Ecosystem recovery: heathland response to a reduction in nitrogen deposition

Atmospheric deposition of nitrogen is responsible for widespread changes in the structure and function of sensitive seminatural ecosystems. The proposed reduction in emissions of nitrogenous pollutants in Europe under the Gothenburg Protocol raises the question of whether affected ecosystems have the potential to recover to their previous condition and, if so, over what timescale. Since 1998, we have monitored the response of a lowland heathland in southern England following the cessation of a long‐term nitrogen addition experiment, and subsequent management, assessing changes in vegetation growth and chemistry, soil chemistry and the soil microbial community. Persistent effects of earlier nutrient loading on Calluna growth and phenology, and on the abundance of lichens, were apparent up to 8 years after nitrogen additions ceased, indicating the potential for long‐term effects of modest nutrient loading (up to 15.4 kg N ha^?1 yr^?1, over 7 years) on heathland ecosystems. The size and activity of the soil microbial community was elevated in former N‐treated plots, 6–8 years after additions ceased, suggesting a prolonged effect on the rate of nutrient cycling. Although habitat management in 1998 reduced nitrogen stores in plant biomass, effects on belowground nitrogen stores were small. Although some parameters (e.g. soil pH) recover pretreatment levels relatively rapidly, others (e.g. vegetation cover and microbial activity) respond much more slowly, indicating that the ecological effects of even small increases in nitrogen deposition will persist for many years after deposition inputs are reduced. Indeed, calculations suggest that the additional soil nitrogen storage associated with 7 years of experimental nitrogen inputs could sustain the observed effects on plant growth and phenology for several decades. Carry over effects on plant phenology and sensitivity to drought suggest that the persistence of vegetation responses to nitrogen deposition should be integrated into long‐term assessments of the impact of global climate change on sensitive ecosystems.     

High retention of 15N‐labeled nitrogen deposition in a nitrogen saturated old‐growth tropical forest

The effects of increased reactive nitrogen (N) deposition in forests depend largely on its fate in the ecosystems. However, our knowledge on the fates of deposited N in tropical forest ecosystems and its retention mechanisms is limited. Here, we report the results from the first whole ecosystem ¹⁵N labeling experiment performed in a N‐rich old‐growth tropical forest in southern China. We added ¹⁵N tracer monthly as ¹⁵NH₄¹⁵NO₃ for 1 year to control plots and to N‐fertilized plots (N‐plots, receiving additions of 50 kg N ha^?1 yr^?1 for 10 years). Tracer recoveries in major ecosystem compartments were quantified 4 months after the last addition. Tracer recoveries in soil solution were monitored monthly to quantify leaching losses. Total tracer recovery in plant and soil (N retention) in the control plots was 72% and similar to those observed in temperate forests. The retention decreased to 52% in the N‐plots. Soil was the dominant sink, retaining 37% and 28% of the labeled N input in the control and N‐plots, respectively. Leaching below 20 cm was 50 kg N ha^?1 yr^?1 in the control plots and was close to the N input (51 kg N ha^?1 yr^?1), indicating N saturation of the top soil. Nitrogen addition increased N leaching to 73 kg N ha^?1 yr^?1. However, of these only 7 and 23 kg N ha^?1 yr^?1 in the control and N‐plots, respectively, originated from the labeled N input. Our findings indicate that deposited N, like in temperate forests, is largely incorporated into plant and soil pools in the short term, although the forest is N‐saturated, but high cycling rates may later release the N for leaching and/or gaseous loss. Thus, N cycling rates rather than short‐term N retention represent the main difference between temperate forests and the studied tropical forest.     

