The influence of temperature and nitrogen source on growth and nitrogen uptake of two polar-desert species,Saxifraga caespitosa and Cerastium alpinum

Polar-desert plants experience low average air temperatures during their short growing season (4–8 °C mean July temperature). In addition, low availability of inorganic nitrogen in the soil may also limit plant growth. Our goals were to elucidate which N sources can be acquired by polar-desert plants, and how growth and N-uptake are affected by low growth temperatures. We compared rates of N-uptake and increases in mass and leaf area of two polar-desert species (Cerastium alpinum L. and Saxifraga caespitosa L.) over a period of 3 weeks when grown at two temperatures (6 °C vs. 15 °C) and supplied with either glycine, NH₄ ⁺ or NO₃ ⁻. At 15 °C, plants at least doubled their leaf area, whereas there was no change in leaf area at 6 °C. Measured mean N-uptake rates varied between 0.5 nmol g⁻¹ root DM s⁻¹ on glycine at 15 °C and 7.5 nmol g⁻¹ root DM s⁻¹ on NH₄ ⁺ at 15 °C. Uptake rates based upon increases in mass and tissue N concentrations showed that plants had a lower N-uptake rate at 6 °C, regardless of N source or species. We conclude that these polar-desert plants can use all three N sources to increase their leaf area and support flowering when grown at 15 °C. Based upon short-term (8 h) uptake experiments, we also conclude that the short-term capacity to take up inorganic or organic N is not reduced by low temperature (6 °C). However, net N-uptake integrated over a three-week period is severely reduced at 6 °C. This revised version was published online in June 2006 with corrections to the Cover Date.     

氮沉降对森林植物的影响

综述了氮沉降对森林植物的影响。氮沉降对森林植物的影响主要表现在以下6个方面：(1)在一定量范围内的氮沉降有利于植物的光合作用,但过量后则会引起植物的光合速率下降;(2)当植物生长受氮限制时,在一定程度上的氮沉降增加植物生产力,但当氮过量后,氮沉降则使植物的生产力下降;(3)过量的氮沉降导致植物体各种营养元素含量的比例失衡;(4)氮沉降会改变植物的形态结构,集中表现为根／冠比减小;(5)氮沉降会增加植物对天然胁迫如干旱、病虫害和风的敏感性,减少其抵御能力;(6)氮沉降会改变植物组成和降低森林植物的多样性。     

A holistic view of nitrogen acquisition in plants

Nitrogen (N) is the mineral nutrient required in the greatest amount and its availability is a major factor limiting growth and development of plants. As sessile organisms, plants have evolved different strategies to adapt to changes in the availability and distribution of N in soils. These strategies include mechanisms that act at different levels of biological organization from the molecular to the ecosystem level. At the molecular level, plants can adjust their capacity to acquire different forms of N in a range of concentrations by modulating the expression and function of genes in different N uptake systems. Modulation of plant growth and development, most notably changes in the root system architecture, can also greatly impact plant N acquisition in the soil. At the organism and ecosystem levels, plants establish associations with diverse microorganisms to ensure adequate nutrition and N supply. These different adaptive mechanisms have been traditionally discussed separately in the literature. To understand plant N nutrition in the environment, an integrated view of all pathways contributing to plant N acquisition is required. Towards this goal, in this review the different mechanisms that plants utilize to maintain an adequate N supply are summarized and integrated.     

Ecological ramifications of the direct foliar uptake of nitrogen

The foliar incorporation of various reactive forms of nitrogen (N) has been identified and studied for nearly 30 years. However, the ecosystem-level ramifications of this uptake pathway have only recently been considered by the scientific community. In this review, I present our current understanding of the foliar uptake process and then discuss why this pathway of N addition to ecosystems should be considered separately from the bulk deposition of N to the soil surface. Direct foliar uptake is a direct addition of N to plant metabolism and could potentially more readily influence plant growth compared to soil-deposited N. Current ecosystem process models do not partition reactive N between foliar and soil entry pathways and the influence of N deposition on ecosystem C sequestration is likely inadequately represented in most models. I also outline several research priorities for the future understanding of the ecological consequences of foliar uptake of reactive N.     

