Soil δ15N Research Articles

Disturbance and Topography Shape Nitrogen Availability and δ15N over Long-Term Forest Succession

Forest disturbance and long-term succession towards old-growth are thought to increase nitrogen (N) availability and N loss, which should increase soil δ15N values. We examined soil and foliar patterns in N and δ15N, and soil N mineralization, across 800 years of forest succession in a topographically complex montane landscape influenced by human logging and wildfire. In contrast to expectations, we found that disturbance caused declines in surface mineral soil δ15N values, both in logged forests measured 40–50 years after disturbance, and in unlogged forests disturbed by severe wildfire within the last 200 years. Both symbiotic N fixation and N transfers from disturbed vegetation and detritus could lower soil δ15N values after disturbance. A more important role for symbiotic N fixation is suggested by lower soil δ15N values in slow-successional sites with slow canopy closure, which favors early-successional N fixers. Soil δ15N values increased only marginally throughout 800 years of succession, reflecting soil N uptake by vegetation and strong overall N retention. Although post-disturbance N inputs lowered surface soil δ15N values, steady-state mass balance calculations suggest that wildfire combustion of vegetation and detritus can dominate long-term N loss and increase whole-ecosystem δ15N. On steeper topography, declining soil δ15N values highlight erosion and accelerated soil turnover as an additional abiotic control on N balances. We conclude for N-limited montane forests that soil δ15N and N availability are less influenced by nitrate leaching and denitrification loss than by interactions between disturbance, N fixation, and erosion.

Nitrogen isotopic patterns in tropical forests along a rainfall gradient in Northeast Brazil

The interpretation of ecosystem nitrogen isotopic patterns is subject to controversies since there are few surveys of signals along climate gradients within the same region. We determined the effects of a rainfall gradient, under high temperatures, on the soil and plant δ15N signals in Northeast Brazil. Nitrogen, carbon and δ15N signals were determined from coastal perennial to subhumid and semiarid deciduous forests to savanna – deciduous forest transition. Soil C and N concentrations were linearly related and both related quadratically with mean annual rainfall while δ15N concentrations decreased with higher rainfall (16 to 4‰). Plant δ15N was 1–3‰ lower than soil δ15N (Δδ15N) in all areas. Only in the deciduous forests, leaves of target species had significantly lower 15 N signals than those of non-fixing species, allowing estimation of fixation. In the perennial forests the low signals indicated that the 15 N natural abundance method was not suitable to estimate fixation, while in the savanna – deciduous forest transition, high signals indicated that the legume species were not fixing. The smaller Δδ15N than those reported in cooler areas and the absence of rainfall effect indicates that, when high, temperature overrides the rainfall effect.

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.

Effects of grazing on carbon and nitrogen in plants and soils in a semiarid desert grassland, China

Grazing can modulate the feedback between vegetation and soil nutrient dynamics (carbon and nitrogen), altering the cycles of these elements in grassland ecosystems. For clarifying the impact of grazing on the C and N in plants and soils in the desert grassland of Ningxia, China, we examined the plant biomass, SOC (soil organic carbon), total soil N and stable isotope signatures of plants and soils from both the grazed and ungrazed sites. Significantly lower aboveground biomass, root biomass, litter biomass and vegetation coverage were found in the grazed site compared to the ungrazed site, with decreases of 42.0%, 16.2%, 59.4% and 30.0%, respectively. The effects of grazing on plant carbon, nitrogen, δ15N and δ13C values were uniform among species. The levels of plant carbon and nitrogen in grasses were greater than those in the forbs (except for the carbon of Cynanchum komarovii and Euphorbia esula). Root 15N and 13C values increased with grazing, while the responses of root carbon and nitrogen to grazing showed no consistent patterns. Root 15N and 13C were increased by 79.0% and 22.4% in the grazed site compared to the ungrazed site, respectively. The values of SOC and total N were significantly lower in the grazed than in the ungrazed sites for all sampling depths (0–10 and 10–20 cm), and values of SOC and total N at the surface (0–10 cm) were lower than those in the deeper soils (10–20 cm). Soil δ15N values were not affected by grazing at any sampling depth, whereas soil δ13C values were significantly affected by grazing and increased by 19.3% and 8.6% in the soils at 0–10 and 10–20 cm, respectively. The soil δ13C values (−8.3‰ to −6.7‰) were higher than those for roots (−20.2‰ to −15.6‰) and plant tissues (−27.9‰ to −13.3‰). Our study suggests that grazing could greatly affect soil organic carbon and nitrogen in contrast to ungrazed grassland and that grazing appears to exert a negative effect on soil carbon and nitrogen in desert grassland.

