Potential canopy influences on the isotopic composition of nitrogen and sulphur in atmospheric deposition

Isotopic studies of nitrogen and sulphur inputs to plant/soil systems commonly rely on limited published data for the ¹⁵N/¹⁴N and ³⁴S/³²S ratios of nitrate, ammonium and sulphate in rainfall. For systems with well-developed plant canopies, however, inputs of these ions from dry deposition or particulates may be more important than rainfall. The manner in which isotopic fractionation between ions and gases may lead to dry deposition and particulates having ¹⁵N/¹⁴N or ³⁴S/³²S ratios different from those of rainfall is considered. Data for rainfall and throughfall in coniferous plantations are then discussed, and suggest that: (1) in line with expectations, nitrate washed from the canopy has ¹⁵N/¹⁴N ratios higher than those in rainfall; (2) the ¹⁵N/¹⁴N ratios of ammonium washed from the canopy are variable, with high ratios being found for canopies of higher pH in conditions of elevated ambient ammonia gas concentrations; and (3) in accord with expectations and previous work, ³⁴S/³²S ratios of sulphate washed from the canopy are not substantially different from those in rainfall. The study suggests that if atmospheric inputs are relevant to isotopic studies of the sources of nitrogen for canopied systems, then confident interpretation will require analysis of these inputs. Received: 3 March 1996 / Accepted: 28 September 1996     

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.     

The stable carbon and nitrogen isotopic composition of vegetation in tropical forests of the Amazon Basin, Brazil

Here we present the within-site, seasonal, and interannual variations of the carbon (δ¹³C) and nitrogen (δ¹⁵N) isotope ratios of leaves, wood, bark and litter from four sites in the Amazon region, Brazil. Samples were collected in Manaus (3° 06′07′′ S; 60°01′30′′ W), Ji-Paraná (10°53′07′′ S; 61°57′06′′ W), and Santarém (2°26′35′′ S; 54°42′30′′ W) with mean annual precipitation of 2207, 2040 and 1909 mm respectively. The overall average for all leaf samples was for δ¹³C and for δ¹⁵N (n=756). The leaf δ values at these sites were often but not always statistically distinct from each other. The δ¹³C values varied from to . Pronounced differences in δ¹³C values occurred with height associated with differences in forest structure. The δ¹³C of leaf dry matter showed seasonal variations associated with the length of the dry season, despite the fact that total annual precipitation was similar among the studied sites. Leaf δ¹⁵N values ranged from to a maximum value of , and the Santarém sites showed more enriched values than Manaus and Ji-Paraná sites. No seasonal variation was detected in the δ¹⁵N of leaves, but significant differences were observed among sites and with changes in canopy height. The isotope ratio data are consistent with our current understanding of the roles of light, water availability, and recycling of soil-respired CO₂ influences on δ¹³C and consistent with our understanding that an open nitrogen cycle can lead to high δ¹⁵N values despite a significant number of legumes in the vegetation.     

Stable isotopic composition of carbon and nitrogen and nitrogen content in vascular epiphytes along an altitudinal transect*

The foliar content of nitrogen and the relative abundances of ¹³C and ¹⁵N were analysed in vascular epiphytes collected from six sites along an altitudinal gradient from tropical dry forests to humid montane forests in eastern Mexico. The proportion of epiphyte species showing crassulacean acid metabolism (CAM) (atmospheric bromeliads, thick-leaved orchids, Cactaceae, and Crassulaceae) decreased with increasing elevation and precipitation from 58 to 6%. Atmospheric bromeliads, almost all of which had δ ¹³C values indicating CAM, were more depleted in ¹⁵N (x = ? 10·9‰ ± 2·11) than the C₃ bromeliads which form water-storing tanks ( ? 6·05‰ ± 2·26). As there was no difference in δ ¹⁵N values between C₃ and CAM orchids, the difference in bromeliads was not related to photosynthetic pathways but to different nitrogen sources. While epiphytes with strong ¹⁵N depletion appear to obtain their nitrogen mainly from direct atmospheric deposition, others have access to nitrogen in intercepted water and from organic matter decomposing on branches and in their phytotelmata. Bromeliads and succulent orchids had a lower foliar nitrogen content than thin-leaved orchids, ferns and Piperaceae. Ground-rooted hemi-epiphytes exhibited the highest nitrogen contents and δ ¹⁵N values.     

