Foliar δ15 N patterns are dependent on seasonal and spatial variations as shown in lowland Scottish scrub

Summary

Patterns of foliar δ¹⁵ N can suggest testable hypotheses concerning N use among and within plant species. However, both spatial and time-series sampling is required to establish how the patterns vary within and among species. On a seasonal basis, foliar δ^l5 Nrankings may change among the species compared. When symbiotic N₂-fixers are among the plants sampled, N₂-fixation may be temporally disjunct from the near-0%c, expected foliar δ¹⁵ N, which is usually attributed to N₂-fixation, and the fates of previously fixed N may not be apparent from a net foliar δ¹⁵ N of either soil or plants.     

Thyme travels: 15N isoscapes of Thymus vulgaris L. invasion in lightly grazed pastoral communities

Alterations to ecosystem nitrogen (N) cycling by introduced plant species may increase the invasibility of habitat providing a positive feedback for the introduced species to become invasive. Spatial patterns of foliar and soil δ¹⁵N ratios reflect variation in rates and process of N‐cycling across invaded landscapes and provide insight into N‐source uptake and utilization strategies of invasive plant species. To evaluate invasion‐associated changes in soil and foliar δ¹⁵N at different scales: regional (among different sites), local (between north‐ and south‐facing aspect at the same site), and microsite (within populations in the same community), we measured foliar and soil δ¹⁵ N, animal faeces cover (as a proxy for grazing intensity) and N₂‐fixing species cover from inside to outside Thymus vulgaris L. (thyme)‐invaded lightly grazed pastoral communities in Central Otago, southern South Island, New Zealand. Mean thyme foliar δ¹⁵N were near‐zero across the invaded landscape, and did not change across the advancing edge of invasion or with aspect. There was no evidence that associations with N₂‐fixing species provide a potential N source. Soil δ¹⁵N was lower inside of thyme compared to at the edge or outside of thyme and was varied between aspects at some sites. Animal faeces cover as a proxy for grazing intensity explained only 23% of this observed variation of soil δ¹⁵N. Thyme invasion may result in lowered soil δ¹⁵N reflecting alterations to N dynamics. Associated invasion‐related impacts of animal grazing may also impact soil δ¹⁵N. Further studies are required to distinguish the underlying mechanism responsible for the observed patterns of foliar and soil δ¹⁵N values across thyme‐invaded Central Otago landscapes.     

15N natural abundance of plant and soil components of a Banksia woodland ecosystem in relation to nitrate utilization, life form, mycorrhizal status and N2-fixing abilities of component species

Studies of the variation in δ¹⁵N values for plants from a fire-prone Banksia woodland in South West Australia showed that pioneer herbaceous, non-mycorrhizal species which were active in nitrate reduction and storage, had the highest values (1.81%c). A detailed study of one such species Ptilotus polystachus demonstrated a close correspondence between the δ¹⁵N values of soil nitrate, xylem nitrate and leaf total nitrogen, suggesting an exclusive reliance on nitrate ions as nitrogen source. These pioneer species also showed a preponderance of the chloroplastic isoform of glutamine synthetase while woody species generally had higher activity associated with the cytosolic isoform. The group comprising monocotyledonous hemicryptophytes and geophytes contained species with slightly positive δ¹⁵N values and moderately active in nitrate reduction and storage. Nitrogen-fixing species had the lowest δ¹⁵N values (–0.36‰), irrespective of their apparent utilisation of nitrate. However, woody resprouter species which had low levels of nitrate reduction and storage had δ¹⁵N values which fell within the range of values obtained for the miscellaneous assemblage of N₂-fixing species. Consequently, ¹⁵N abundance values failed to distinguish N₂ fixing from non-fixing woody species, and therefore, could not be used in the ecosystem to determine the dependence of putative nitrogen fixing species on N₂ fixation. The study demonstrated complex patterns of nitrogen utilization in the ecosystem in which exploitation of different nitrogen resources related to plant life form and the physiological attributes of nitrogen assimilation by component species.     

