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.     

Foliar chemistry and tree ring δ13C of Pinus densiflora in relation to tree growth along a soil pH gradient

Background and aims

Soil acidification is known to be one of the constraints of tree growth; however, it is unclear how it affects tree growth at photosynthesis level (i.e., through affecting stomatal conductance vs. carboxylation rate) during the growth of trees. This paper studied the effects of soil acidification on Pinus densiflora foliar chemistry and tree ring C isotope ratio (¹³C/¹²C, expressed as δ¹³C) and their relationship with tree growth.

Methods

Tree growth (diameter, annual growth ring area, and root biomass), soil chemistry (pH, mineral N, and exchangeable Ca and Al), foliage chemistry (N, Ca/Al, and δ¹³C), and tree ring δ¹³C in P. densiflora stands along a soil pH gradient (from 4.38 to 4.83, n?=?9) in southern Korea were investigated.

Results

Overall, trees with relatively poor growth under more acidic soil conditions (low pH and Ca/Al) had lower values of foliar N concentration and δ¹³C and tree ring δ¹³C, suggesting that restricted N uptake under more acidic soil conditions caused N limitation for photosynthesis, leading to poor tree growth. In addition, relationships between mean annual area increment and carbon isotope discrimination of tree rings at five-yr intervals from 1968 to 2007 revealed that the impact of soil acidification on tree growth became severer during the last 15 yrs as negative correlations between them became significant after 1993.

Conclusions

Reduced N uptake under acidic soil conditions resulted in lower radial growth of P. densiflora via non-stomatal limitation of photosynthesis.     

Nitrogen form, availability, and mycorrhizal colonization affect biomass and nitrogen isotope patterns in Pinus sylvestris

Nitrogen (N) isotope patterns are useful for understanding carbon and nitrogen dynamics in mycorrhizal systems but questions remain about how different N forms, fungal symbionts, and N availabilities influence δ¹⁵N signatures. Here, we studied how biomass allocation and δ¹⁵N patterns in Pinus sylvestris L. cultures were affected by nitrogen supply rate (3% per day or 4% per day relative to the nitrogen already present), nitrogen form (ammonium versus nitrate), and mycorrhizal colonization by fungi with a greater (Laccaria laccata) or lesser (Suillus bovinus) ability to assimilate nitrate. Mycorrhizal (fungal) biomass was greater with ammonium than with nitrate nutrition for Suillus cultures but similar for Laccaria cultures. Total biomass was less with nitrate nutrition than with ammonium nutrition for nonmycorrhizal cultures and was less in mycorrhizal cultures than in nonmycorrhizal cultures. The sequestration of available N by mycorrhizal fungi limited plant N supply. This limitation and the higher energetic cost of nitrate reduction than ammonium assimilation appeared to control plant biomass accumulation. Colonization decreased foliar δ¹⁵N by 0.5 to 2.2‰ (nitrate) or 1.7 to 3.5‰ (ammonium) and increased root tip δ¹⁵N by 0 to 1‰ (nitrate) or 0.6 to 2.3‰ (ammonium). Root tip δ¹⁵N and fungal biomass on root tips were positively correlated in ammonium treatments (r ²?=?0.52) but not in nitrate treatments (r ²?=?0.00). Fungal biomass on root tips was enriched in ¹⁵N an estimated 6–8‰ relative to plant biomass in ammonium treatments. At high nitrate availability, Suillus colonization did not reduce plant δ¹⁵N. We conclude that: (1) transfer of ¹⁵N-depleted N from mycorrhizal fungi to plants produces low plant δ¹⁵N signatures and high root tip and fungal δ¹⁵N signatures; (2) limited nitrate reduction in fungi restricted transfer of ¹⁵N-depleted N to plants when nitrate is supplied and may account for many field observations of high plant δ¹⁵N under such conditions; (3) plants could transfer assimilated nitrogen to fungi at high nitrate supply but such transfer was without ¹⁵N fractionation. These factors probably control plant δ¹⁵N patterns across N availability gradients and were here incorporated into analytical equations for interpreting nitrogen isotope patterns in mycorrhizal fungi and plants.     