Soil and plant nitrogen pools in paddy and upland ecosystems have contrasting δ15N

Waterlogged paddy and water-unsaturated upland ecosystems have contrasting soil nitrogen (N) processes that affect the natural 15N abundance (15N/14N, expressed as δ15N) in different N pools. In this study, we investigated the δ15N patterns in soil and plant N pools of paddy and upland ecosystems. Samples were collected from 20 each of paddy and upland fields at the active growing season and analyzed for N concentration and δ15N. The higher (P < 0.001) concentration (22.1 mg N kg−1) and lower δ15N (6.9 ‰) of NH4 + in paddy than in upland soils (6.1 mg N kg−1 and 9.2 ‰, respectively) likely reflected the lower nitrification potential in paddy soils. On the other hand, a higher (P < 0.001) δ15N of NO3 − in paddy (12.7 ‰) than in upland soils (4.7 ‰) indicated higher denitrification rates in paddy soils. The positive (P < 0.01) correlation of the δ15N of soil organic N with the δ15N of NH4 + rather than with that of NO3 − suggested a tight linkage between organic N and NH4 + as a result of immobilization-mineralization turnover of NH4 +. Therefore, in waterlogged paddy soils where nitrification was restricted, a high rate of microbial immobilization-mineralization turnover might lead to a lower δ15N of soil N (total N, organic N, and NH4 +) than those for upland. The δ15N in plant N pools also reflected the dominant mineral N species in paddy (NH4 +) and upland soils (NO3 −). We conclude that contrasting soil N processes (particularly nitrification) leave distinct δ15N signatures in the soil and plant N pools between paddy and upland fields.

Land management between crops affects soil inorganic nitrogen balance in a tropical rice system

Sustainable production of lowland rice (Oryza sativa L.) requires minimising undesirable soil nitrogen (N) losses via nitrate (NO3 −) leaching and denitrification. However, information is limited on the N transformations that occur between rice crops (fallow and land preparation), which control indigenous N availability for the subsequent crop. In order to redress this knowledge gap, changes in NO3 − isotopic composition (δ15N and δ18O) in soil and water were measured from harvest through fallow, land preparation, and crop establishment in a 7 year old field trial in the Philippines. During the period between rice crops, plots were maintained either, continuously flooded, dry, or alternately wet and dry from rainfall. Plots were split with addition or removal of residue from the previous rice crop. No N fertilizer was applied during the experimental period. Nitrogen accumulated during the fallow (20 kg NH4 +–N ha−1 in flooded treatments and 10 kg NO3 −–N ha−1 in treatments with drying), but did not influence N availability for the subsequent crop. Nitrate isotope fractionation patterns indicated that denitrification drove this homogenisation: during land preparation ~50 % of inorganic N in the soil (top 10 cm) was denitrified, and by 2 weeks after transplanting this increased to >80 % of inorganic N, regardless of fallow management. The 17 days between fallow and crop establishment controlled not only N attenuation (3–7 kg NO3 −–N ha−1 denitrified), but also N inputs (3–14 kg NO3 −–N ha−1 from nitrification), meaning denitrification was dependent on soil nitrification rates. While crop residue incorporation delayed the timing of N attenuation, it ultimately did not impact indigenous N supply. By measuring NO3 − isotopic composition over depth and time, this study provides unique in situ measurements of the pivotal role of land preparation in determining paddy soil indigenous N supply.

Soil nitrogen dynamics within profiles of a managed moist temperate forest chronosequence consistent with long‐term harvesting‐induced losses

AbstractThis study investigates whether clear‐cut forest harvesting leads to alterations in the decadal‐scale biogeochemical nitrogen (N) cycles of moist temperate forest ecosystems. Using a harvested temperate red spruce (Picea rubens Sarg.) forest chronosequence in Nova Scotia, Canada, representing &lt;1 to &gt;80 year old postharvest conditions, alongside a reference old‐growth (125+ year old) site with no documented history of disturbance, we examine harvesting‐related changes in soil N pools and fluxes. Specifically, we quantify soil N storage with depth and age across the forest chronosequence, examine changes in physical fractions and δ15N of soil N through depth and time, and quantify gross soil N transformation rates through depth and time using a 15N isotope dilution technique. Our findings point to a large loss of total N in the soil pool, particularly within the deep soil (&gt;20 cm) and organomineral fractions. A pulse of available mineralized N (as ammonium) was observed following harvesting (mean residence time (MRT) &gt; 6 days), but its MRT dropped to &lt;1 day 80 years following harvesting, in contrast to the MRT of 2–3 days observed in the reference old‐growth forest site. These mineralization patterns coupled with inferred leaching losses to groundwater are consistent with storage estimates that suggest soil N may not reaccrue for almost a century following this disturbance.

