseous emissions (which increases delta15N) and N2 fixation other than that associated with woody legumes (which lowers delta15N) on isotopic signatures.     

Human and climate impact on 15N natural abundance of plants and soils in high-mountain ecosystems: a short review and two examples from the Eastern Pamirs and Mt. Kilimanjaro

Population pressure increasingly endangers high-mountain ecosystems such as the pastures in the Eastern Pamirs and the mountain forests on Mt. Kilimanjaro. At the same time, these ecosystems constitute the economic basis for millions of people living there. In our study, we, therefore, aimed at characterising the land-use effects on soil degradation and N-cycling by determining the natural abundance of ¹⁵N. A short review displays that δ¹⁵N of plant–soil systems may often serve as an integrated indicator of N-cycles with more positive δ¹⁵N values pointing towards N-losses. Results for the high-mountain pastures in the Eastern Pamirs show that intensively grazed pastures are significantly enriched in ¹⁵N compared to the less-exploited pastures by 3.5 ‰, on average. This can be attributed to soil organic matter degradation, volatile nitrogen losses, nitrogen leaching and a general opening of the N-cycle. Similarly, the intensively degraded savanna soils, the cultivated soils and the soils under disturbed forests on the foothill of Mt. Kilimanjaro reveal very positive δ¹⁵N values around 6.5 ‰. In contrast, the undisturbed forest soils in the montane zone are more depleted in ¹⁵N, indicating that here the N-cycle is relatively closed. However, significantly higher δ¹⁵N values characterise the upper montane forest zone at the transition to the subalpine zone. We suggest that this reflects N-losses by the recently monitored and climate change and antropogenically induced increasing fire frequency pushing the upper montane rainforest boundary rapidly downhill. Overall, we conclude that the analysis of the ¹⁵N natural abundance in high-mountain ecosystems is a purposeful tool for detecting land-use- or climate change-induced soil degradation and N-cycle opening.     

Independence of alpine meadow plants from soil organic matter for nitrogen supply: evidence from stable nitrogen isotopes

Stable nitrogen isotope signatures of major sources of mineral nitrogen (mineralization of soil organic nitrogen, biological N(2) fixation by legumes, annual precipitation and plant litter decomposition) were measured to relatively define their individual contribution to grass assimilation at the Haibei Alpine Meadow Ecosystem, Qinghai, China. The results indicated that delta(15)N values (-2.40 per thousand to 0.97 per thousand) of all grasses were much lower than those of soil organic matter (3.4+/-0.18 per thousand) and mineral nitrogen (ammonium and nitrate together, 7.8+/-0.57 per thousand). Based on the patterns of stable nitrogen isotopes, soil organic matter (3.4+/-0.18 per thousand), biological N(2) fixation (0 per thousand), and precipitation (-6.34+/-0.24 per thousand) only contributed to a small fraction of nitrogen requirements of grasses, but plant litter decomposition (-1.31+/-1.01 per thousand) accounted for 67 %.     

Stable isotopic studies of earthworm feeding ecology in tropical ecosystems of Puerto Rico

Feeding strategies of earthworms and their influence on soil processes are often inferred from morphological, behavioral and physiological traits. We used (13)C and (15)N natural abundance in earthworms, soils and plants to explore patterns of resource utilization by different species of earthworms in three tropical ecosystems in Puerto Rico. In a high altitude dwarf forest, native earthworms Trigaster longissimus and Estherella sp. showed less (15)N enrichment ((15)N = 3-6 per thousand) than exotic Pontoscolex corethrurus ((15)N =7-9 per thousand) indicating different food sources or stronger isotopic discrimination by the latter. Conversely, in a lower altitude tabonuco forest, Estherella sp. and P. corethrurus overlapped completely in (15)N enrichment ((15)N = 6-9 per thousand), suggesting the potential for interspecific competition for N resources. A tabonuco forest converted to pasture contained only P. corethrurus which were less enriched in (15)N than those in the forest sites, but more highly enriched in (13)C suggesting assimilation of C from the predominant C(4) grass. These results support the utility of stable isotopes to delineate resource partitioning and potential competitive interactions among earthworm species. Copyright 1999 John Wiley & Sons, Ltd.     