Patterns of foliar δ15N and their control in Eastern Asian forests

Foliar δ¹⁵N has been used increasingly in research on ecosystem nitrogen (N) cycling, because it can serve as an integrator of ecosystem N cycling and thus has a potential to reveal temporal and spatial patterns of N cycling as well as how the N cycle is altered by disturbances. However, the current understanding on controls of foliar δ¹⁵N is based principally on studies from America, Europe, Australia and Africa. Here we compiled data from 65 forests at 33 sites across East Asia to explore regional patterns and what controls foliar δ¹⁵N by linking it to climate, species composition, soil depth, slope position, N deposition, and soil N availability. In East Asia, foliar δ¹⁵N ranged from ?7.1 to +2.7‰. Mean foliar δ¹⁵N values for tropical, subtropical and temperate forests were all ?3.1‰, which was unexpected. The patterns of foliar δ¹⁵N with precipitation, temperature and altitude were not clear. The variation in foliar δ¹⁵N among species and between different slope positions appeared to be small within a given forest. The δ¹⁵N for both bulk soil N and extractable inorganic N generally increased with soil depth as expected, strengthening the idea that deep-rooted trees may have access to ¹⁵N-enriched N. Different from the positive correlations reported across America and Europe, in East Asia we found that foliar δ¹⁵N decreased with increasing N deposition and did not relate to soil N availability. These discrepancies deserve more research to elucidate the mechanisms by which foliar δ¹⁵N is affected by ecosystem N availability at a regional scale.     

Carbon and nitrogen parasitism by a xylem‐tapping mistletoe (Tapinanthus oleifolius) along the Kalahari Transect: a stable isotope study

The present study explores the xylem‐tapping parasitism by mistletoe (Tapinanthus oleifolius) on native tree species along the Kalahari Transect (KT) using the stable isotopes of carbon and nitrogen. Mistletoe‐host pairs were collected at three geographical locations along the KT rainfall gradient in the 2005 and 2006 wet seasons. Foliar total carbon, total nitrogen and their stable isotope compositions (δ¹³C and δ¹⁵N) were measured. Heterotrophy (H) was calculated using foliar δ¹³C values of mistletoes and their hosts as an indicator of proportion of carbon in the mistletoes derived from host photosynthate. Based on the mistletoe H‐value and relationship between the mistletoe foliar δ¹⁵N and their host foliar δ¹⁵N, the results showed that mistletoes along the KT derived both nitrogen and carbon from their hosts. Mistletoes may regulate water use in relation to nitrogen supply. The proportion of carbon in the mistletoes derived from host photosynthate was between 35% and 78%, and the degree of heterotrophy was species‐specific with only limited annual variation. The study emphasizes the importance of incorporating parasitic associations in future studies on studying carbon, water and nutrient cycling along the Kalahari.     

Stable isotopes as indicators for seasonally dominant nitrogen cycling processes in a subarctic lake

Nitrogen stable isotopes (δ¹⁵N) of dissolved inorganic nitrogen (DIN = NH₄⁺ and NO₃^–), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) were measured in Smith Lake, Alaska to assess their usefulness as proxies for the biological nitrogen cycling processes, nutrient concentration, and lake productivity. Large seasonal variations in δ¹⁵NH₄⁺, δ¹⁵NO₃^– and δ¹⁵N_PON occurred in response to different processes of nitrogen transformation that dominated a specific time period of the annual production cycle. In spring, ¹⁵N depletion in all three pools was closely related to the occurrences of a N₂‐fixing cyanobacterial bloom (Anabaena flos‐aquae). In summer, δ¹⁵N_PON increased as phytoplankton community shifted to use NH₄⁺ and decreased as a brief N₂‐fixing bloom (Aphanizomenon flos‐aquae) occurred in August. In early and mid‐winter, microbial nitrogen processes were dominated by nitrification that resulted in the largest isotope fractionation between NO₃^– and NH₄⁺ in the annual cycle. This was followed by denitrification that led to the highest ¹⁵N enrichment in NO₃^–. A peak of NH₄⁺ assimilation by phytoplankton along with the elevated δ¹⁵N_PON and Chl a concentration occurred just before the ice break due to increased light penetration. The δ¹⁵N_DON displayed little temporal and spatial variations. This suggests that the DON pool was not altered by biological transformations of nitrogen as the results of its large size and possibly refractory nature. There was a positive correlation between Chl a concentration and δ¹⁵N_PON, and a negative correlation between NH₄⁺ and δ¹⁵N_PON, suggesting that δ¹⁵N_PON is a useful proxy for nitrogen productivity and ammonium concentration. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Variations in nitrogen-15 natural abundance of plant and soil systems in four remote tropical rainforests, southern China

