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Understanding how foliar δ13C and δ15N vary with the environment is crucial for elucidating the carbon and nitrogen cycle dynamics within ecosystems. Yet, there is limited knowledge about these variations among different plant life types along altitudinal gradients, particularly in subtropical forest ecosystems found in the southeastern Tibetan Plateau. Therefore, we collected leaves from 67 species, including 20 trees, 24 shrubs, and 23 herbs along an altitudinal gradient of 834 m to 3105 m in the subtropical forest of Medog, and investigated their patterns of variation along the altitudinal gradient. Our findings revealed a positive relationship between foliar δ13C and altitude, as well as a negative relationship between foliar δ13C and soil temperature. There was no significant correlation between foliar δ15N and altitude, but a hump-shaped relationship was observed between foliar δ15N and soil moisture. Among the different plant functional groups, the δ13C of shrubs is more sensitive and representative of environmental changes. In addition, we discovered no difference in the δ13C (δ15N) values of leaves among trees, shrubs, and herbs in this study. Still, there were significant differences when compared to the δ13C (δ15N) values of leaves in other typical climatic zones, indicating that climatic zones influenced the δ13C and δ15N values of leaves more than plant functional groups. In conclusion, our results provide new insights into the climatic drivers of the changes in δ13C and δ15N of the different plant life types along the altitudinal gradient in the southeastern Tibetan Plateau.

Abstract Background Forest succession is an important ecological process and has been studied for more than a century. However, changes in nitrogen (N) availability during succession remain unclear as they may lead to either N saturation or N limitation. Here, we propose a conceptual model to illustrate changes in N availability during four stages of secondary succession using the natural abundance of 15N in plant leaves (foliar δ15N). We predicted that N availability would decline in the early stages of succession and then increase in late stages, coinciding with the changes in foliar δ15N, with the inflection point varying in different climate zones. Data on foliar δ15N from 16 succession sequences were synthesized to explore changes in N availability during forest succession. Results The compiled data were consistent with the proposed conceptual model. Foliar δ15N in boreal and temperate forests decreased significantly in the first two stages of succession (estimated to last at least 66 years in temperate forests), at a rate of 0.18‰ and 0.38‰ per decade, respectively, and decreased slightly in tropical forests in the first 23 years. Foliar δ15N is projected to increase in later stages in all forests, which is supported by observations in both temperate and tropical forests. The inflection points of N availability when N limitation peaked during succession were different in different climate zones, implying different ecosystem N turnovers. Conclusions Our study reconciles the controversies regarding changes in N availability during forest secondary succession. Our findings are also useful for predicting the recovery of N and carbon accumulation during succession. Nonetheless, studies on forest secondary succession using foliar δ15N have thus far been limited, and more research should be conducted to further verify the conceptual model proposed here.

Background: Forest succession is an important ecological process and has been studied for more than a century. However, changes in nitrogen (N) availability during succession remain unclear as they may lead to either N saturation or N limitation. Here, we propose a conceptual model to illustrate changes in N availability during four stages of secondary succession using the natural abundance of 15N in plant leaves (foliar δ15N). We predicted that N availability would decline in the early stages of succession and then increase in late stages, coinciding with the changes in foliar δ15N, with the inflection point varying in different climate zones. Data on foliar δ15N from 16 succession sequences were synthesized to explore changes in N availability during forest succession. Results: The compiled data were consistent with the proposed conceptual model. Foliar δ15N in boreal and temperate forests decreased significantly in the first two stages of succession (estimated to last at least 66 years in temperate forests), at a rate of 0.18‰ and 0.38‰ per decade, respectively, and decreased slightly in tropical forests in the first 23 years. Foliar δ15N is projected to increase in later stages in all forests, which is supported by observations in both temperate and tropical forests. The inflection points of N availability when N limitation peaked during succession were different in different climate zones, implying different ecosystem N turnovers. Conclusions: Our study reconciles the controversies regarding changes in N availability during forest secondary succession. Our findings are also useful for predicting the recovery of N and carbon accumulation during succession. Nonetheless, studies on forest secondary succession using foliar δ15N have thus far been limited, and more research should be conducted to further verify the conceptual model proposed here. [ABSTRACT FROM AUTHOR]