Effects of hydrolysable tannin-treated grass silage on milk yield and composition,nitrogen partitioning and nitrogen isotopic discrimination in lactating dairy cows

The objective of this study was to evaluate the effects of oak tannin extract (OTE) added in forage before ensiling on dairy cows fed at 92% of their digestible protein requirements. Six multiparous lactating Holstein cows were used in a crossover design (two treatments × two periods). The control treatment (CON) was based on a diet including 50% of grass silage, whereas the experimental treatment (TAN) included grass silage sprayed with OTE (26 g/kg DM) just before baling. Milk yield (on average 24 kg fat protein corrected milk per day) was not affected, but both milk and rumen fatty acids profiles were impacted by OTE. Nitrogen intake (415 g N per cow per day) and nitrogen use efficiency (NUE; 0.25 on average) were not affected, but a shift from urine (−8% of N intake relatively to control, P = 0.06) to faecal N (+5%; P = 0.004) was observed with the TAN diet (P ≤ 0.05). Nitrogen apparent digestibility was thus reduced for TAN (−3%; P ≤ 0.05). The effect of OTE on ruminal and milk FA profiles suggests an impact on rumen microbiota. Nitrogen isotopic discrimination between animal proteins and diet (Δ¹⁵N) was evaluated as a proxy for NUE. While no differences in NUE were observed across diets, a lower Δ¹⁵N of plasma proteins was found when comparing TAN v. CON diets. This finding supports the concept that Δ¹⁵N would mainly sign the N partitioning at the metabolic level rather than the overall NUE, with the latter also being impacted by digestive processes. Our results agree with a N shift from urine to faeces, and this strategy can thus be adopted to decrease the environmental impact of ruminant protein feeding.     

Retention of nitrogen and phosphorus in a Danish lowland river system: implications for the export from the watershed

In a Danish lowland river system intensive measurements were made, in four 80 m reaches, of the nitrogen (N) and phosphorus (P) stored in the stream sediment. The results were used for calculation of the total retention in the river system during two summers (June to August). In addition, the mobilization of nutrients from the stream bottom in autumn 1987 was compared with the export from the watershed.During the study period (June 1987 to September 1988) the amounts of N and P stored in stream reaches were determined fortnightly using a core-sample technique. In reaches dominated by submersed macrophytes, 25–40 g N m^–2 and 20–30 g P m^–2 were stored during two summers, against only 10–15g N and P m^–2 for sandy and gravely reaches. In riparian zones with emergent macrophytes the retention was even higher than in the submersed macrophytes. Gross retention exceeded net retention by a factor of two to three.Net retention of P in the river system during the summer of 1987 was equal to the summer export from the watershed. On an annual basis, retention in the summer constituted 20% of the P export. In contrast, retention in the summer of 1988 amounted to 60% of the total P export during the same period (38% reduction) and 22% in comparison with the annual export. The corresponding figures for N were lower, showing reductions of 16% and 12% of the export of total N in the two summer periods, and about 1% of the annual exports.In September 1987 6.4% of the total N export and 65% of the total P export from the watershed consisted of resuspended material. In 1987 the N and P retained during the summer was almost completely resuspended during storm events during September to November.     