The foliar stable N isotope ratio (δ¹⁵N) can provide integrated information on ecosystem N cycling. Here we present the δ¹⁵N of plant and soil in four remote typical tropical rainforests (one primary and three secondary) of southern China. We aimed to examine if (1) foliar δ¹⁵N in the study forests is negative, as observed in other tropical and subtropical sites in eastern Asia; (2) variation in δ¹⁵N among different species is smaller compared to that in many N-limited temperate and boreal ecosystems; and (3) the primary forest is more N rich than the younger secondary forests and therefore is more ¹⁵N enriched. Our results show that foliar δ¹⁵N ranged from ?5.1 to 1.3 ‰ for 39 collected plant species with different growth strategies and mycorrhizal types, and that for 35 species it was negative. Soil NO₃ ^? had low δ¹⁵N (?11.4 to ?3.2 ‰) and plant NO₃ ^? uptake could not explain the negative foliar δ¹⁵N values (NH₄ ⁺ was dominant in the soil inorganic-N fraction). We suggest that negative values might be caused by isotope fractionation during soil NH₄ ⁺ uptake and mycorrhizal N transfer, and by direct uptake of atmospheric NH₃/NH₄ ⁺. The variation in foliar δ¹⁵N among species (by about 6 ‰) was smaller than in many N-limited ecosystems, which is typically about or over 10 ‰. The primary forest had a larger N capital in plants than the secondary forests. Foliar δ¹⁵N and the enrichment factor (foliar δ¹⁵N minus soil δ¹⁵N) were higher in the primary forest than in the secondary forests, albeit differences were small, while there was no consistent pattern in soil δ¹⁵N between primary and secondary forests.     

The influence of weed control on foliar δ<Superscript>15</Superscript>N, δ<Superscript>13</Superscript>C and tree growth in an 8 year-old exotic pine plantation of subtropical Australia

Background and aims

The aim of weed control and fertilization in forest plantations was to increase tree growth by reducing competition for available nutrients and water. However, treatments that influence weed biomass can also have significant impacts on soil carbon (C) and nitrogen (N) cycling which can in turn lead to changes in the dynamics of stable C (δ¹³C) and N (δ¹⁵N) isotope compositions in soils and tree foliage.

Methods

We examined the key C and N cycling processes influenced by routine and luxury weed control and fertilization treatments as reflected by soil and foliar δ¹³C and δ¹⁵N and long-term tree growth in an 8-year old F₁ hybrid pine (Pinus elliottii x P. caribaea) plantation in southeast Queensland, Australia. Weed control treatments varied by treatment frequency and intensity while fertilization treatments varied by the application of N, phosphorus (P), potassium (K) and micronutrients. Different soil and canopy sampling positions were assessed to determine if sampling position enhanced the relationships among soil N transformations and tree N use, water use efficiency and carbon gain under the early establishment silviculture.