Enhanced foliar 15N enrichment with increasing nitrogen addition rates: Role of plant species and nitrogen compounds

Determining the abundance of N isotope (δ¹⁵N) in natural environments is a simple but powerful method for providing integrated information on the N cycling dynamics and status in an ecosystem under exogenous N inputs. However, whether the input of different N compounds could differently impact plant growth and their ¹⁵N signatures remains unclear. Here, the response of ¹⁵N signatures and growth of three dominant plants (Leymus chinensis, Carex duriuscula, and Thermopsis lanceolata) to the addition of three N compounds (NH₄HCO₃, urea, and NH₄NO₃) at multiple N addition rates were assessed in a meadow steppe in Inner Mongolia. The three plants showed different initial foliar δ¹⁵N values because of differences in their N acquisition strategies. Particularly, T. lanceolata (N₂-fixing species) showed significantly lower ¹⁵N signatures than L. chinensis (associated with arbuscular mycorrhizal fungi [AMF]) and C. duriuscula (associated with AMF). Moreover, the foliar δ¹⁵N of all three species increased with increasing N addition rates, with a sharp increase above an N addition rate of ~10 g N m⁻² year⁻¹. Foliar δ¹⁵N values were significantly higher when NH₄HCO₃ and urea were added than when NH₄NO₃ was added, suggesting that adding weakly acidifying N compounds could result in a more open N cycle. Overall, our results imply that assessing the N transformation processes in the context of increasing global N deposition necessitates the consideration of N deposition rates, forms of the deposited N compounds, and N utilization strategies of the co-existing plant species in the ecosystem.     

氮添加对中国陆地植被地上-地下生物量分配的影响

大气氮沉降水平持续升高导致的外源氮输入增加,强烈影响了陆地生态系统的碳循环。目前,已有大量报道证实了氮沉降升高对全球陆地植被固碳的积极影响。虽然之前大部分研究将这一结果归因于光合作用增强导致的地上生物量增加,但最近的研究发现长期氮添加对植物地下根系的影响也同样重要。归纳整理了181篇公开发表的我国野外模拟氮沉降试验结果,采用整合分析(Meta-analysis)方法,定量评估了氮添加对我国陆地植被地上-地下生物量分配的影响特征和不同生态系统类型及施氮方式之间的影响差异。通过分析地上-地下生物量分配对氮添加的响应差异来探究植被碳增益对长期大气氮沉降增加的潜在响应机制。结果表明,氮添加显著增强了我国陆地植被的光合作用及碳固存,且植物碳增益在不同生态系统类型及施氮制度间有所差异。植物叶片的氮含量显著增加,使得叶片碳氮比及凋落物碳氮比显著降低,但并未显著影响细根的碳氮比。氮添加总体上显著提高了植物的净光合速率,但降低了光合利用效率。地上生物量,凋落物产量和根生物量平均分别显著增加了38%,17%和18%,总体上植物地上部分对氮添加的响应程度比地下部分更高。然而,不同生态系统类型的地上-地下生物...     

Plant root distributions and nitrogen uptake predicted by a hypothesis of optimal root foraging

CO(2)-enrichment experiments consistently show that rooting depth increases when trees are grown at elevated CO(2) (eCO(2)), leading in some experiments to increased capture of available soil nitrogen (N) from deeper soil. However, the link between N uptake and root distributions remains poorly represented in forest ecosystem and global land-surface models. Here, this link is modeled and analyzed using a new optimization hypothesis (MaxNup) for root foraging in relation to the spatial variability of soil N, according to which a given total root mass is distributed vertically in order to maximize annual N uptake. MaxNup leads to analytical predictions for the optimal vertical profile of root biomass, maximum rooting depth, and N-uptake fraction (i.e., the proportion of plant-available soil N taken up annually by roots). We use these predictions to gain new insight into the behavior of the N-uptake fraction in trees growing at the Oak Ridge National Laboratory free-air CO(2)-enrichment experiment. We also compare MaxNup with empirical equations previously fitted to root-distribution data from all the world's plant biomes, and find that the empirical equations underestimate the capacity of root systems to take up N.     