Isotopic composition of carbon (δ13C) and nitrogen (δ15N) in foliage and soil as a function of tree species.

Isotopic composition of carbon (δ13C) and nitrogen (δ15N) in foliage and soil as a function of tree species.

Contribution of allochthonous organic carbon across the Serrano River Basin and the adjacent fjord system in Southern Chilean Patagonia: Insights from the combined use of stable isotope and fatty acid biomarkers

Chilean Patagonia is characterized by an irregular geography involving many islands, peninsulas, channels, sounds and fjords, that prevent direct interaction between oceanic water masses and freshwater river discharges at the head of the continental fjords. In this paper, we evaluate the potential sources and composition of organic matter along the Serrano River basin and the adjacent channels and fjords in Southern Chilean Patagonia (51–52°S), as well as their importance for marine planktonic organisms. In spring of 2009, evidence of C:N ratio, δ13C, δ15N and fatty acids composition in particulate organic carbon (POC), surface sediment, soil, plankton, and vegetal tissue, as well some physical and chemical characteristics (i.e. salinity, dissolved oxygen, NO3−, NH4+, PO4−3, Si(OH)4), were measured in samples collected during the CIMAR 14 Fiordos oceanographic cruise. Significant differences in δ13C-POC were found between the terrestrial and marine environments but not within fjord stations. Along the fjord region, the high C:N ratio and depleted δ13C values in POC samples suggest that particulate organic matter (POM) in the upper level of the water column (0–10m depth) is supported by different sources. Terrestrial organic carbon exported by rivers may constitute a significant subsidy, up to 70% based on two end-member mixing model, to the fjord ecosystem. Furthermore, terrestrial carbon might account for a significant percentage of the zooplankton body carbon, estimated both by using isotopic (∼24–61%) and fatty acid analysis (∼14–61%). Isotopic analyses in marine sediment samples suggest that POC seems to be decoupled from terrestrial-influenced surface sources at the fjord stations, and the contribution of surrounding vegetation seemingly unimportant for carbon export to the benthos. Local hydrographic and geomorphological characteristics might determine the presence of oceanographic frontal zones, which in turn might explain differences in carbon sources found between POM and the surface sediments.

Patterns of Soil 15N and Total N and Their Relationships with Environmental Factors on the Qinghai-Tibetan Plateau

The patterns of soil nitrogen (N) isotope composition at large spatial and temporal scales and their relationships to environmental factors illustrate N cycle and sources of N, and are integrative indicators of the terrestrial N cycle and its response to global change. The objectives of this study were: i) to investigate the patterns of soil N content and natural abundance of 15N (δ15N) values in different ecosystem types and soil profiles on the Qinghai-Tibetan Plateau; ii) to examine the effects of climatic factors and soil characteristics on the patterns of soil N content and soil δ15N values; and iii) to test the relationship between soil δ15N values and soil C/N ratios across ecosystems and soil profiles. Soil profiles were sampled at 51 sites along two transects 1 875 km in length and 200 km apart and distributed in forest, meadow and steppe on the Qinghai-Tibetan Plateau. Each site was sampled every 10 cm from a soil depth of 0 to 40 cm and each sample was analyzed for soil N content and δ15N values. Our results indicated that soil N and δ15N values (0–40 cm) in meadows were much higher than in desert steppe. Soil N decreased with soil depth for each ecosystem, while variations of soil δ15N values along soil profiles were not statistically significant among most ecosystems but for mountain meadow, lowland meadow, and temperate steppe where soil δ15N values tended to increase with soil depth. The parabolic relationship between soil δ15N values and mean annual precipitation indicated that soil δ15N values increased with increasing precipitation in desert steppe up to 500 mm, and then decreased with increasing precipitation across all other ecosystems. Moreover, the parabolic relationship between δ15N values and mean annual temperature existed in all individual ecosystem types. Soil N and δ15N values (0–40 cm) increased with an increase in soil silt and clay contents. Furthermore, a threshold of C/N ratio of about 11 divided the parabolic relationship between soil δ15N values and soil C/N ratios into positive (C/N < 11) and negative (C/N > 11) parts, which was valid across all ecosystems and soil profiles. The large explanatory power of soil C/N ratios for soil δ15N values suggested that C and N concentrations, being strongly controlled by precipitation and temperature, were the primary factors determining patterns of soil δ15N on the Qinghai-Tibetan Plateau.