Results

Routine weed control was associated with increased weed biomass returned to the soil, compared with luxury weed control. Soil δ¹³C increased at the 0–5 cm soil sampling depth in both the inter-planting (IPR) and planting row (PR) as a result of the routine weed control treatments. In addition, soil δ¹³C was significantly higher as a result of fertilisation treatment in the 0–5 cm soil sampling depth in the PR. Soil δ¹³C was negatively correlated to soil δ¹⁵N at the 0–5 cm soil sampling depth in the IPR. Soil δ¹⁵N increased in the 0–5 and 5–10 cm soil sampling depths in the IPR, as a result of more frequent (luxury) weed control. Foliar δ¹⁵N and tree water use efficiency (WUE) (as indicated by foliar δ¹³C) were positively correlated with tree growth at age 8 years. While relationships between δ¹³C and δ¹⁵N in the soil and foliage varied depending on soil sampling depth and position, and with canopy sampling position where there were consistent relationships between soil δ¹³C (or δ¹⁵N) and foliar δ¹⁵N.

Conclusions

This study demonstrates how early establishment silviculture has important implications for soil C and N cycling and how soil δ¹³C and δ¹⁵N were consistent with changes in soil C cycling and N transformations as a result of weed control treatments, while foliar δ¹⁵N was linked to more rapid N cycling as reflected in the soil δ¹⁵N, which increased tree growth and tree WUE (as reflected by foliar δ¹³C).

     

Waterlogging and fire impacts on nitrogen availability and utilization in a subtropical wet heathland (wallum)

Protein, amino acids and ammonium were the main forms of soluble soil nitrogen in the soil solution of a subtropical heathland (wallum). After fire, soil ammonium and nitrate increased 90- and 60-fold, respectively. Despite this increase in nitrate availability after fire, wallum species exhibited uniformly low nitrate reductase activities and low leaf and xylem nitrate. During waterlogging soil amino acids increased, particularly γ-aminobutyric acid (GABA) which accounted for over 50% of amino nitrogen. Non-mycorrhizal wallum species were significantly (P < 0.05) ¹⁵N-enriched (0.3–4.3‰) compared to species with mycorrhizal associations (ericoid-type, ecto-, va-mycorrhizal) which were strongly depleted in ¹⁵N (-6.3 to -1.8‰). Lignotubers and roots had δ¹⁵N signatures similar to that of the leaves of respective species. The exceptions were fine roots of ecto-, ecto/va-, and ericoid type mycorrhizal species which were enriched in ¹⁵N (0.1–2.4‰). The 5¹⁵N signatures of δ¹⁵N_{total soil N} and δ¹⁵N_{soil NH4+} were in the range 3.7–4.5‰, whereas δ¹⁵N_{soil NO3?} was significantly (P < 0.05) more enriched in ¹⁵N (9.2–9.8‰). It is proposed that there is discrimination against ¹⁵N during transfer of nitrogen from fungal to plant partner. Roots of selected species incorporated nitrogen sources in the order of preference: ammonium > glycine > nitrate. The exception were proteoid roots of Hakea (Proteaceae) which incorporated equal amounts of glycine and ammonium.     

Environmental and species‐specific controls on δ13C and δ15N in dominant woody plants from central‐western Argentinian drylands

Spatial variation in mean annual precipitation is the principal driver of plant water and nitrogen status in drylands. The natural abundance of carbon stable isotopes (δ¹³C) in photosynthetic tissues of C3 plants is an indicator of time‐integrated behaviour of stomatal conductance; while that of nitrogen stable isotopes (δ¹⁵N) is an indicator of the main source of plant N (soil N vs. atmospheric N₂). Previous studies in drylands have documented that plant δ¹³C and δ¹⁵N values increase with decreasing mean annual precipitation due to reductions in stomatal conductance, and soil enriched in ¹⁵N, respectively. However, evidence for this comes from studies focused on stable isotopes measurements integrated at the plant community level or on dominant plants at the site level, but little effort has been made to study C and N isotope variations within a species growing along rainfall gradients. We analysed plant δ¹³C, δ¹⁵N and C/N values of three woody species having different phenological leaf traits (deciduous, perennial and aphyllous) along a regional mean annual precipitation gradient from the central‐western Argentinian drylands. Noticeably, plant δ¹³C and δ¹⁵N values in the three woody species did not increase towards sites with low precipitation or at the start of the growing season (drier period), as we expected. These results suggest that environmental factors other than mean annual precipitation may be affecting plant δ¹³C and δ¹⁵N. The short‐term environmental conditions may interact with species‐specific plant traits related to water and nitrogen use strategies and override the predictive influence of the mean annual precipitation on plant δ¹³C and δ¹⁵N widely reported in drylands.     