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。     

Nutrient leaching in dry heathland ecosystems: effects of atmospheric deposition and management

Atmospheric nutrient deposition has contributed to widespread changes in sensitive seminatural ecosystems throughout Europe. For an understanding of underlying processes it is important to quantify input–output flows in relation to ongoing atmospheric inputs and current management strategies. In this study we quantified losses of N, P, Ca, Mg, and K via leaching in heathland ecosystems (Lüneburger Heide, NW Germany) as a function of current deposition rates and different management measures (mowing, prescribed burning, choppering, sod-cutting) which aim to prevent shrub and tree encroachment. Leaching was only moderately related to atmospheric input rates, indicating that leaching was mostly affected by internal turnover processes. Leaching significantly increased for most of the nutrients after the application of management measures, particularly in the choppered and sod-cut plots. However, atmospheric nutrient inputs exceeded leaching outputs for most of the nutrients, even in the plots subjected to management. Despite high deposition rates (20–25 kg N ha⁻¹ year⁻¹), retention of atmospheric N input ranged between 74% and 92% in the control plots. In the treated plots, N retention decreased to 59–80%. However, in the study area mean N leaching in the controls has almost doubled since 1980 and currently amounts to 3.7 kg ha⁻¹ year⁻¹, indicating an early stage of N saturation. Our study provides evidence that leaching did not compensate for atmospheric nutrient deposition, particularly as regards N. Management, thus, will be an indispensable tool for the maintenance of the low-nutrient status as a prerequisite for the long-term preservation of heathland ecosystems.     

Impacts of elevated nitrogen inputs on oak reproductive and seed ecology

The effects of increased anthropogenic inputs of reactive nitrogen (N) have been studied at the Harvard Forest Chronic N Experiment, where NH₄NO₃ has been applied experimentally since 1988 to increase atmospheric deposition rates ～6‐ and ～18‐fold above ambient. This paper asks whether conditions favorable to primary production also resulted in plastic increases to flower, fruit, and seedling traits, and focuses primarily on the oaks that dominate the hardwood stands of the Harvard Forest experiment. Litterfall samples collected between 1996 and 2001 revealed that flowers and fruits were significantly more abundant in N‐treated plots, and an analysis of oak tree abundance found significant variation both among and within plots. Acorn samples collected during 2003 and 2004 (a mast and a postmast year) were therefore analyzed using ancova models that included an estimate of oak tree abundance. This tree abundance estimate was the only significant driver of increased acorn production during the mast year, and in both years it was a significant factor on plots receiving the highest levels of N. In the postmast year, acorn production was increased in direct response to N‐related factors other than tree abundance. Our comparisons of control and N‐treated plots for acorn quality traits (e.g. rates of acorn damage, germination percentage, seedling growth) revealed negligible or only transient differences. Shifts in overall acorn abundance – particularly disproportionate N‐mediated increases during nonmast years – could have a wide range of ecological consequences beyond the more frequently examined impacts of N deposition on primary production and carbon sequestration.     

Global patterns and substrate‐based mechanisms of the terrestrial nitrogen cycle

Nitrogen (N) deposition is impacting the services that ecosystems provide to humanity. However, the mechanisms determining impacts on the N cycle are not fully understood. To explore the mechanistic underpinnings of N impacts on N cycle processes, we reviewed and synthesised recent progress in ecosystem N research through empirical studies, conceptual analysis and model simulations. Experimental and observational studies have revealed that the stimulation of plant N uptake and soil retention generally diminishes as N loading increases, while dissolved and gaseous losses of N occur at low N availability but increase exponentially and become the dominant fate of N at high loading rates. The original N saturation hypothesis emphasises sequential N saturation from plant uptake to soil retention before N losses occur. However, biogeochemical models that simulate simultaneous competition for soil N substrates by multiple processes match the observed patterns of N losses better than models based on sequential competition. To enable better prediction of terrestrial N cycle responses to N loading, we recommend that future research identifies the response functions of different N processes to substrate availability using manipulative experiments, and incorporates the measured N saturation response functions into conceptual, theoretical and quantitative analyses.     