Characteristics of suspended matter in the River Sava watershed,Slovenia

A combination of C/N ratios, δ¹³C and δ¹⁵N values in suspended matter was used to examine the seasonal (late summer 2004 and spring 2005) relationship with hydrological characteristics of the River Sava watershed in Slovenia. The values of C/N ratios range from 1.2 to 19.1, δ¹³C values range from?29.2 to?23.0 ‰ and δ¹⁵N values from 0.5 to 16.7 ‰ and indicate that the samples are a mixture of two end members: modern soils and plant litter. A simple mixing model was used to indicate that soil organic carbon prevails over plant litter and contributes more than 50% of suspended material. The calculated annual particulate organic carbon flux is estimated as 5.2×10¹⁰ g C/year, the annual particulate nitrogen flux 8.5×10⁹ g N/year and the total suspended solid flux is estimated to be 1.3×10¹² g/year. Anthropogenic impact was detected only in a tributary stream of the River Sava, which is located in an agriculture–industrial area and is reflected in higher δ¹⁵N values in suspended matter and high nitrate concentrations in the late summer season.     

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.     

Use of 13C and 15N mass spectrometry to study the decomposition of Calamagrostis epigeios in soil column experiments with and without ash additions

The dynamics of C and N in terrestrial ecosystems are not completely understood and the use of stable isotopes may be useful to gain further insight in the pathways of CO2 emissions and leaching of dissolved organic carbon (DOC) and nitrogen (DON) during decomposition of litter. Objectives were (i) to study the decomposition dynamics of Calamagrostis epigeios, a common grass species in forests, using 13C-depleted and 15N-enriched plants and (ii) to quantify the effect wood ash addition on the decomposition and leaching of DOC and DON. Decomposition was studied for 128 days under aerobic conditions at 8 degrees C and moisture close to field capacity in a spodic dystric Cambisol with mor-moder layer. Variants included control plots and additions of (i) Calamagrostis litter and (ii) Calamagrostis litter plus 4 kg ash m-2. (i) Decomposition of Calamagrostis resulted in a CO2 production of 76.2 g CO2-C m-2 (10% of added C) after 128 days and cumulative DOC production was 14.0 g C m-2 out of which 0.9 g C m-2 was Calamagrostis-derived (0.1% of added C). The specific CO2 formation and specific DOC production from Calamagrostis were 6 times higher (CO2) and 4 times smaller (DOC) than those from the organic layer. The amount of Calamagrostis-derived total N (NH4+, NO3-, DON) leached was 0.7 g N m-2 (4.8% of added N). Cumulative DON production was 0.8 g N m-2 which was slightly higher than for the control. During soil passage, much of the DOC and DON was removed due to sorption or decomposition. DOC and DON releases from the mineral soil (17 cm depth) were 6.3 g C m-2 and 0.5 g N m-2. (ii) Addition of ash resulted in a complete fixing of CO2 for 40 days due to carbonatisation. Afterwards, the CO2 production rates were similar to the variant without ash addition. Production of DOC (98.6 g C m-2) and DON (2.5 g N m-2) was marked, mainly owing to humus decay. However, Calamagrostis-derived DOC and Calamagrostis-derived total N were only 3.9 g C m-2 (0.5% of added C) and 0.5 g N m-2 (3.4% of added N). The specific DOC production rate from the organic layer was 6 times higher than that from Calamagrostis. The results suggest that with increasing humification from fresh plant residues to more decomposed material (OF and OH layers) the production ratio of DOC/CO2-C increases. Addition of alkaline substances to the forest floor can lead to a manifold increase in DOC production.     

Using natural 15N abundances to trace the fate of waste-derived nitrogen in forest ecosystems: New Zealand case studies

Treatment of wastewater generally results in elevated natural 15N abundance (delta15N) in the effluent and sludges. For example, high delta15N values are found in treated sewage effluent, biosolids, and other wastes that are commonly applied to land. In contrast, N deficient coniferous forest soils usually have a low delta15N. When wastes with high delta15N values are applied to land, their distinctive delta15N signature can potentially be used to trace the fate of waste-derived N in the ecosystem. In this paper, we provide an overview of the use of delta15N in land application of wastes, including New Zealand case studies on tracing nitrogen in forest ecosystems.     