Stable nitrogen isotope patterns of trees and soils altered by long-term nitrogen and phosphorus addition to a lowland tropical rainforest

Foliar nitrogen (N) isotope ratios (δ15N) are used as a proxy for N-cycling processes, including the “openness” of the N cycle and the use of distinct N sources, but there is little experimental support for such proxies in lowland tropical forest. To address this, we examined the δ15N values of soluble soil N and canopy foliage of four tree species after 13 years of factorial N and P addition to a mature lowland rainforest. We hypothesized that N addition would lead to 15N-enriched soil N forms due to fractionating losses, whereas P addition would reduce N losses as the plants and microbes adjusted their stoichiometric demands. Chronic N addition increased the concentration and δ15N value of soil nitrate and δ15N in live and senesced leaves in two of four tree species, but did not affect ammonium or dissolved organic N. Phosphorus addition significantly increased foliar δ15N in one tree species and elicited significant N × P interactions in two others due to a reduction in foliar δ15N enrichment under N and P co-addition. Isotope mixing models indicated that three of four tree species increased their use of nitrate relative to ammonium following N addition, supporting the expectation that tropical trees use the most available form of mineral N. Previous observations that anthropogenic N deposition in this tropical region have led to increasing foliar δ15N values over decadal time-scales is now mechanistically linked to greater usage of 15N-enriched nitrate.

C-N Isotope Coupling along the Vertical Profiles under Different Land Use in a Typical Karst Area, Guizhou, Southwest China

The soil stable carbon and nitrogen isotopic compositions characterize its fate and identify the different land use. In this study, carbon concentration and isotope component, nitrogen concentration and isotope component, and C/N ratios in soils collected from Puding, southwest of China were determined. The carbon and nitrogen content data range from 7.92% to o.48%, and 0.77% to 0.05%, respectively. Soil δ13C ranged from -25.49‰ to -17‰, soil δ15N varied from 1.11‰ to 9.46‰. The concentrations of SOC and SON decrease with depth along the vertical profiles. In addition, combined the SOC and δ13C or SON and δ15N could obtained the possibility of distinguish the replaced of C3 and C4 plants. However the pattern of distribution in soil organic δ13C is different from that in soil organic δ15N at the subsoil (deeper than 30cm). With correlation analyses with C/N ratios and δ13C, it reveals that subsoil carbon microbial derived carbon due to root activity.

Natural 15N Abundance in Winter Wheat Amended with Urea and Compost: A Long-Term Experiment

We investigated 15N abundance (δ15N) of winter wheat (Triticum aestivum cv. Jinmai 1) plants and soil at different growth stages in a field with a 13-year fertilization history of urea and compost, to determine whether or not the δ15N of plant parts can be used as an indicator of organic amendment with compost. Plant parts (roots, leaves, stems and grains) and soil were sampled at re-greening, jointing, grain filling and mature growth stages of winter wheat. There were significant differences between the urea and compost treatments in δ15N of whole plants, plant parts and soil over the whole growing season. Determination of the δ15N of plant parts was more convenient than that of whole plant to distinguish between the application of organic amendment and synthetic N fertilizer.

Organic vs. Conventional Grassland Management: Do 15N and 13C Isotopic Signatures of Hay and Soil Samples Differ?