Stable nitrogen isotopic composition of amino acids reveals food web structure in stream ecosystems

The stable N isotopic composition of individual amino acids (SIAA) has recently been used to estimate trophic positions (TPs) of animals in several simple food chain systems. However, it is unknown whether the SIAA is applicable to more complex food web analysis. In this study we measured the SIAA of stream macroinvertebrates, fishes, and their potential food sources (periphyton and leaf litter of terrestrial C3 plants) collected from upper and lower sites in two streams having contrasting riparian landscapes. The stable N isotope ratios of glutamic acid and phenylalanine confirmed that for primary producers (periphyton and C3 litter) the TP was 1, and for primary consumers (e.g., mayfly and caddisfly larvae) it was 2. We built a two-source mixing model to estimate the relative contributions of aquatic and terrestrial sources to secondary and higher consumers (e.g., stonefly larva and fishes) prior to the TP calculation. The estimated TPs (2.3–3.5) roughly corresponded to their omnivorous and carnivorous feeding habits, respectively. We found that the SIAA method offers substantial advantages over traditional bulk method for food web analysis because it defines the food web structure based on the metabolic pathway of amino groups, and can be used to estimate food web structure under conditions where the bulk method cannot be used. Our result provides evidence that the SIAA method is applicable to the analysis of complex food webs, where heterogeneous resources are mixed.     

Stable carbon and nitrogen isotopic composition of diet and hair of Gidra-speaking Papuans

The carbon and nitrogen stable isotopic composition of the scalp hair and diet of Gidra-speaking people in four villages in Papua New Guinea is presented. The isotopic composition of hair was measured, while that of the diet was estimated from food consumption survey data and the measured isotopic composition and protein and carbohydrate contents of food items. The average isotopic ratios of the hair samples and of the diet varied among the four study villages, which were selected because of their diverse ecological settings. Comparison was made between hair and calculated dietary isotopic compositions. Two of the four diet-hair enrichment values obtained for ¹³C (+1.8 and 2.2%‰) were similar to those previously reported (1.4–2.0%‰), but the other two values (3.7 and 4.8%) were greater than in earlier reports. ¹⁵N enrichment was systematically greater (by 1%‰) than reported values (∼4.3%‰) except for one village, where a much greater enrichment (6.9%‰) was found. The factors potentially relevant to these deviations are discussed. Possible errors in estimating the dietary isotopic composition and minor modifications of dietary habits revealed by food consumption surveys could explain most of the discrepancies. However, the great enrichment of ¹⁵N found in one of the villages remains unexplained. © 1996 Wiley-Liss, Inc.     

High‐resolution food webs based on nitrogen isotopic composition of amino acids

Food webs are known to have myriad trophic links between resource and consumer species. While herbivores have well‐understood trophic tendencies, the difficulties associated with characterizing the trophic positions of higher‐order consumers have remained a major problem in food web ecology. To better understand trophic linkages in food webs, analysis of the stable nitrogen isotopic composition of amino acids has been introduced as a potential means of providing accurate trophic position estimates. In the present study, we employ this method to estimate the trophic positions of 200 free‐roaming organisms, representing 39 species in coastal marine (a stony shore) and 38 species in terrestrial (a fruit farm) environments. Based on the trophic positions from the isotopic composition of amino acids, we are able to resolve the trophic structure of these complex food webs. Our approach reveals a high degree of trophic omnivory (i.e., noninteger trophic positions) among carnivorous species such as marine fish and terrestrial hornets.This information not only clarifies the trophic tendencies of species within their respective communities, but also suggests that trophic omnivory may be common in these webs.     

Recent and historical discharge of a large European river system - oxygen isotopic composition of river water and skeletal aragonite of Unionidae in the Rhine

Seasonal variations in the oxygen isotopic composition of Rhine River water were analyzed in detail and compared with the oxygen isotopic record from recent and historical specimens of freshwater bivalves (Unionidae). The purpose of this study was to investigate the potential use of these aragonitic bivalves as proxy recorders for varying amounts and sources of discharge, and thereby infer climate change. Seasonal variations in the river water δ¹⁸O are on the order of 1-2‰. During summer, Alpine melt-water contributes significantly to the total discharge, resulting in average values of −10 to −10.5‰, whereas the non-Alpine contribution is higher during winter, as indicated by mean δ¹⁸O values of −8.5 to −9‰. The basic pattern of the modern seasonal variation of δ¹⁸O of river water can be described by a numerical mass balance approximation of the various contributions from the Alpine and non-Alpine catchments with their average δ¹⁸O composition. The δ¹⁸O of growth increments in the prismatic shell layer of Anodonta corresponds perfectly to what is predicted by known fractionation of ¹⁸O between water and aragonite. Shell growth is restricted to water temperatures above 8-10°C, so variations in δ¹⁸O and the river water temperature are faithfully recorded by relatively large growth increments during summer. The distinctive isotopic signatures of individual flood events during summer and autumn are also recorded in the shells.     