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.     

Carbon, nitrogen, and hydrogen isotope ratios in creekside trees in western Kansas

Three species of creekside trees were monitored weekly during the 2007 and 2008 growing seasons. The 2007 growing season was wet early, but became drier as the season progressed. In contrast, the 2008 growing season was dry early, but became wetter as the season progressed. Creekside trees were measured to determine effects of changing water regimes on leaf-level processes. Lonicera tatarica plants were compared to Morus alba and Celtis occidentalis trees. Leaves were monitored for changes in stomatal conductance, transpiration, δ¹³C, δ¹⁵N, δD, leaf temperature, and heat losses via latent, sensible, and radiative pathways. δD of creek water was more similar to ground water than to rain water, but the creek was partially influenced by summer rains. δD of bulk leaf material was significantly higher in individuals of C. occidentalis compared to the other species, consistent with source water coming from shallower soil layers. Despite decreasing water levels, none of these tree species showed signs of water stress. There were no significant differences between species in stomatal conductance or transpiration. Leaf δ¹³C was significantly lower in individuals of L. tatarica compared to the other species. Differences in δ¹³C were attributed to a lower carboxylation capacity, consistent with lower leaf nitrogen content in L. tatarica plants. Leaf δ¹⁵N was significantly lower in individuals of L. tatarica compared to the other species, consistent with uptake of a different N source. Two of the three sites appeared to be affected by inorganic N from fertilizer run-off. Evidence is presented that these species acquired water and nitrogen from different sources, resulting from differences in root uptake patterns.     

A bioassay of the availability of residual 15N fertilizer eight years after application to a forest soil in interior British Columbia

Although a high proportion of fertilizer N may be immobilized in organic forms in the soil, no studies have examined the long-term availability of residual fertilizer ¹⁵N in forestry situations. We investigated this by growing lodgepole pine (Pinus contorta) seedlings in surface (0–10 cm) soil sample eight years after application of ¹⁵N-urea, ¹⁵NH₄NO₃ and NH₄ ¹⁵NO₃ to lodgepole pine in interior British Columbia. After nine months of growth in the greenhouse, seedlings took up an average of 8.5% of the ¹⁵N and 4.6% of the native N per pot. Most of the mineral N in the pots without seedlings was in the form of nitrate, while pots with seedlings had very low levels of mineral N. In contrast to the greenhouse study, there was no significantuptake of ¹⁵N by trees in the field study after the first growing season, although half of the soil organic ¹⁵N was lost between one and eight years after fertilization. This indicates the need to understand the mechanisms which limit the uptake of mineral N by trees in the field, and the possible mismatch of tree demand and mineral N availability.     

Carbon and nitrogen stable isotope ratios of macroinvertebrates in the littoral zone of Lake Biwa as indicators of anthropogenic activities in the watershed

Carbon and nitrogen stable isotope ratios (δ¹³C and δ¹⁵N) of macroinvertebrates inhabiting littoral zones of lakes can serve as useful indicators of material loading from the watershed. We collected snails (Semisulcospira spp.) and bivalves (Unio douglasiae biwae Kobelt) from 29 littoral sites in Lake Biwa near the mouths of river tributaries with various human population density (HPD) and land-use patterns. The δ¹³C and δ¹⁵N signatures were determined for three potential food sources: particulate organic matter in the pelagic zone (PPOM), riverine particulate organic matter from tributaries (RPOM) and epilithic organic matter in the littoral zone (EOM). The stable isotope mixing model revealed that snails relied mainly on EOM, and bivalves on PPOM and RPOM. Multiple regression analysis showed that intersite variation in δ¹⁵N for snails was best explained by HPD, while variation in δ¹⁵N of EOM and nitrate was explained to a lesser extent by HPD. Comparison with isotope signatures of their food sources and riverine nutrients revealed that snails assimilated anthropogenic nitrogen from wastewater in the watershed. Our results also showed that the δ¹³C value of bivalves was marginally related to the fraction of paddy fields in the watersheds. In conclusion, the isotope signatures of macroinvertebrates inhabiting the littoral zone can be useful indicators of anthropogenic impacts from the watershed.     