Recovery of soil nitrogen pools in species‐rich grasslands after 12 years of simulated pollutant nitrogen deposition: a 6‐year experimental analysis

Nitrogen (N) deposition from anthropogenic sources is a global problem that can reduce biodiversity and impair ecosystem functioning through effects on soil eutrophication and acidification. While increasing controls on emissions of oxides of nitrogen (NO_x) have reduced European N deposition rates from their peak in the late 20th Century, little is known about the legacy effects of N deposition in soils or the reversibility of N‐induced shifts in ecosystem processes. We studied species‐rich limestone and acidic grasslands, located in a highly polluted region that received over 3000 kg N deposition ha^?1 throughout the 20th Century, followed by a decline of ～50% in NO_x deposition rate in the past two decades. We investigated the effects on seasonal and annual mean concentrations of soil mineral N in experimental plots established in 1990 receiving simulated enhanced N deposition (0–140 kg N ha^?1 yr^?1) until 2002, both in the final year of treatment, and the subsequent 5 years of ‘recovery’ following cessation of treatments. Winter–summer cycles of N mineralization–immobilization were strongly amplified by simulated N deposition rates through the final year of treatments and into the first year of recovery, with winter concentrations of ammonium‐N in the acidic grassland and nitrate in the limestone grassland enhanced by up to 360% and 450%, respectively. Both the magnitude of the seasonal variations and the residual effects of the treatments on soil mineral N concentrations decreased progressively in the first 5 years after treatments ceased, although dose‐dependent trends remained in the acidic grassland. This study establishes that reducing N deposition rates in species‐rich grasslands can reverse eutrophication, even in soils that have experienced prolonged high rates of deposition. It provides new insight into the rates of recovery following, and effects of, declining N deposition rates with implications for restoration of species‐rich grasslands.     

Sources,fates, and impacts of nitrogen inputs to terrestrial ecosystems: review and synthesis

The relative importance of nitrogen inputs from atmospheric deposition and biological fixation is reviewed in a number of diverse, non-agricultural terrestrial ecosystems. Bulk precipitation inputs of N (l–l2 kg N ha^–1 yr^–1) are the same order of magnitude as, or frequently larger than, the usual range of inputs from nonsymbiotic fixation (< 1=" –=" 5=" kg=" n=">^–1 yr^–1), especially in areas influenced by industrial activity. Bulk precipitation measurements may underestimate total atmospheric deposition by 30–40% because they generally do not include all forms of wet and dry deposition. Symbiotic fixation generally ranges from 10–160 kg N ha^–1 yr^–1) in ecosystems where N-fixing species are present during early successional stages, and may exceed the range under unusual conditions.Rates of both symbiotic and nonsymbiotic fixation appear to be greater during early successional stages of forest development, where they have major impacts on nitrogen dynamics and ecosystem productivity. Fates and impacts of these nitrogen inputs are important considerations that are inadequately understood. These input processes are highly variable in space and time, and few sites have adequate comparative information on both nitrogen deposition and fixation.

Anthropogenic N deposition and the fate of 15NO 3 − in a northern hardwood ecosystem

Human activity has substantially increased atmospheric NO ₃ ^– deposition in many regions of the Earth, which could lead to the N saturation of terrestrial ecosystems. Sugar maple (Acer saccharum Marsh.) dominated northern hardwood forests in the Upper Great Lakes region may be particularly sensitive to chronic NO ₃ ^– deposition, because relatively moderate experimental increases (three times ambient) have resulted in substantial N leaching over a relatively short duration (5–7 years). Although microbial immobilization is an initial sink (i.e., within 1–2 days) for anthropogenic NO ₃ ^– in this ecosystem, we have an incomplete understanding of the processes controlling the longer-term (i.e., after 1 year) retention and flow of anthropogenic N. Our objectives were to determine: (i) whether chronic NO ₃ ^– additions have altered the N content of major ecosystem pools, and (ii) the longer-term fate of ¹⁵NO ₃ ^– in plots receiving chronic NO ₃ ^– addition. We addressed these objectives using a field experiment in which three northern hardwood plots receive ambient atmospheric N deposition (ca. 0.9 g N m^–2 year^–1) and three plots which receive ambient plus experimental N deposition (3.0 g NO₃ ^–-N m^–2 year^–1). Chronic NO ₃ ^– deposition significantly increased the N concentration and content (g N/m²) of canopy leaves, which contained 72% more N than the control treatment. However, chronic NO ₃ ^– deposition did not significantly alter the biomass, N concentration or N content of any other ecosystem pool. The largest portion of ¹⁵N recovered after 1 year occurred in overstory leaves and branches (10%). In contrast, we recovered virtually none of the isotope in soil organic matter (SOM), indicating that SOM was not a sink for anthropogenic NO ₃ ^– over a 1 year duration. Our results indicate that anthropogenic NO ₃ ^– initially assimilated by the microbial community is released into soil solution where it is subsequently taken up by overstory trees and allocated to the canopy. Anthropogenic N appears to be incorporated into SOM only after it is returned to the forest floor and soil via leaf litter fall. Short- and long-term isotope tracing studies provided very different results and illustrate the need to understand the physiological processes controlling the flow of anthropogenic N in terrestrial ecosystems and the specific time steps over which they operate.     