Plankton composition and water chemistry in the mixing zone of the Selenga River with Lake Baikal

Chemical and biological components of the Selenga River waters, the largest tributary of Lake Baikal, differ significantly from the lake waters. Active transformation processes of river waters into the lake ones occur in the vast barrier-like zone in the river-sea boundary areas. This study presents results on the spatial distribution and dynamics of water chemistry as well as the quantity and diversity of phyto- and bacterioplankton at a distance of 14 km off the Selenga River mouth. The most representative tracers of river and lake waters are total amount of ions and sulphates. Principal changes of chemical and biological parameters were fixed at 1–3 km off the Selenga River mouth that was determined as a mixing zone. Intense development of phytoplankton and eukaryotic picoplankton causing the decrease of nitrate and phosphate concentrations and organic matter rise were registered in this area. Gradual replacement of river phytoplankton by the lake one, abundance reduction of microorganisms and organotrophic bacteria and percentage increase of oligotrophic and psychrotolerant bacteria occurred in the mixing zone. Replacement of PC-rich picocyanobacteria by PE-rich ones was also recorded here. At a distance of 5–7 km off the shore, nutrient concentration and plankton composition were similar to those of Lake Baikal.     

Effects of elemental composition on the incorporation of dietary nitrogen and carbon isotopic signatures in an omnivorous songbird

The use of stable isotopes to infer diet requires quantifying the relationship between diet and tissues and, in particular, knowing of how quickly isotopes turnover in different tissues and how isotopic concentrations of different food components change (discriminate) when incorporated into consumer tissues. We used feeding trials with wild-caught yellow-rumped warblers (Dendroica coronata) to determine delta15N and delta13C turnover rates for blood, delta15N and delta13C diet-tissue discrimination factors, and diet-tissue relationships for blood and feathers. After 3 weeks on a common diet, 36 warblers were assigned to one of four diets differing in the relative proportion of fruit and insects. Plasma half-life estimates ranged from 0.4 to 0.7 days for delta13C and from 0.5 to 1.7 days for delta15N . Half-life did not differ among diets. Whole blood half-life for delta13C ranged from 3.9 to 6.1 days. Yellow-rumped warbler tissues were enriched relative to diet by 1.7-3.6% for nitrogen isotopes and by -1.2 to 4.3% for carbon isotopes, depending on tissue and diet. Consistent with previous studies, feathers were the most enriched and whole blood and plasma were the least enriched or, in the case of carbon, slightly depleted relative to diet. In general, tissues were more enriched relative to diet for birds on diets with high percentages of insects. For all tissues, carbon and nitrogen isotope discrimination factors increased with carbon and nitrogen concentrations of diets. The isotopic signature of plasma increased linearly with the sum of the isotopic signature of the diet and the discrimination factor. Because the isotopic signature of tissues depends on both elemental concentration and isotopic signature of the diet, attempts to reconstruct diet from stable isotope signatures require use of mixing models that incorporate elemental concentration.     