Nitrogen Isotope Patterns in Alaskan Black Spruce Reflect Organic Nitrogen Sources and the Activity of Ectomycorrhizal Fungi

Global patterns in soil, plant, and fungal stable isotopes of N (δ¹⁵N) show promise as integrated metrics of N cycling, particularly the activity of ectomycorrhizal (ECM) fungi. At small spatial scales, however, it remains difficult to differentiate the underlying causes of plant δ¹⁵N variability and this limits the application of such measurements to better understand N cycling. We conducted a landscape-scale analysis of δ¹⁵N values from 31 putatively N-limited monospecific black spruce (Picea mariana) stands in central Alaska to assess the two main hypothesized sources of plant δ¹⁵N variation: differing sources and ECM fractionation. We found roughly 20% of the variability in black spruce foliar N and δ¹⁵N values to be correlated with the concentration and δ¹⁵N values of soil NH₄ ⁺ and dissolved organic N (DON) pools, respectively. However, ¹⁵N-based mixing models from 24 of the stands suggested that fractionation by ECM fungi obscures the ¹⁵N signature of soil N pools. Models, regressions, and N abundance data all suggested that increasing dependence on soil DON to meet black spruce growth demands predicates increasing reliance on ECM-derived N and that black spruce, on average, received 53% of its N from ECM fungi. Future research should partition the δ¹⁵N values within the soil DON pool to determine how choice of soil δ¹⁵N values influence modeled ECM activity. The C balance of boreal forests is tightly linked to N cycling and δ¹⁵N values may be useful metrics of changes to these connections.     

Carbon (δ13C) and nitrogen (δ15N) stable isotope composition in plant and soil in Southern Patagonia's native forests

Stable isotope natural abundance measurements integrate across several biogeochemical processes in ecosystem N and C dynamics. Here, we report trends in natural isotope abundance (δ¹³C and δ¹⁵N in plant and soil) along a climosequence of 33 Nothofagus forest stands located within Patagonia, Southern Argentina. We measured 28 different abiotic variables (both climatic variables and soil properties) to characterize environmental conditions at each of the 33 sites. Foliar δ¹³C values ranged from ?35.4‰ to ?27.7‰, and correlated positively with foliar δ¹⁵N values, ranging from ?3.7‰ to 5.2‰. Soil δ¹³C and δ¹⁵N values reflected the isotopic trends of the foliar tissues and ranged from ?29.8‰ to ?25.3‰, and ?4.8‰ to 6.4‰, respectively, with no significant differences between Nothofagus species (Nothofagus pumilio, Nothofagus antarctica, Nothofagus betuloides). Principal component analysis and multiple regressions suggested that mainly water availability variables (mean annual precipitation), but not soil properties, explained between 42% and 79% of the variations in foliar and soil δ¹³C and δ¹⁵N natural abundance, which declined with increased moisture supply. We conclude that a decline in water use efficiency at wetter sites promotes both the depletion of heavy C and N isotopes in soil and plant biomass. Soil δ¹³C values were higher than those of the plant tissues and this difference increased as annual precipitation increased. No such differences were apparent when δ¹⁵N values in soil and plant were compared, which indicates that climatic differences contributed more to the overall C balance than to the overall N balance in these forest ecosystems.     