Changes in Canopy Processes Following Whole-Forest Canopy Nitrogen Fertilization of a Mature Spruce-Hemlock Forest

Abstract Most experimental additions of nitrogen to forest ecosystems apply the N to the forest floor, bypassing important processes taking place in the canopy, including canopy retention of N and/or conversion of N from one form to another. To quantify these processes, we carried out a large-scale experiment and determined the fate of nitrogen applied directly to a mature coniferous forest canopy in central Maine (18–20 kg N ha⁻¹ y⁻¹ as NH₄NO₃ applied as a mist using a helicopter). In 2003 and 2004 we measured NO₃ ⁻, NH₄ ⁺, and total dissolved N (TDN) in canopy throughfall (TF) and stemflow (SF) events after each of two growing season applications. Dissolved organic N (DON) was greater than 80% of the TDN under ambient inputs; however NO₃ ⁻ accounted for more than 50% of TF N in the treated plots, followed by NH₄ ⁺ (35%) and DON (15%). Although NO₃ ⁻ was slightly more efficiently retained by the canopy under ambient inputs, canopy retention of NH₄ ⁺as a percent of inputs increased markedly under fertilization. Recovery of less than 30% of the fertilizer N in TF suggested that the forest canopy retained more than 70% of the applied N (>80% when corrected for N which bypassed tree surfaces at the time of fertilizer addition). Results from plots receiving ¹⁵N enriched NO₃ ⁻ and NH₄ ⁺ confirmed bulk N estimations that more NO₃ ⁻ than NH₄ ⁺ was washed from the canopy by wet deposition. The isotope data did not show evidence of canopy nitrification, as has been reported in other spruce forests receiving much higher N inputs. Conversions of fertilizer-N to DON were observed in TF for both ¹⁵NH₄ ⁺ and ¹⁵NO₃ ⁻ additions, and occurred within days of the application. Subsequent rain events were not significantly enriched in ¹⁵N, suggesting that canopy DON formation was a rapid process related to recent N inputs to the canopy. We speculate that DON may arise from lichen and/or microbial N cycling rather than assimilation and re-release by tree tissues in this forest. Canopy retention of experimentally added N may meet and exceed calculated annual forest tree demand, although we do not know what fraction of retained N was actually physiologically assimilated by the plants. The observed retention and transformation of DIN within the canopy demonstrate that the fate and ecosystem consequences of N inputs from atmospheric deposition are likely influenced by forest canopy processes, which should be considered in N addition studies. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