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.     

Can tree-ring δ<Superscript>15</Superscript>N be used as a proxy for foliar δ<Superscript>15</Superscript>N in European beech and Norway spruce?

Key message

For long-term environmental investigations, tree-ring δ ¹⁵ N values are inappropriate proxies for foliar δ ¹⁵ N for both Fagus sylvatica and Picea abies under moderate N loads.

Abstract

Currently it is unclear whether stable nitrogen isotope signals of tree-rings are related to those in foliage, and whether they can be used to infer tree responses to environmental changes. We studied foliar and tree-ring nitrogen (δ¹⁵N) and carbon (δ¹³C) isotope ratios in European beech (Fagus sylvatica L.) and Norway spruce (Picea abies L.) from six long-term forest monitoring sites in Switzerland together with data on N deposition and soil N availability, as well as a drought response index over the last two decades. For both species, tree-ring δ¹⁵N and δ¹³C values were less negative compared to foliar δ¹⁵N and δ¹³C values, most likely due to recycling and reallocation of N within the tree and fractionation processes associated with the transport of sucrose and the formation of tree-rings, respectively. Temporal trends recorded in foliar δ¹⁵N were not reflected in tree-ring δ¹⁵N, with much higher variations in tree-rings compared to foliage. Soil N availability and N deposition were partially able to explain changes in foliar δ¹³C, while there were no significant correlations between environmental variables and either tree-ring or foliar δ¹⁵N. Our results suggest an uncoupling between the N isotopic composition of tree-rings and foliage. Consequently, tree-ring δ¹⁵N values are inappropriate proxies of foliar δ¹⁵N values under low-to-moderate N deposition loads. Furthermore, at such low levels of deposition, tree-ring δ¹⁵N values are not recommended as archives of tree responses to soil C/N or bulk N deposition.

     

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).

     

Ectomycorrhizal impacts on plant nitrogen nutrition: emerging isotopic patterns,latitudinal variation and hidden mechanisms

Ectomycorrhizal (EcM)‐mediated nitrogen (N) acquisition is one main strategy used by terrestrial plants to facilitate growth. Measurements of natural abundance nitrogen isotope ratios (denoted as δ¹⁵N relative to a standard) increasingly serve as integrative proxies for mycorrhiza‐mediated N acquisition due to biological fractionation processes that alter ¹⁵N:¹⁴N ratios. Current understanding of these processes is based on studies from high‐latitude ecosystems where plant productivity is largely limited by N availability. Much less is known about the cause and utility of ecosystem δ¹⁵N patterns in the tropics. Using structural equation models, model selection and isotope mass balance we assessed relationships among co‐occurring soil, mycorrhizal plants and fungal N pools measured from 40 high‐ and 9 low‐latitude ecosystems. At low latitudes ¹⁵N‐enrichment caused ecosystem components to significantly deviate from those in higher latitudes. Collectively, δ¹⁵N patterns suggested reduced N‐dependency and unique sources of EcM ¹⁵N‐enrichment under conditions of high N availability typical of the tropics. Understanding the role of mycorrhizae in global N cycles will require reevaluation of high‐latitude perspectives on fractionation sources that structure ecosystem δ¹⁵N patterns, as well as better integration of EcM function with biogeochemical theories pertaining to climate‐nutrient cycling relationships.     

Carbon and nitrogen acquisition strategies by wood decay fungi influence their isotopic signatures in Picea abies forests