Human and climate impact on ¹⁵N natural abundance of plants and soils in high-mountain ecosystems: a short review and two examples from the Eastern Pamirs and Mt. Kilimanjaro

Population pressure increasingly endangers high-mountain ecosystems such as the pastures in the Eastern Pamirs and the mountain forests on Mt. Kilimanjaro. At the same time, these ecosystems constitute the economic basis for millions of people living there. In our study, we, therefore, aimed at characterising the land-use effects on soil degradation and N-cycling by determining the natural abundance of (15)N. A short review displays that δ(15)N of plant-soil systems may often serve as an integrated indicator of N-cycles with more positive δ(15)N values pointing towards N-losses. Results for the high-mountain pastures in the Eastern Pamirs show that intensively grazed pastures are significantly enriched in (15)N compared to the less-exploited pastures by 3.5 ‰, on average. This can be attributed to soil organic matter degradation, volatile nitrogen losses, nitrogen leaching and a general opening of the N-cycle. Similarly, the intensively degraded savanna soils, the cultivated soils and the soils under disturbed forests on the foothill of Mt. Kilimanjaro reveal very positive δ(15)N values around 6.5 ‰. In contrast, the undisturbed forest soils in the montane zone are more depleted in (15)N, indicating that here the N-cycle is relatively closed. However, significantly higher δ(15)N values characterise the upper montane forest zone at the transition to the subalpine zone. We suggest that this reflects N-losses by the recently monitored and climate change and antropogenically induced increasing fire frequency pushing the upper montane rainforest boundary rapidly downhill. Overall, we conclude that the analysis of the (15)N natural abundance in high-mountain ecosystems is a purposeful tool for detecting land-use- or climate change-induced soil degradation and N-cycle opening.     

Geochemical signatures (C, N, delta13C, delta15N, metals) of suspended matter in the river Weisse Elster (central Germany): their seasonal and flow-related distribution 1997-2001

Geochemical studies of carbon, nitrogen, delta13C, delta15N as well as Fe, Mn, Cd, Zn, Cr and Hg in suspended matter taken from the river Weisse Elster (central Germany) between 1997 and 2001 reveal significant changes to the composition of the organic sediment load, which correlate with the hydrological period and flow rate. Using C/N ratios and the isotope values of carbon and nitrogen as source indicators, it was found that the organic suspended matter fractions in hydrological winter periods comprise both resuspended mortal plankton material from the riverbed and terrigenous C3 plant material from the clastic input. During the 6 month summer periods, increased bioproductivity results in more dissolved carbon and mineral nitrogen compounds being taken up by the freshly formed aquatic organic substance (freshwater plankton). These compounds stem from bacterial breakdown processes affecting organic components of the river sediment and/or the peripheral soil zone. Increasing fractions of freshwater plankton during the summer period are accompanied by an increase in the nitrogen content and by isotope signatures shifting (delta13C to lower but delta15N to higher values) in the suspended matter. Seasonally opposite correlations between metal contents (e.g. Cd, Zn, Cr, Hg and Fe) and the carbon and nitrogen levels of suspended matter (significantly positive in winter and significantly negative in summer) show that in suspended matter these elements mostly bind to resuspended mortal (rather than the freshly formed living aquatic) organic substance. According to long-term measuring series, between 1993 and 2002 the levels of heavy metals (especially cadmium) in the suspended matter of the river Weisse Elster decreased. Similarly, between 1997 and 2001 the oxygen level in the river Weisse Elster improved. This caused the faster breakdown of organic substance on the riverbed, resulting in the increased uptake of 15N-rich nitrogen compounds into the fresh aquatic organic substance formed every year, and an increase in the conversion of dissolved manganese in the water into insoluble manganese compounds in the river sediment.     

Contribution of collagen stable isotope biogeochemistry to the paleobiology of extinct endemic vertebrates from Tenerife (Canary Islands, Spain)

The paleodiet and paleoenvironmental context of two extinct species from Tenerife island, one giant rat Canariomys bravoi and one giant lizard Gallotia goliath, have been investigated using carbon and nitrogen isotopic compositions of fossil bone collagen. Preliminary to this study, a calibration of the isotopic variations of bone collagen from modern Rat Rattus rattus, Rabbit Oryctolagus cuniculus and Lizard Gallotia galotti relative to environmental conditions on Tenerife Islands has been attempted. No clear relationship could be found between collagen delta13C and delta15N values and aridity; the only relevant factors seem to be seashore proximity for rat, and the relative amount of C3 and CAM plants. It seems that anthropic activities have interfered with the expected relationships between collagen isotopic compositions and environmental conditions. Most fossil specimens yielded well preserved collagen. The isotopic composition of giant rat and giant lizard collagen suggest a purely C3 environment, possibly more humid than today on Tenerife. Large ranges of nitrogen isotopic compositions, especially within giant rats, may be due to local environmental conditions. Further work is needed in order to provide more valuable paleobiological information in order to better understand the role of environmental factors in the evolution and extinction of insular endemic species on Tenerife.