1961-2010年中国区域氮沉降时空格局模拟研究

由于人类活动的干扰,近年来,通过沉降和施肥形式进入陆地生态系统的氮素持续增加,众多研究表明,中国已经成为继欧洲和北美之后的第三大氮沉降区。氮与陆地生态系统生物地球化学循环的一系列过程都相互联系,碳循环及其格局也受到氮的影响,因此大气氮沉降的变化受到广泛关注,探明区域大气氮沉降的时空格局对评估氮沉降对陆地生态系统碳循环的影响具有重要意义。构建了一个基于降水、能源消费和施肥数据的氮沉降时空格局模拟方法,通过与观测数据的比较说明该模拟方法能够较好地模拟氮沉降的时空变化,在此基础上,利用该方法模拟了1961-2010年中国区域氮沉降的时空格局。结果表明:(1)1961-2010年中国区域年平均氮沉降速率为0.81 g N m^-2 a^-1,由20世纪60年代的0.31 g N m^-2 a^-1增加到21世纪初的1.71 g N m^-2 a^-1,年增长率为0.04 g N m^-2 a^-1。总氮沉降量由20世纪60年代的2.85 TgN/a增加至15.68 TgN/a。(2)NH_x-N的沉降速率大约是NO_y-N的4倍,是主要的氮沉降形式。1961-2010年我国湿沉降平均速率为0.63 g N m^-2 a^-1,是干沉降速率(0.17 g N m^-2 a^-1)的3.63倍,是氮素进入陆地生态系统的重要途径。(3)在空间上,我国的大气氮沉降速率呈现出由东南向西北梯度递减的格局,华北、华中和东北的农田是氮沉降速率最大的区域,同时也是氮沉降速率增长最快的区域。     

Long‐term nitrogen addition causes the evolution of less‐cooperative mutualists

Human activities have altered the global nitrogen (N) cycle, and as a result, elevated N inputs are causing profound ecological changes in diverse ecosystems. The evolutionary consequences of this global change have been largely ignored even though elevated N inputs are predicted to cause mutualism breakdown and the evolution of decreased cooperation between resource mutualists. Using a long‐term (22 years) N‐addition experiment, we find that elevated N inputs have altered the legume–rhizobium mutualism (where rhizobial bacteria trade N in exchange for photosynthates from legumes), causing the evolution of less‐mutualistic rhizobia. Plants inoculated with rhizobium strains isolated from N‐fertilized treatments produced 17–30% less biomass and had reduced chlorophyll content compared to plants inoculated with strains from unfertilized control plots. Because the legume–rhizobium mutualism is the major contributor of naturally fixed N to terrestrial ecosystems, the evolution of less‐cooperative rhizobia may have important environmental consequences.     

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。     

What Have Stable Isotope Studies Revealed About the Nature and Mechanisms of N Saturation and Nitrate Leaching from Semi-Natural Catchments?

Various studies over the last 15 years have attempted to describe the processes of N retention, saturation and NO₃ ⁻ leaching in semi-natural ecosystems based on stable isotope studies. Forest ecologists and terrestrial biogeochemists have used ¹⁵N labelled NO₃ ⁻ and NH₄ ⁺ tracers to determine the fate of atmospheric deposition inputs of N to terrestrial ecosystems, with NO₃ ⁻ leaching to surface waters being a key output flux. Separate studies by aquatic ecologists have used similar isotope tracer methods to determine the fate and impacts of inorganic N species, leached from terrestrial ecosystems, on aquatic ecosystems, usually without reference to comparable terrestrial studies. A third group of isotopic studies has employed natural abundances of ¹⁵N and ¹⁸O in precipitation and surface water NO₃ ⁻ to determine the relative contributions of atmospheric and microbial sources. These three sets of results often appear to conflict with one another. Here we attempt to synthesize and reconcile the results of these differing approaches to identifying both the source and the fate of inorganic N in natural or semi-natural ecosystems, and identify future research priorities. We conclude that the results of different studies conform to a consistent conceptual model comprising: (1) rapid microbial turnover of atmospherically deposited NO₃ ⁻ at multiple biologically active locations within both terrestrial and aquatic ecosystems; (2) maximum retention and accumulation of N in carbon-rich ecosystems and (3) maximum leaching of NO₃ ⁻, most of which has been microbially cycled, from carbon-poor ecosystems exposed to elevated atmospheric N inputs.     