Distinguishing organic and conventional products is a major issue of food security and authenticity. Previous studies successfully used stable isotopes to separate organic and conventional products, but up to now, this approach was not tested for organic grassland hay and soil. Moreover, isotopic abundances could be a powerful tool to elucidate differences in ecosystem functioning and driving mechanisms of element cycling in organic and conventional management systems. Here, we studied the δ15N and δ13C isotopic composition of soil and hay samples of 21 organic and 34 conventional grasslands in two German regions. We also used Δδ15N (δ15N plant - δ15N soil) to characterize nitrogen dynamics. In order to detect temporal trends, isotopic abundances in organic grasslands were related to the time since certification. Furthermore, discriminant analysis was used to test whether the respective management type can be deduced from observed isotopic abundances.Isotopic analyses revealed no significant differences in δ13C in hay and δ15N in both soil and hay between management types, but showed that δ13C abundances were significantly lower in soil of organic compared to conventional grasslands. Δδ15N values implied that management types did not substantially differ in nitrogen cycling. Only δ13C in soil and hay showed significant negative relationships with the time since certification. Thus, our result suggest that organic grasslands suffered less from drought stress compared to conventional grasslands most likely due to a benefit of higher plant species richness, as previously shown by manipulative biodiversity experiments. Finally, it was possible to correctly classify about two third of the samples according to their management using isotopic abundances in soil and hay. However, as more than half of the organic samples were incorrectly classified, we infer that more research is needed to improve this approach before it can be efficiently used in practice.

Classification of organic and conventional sweet peppers and lettuce using a combination of isotopic and bio-markers with multivariate analysis

In order to verify the authenticity of organic products, multivariate data (δ15N values, key-metabolites and mineral content) were used to classify organic and conventional peppers and lettuce by means of canonical discriminant analysis. Different origins and types of fertilisers were considered. This is the first paper to look into the influence of biosolarisation, a soil disinfection technique widely used in organic cultivation, on the differentiation between organic and conventional crops. The use of pre-harvest parameters (plant and soil δ15N and plant weight) allowed 78.8% and 65.4% of pepper and lettuce samples, respectively, to be correctly classified. In pepper, this percentage was reduced (69.2%) when only post-harvest sampling parameters were used to build the classification model. In lettuce, the use of biomarkers significantly improved the model, allowing 90.4% of cases to be correctly classified. This methodology showed a high potential for use as an authenticity control tool for organic cultivation.

Effects of weed control and fertilization at early establishment on tree nitrogen and water use in an exotic F1 hybrid pine of subtropical Australia

Purpose We investigated the effects of weed control and fertilization at early establishment on foliar stable carbon (δ13C) and nitrogen (N) isotope (δ15N) compositions, foliar N concentration, tree growth and biomass, relative weed cover and other physiological traits in a 2-year old F1 hybrid (Pinus elliottii var. elliottii (Engelm) × Pinus caribaea var. hondurensis (Barr. ex Golf.)) plantation grown on a yellow earth in southeast Queensland of subtropical Australia. Materials and methods Treatments included routine weed control, luxury weed control, intermediate weed control, mechanical weed control, nil weed control, and routine and luxury fertilization in a randomised complete block design. Initial soil nutrition and soil fertility parameters included (hot water extractable organic carbon (C) and total nitrogen (N), total C and N, C/N ratio, labile N pools (nitrate (NO3 −) and ammonium (NH4 +)), extractable potassium (K+)), soil δ15N and δ13C. Relative weed cover, foliar N concentrations, tree growth rate and physiological parameters including photosynthesis, stomatal conductance, photosynthetic nitrogen use efficiency, foliar δ15N and foliar δ13C were also measured at early establishment. Results and discussion Foliar N concentration at 1.25 years was significantly different amongst the weed control treatments and was negatively correlated to the relative weed cover at 1.1 years. Foliar N concentration was also positively correlated to foliar δ15N and foliar δ13C, tree height, height growth rates and tree biomass. Foliar δ15N was negatively correlated to the relative weed cover at 0.8 and 1.1 years. The physiological measurements indicated that luxury fertilization and increasing weed competition on these soils decreased leaf xylem pressure potential (Ψxpp) when compared to the other treatments. Conclusions These results indicate how increasing N resources and weed competition have implications for tree N and water use at establishment in F1 hybrid plantations of southeast Queensland, Australia. These results suggest the desirability of weed control, in the inter-planting row, in the first year to maximise site N and water resources available for seedling growth. It also showed the need to avoid over-fertilisation, which interfered with the balance between available N and water on these soils.