Effects of nitrogen addition on the abundance and composition of soil ammonia oxidizers in Inner Mongolia Grassland

Nitrogen accumulation in soil is increasing in Inner Mongolia which is resulted mainly from fertilization accompanied by conversion of large area of grasslands to croplands. Ammonia-oxidation is the key step of nitrification which is driven by ammonia-oxidizing microorganisms, and study on the response of ammonia-oxidizing microorganisms is necessary for understanding the effects of nitrogen fertilization on ecosystem functions. In this study, the abundance and community structure of soil ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) under long-term N addition of different rates (0, 1.75, 5.25, 10.5, 17.5, and 28 g N m^?2 yr^?1) in a typical steppe of the Inner Mongolia Grassland were investigated using quantitative real-time PCR, cloning and sequencing based on amoA gene. In addition, soil potential ammonia oxidation rate was analyzed. Our results demonstrated that, with the increase of nitrogen addition rate, soil pH declined gradually from 6.6 to 4.9, and potential ammonia oxidation rate also declined which was positively correlated with soil pH (P < 0.01), while the copy number of bacterial amoA gene increased and positively (P < 0.01) correlated with ammonia concentration in soil. The archaeal amoA gene copy number did not change a lot with N nitrogen addition rate below 10.5 g N/m², but significantly decreased with addition of 17.5 and 28 g N m^?2 yr^?1. Sequencing of clone libraries of treatments revealed that in the treatment without N addition, AOB was dominated by Cluster 3a1 of Nitrosospira with a proportion of 87%, while in the treatment with N addition of 28 g N m^?2 yr^?1, proportion of Cluster 2 increased significantly to 41%. All archaeal amoA sequences were affiliated with the soil/sediment clade, and no significant variation of community structure was found between the treatments without N addition and with 28 g N m^?2 yr^?1 addition rate. In conclusion, this study demonstrated significant effects of nitrogen addition on potential ammonia oxidation rate and compositions of ammonia-oxidation microorganisms, which may have important implications for evaluating the impacts of N accumulation on ecosystem functioning. Further, the effects of pH and ammonia concentration on the ammonia oxidation rate and compositions of ammonia-oxidation microorganisms were discussed.     

Effects of nitrogen addition on the abundance and composition of soil ammonia oxidizers in Inner Mongolia Grassland

Nitrogen accumulation in soil is increasing in Inner Mongolia which is resulted mainly from fertilization accompanied by conversion of large area of grasslands to croplands. Ammonia-oxidation is the key step of nitrification which is driven by ammonia-oxidizing microorganisms, and study on the response of ammonia-oxidizing microorganisms is necessary for understanding the effects of nitrogen fertilization on ecosystem functions. In this study, the abundance and community structure of soil ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) under long-term N addition of different rates (0, 1.75, 5.25, 10.5, 17.5, and 28 g N m^?2 yr^?1) in a typical steppe of the Inner Mongolia Grassland were investigated using quantitative real-time PCR, cloning and sequencing based on amoA gene. In addition, soil potential ammonia oxidation rate was analyzed. Our results demonstrated that, with the increase of nitrogen addition rate, soil pH declined gradually from 6.6 to 4.9, and potential ammonia oxidation rate also declined which was positively correlated with soil pH (P < 0.01), while the copy number of bacterial amoA gene increased and positively (P < 0.01) correlated with ammonia concentration in soil. The archaeal amoA gene copy number did not change a lot with N nitrogen addition rate below 10.5 g N/m², but significantly decreased with addition of 17.5 and 28 g N m^?2 yr^?1. Sequencing of clone libraries of treatments revealed that in the treatment without N addition, AOB was dominated by Cluster 3a1 of Nitrosospira with a proportion of 87%, while in the treatment with N addition of 28 g N m^?2 yr^?1, proportion of Cluster 2 increased significantly to 41%. All archaeal amoA sequences were affiliated with the soil/sediment clade, and no significant variation of community structure was found between the treatments without N addition and with 28 g N m^?2 yr^?1 addition rate. In conclusion, this study demonstrated significant effects of nitrogen addition on potential ammonia oxidation rate and compositions of ammonia-oxidation microorganisms, which may have important implications for evaluating the impacts of N accumulation on ecosystem functioning. Further, the effects of pH and ammonia concentration on the ammonia oxidation rate and compositions of ammonia-oxidation microorganisms were discussed.     