Effects of nitrogen deposition and insect herbivory on patterns of ecosystem-level carbon and nitrogen dynamics: results from the CENTURY model

Atmospheric nitrogen deposition may indirectly affect ecosystems through deposition-induced changes in the rates of insect herbivory. Plant nitrogen (N) status can affect the consumption rates and population dynamics of herbivorous insects, but the extent to which N deposition-induced changes in herbivory might lead to changes in ecosystem-level carbon (C) and N dynamics is unknown. We created three insect herbivory functions based on empirical responses of insect consumption and population dynamics to changes in foliar N and implemented them into the CENTURY model. We modeled the responses of C and N storage patterns and flux rates to N deposition and insect herbivory in an herbaceous system. Results from the model indicate that N deposition caused a strong increase in plant production, decreased plant C : N ratios, increased soil organic C (SOC), and enhanced rates of N mineralization. In contrast, herbivory decreased both vegetative and SOC storage and depressed N mineralization rates. The results suggest that herbivory plays a particularly important role in affecting ecosystem processes by regulating the threshold value of N deposition at which ecosystem C storage saturates; C storage saturated at lower rates of N deposition with increasing intensity of herbivory. Differences in the results among the modeled insect herbivory functions suggests that distinct physiological and population response of insect herbivores can have a large impact on ecosystem processes. Including the effects of herbivory in ecosystem studies, particularly in systems where rates of herbivory are high and linked to plant C : N, will be important in generating accurate predictions of the effects of atmospheric N deposition on ecosystem C and N dynamics.     

Reliance on prey-derived nitrogen by the carnivorous plant Drosera rotundifolia decreases with increasing nitrogen deposition

? Carnivory in plants is presumed to be an adaptation to a low-nutrient environment. Nitrogen (N) from carnivory is expected to become a less important component of the N budget as root N availability increases. ? Here, we investigated the uptake of N via roots versus prey of the carnivorous plant Drosera rotundifolia growing in ombrotrophic bogs along a latitudinal N deposition gradient through Sweden, using a natural abundance stable isotope mass balance technique. ? Drosera rotundifolia plants receiving the lowest level of N deposition obtained a greater proportion of N from prey (57%) than did plants on bogs with higher N deposition (22% at intermediate and 33% at the highest deposition). When adjusted for differences in plant mass, this pattern was also present when considering total prey N uptake (66, 26 and 26 μg prey N per plant at the low, intermediate and high N deposition sites, respectively). The pattern of mass-adjusted root N uptake was opposite to this (47, 75 and 86 μg N per plant). ? Drosera rotundifolia plants in this study switched from reliance on prey N to reliance on root-derived N as a result of increasing N availability from atmospheric N deposition.     

Atmospheric nitrogen deposition on petals enhances seed quality of the forest herb Anemone nemorosa

• Elevated atmospheric input of nitrogen (N) is currently affecting plant biodiversity and ecosystem functioning. The growth and survival of numerous plant species is known to respond strongly to N fertilisation. Yet, few studies have assessed the effects of N deposition on seed quality and reproductive performance, which is an important life‐history stage of plants.
• Here we address this knowledge gap by assessing the effects of atmospheric N deposition on seed quality of the ancient forest herb Anemone nemorosa using two complementary approaches.
• By taking advantage of the wide spatiotemporal variation in N deposition rates in pan‐European temperate and boreal forests over 2 years, we detected positive effects of N deposition on the N concentration (percentage N per unit seed mass, increased from 2.8% to 4.1%) and N content (total N mass per seed more than doubled) of A. nemorosa seeds. In a complementary experiment, we applied ammonium nitrate to aboveground plant tissues and the soil surface to determine whether dissolved N sources in precipitation could be incorporated into seeds. Although the addition of N to leaves and the soil surface had no effect, a concentrated N solution applied to petals during anthesis resulted in increased seed mass, seed N concentration and N content.
• Our results demonstrate that N deposition on the petals enhances bioaccumulation of N in the seeds of A. nemorosa. Enhanced atmospheric inputs of N can thus not only affect growth and population dynamics via root or canopy uptake, but can also influence seed quality and reproduction via intake through the inflorescences.