Foliar δ15N is affected by foliar nitrogen uptake, soil nitrogen, and mycorrhizae along a nitrogen deposition gradient

Foliar nitrogen isotope (δ¹⁵N) composition patterns have been linked to soil N, mycorrhizal fractionation, and within-plant fractionations. However, few studies have examined the potential importance of the direct foliar uptake of gaseous reactive N on foliar δ¹⁵N. Using an experimental set-up in which the rate of mycorrhizal infection was reduced using a fungicide, we examined the influence of mycorrhizae on foliar δ¹⁵N in potted red maple (Acer rubrum) seedlings along a regional N deposition gradient in New York State. Mycorrhizal associations altered foliar δ¹⁵N values in red maple seedlings from 0.06 to 0.74 ‰ across sites. At the same sites, we explored the predictive roles of direct foliar N uptake, soil δ¹⁵N, and mycorrhizae on foliar δ¹⁵N in adult stands of A. rubrum, American beech (Fagus grandifolia), black birch (Betula lenta), and red oak (Quercus rubra). Multiple regression analysis indicated that ambient atmospheric nitrogen dioxide (NO₂) concentration explained 0, 69, 23, and 45 % of the variation in foliar δ¹⁵N in American beech, red maple, red oak, and black birch, respectively, after accounting for the influence of soil δ¹⁵N. There was no correlation between foliar δ¹³C and foliar %N with increasing atmospheric NO₂ concentration in most species. Our findings suggest that total canopy uptake, and likely direct foliar N uptake, of pollution-derived atmospheric N deposition may significantly impact foliar δ¹⁵N in several dominant species occurring in temperate forest ecosystems.     

Variation of 15N natural abundance in leaves and nodules of actinorhizal shrubs in Northwest Patagonia

This study was performed to assess the N₂-fixing capability of the native actinorhizal species Ochetophila trinervis (sin. Discaria trinervis) and Discaria chacaye (Rhamnaceae) in Northwest Patagonia. We measured the N concentration and ¹⁵N natural abundance in leaves and nodules of O. trinervis and D. chacaye, in leaves of associated non-actinorhizal vegetation, and in the soils under each sampled plant. O. trinervis and D. chacaye had foliar N concentrations that were about twice that of non-actinorhizal shrubs growing at the same sites, even though soils varied four-fold in total N across the sites. Leaves of both actinorhizal plants had a similar δ¹⁵N at any site and were close to atmospheric values. The foliar δ¹⁵N of non-actinorhizal plants and soil δ¹⁵N were strongly correlated across the sites. Nodules were depleted in δ¹⁵N relative to the foliage of the respective actinorhizal species. In conjunction with the uniformly high foliage N concentration of these actinorhizal plants and the universal presence of vesicles observed in root nodules, these data strongly suggest that O. trinervis and D. chacaye obtain a significant amount of their N from N₂ fixation. To calculate the proportion of N derived from atmosphere, theoretical B-values were estimated. In all cases where the δ¹⁵N of fixing and reference foliage were significantly different, O. trinervis and D. chacaye obtained almost all of their N from N₂ fixation. These results are the first to demonstrate N₂ fixation by O. trinervis and D. chacaye in the field and therefore suggest an important role for these actinorhizal plants in the N economy of ecosystems in northwest Patagonia as well as their potential use for restoration of degraded lands in this region.     

Exploring cycad foliage as an archive of the isotopic composition of atmospheric nitrogen

Molecular nitrogen (N₂) constitutes the majority of Earth's modern atmosphere, contributing ~0.79 bar of partial pressure (pN₂). However, fluctuations in pN₂ may have occurred on 10⁷–10⁹ year timescales in Earth's past, perhaps altering the isotopic composition of atmospheric nitrogen. Here, we explore an archive that may record the isotopic composition of atmospheric N₂ in deep time: the foliage of cycads. Cycads are ancient gymnosperms that host symbiotic N₂‐fixing cyanobacteria in modified root structures known as coralloid roots. All extant species of cycads are known to host symbionts, suggesting that this N₂‐fixing capacity is perhaps ancestral, reaching back to the early history of cycads in the late Paleozoic. Therefore, if the process of microbial N₂ fixation records the δ¹⁵N value of atmospheric N₂ in cycad foliage, the fossil record of cycads may provide an archive of atmospheric δ¹⁵N values. To explore this potential proxy, we conducted a survey of wild cycads growing in a range of modern environments to determine whether cycad foliage reliably records the isotopic composition of atmospheric N₂. We find that neither biological nor environmental factors significantly influence the δ¹⁵N values of cycad foliage, suggesting that they provide a reasonably robust record of the δ¹⁵N of atmospheric N₂. Application of this proxy to the record of carbonaceous cycad fossils may not only help to constrain changes in atmospheric nitrogen isotope ratios since the late Paleozoic, but also could shed light on the antiquity of the N₂‐fixing symbiosis between cycads and cyanobacteria.     