We examined whether sporocarp carbon and nitrogen isotope ratios (δ¹³C and δ¹⁵N values) reflected different functional strategies in 15 species of wood decay fungi. In Finnish Picea abies forests, we compared sporocarp δ¹³C and δ¹⁵N against log diameter, proximity to ground, and three wood decay types, specifically brown rot, nonselective white rot, and selective white rot (targeting hemicellulose and lignin preferentially). In regression analysis (adjusted r² = 0.576), species accounted for 31% of variability in δ¹³C, with factors influencing wood δ¹³C accounting for the remainder. Brown rot fungi and three white rot fungi that selectively attacked hemicellulose (Heterobasidion parviporum, Phellopilus nigrolimitatus, and Trichaptum abietinum) were higher in δ¹³C than nonselective white rot fungi. This was attributed to greater assimilation of ¹³C-enriched pentoses from hemicellulose by these fungi. The pathogenic white rot fungus Heterobasidion parviporum had higher δ¹⁵N with proximity to ground and increasing log diameter. This suggested that ¹⁵N-enriched soil N contributed to decomposing logs and that Heterobasidion growing from a bigger resource base had increased access to soil N. These isotopic patterns accordingly reflected both functional diversity of wood decay fungi and site-specific factors.     

Nitrogen uptake strategies of edaphically specialized Bornean tree species

The association of tree species with particular soil types contributes to high β diversity in forests, but the mechanisms producing such distributions are still debated. Soil nitrogen (N) often limits growth and occurs in differentially available chemical forms. In a Bornean forest where tree species composition changes dramatically along a soil gradient varying in supplies of different N-forms, we investigated whether tree species’ N-uptake and soil specialization strategies covaried. We analyzed foliar ¹⁵N natural abundance for a total of 216 tree species on clay or sandy loam (the soils at the gradient’s extremes) and conducted a ¹⁵N-tracer experiment with nine specialist and generalist species to test whether species displayed flexible or differential uptake of ammonium and nitrate. Despite variation in ammonium and nitrate supplies and nearly 4 ‰ difference in foliar δ¹⁵N between most soil specialists and populations of generalists on these soils, our ¹⁵N tracer experiment showed little support for the hypothesis that soil specialists vary in N-form use or the ratios in which they use these forms. Instead, our results indicate that these species possess flexible capacities to take up different inorganic N forms. Variation between soil specialists in uptake of different N forms is thus unlikely to cause the soil associations of tree species and high β diversity characteristic of this Bornean rain forest. Flexible uptake strategies would facilitate N-acquisition when supply rates of N-forms exhibit spatiotemporal variation and suggest that these species may be functionally redundant in their responses to N gradients and influences on ecosystem N-cycles.     

The natural abundance of 15N in litter and soil profiles under six temperate tree species: N cycling depends on tree species traits and site fertility

Aims

We investigated the influence of tree species on the natural ¹⁵N abundance in forest stands under elevated ambient N deposition.

Methods

We analysed δ¹⁵N in litter, the forest floor and three mineral soil horizons along with ecosystem N status variables at six sites planted three decades ago with five European broadleaved tree species and Norway spruce.

Results

Litter δ¹⁵N and ¹⁵N enrichment factor (δ¹⁵N_litter–δ¹⁵N_soil) were positively correlated with N status based on soil and litter N pools, nitrification, subsoil nitrate concentration and forest growth. Tree species differences were also significant for these N variables and for the litter δ¹⁵N and enrichment factor. Litter from ash and sycamore maple with high N status and low fungal mycelia activity was enriched in ¹⁵N (+0.9 delta units) relative to other tree species (European beech, pedunculate oak, lime and Norway spruce) even though the latter species leached more nitrate.

Conclusions

The δ¹⁵N pattern reflected tree species related traits affecting the N cycling as well as site fertility and former land use, and possibly differences in N leaching. The tree species δ¹⁵N patterns reflected fractionation caused by uptake of N through mycorrhiza rather than due to nitrate leaching or other N transformation processes.     

Is the high 15N natural abundance of trees in N-loaded forests caused by an internal ecosystem N isotope redistribution or a change in the ecosystem N isotope mass balance?