The increase in soil organic matter mineralization rate in boreal forests that may result from global warming is a major concern as it could release large amounts of C to the atmosphere. On the other hand, this may also release N to the soil and stimulate tree growth, which could partly offset the C losses from the soil. The long-term interaction between increased N deposition and soil warming on soil organic N mineralization and tree growth is still uncertain in the boreal zone. In this study, soil temperature was increased by +2–4°C from April to July with heating cables for a period of 9 years and N was applied above the canopy of mature stands from June to September for 7–9 years in two eastern Canada boreal forests [Bernatchez (BER) and Simoncouche (SIM)]. We assessed the effects of these treatments on organic N mineralization rate and on the growth, phenology and foliar N of Abies balsamea (L.) Mill. (balsam fir; BF) and Picea mariana (Mill.) BSP (black spruce; BS) seedlings. The soil warming (SW) treatment had no significant effect on N mineralization rates, whereas canopy N addition (CNA) decreased gross N mineralization rate by 23% and forest floor’s alkyl/O-alkyl C ratio by ∼15% relative to unfertilized plots. Foliar δ15N in the control plots was markedly lower in BS than in BF and at BER than at SIM (–4.8 and –2.9‰ in BS at BER and SIM, respectively; 1.6 and 3.8‰ in BF at BER and SIM, respectively) likely due to a higher contribution of 15N-depleted N derived from mycorrhizal fungi in BS and at BER, the colder and the more N-depleted site. The treatments had non-significant effects on seedling growth and foliar chemistry but SW caused a premature bud outbreak and faster bud development for both species at both sites. Overall, our results show that increased soil temperature and N deposition in boreal forests may not impact soil fertility and vegetation growth as much as previously thought, but climate warming, by initiating earlier and faster bud development, may however expose seedlings to late spring frosts in the future.

Legumes that possess the capacity to fix atmospheric nitrogen (N) via symbiotic N2 fixation are widely considered to have the potential to regulate ecosystem N cycling. However, the N dynamics of those N2-fixing species in response to changes of climatic, edaphic and biotic factors are not well-understood, especially for alpine plants that are particularly susceptible to climate change. In this work, foliar and soil N isotopic composition (δ15N) of Caragana jubata, a legume shrub species mainly growing in alpine regions, was measured from 35 sites across a 2666 km east–west transect. Results showed that there were significant differences between foliar δ15N (ranging from − 0.67 to 0.60‰) and soil δ15N (ranging from 0.60 to 5.71‰), but no correlation was observed between them. Foliar δ15N increased with growing season temperature, foliar N concentration, and decreased with altitude, foliar carbon (C) concentration, foliar carbon to nitrogen ratio (C/N), and soil pH, whereas soil δ15N increased with altitude and decreased with soil C, soil N, soil C/N, and soil electrical conductivity. A principal component analysis indicated that the significant influential factors on foliar δ15N were foliar N, foliar C/N, growing season precipitation, growing season temperature and soil pH, whereas those on soil δ15N were soil electrical conductivity, soil C, soil N, soil C/N, foliar C, growing season precipitation and growing season temperature. Structural equation modeling indicated that the temperature and precipitation during the growing season affected foliar δ15N mainly via their effects on soil pH and foliar N, while affected soil δ15N mainly by their effects on soil electrical conductivity and soil C/N. This study provides strong evidence that C. jubata in the alpine regions obtains N mainly by symbiotic N2 fixation, which has resulted in the decoupling of N dynamics between plant and soil, and in different patterns and controls of foliar and soil δ15N across climatic, edaphic and biotic gradients. [ABSTRACT FROM AUTHOR]

Variation in foliar and soil nitrogen (N) isotope composition (δ15N) across environmental gradients has been widely studied at various scales. However, much less is known about the patterns of N2-fixing species that are presumed to be independent of climatic factors due to their capacity to fix atmospheric N2. In this study, the foliar and soil δ15N in Caragana microphylla, a leguminous shrub important for its role in controlling soil erosion and land desertification in northern China, were determined from 23 sites along an east–west transect across the species distribution range. The soil δ15N tended to be significantly higher than the foliar δ15N and there was a positive correlation between them (r2 = 0.58, p < 0.0001), though substantial variability was found in foliar and soil δ15N across all sites. Both soil and foliar δ15N were negatively correlated with precipitation, temperature, dry N deposition (DND) and longitude, but positively correlated with foliar N, soil N and altitude. The structural equation modeling further suggested growing season precipitation indirectly influenced foliar δ15N by its direct impact on soil δ15N and by its indirect impact on soil δ15N via DND and total N deposition (TND), whereas growing season temperature affected foliar δ15N indirectly both by its effect on foliar N and by its effect on soil δ15N via WND, TND and foliar N. Altogether, our results indicated that foliar δ15N also shifted along environmental gradients in leguminous plant species and showed primary dependence on soil δ15N, confirming previous findings in non-leguminous plants. These results augment our common understanding of foliar and soil δ15N patterns across climatic gradients in plants. [ABSTRACT FROM AUTHOR]

Highlights: [•] Tillage practices strongly affected plant nutrition in a short-term period. [•] Transition from reduced to no-tillage decrease nitrogen status and almond crop production. [•] Almond tree productivity is strongly co-limiting by nutrients in normal or wet years. [•] δ15N was a good indicator of fertility in dryland agroecosystems. [Copyright &y& Elsevier]