     

Decoupling the direct and indirect effects of nitrogen deposition on ecosystem function

Elevated nitrogen (N) inputs into terrestrial ecosystems are causing major changes to the composition and functioning of ecosystems. Understanding these changes is challenging because there are complex interactions between 'direct' effects of N on plant physiology and soil biogeochemistry, and 'indirect' effects caused by changes in plant species composition. By planting high N and low N plant community compositions into high and low N deposition model terrestrial ecosystems we experimentally decoupled direct and indirect effects and quantified their contribution to changes in carbon, N and water cycling. Our results show that direct effects on plant growth dominate ecosystem response to N deposition, although long-term carbon storage is reduced under high N plant-species composition. These findings suggest that direct effects of N deposition on ecosystem function could be relatively strong in comparison with the indirect effects of plant community change.     

Total nitrogen deposition at key growing stages of maize and wheat as affected by pot surface area and crop variety

Atmospheric nitrogen (N) deposition is a serious problem on the North China Plain (NCP) because it imposes a considerable nutrient burden on the local environment. However, it also makes a substantial contribution to agricultural crop N requirements. The integrated total N input (ITNI) system is a method to quantify total atmospheric N deposition by using ¹⁵N-labeled monitor plants grown in pots. The effect of pot surface area and variety of indicator plant on the amount of airborne N input quantified by the ITNI system was investigated in this study. Total N deposition to the soil-maize/soil-wheat plant system at key growth stages was also quantified to improve N-fertilizer recommendations. When indicator plants having a high space requirement were used a correction factor was needed and this could be obtained only by simulating commercial field conditions, especially plant density, because the factor depends largely on pot area or the difference in plant density between pot conditions and field conditions. The total airborne N input measured by the ITNI system was not influenced by the variety of monitoring plant. N deposition was 20?C25 kg N ha^?1 during growth from three expanded leaf to ten expanded leaf and also from ten expanded leaf to maturity of maize. N deposition was 29.1 kg N ha^?1 between planting and the jointing stage and 10.1 kg N ha^?1 from jointing to maturity of wheat. This high measured N deposition indicates that N deposition should be taken into account when calculating the N fertilizer requirements of maize and wheat in this region.     

Mycorrhizal‐mediated nitrogen acquisition in switchgrass under elevated temperatures and N enrichment

Arbuscular mycorrhizal fungi (AMF) can perform key roles in ecosystem functioning through improving host nutrient acquisition. Nitrogen (N) is an essential nutrient for plant growth, however, anthropogenic N loading (e.g. crop fertilization and deposition from combustion sources) is increasing so that N now threatens ecosystem sustainability around the world by causing terrestrial and aquatic eutrophication and acidification. It is important to better understand the capacity of AMF to directly uptake N from soils and transfer it to host plants because this process may increase N recycling and retention within ecosystems. In addition to understanding the role of AMF in the N cycle in the present day it is important to understand how AMF function may change as global change proceeds. Currently the net effects of N enrichment and elevated temperature predicted with global change on AMF are unknown. In this study, we examined the effects of N enrichment by simulated N‐deposition loading, elevated temperatures expected by future global changes and their interactions on growth and AMF‐mediated N acquisition of switchgrass (Panicum virgatum var. Alamo), an important species for biofuel production. Switchgrass plants were grown in microcosm units that divided mycorrhizal roots from AMF hyphae and organic residues enriched with ¹⁵N by compartments separated by an air gap to reduce N diffusion. While AMF did not enhance switchgrass biomass, mycorrhizas significantly increased ¹⁵N in shoots and total shoot N. Neither N enrichment nor elevated temperatures influenced this mycorrhizal‐mediated N uptake and transfer. Results from this study can aid in developing sustainable bioethanol and switchgrass production practices that are less reliant on synthetic fertilizers and more dependent on internal N recycling from AMF.     