High δ¹⁵N of tree foliage in forests subject to high N supply has been attributed to ¹⁵N enrichment of plant available soil N pools after losses of N through processes involving N isotope fractionation (ammonia volatilization, nitrification followed by leaching and denitrification, and denitrification in itself). However, in a long-term experiment with high annual additions of NH₄NO₃, we found no change in the weighted average δ¹⁵N of the soil, but attributed the high δ¹⁵N of trees to loss of ectomycorrhizal fungi and their function in tree N uptake, which involves redistribution of N isotopes in the ecosystem (Högberg et al. New Phytol 189:515–525, 2011), rather than a loss of isotopically light N. Here, we compare the effects of additions of urea and NH₄NO₃ on the δ¹⁵N of trees and the soil profile, because we have previously found higher δ¹⁵N in tree foliage in trees in the urea plots. Doing this, we found no differences between the NH₄NO₃ and urea treatments in the concentration of N in the foliage, or the amounts of N in the organic mor-layer of the soil. However, the foliage of trees receiving the highest N loads in the urea treatment were more enriched in ¹⁵N than the corresponding NH₄NO₃ plots, and, importantly, the weighted average δ¹⁵N of the soil showed that N losses had been associated with fractionation against ¹⁵N in the urea plots. Thus, our results in combination with those of Högberg et al. (New Phytol 189:515–525, 2011) show that high δ¹⁵N of the vegetation after high N load may be caused by both an internal redistribution of the N isotopes (as a result of change of the function of ectomycorrhiza) and by losses of isotopically light N through processes fractionating against ¹⁵N (in case of urea ammonia volatilization, nitrification followed by leaching and denitrification).     

Overstorey and juvenile response to thinning and drought in a jarrah (Eucalyptus marginata Donn ex Sm.) forest of southwestern Australia

Aims

Forest thinning is expected to affect tree water use and carbon assimilation, but the related influence from climate variability is little known. Recent forest thinning in the Wungong catchment coincided with a record dry year following the thinning, which provides a rare opportunity to understand the climate influence on the thinning effect.

Methods

A field experiment was conducted to examine changes before and after thinning, especially the rainfall, soil moisture, leaf water status, tissue isotope signature (¹³?C and ¹⁵?N) and N concentration of overstorey and understorey juvenile trees of Eucalyptus marginata (Donn ex Sm.).

Results

Despite the post-thinning drought, surface soil was moister and juvenile jarrah plants were less water stressed, attributable to reduced rain interception and transpiration as a result of less canopy cover. The overstorey was under stress but mainly due to drought rather than by thinning. The concentration of N declined in both tree stems and juvenile leaves along with available N in soil, suggesting a soil N limitation. No treatment effects were detected from leaf relative water content and tissue isotope signature (¹³?C and ¹⁵?N).

Conclusions

The drought effects were superimposed over the thinning effects on overstorey growth, with stemwood δ¹³C being a major indicator of water stress. The water relations and carbon assimilation of understorey juveniles were however dependent more on topsoil moisture, and the wetter soil during the year following thinning enhanced growth activity and hence the depletion of ¹³?C (more negative δ¹³C) in juvenile leaves.     

The use of different soil nitrogen sources by young Norway spruce plants

 Three-year-old Norway spruce trees were planted into a low-nitrogen mineral forest soil and supplied either with two different levels of mineral nitrogen (NH₄NO₃) or with a slow-release form of organic nitrogen (keratin). Supply of mineral nitrogen increased the concentrations of ammonium and nitrate in the soil solution and in CaCl₂-extracts of the rhizosphere and bulk soil. In the soil solution, in all treatments nitrate concentrations were higher than ammonium concentrations, while in the soil extracts ammonium concentrations were often higher than nitrate concentrations. After 7 months of growth, ¹⁵N labelled ammonium or nitrate was added to the soil. Plants were harvested 2 weeks later. Keratin supply to the soil did not affect growth and nitrogen accumulation of the trees. In contrast, supply of mineral nitrogen increased shoot growth and increased the ratio of above-ground to below-ground growth. The proportion of needle biomass to total above-ground biomass was not increased by mineral N supply. The atom-% ¹⁵N was higher in younger needles than in older needles, and in younger needles higher in plants supplied with ¹⁵N-nitrate than in plants supplied with ¹⁵N-ammonium. The present data show that young Norway spruce plants take up nitrate even under conditions of high plant internal N levels. Received: 1 April 1998 / Accepted: 9 October 1998     

Does geographic origin dictate ecological strategies in Acacia senegal (L.) Willd.? Evidence from carbon and nitrogen stable isotopes

Background and aims

Acacia senegal, a leguminous dryland tree, is economically and ecologically important to sub-Saharan Africa. Water-use efficiency (WUE) and biological nitrogen fixation (BNF) are fundamental to plant productivity and survival. We quantify provenance differences in WUE, BNF, photosynthesis, biomass and gum arabic production from A. senegal assessing genetic improvement potential.