Ecosystem development in short-term postglacial chronosequences: N and P limitation in glacier forelands from Santa Inés Island, Magellan Strait

Glacier foreland moraines provide an ideal model to examine the patterns of ecosystem development and the evolution of nitrogen and phosphorous limitation over successional time. In this paper, we focus on a 400-year soil chronosequence in the glacier forelands of Santa Ines Island in the Magellan Strait, southern Chile by examining forest development on phosphorus (P)-poor substrates in a uniquely unpolluted region of the world. Results show a steady increase in tree basal area and a humped trend in tree species richness over four centuries of stand development. The increase in basal area suggests that the late successional tree species were more efficient nutrient users than earlier successional ones. Total contents of carbon (C) and nitrogen (N) in soils increased during the chronosequence, reaching an asymptote in late succession. The net increases in soil C : N, C : P and N : P ratios observed over successional time suggest that nutrient limitation is maximal in 400-year-old substrates. Foliar C : N and C : P ratios also increased over time to reach an asymptote in old-growth stages, following soil stoichiometric relationships; however the foliar N-to-P ratio remained constant throughout the chronosequence. Biological N fixation was greater in early postglacial succession, associated with the presence of the symbiotic N-fixer Gunnera magellanica. Declining trends of δ15N in surface soils through the 400-year chronosequence are evidence of decreasing N losses in old-growth forests. In synthesis, glacier foreland chronosequences at this high South American latitude provide evidence for increasing efficiency of N and P use in the ecosystem, with the replacement of shade-intolerant pioneers by more efficient, shade-tolerant tree species. This pattern of ecosystem development produces a constant foliar N : P ratio, regardless of variation in soil N-to-P ratio over four centuries.

Foliar δ15N variations with stand ages in temperate secondary forest ecosystems, Northeast China

Natural abundance of 15N changes strongly with gradients in soil or environmental conditions across large spatial and temporal scales, but variation in δ15N with stand age at intermediate scales is poorly understood. We analyzed soil and foliar δ15N along a secondary forest chronosequence in Northeast China in mid-growing seasons 2008 and 2009, to address two questions: how does foliar δ15N vary with stand age; and are the variations driven by compositional differences in species among stands, or by consistent changes in δ15N with plant growth forms, and within species. The results showed that community-level foliar δ15N depleted as stand age increased, and these responses were remarkably consistent within three contrasting plant growth forms (herbs, shrubs, and trees), and within individual species. In spite of the three plant growth forms sharing similar responses to the stand age, tree species showed smaller variations in foliar δ15N along stand ages than herbs and shrubs. Soil δ15N also significantly depleted with increasing stand age, which may drive the variations of foliar δ15N. In addition, mycorrhizal fungi discrimination against 15N may also partly affect the patterns of foliar δ15N along stand ages. The results clearly indicate that differences in foliar δ15N among different stand ages are dominantly driven by the δ15N variations at the species levels, which reflects the variations of soil δ15N and mycorrhizal association intensity and association type, not by compositional difference in species among stands.

A positive relationship between the abundance of ammonia oxidizing archaea and natural abundance δ15N of ecosystems

We present a significant relationship between the natural abundance isotopic composition of ecosystem pools and the abundance of a microbial gene. Natural abundance 15N of soils and soil DNA were analysed and compared with archaeal ammonia oxidizer abundance along an elevation gradient in northern Arizona and along a substrate age gradient in Hawai'i. There was a significant positive correlation between the abundance of archaeal amoA genes and natural abundance δ15N of total soil or DNA suggesting that ammonia oxidizing archaea play an important role in ecosystem N release.

Vegetation and Soil 15N Natural Abundance in Alpine Grasslands on the Tibetan Plateau: Patterns and Implications

The natural abundance of nitrogen (N) stable isotopes (δ15N) has the potential to enhance our understanding of the ecosystem N cycle at large spatial scales. However, vegetation and soil δ15N patterns along climatic and edaphic gradients have not yet been fully understood, particularly for high-altitude ecosystems. Here we determined vegetation and soil δ15N in alpine grasslands on the Tibetan Plateau by conducting four consecutive regional surveys during 2001–2004, and then examined their relationships with both climatic and edaphic variables. Our results showed that both vegetation and soil N in Tibetan alpine grasslands were more 15N-enriched than global averages. Vegetation δ15N did not exhibit any significant trend along the temperature gradient, but decreased significantly with an increase in precipitation amount. In contrast, soil δ15N did not vary with either mean annual temperature or precipitation. Our results also indicated that soil δ15N exhibited a slight increase with clay content, but decreased with soil carbon:nitrogen ratio. A general linear model analysis revealed that variations in vegetation δ15N were dominantly determined by climatic variables, whereas soil δ15N was related to edaphic variables. These results provide clues for potential climatic and edaphic regulations on ecosystem N cycle in these high-altitude regions.