Ecological effects of atmospheric reactive nitrogen deposition on semi-natural terrestrial ecosystems

Evidence that enhanced reactive nitrogen deposition is affecting semi-natural terrestrial ecosystems comes from historic increases in plant tissue N concentrations, correlations between tissue N concentrations and present-day total atmospheric N deposition, changes in plant amino-acid composition and effects on N assimilation. The ecological significance of such changes in biomarkers is uncertain. This paper explores the ecological significance of reactive atmospheric N deposition through a review of previous experimental findings and new experimental evidence from an acidic and a calcareous grassland, both showing phosphorus limitation, and a N-limited Calluna vulgaris (L.) Hull heathland in upland Britain. Nitrogen addition in the range 0–20 g N m⁻² yr⁻¹ initially (years 0–4) increased the growth of Calluna and a decline in some subordinate species. In subsequent years, shoot extension was not stimulated, but winter injury was observed from 1993 onwards, suggesting a strong interaction between N supply and climatic conditions. By contrast, the grasslands showed a small decrease in the cover of higher plants in later years (6–7) of the experimental treatments (0–14 g N m⁻² yr⁻¹) and no growth stimulation. All N treatments reduced the bryophyte cover in the acidic grassland. There were marked effects on below-ground processes, including a sustained stimulation of N mineralization in the grassland soils, and an increase in the bacterial utilization of organic substrates in the heathland, as measured in BIOLOG plates. The results strongly suggest the importance of atmospheric N deposition on microbially driven processes in soils, and are discussed in relation to the scale of potential ecosystem changes and their reversibility by pollution abatement.     

Convergent responses of nitrogen and phosphorus resorption to nitrogen inputs in a semiarid grassland

Human activities have significantly altered nitrogen (N) availability in most terrestrial ecosystems, with consequences for community composition and ecosystem functioning. Although studies of how changes in N availability affect biodiversity and community composition are relatively common, much less remains known about the effects of N inputs on the coupled biogeochemical cycling of N and phosphorus (P), and still fewer data exist regarding how increased N inputs affect the internal cycling of these two elements in plants. Nutrient resorption is an important driver of plant nutrient economies and of the quality of litter plants produce. Accordingly, resorption patterns have marked ecological implications for plant population and community fitness, as well as for ecosystem nutrient cycling. In a semiarid grassland in northern China, we studied the effects of a wide range of N inputs on foliar nutrient resorption of two dominant grasses, Leymus chinensis and Stipa grandis. After 4 years of treatments, N and P availability in soil and N and P concentrations in green and senesced grass leaves increased with increasing rates of N addition. Foliar N and P resorption significantly decreased along the N addition gradient, implying a resorption‐mediated, positive plant–soil feedback induced by N inputs. Furthermore, N : P resorption ratios were negatively correlated with the rates of N addition, indicating the sensitivity of plant N and P stoichiometry to N inputs. Taken together, the results demonstrate that N additions accelerate ecosystem uptake and turnover of both N and P in the temperate steppe and that N and P cycles are coupled in dynamic ways. The convergence of N and P resorption in response to N inputs emphasizes the importance of nutrient resorption as a pathway by which plants and ecosystems adjust in the face of increasing N availability.     