Variations of the nitrate isotopic composition in the St. Lawrence River caused by seasonal changes in atmospheric nitrogen inputs

We present 42 dual-isotope nitrate analyses of fresh water samples collected in the St. Lawrence River between June 2006 and July 2008. Measured δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? values correlate negatively, while δ¹⁸O–NO₃ ^? displays no negative correlation with nitrate concentration. This suggests that nitrate uptake and/or elimination by denitrification is not the main driver of observed variations in nitrate concentration and isotopic signature in the St. Lawrence River. In addition, δ¹⁸O–NO₃ ^? is negatively correlated with the seasonally variable δ¹⁸O of ambient water, indicating that the variation in the isotopic signature of nitrate is barely modulated by in-stream nitrate regeneration (nitrification). It rather is constrained by along-river changes in the external sources of nitrate. Given the distinct nitrogen (N) and oxygen (O) isotopic signature of atmospheric nitrate, we argue that observed seasonal variations of δ¹⁵N–NO₃ ^? and δ¹⁸O–NO₃ ^? in the St. Lawrence River are due to variable contributions of snowmelt-derived water. Based on a N and O isotope mass balance, we show that total nitrate loading in the St. Lawrence River is dominated by a N input from the Great Lakes (47 ± 28 %) and from nitrate regeneration of both internal and external N (48 ± 22 %). While temporal nitrate N and O isotope dynamics in the St. Lawrence River are mainly influenced by the atmospheric N input fluctuations, with an increase in atmospheric loading during spring, atmospheric N plays overall a rather insignificant role with regards to the N budget (5 ± 4 %).     

香溪河流域水体环境因子研究

基于香溪河及其三大支流古夫河、高岚河与九冲河的8年监测数据,对该流域水体的环境因子及其时空动态进行了比较分析。结果显示,香溪河流域的水体呈中性偏弱碱性,属于中等硬度水,水体溶解氧丰富,还原性物质以及氯化物水平不高,具有较强的缓冲能力。多数环境因子具有明显的季节变化,而年际变化规律不显著。香溪河流域水体的主要营养元素氮含量较高,并以有城镇影响的古夫河为最;高岚河由于受到生活污水与农业活动的影响,水质相对较差;香溪河的磷污染严重;森林覆盖率较高的九冲河的水质最佳。在香溪河流域,氮主要以面源污染的方式进入河道,而香溪河的磷则主要以点源污染的方式进入河道。水土流失是香溪河流域面源污染的主要途径,磷矿和磷化工厂的排污则是磷点源污染的主要途径。从保护三峡水库香溪河库湾的生态学角度考虑,应重点控制香溪河磷的点源污染。     

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.     

华北低丘山区栓皮栎生态系统氧同位素日变化及蒸散定量区分

利用稳定同位素技术对华北低丘山区栓皮栎生态系统氧同位素日变化及蒸散定量区分进行研究,为华北低丘山区森林生态系统水汽交换研究提供基础。试验采用离轴积分腔输出光谱技术(OA-ICOS)连续测定生态系统不同高度水汽浓度和δ18O值,同时采用真空提取和液态水同位素分析仪测定枝条和土壤的δ18O值。结果显示,4个晴天中大气水汽浓度日变化复杂,变化趋势差异大,而δ18O日变化均成高-低-高的"V"型变化,最小值出现在12:00—18:00。Keeling方程在10:00—12:00的相关系数R2均大于0.71,方程达到极显著水平,表明此时段蒸腾速率较高,满足植物蒸腾的同位素稳定态假设。利用Keeling方程估算的栓皮栎生态系统δET值有相似的低-高-低日变化,与大气的δv值变化趋势相反。同位素分割结果显示栓皮栎生态系统中蒸腾占蒸散比例日变化呈现低-高-低的趋势,10:00—14:00蒸腾占蒸散比例达到90%以上,尽管6:00—10:00和14:00—18:00的蒸腾占蒸散比例下降,但平均值仍高达69.38%,表明华北低丘山区栓皮栎生态系统的蒸散主要来源于植物蒸腾。