Nitrogen stable isotopes indicate differences in nitrogen cycling between two contrasting Jamaican montane forests

Background and aims

The aim of this study is to enhance our knowledge of nitrogen (N) cycling and N acquisition in tropical montane forests through analysis of stable N isotopes (δ¹⁵N).

Methods

Leaves from eight common tree species, leaf litter, soils from three depths and roots were sampled from two contrasting montane forest types in Jamaica (mull ridge and mor ridge) and were analysed for δ¹⁵N.

Results

All foliar δ¹⁵N values were negative and varied among the tree species but were significantly more negative in the mor ridge forest (by about 2 ‰). δ¹⁵N of soils and roots were also more negative in mor ridge forests by about 3 ‰. Foliar δ¹⁵N values were closer to that of soil ammonium than soil nitrate suggesting that trees in these forests may have a preference for ammonium; this may explain the high losses of nitrate from similar tropical montane forests. There was no correlation between the rankings of foliar δ¹⁵N in the two forest types suggesting a changing uptake ratio of different N forms between forest types.

Conclusions

These results indicate that N is found at low concentrations in this ecosystem and that there is a tighter N cycle in the mor ridge forest, confirmed by reduced nitrogen availability and lower rates of nitrification. Overall, soil or root δ¹⁵N values are more useful in assessing ecosystem N cycling patterns as different tree species showed differences in foliar δ¹⁵N between the two forest types.     

Tree ring δ<Superscript>15</Superscript>N as validation of space-for-time substitution in disturbance studies of forest nitrogen status

Forest ecosystem nitrogen (N) response to disturbance has often been examined by space-for-time substitution, but there are few objective tests of the possible variation in disturbance type and intensity across chronosequence sites. We hypothesized that tree ring δ¹⁵N, as a record of ecosystem N status, could validate chronosequence assumptions and provide isotopic evidence to corroborate N trends. To test this we measured soil N availability, soil δ¹⁵N, and foliar N attributes of overstory Douglas-fir (Pseudotsuga menziesii) and understory western hemlock (Tsuga heterophylla) across three old-growth stands and nine second-growth plantations on southeast Vancouver Island, British Columbia (Canada). Increment cores for wood δ¹⁵N were retrieved from three co-dominant Douglas-fir per plot. Bulk soil δ¹⁵N was well aligned with both foliar and recent wood δ¹⁵N, demonstrating the utility of wood δ¹⁵N in monitoring ecosystem N status. Strongly contrasting trends in tree ring δ¹⁵N were evident among second-growth stands, with most trees from plantations older than 50 years exhibiting steep declines (3–4‰) in δ¹⁵N but with no temporal trends detected for younger plantations. The discrepancy in tree ring δ¹⁵N suggests disturbance history varied considerably among second-growth sites, likely because of greater slash loads and hotter broadcast burns on older cutblocks. As a consequence, the pattern of increased soil N availability and foliar N concentration with time since disturbance derived from the chronosequence could not be validated. Tree ring δ¹⁵N may provide insights into disturbance intensity, especially fire, and correlations with foliar N concentration could inform the extent of changes in stand nutrition.     