Positive and negative impacts of insect frass quality on soil nitrogen availability and plant growth

Frass deposition to soil is an important pathway by which herbivorous insects impact decomposition and soil nutrient availability. However, little is known about how frass quality influences ecosystem properties. Here, we examined the effects of frass quality on the decomposition process, soil nitrogen (N) availability, and plant growth, using frass of Mamestra brassicae (L.) that fed on fertilized or unfertilized Brassica rapa L. var. perviridis Bailey. The frass quality was largely dependent on the host plant quality. Frass excreted by larvae that fed on the fertilized plants had higher N than that of larvae that fed on the unfertilized plants. The decomposition rate of the frass did not differ between N-rich and N-poor frass, except during the early decomposition period. The inorganic N concentration decreased during decomposition in both frass types. However, difference in the initial inorganic N concentration led to different consequences regarding soil N availability. Furthermore, addition of frass to the soil differently influenced the growth of B. rapa plants depending on the frass quality: plant biomass was increased by N-rich frass addition but decreased by N-poor frass addition, compared to the biomass without frass addition. These results indicate that frass quality is an important factor in determining the impact of herbivorous insects on nutrient dynamics, and that frass positively or negatively influences soil N availability and plant growth, depending on its quality.     

森林生态系统碳循环对全球氮沉降的响应

森林土壤和植被储存着全球陆地生态系统大约46%的碳,在全球碳平衡中起着非常重要的作用。过去几十年来,森林生态系统的碳循环和碳吸存受到了全球氮沉降的深刻影响,因为氮沉降改变了陆地生态系统的生产力和生物量积累。以欧洲和北美温带森林区域开展的研究为基础,综述了氮沉降对植物光合作用、土壤呼吸、土壤DOM及林木生长的影响特征和机理,探讨了森林生态系统碳动态对氮沉降响应的不确定性因素。热带森林C、N循环与大部分温带森林不同,人为输入的氮对热带生态系统过程的影响也可能不同,因此指出了在热带地区开展碳氮循环耦合研究的必要性和紧迫性。     

Modeling of nitrogen transformations and translocations

Different submodels within complex model packages on N regimes-for plant N-uptake, net N-mineralization, nitrate leaching and microbial N immobilization-are critically reviewed mainly with regard to their prediction ability on the basis of three comparative papers. Only for some of the processes adequate statistical evaluation of the models was possible. Compared to the other statistically evaluable process, nitrate leaching, modeling of plant N-uptake yields the better results. Most models for mineralization use arbitrary approaches rather than empirical ones. Although only approximate estimates of N mineralisation were at hand, the models generally behave expectedly poor. Only one model-DAISY-out of 16 involved in the comparison uses an explicit microbial biomass sub-model including microbial growth, decline and maintenance terms. So DAISY is the only model coupling C and N cycles. But what is true for an individual model describing the C and N transformation of a lab incubation experiment seems to be valid for most of the complex simulation work on the C and N regimes: this model was said to be overparameterized with respect to the available data.     

Arbuscular mycorrhizal fungi and nitrogen uptake

Nitrogen (N) is among the most important macro-nutrients significantly affecting plant growth and yield production. Accordingly, N must be supplied adequately so that optimum amounts of yield are resulted. There are different ways of supplying N to the plant including the use of chemical and biological fertilization. The chemical properties of N make it very mobile, especially under humid conditions. Hence, N must not be overfertilized with respect to the economical and environmental points of view. N Biological fertilization includes the use of plant growth-promoting rhizobacteria (PGPR) including the N-fixing bacteria, rhizobium. There are also arbuscular mycorrhizal (AM) fungi in the soil, which are symbiotic to most terrestrial plants enhancing plant growth and yield production through increasing the uptake of water and nutrients by the host plant. Numerous experiments have indicated the important role of AM fungi in enhancing P uptake by plant. However, it is yet a matter of debate that how AM fungi may affect soil N dynamic and hence plant N uptake. Some of the most important and recent aspects regarding such effects by AM fungi are highlighted, which can be of significance to health and productivity of the ecosystem.