Methods

Using stable isotope ratios, we determined WUE (δ¹³C) and BNF (δ¹⁵N) from provenances of mature A. senegal in field-trials (Senegal), sampling leaves at the beginning (wet) and end (dry) of the rainy season. Seedling provenance trials (UK) determined photosynthesis, and biomass and δ¹³C in relation to water table. Environmental data were characterised for all provenances at their sites of origin.

Results

Provenances differed in both δ¹³C and δ¹⁵N. Gum yield declined with increasing WUE. Virtually no BNF was detected during the dry season and seedlings and mature trees may have different WUE strategies. Wind speed and soil characteristics at provenance origin were correlated with isotope composition and gum production.

Conclusion

Provenance differences suggest that selection for desirable traits, e.g., increased gum production, may be possible. As ecological strategies relate to native locality, the environmental conditions at plantation site and provenance origin are important in assessing selection criteria.     

Experimental evidence for diel δ15N‐patterns in different tissues,xylem and phloem saps of castor bean (Ricinus communis L.)

Nitrogen isotope signatures in plants might give insights in the metabolism and allocation of nitrogen. To obtain a deeper understanding of the modifications of the nitrogen isotope signatures, we determined δ¹⁵N in transport saps and in different fractions of leaves, axes and roots during a diel course along the plant axis. The most significant diel variations were observed in xylem and phloem saps where δ¹⁵N was significantly higher during the day compared with during the night. However in xylem saps, this was observed only in the canopy, but not at the hypocotyl positions. In the canopy, δ¹⁵N was correlated fairly well between phloem and xylem saps. These variations in δ¹⁵N in transport saps can be attributed to nitrate reduction in leaves during the photoperiod as well as to ¹⁵N‐enriched glutamine acting as transport form of N. δ¹⁵N of the water soluble fraction of roots and leaves partially affected δ¹⁵N of phloem and xylems saps. δ¹⁵N patterns are likely the result of a complex set of interactions and N‐fluxes between plant organs. Furthermore, the natural nitrogen isotope abundance in plant tissue is not constant during the diel course – a fact that needs to be taken into account when sampling for isotopic studies.     

Patterns of soil and plant nitrogen isotope composition and their relationships with climate factors in Xilingol League,Inner Mongolia,China

Background: Plant and soil nitrogen stable isotope (δ¹⁵N) can integrate several fundamental biogeochemical processes in ecosystem nitrogen dynamics, and reflect characteristics of ecosystem nitrogen cycling.

Aims: We investigated how climate change influenced plant-soil nitrogen cycling by relating soil δ¹⁵N, plant δ¹⁵N and Δδ¹⁵N (difference between soil and plant δ¹⁵N) with climatic factors.

Methods: Field investigation was conducted in temperate grasslands in Inner Mongolia during August 2015. Plant δ¹⁵N, soil δ¹⁵N and Δδ¹⁵N were determined, and their relationships with climatic factors were examined by simple regression analyses and general linear models.

Results: Soil δ¹⁵N was significantly higher than plant δ¹⁵N, and there was a positive linear correlation between them. Soil and plant δ¹⁵N were negatively related with mean annual precipitation (MAP) and positively with mean annual temperature (MAT); conversely, Δδ¹⁵N was positively related with MAP and negatively with MAT.

Conclusion: Soil δ¹⁵N was dominantly controlled by MAT, while it was MAP for plant δ¹⁵N. Climate factors influenced plant δ¹⁵N not only through their effects on soil nitrogen dynamics but also strategies of plant nitrogen acquisition. Thus, compared with plant δ¹⁵N, soil δ¹⁵N can more accurately reflect soil nitrogen dynamics, while plant δ¹⁵N may integrate soil nitrogen dynamics and plant nitrogen acquisition.