Soil 13C–15N dynamics in an N2-fixing clover system under long-term exposure to elevated atmospheric CO2

Reduced soil N availability under elevated CO₂ may limit the plant's capacity to increase photosynthesis and thus the potential for increased soil C input. Plant productivity and soil C input should be less constrained by available soil N in an N₂‐fixing system. We studied the effects of Trifolium repens (an N₂‐fixing legume) and Lolium perenne on soil N and C sequestration in response to 9 years of elevated CO₂ under FACE conditions. ¹⁵N‐labeled fertilizer was applied at a rate of 140 and 560 kg N ha^?1 yr^?1 and the CO₂ concentration was increased to 60 Pa pCO₂ using ¹³C‐depleted CO₂. The total soil C content was unaffected by elevated CO₂, species and rate of ¹⁵N fertilization. However, under elevated CO₂, the total amount of newly sequestered soil C was significantly higher under T. repens than under L. perenne. The fraction of fertilizer‐N (f_N) of the total soil N pool was significantly lower under T. repens than under L. perenne. The rate of N fertilization, but not elevated CO₂, had a significant effect on f_N values of the total soil N pool. The fractions of newly sequestered C (f_C) differed strongly among intra‐aggregate soil organic matter fractions, but were unaffected by plant species and the rate of N fertilization. Under elevated CO₂, the ratio of fertilizer‐N per unit of new C decreased under T. repens compared with L. perenne. The L. perenne system sequestered more ¹⁵N fertilizer than T. repens: 179 vs. 101 kg N ha^?1 for the low rate of N fertilization and 393 vs. 319 kg N ha^?1 for the high N‐fertilization rate. As the loss of fertilizer‐¹⁵N contributed to the ¹⁵N‐isotope dilution under T. repens, the input of fixed N into the soil could not be estimated. Although N₂ fixation was an important source of N in the T. repens system, there was no significant increase in total soil C compared with a non‐N₂‐fixing L. perenne system. This suggests that N₂ fixation and the availability of N are not the main factors controlling soil C sequestration in a T. repens system.     

Do grazers alter nitrogen dynamics on grazing lawns in a South African savannah?

The presence of grazers on grazing lawns in East Africa and North America often alters nitrogen cycling and availability. Grazing lawns can be defined as areas where grasses are kept in a short, actively growing, palatable state by the action of grazers. Our aim was to test whether lawns have enhanced leaf nitrogen (N) concentrations, total soil N and δ¹⁵N when compared to tall grass areas in a South African savannah. Previous studies have used ecosystem δ¹⁵N as a proxy of N availability, and enriched δ¹⁵N values have been suggested to indicate higher N availability or higher N transformation rates. Across all sites, foliar N concentrations (but not soil N) were higher when compared to tall grass areas, and evidence of enriched foliar and soil δ¹⁵N values was found on the lawns. These results suggest that grazers may be involved in altering the rates of N transformations directly on grazing lawns. Regardless of whether these N transformations included increased net N mineralization, higher N concentrations in above‐ground foliage attract grazers back to the lawns, encouraging their maintenance.     

N-fertilization does not alleviate grass competition induced reduction of growth of African savanna species

Background and aims

Below-ground grass competition limits woody establishment in savannas. N₂-fixing legumes may, however, have a nutritional advantage over broad-leaved species. We hypothesised that broad-leaved non-legume savanna thicket species would be more severely constrained by grass competition for N and consequently respond more to N-fertilization than the legume, Acacia karroo.

Methods

A. karroo and five non-legume thicket species (Maytenus senegalensis, M. heterophylla, Euclea divinorum, Ziziphus mucronata, Schotia brachypetala) were grown together in an irrigated competition experiment with clipped-, unclipped-grass and without grass with/without N-fertilizer. The biomass, foliar nutrient, δ¹³C and δ¹⁵N of grasses and woody species were determined.

Results

Growth of both A. karroo and the non-legume species was equally sensitive (c. 90 % reduction) to both clipped- and unclipped-grass competition, regardless of N-fertilization. With grass competition, however, foliar [N] increased and δ¹⁵N decreased in response to N-fertilization. Grass biomass accumulation was also unchanged by fertilisation, despite increases in foliar [N] and decreases in δ¹⁵N.

Conclusions

The N₂-fixation capacity of A. karroo provided no growth advantage over non-legumes. The lack of responsiveness of biomass accumulation by both the woody species and the grasses to N-fertilization, despite evidence that plants accessed the N-fertilizer, indicates limitation by other nutrients.