Questions What is the role of nutrient limitation of dominant species in maintaining plant species diversity at small spatial scales, particularly in biodiversity hotspots? Location Southwest of Western Australia. Methods The extent of nutrient limitation was determined by measuring variation in the foliar nutrients of dominant plant species and total above-ground biomass in relation to soil N and P across 16 plots. Plant species richness and evenness for the same plots were then regressed against foliar N:P and δ15N of the dominant species. Results Foliar N and P content as well as above-ground biomass increased, while foliar N:P decreased with increasing soil N and P, suggesting that dominant species are limited by both N and P in southwest Australia. In contrast, foliar δ15N values only increased with decreasing soil P, indicating that δ15N enrichment reflects low P availability. Species richness increased at sites where foliar N:P was higher and δ15N more enriched, suggesting that a relatively greater number of rare plant species co-exist where the growth of dominant vegetation is more nutrient-limited. Conclusions The mechanisms that allow an exceptionally high number of plant species to co-exist at small spatial scales in a southwest Australian biodiversity hotspot are linked to the extent to which the dominant plant species are limited in N and P. In addition, on nutrient-impoverished soils, a foliar N:P ratio >16 can reflect N as well as P limitation. While foliar δ15N also reflects availability of soil P, it is not necessarily a reliable indicator of nutrient limitation in southwest Australia, and perhaps in other environments where plants are highly adapted to low-nutrient conditions. [ABSTRACT FROM AUTHOR]

Nitrogen (N) oligotrophication is increasing globally across terrestrial ecosystems and manifested in decreasing nitrogen concentration ([N]) and changes in the stable nitrogen isotope composition (δ 15N) of foliage. Heterogeneity in plant nitrogen sources makes it challenging to detect the effects of N oligotrophication even at a small catchment scale with complex topography. Understanding the spatial and temporal variation of foliar δ 15N and [N] at such a scale is required to develop useful ecological indicators and monitoring methods to support catchment management with a potential N oligotrophication problem. This study examined spatial and high-resolution temporal variation of foliar δ 15N and [N] and their influencing factors in ten trees grouped by Eucalyptus and Acacia in a native forest vegetation catchment. Over 16 sampling campaigns within a 12-month period, foliar δ 15N and [N] increased in Eucalyptus but were constant in the N 2 -fixing Acacia. The higher foliar [N] and δ 15N in Acacia reflected its N 2 -fixation ability. Topographic flow accumulation area (NDVI) explained 46% (77%) of spatial variation in dry-season Eucalyptus foliar δ 15N ([N]). For Eucalyptus , foliar δ 15N was higher at the downslope than the upslope locations, but no hillslope location differences were observed for foliar [N]. These results suggest that in the non-N 2 -fixing Eucalyptus , seasonal water stress related nitrogen availability may be reflected in foliar δ 15N rather than foliar [N]. As such, foliar δ 15N of non-N 2 -fixing plants potentially is a more sensitive indicator of seasonal or topographical N availability than foliar [N]. [Display omitted] • Foliar δ 15N and [N] of Eucalyptus varied in a year as those of Acacia kept constant. • Foliar δ 15N and [N] are positively correlated for both Eucalyptus and Acacia. • Eucalyptus foliar δ 15N was higher at the downslope than the upslope locations. • Eucalyptus foliar [N] was independent of slope locations. • Drainage area explained 46% spatial variation of Eucalyptus dry-season foliar δ 15N. [ABSTRACT FROM AUTHOR]

• The magnitude and impact of gaseous nitrogen dioxide (NO2) directly entering the leaves were investigated using foliar nitrogen isotopic composition (δ15N) values in tomato ( Lycopersicon esculentum) and tobacco ( Nicotiana tabacum). • Using a hydroponics–fumigation system, 15NO2 (20 and 40 ppb) was supplied to shoot systems and (50 and 500 µM) was supplied to root systems. Morphological, stable isotope and nitrate reductase activity (NRA) analyses were used to quantify foliar NO2 uptake and to examine whether realistic concentrations of NO2 influenced plant metabolism. • Nicotiana tabacum and L. esculentum incorporated 15 and 11%, respectively, of 15NO2-N into total biomass via foliar uptake under low supply. On a mass basis, N. tabacum and L. esculentum incorporated 3.3 ± 0.9 and 3.1 ± 0.8 mg of 15NO2-N into biomass, respectively, regardless of availability. There were no strong effects on biomass accumulation or allocation, leaf δ13C values, or leaf or root NRA in response to NO2 exposure. • Foliar NO2 uptake may contribute a significant proportion of N to plant metabolism under N-limited conditions, does not strongly influence growth at 40 ppb, and may be traced using foliar δ15N values. [ABSTRACT FROM AUTHOR]