Atmospheric nitrogen deposition in world biodiversity hotspots: the need for a greater global perspective in assessing N deposition impacts

Increased atmospheric nitrogen (N) deposition is known to reduce plant diversity in natural and semi‐natural ecosystems, yet our understanding of these impacts comes almost entirely from studies in northern Europe and North America. Currently, we lack an understanding of the threat of N deposition to biodiversity at the global scale. In particular, rates of N deposition within the newly defined 34 world biodiversity hotspots, to which 50% of the world's floristic diversity is restricted, has not been quantified previously. Using output from global chemistry transport models, here we provide the first estimates of recent (mid‐1990s) and future (2050) rates and distributions of N deposition within biodiversity hotspots. Our analysis shows that the average deposition rate across these areas was 50% greater than the global terrestrial average in the mid‐1990s and could more than double by 2050, with 33 of 34 hotspots receiving greater N deposition in 2050 compared with 1990. By this time, 17 hotspots could have between 10% and 100% of their area receiving greater than 15 kg N ha^?1 yr^?1, a rate exceeding critical loads set for many sensitive European ecosystems. Average deposition in four hotspots is predicted to be greater than 20 kg N ha^?1 yr^?1. This elevated N deposition within areas of high plant diversity and endemism may exacerbate significantly the global threat of N deposition to world floristic diversity. Overall, we highlight the need for a greater global approach to assessing the impacts of N deposition.     

氮沉降对荒漠化草原草本植物物种多样性和群落组成的影响

采用野外条件下人工外源施加氮素模拟氮沉降的方式,设置了0、6、12和24g.m-2 4个纯氮素投加水平,在试验第一年(2007年)将不同水平的氮素随机投加到样方中,随后两年(2008～2009年)不再继续投加,研究氮沉降对荒漠化草原草本植物物种多样性和群落物种组成的影响。结果表明:植物物种丰富度和多度在年内均随着氮素投加水平的增大而降低,且物种多度比丰富度的降低程度更大;植物物种丰富度和多度年际间的变化则表现为低氮水平下差异显著(P<0.05),高氮水平下差异不显著的规律(P>0.05)。氮沉降改变了草本层片植物群落的物种组成,相对于多年生禾本科植物,多年生非禾本科植物在氮素处理下消失的概率更大。可见,氮沉降会降低荒漠化草原草本植物的物种多样性,改变草本植物群落的物种组成,且对荒漠化草原草本植物群落的影响是一个长期的过程。     

Effects of long-term nitrogen deposition on phosphorus leaching dynamics in a mature tropical forest

Elevated anthropogenic nitrogen (N) deposition is suggested to affect ecosystem phosphorus (P) cycling through altered biotic P demand and soil acidification. To date, however, there has been little information on how long-term N deposition regulates P fluxes in tropical forests, where P is often depleted. To address this question, we conducted a long-term N addition experiment in a mature tropical forest in southern China, using the following N treatments: 0, 50, 100, and 150 kg N ha^?1 year^?1. We hypothesized that (i) tropical forest ecosystems have conservative P cycling with low P output, and (ii) long-term N addition decreases total dissolved phosphorus (TDP) leaching losses due to reduced litter decomposition rates and stimulated P sorption deriving from accelerated soil acidification. As hypothesized, we demonstrated a closed P cycling with low leaching outputs in our forest. Under experimental N addition, TDP flux in throughfall was significantly reduced, suggesting that N addition may result in a less internal P recycling. Contrary to our hypothesis, N addition did not decrease TDP leaching, despite reduced litter decomposition and accelerated soil acidification. We find that N addition might have negative impacts on biological P uptake without affecting TDP leaching, and that the amount of TDP leaching from soil could be lower than a minimum concentration for TDP retention. Overall, we conclude that long-term N deposition does not necessarily decrease P effluxes from tropical forest ecosystems with conservative P cycling.     

Retention and leaching losses of atmospherically-derived nitrogen in the aggrading coastal watershed of Waquoit Bay,MA

Extensive areas of the eastern United States are being exposed to elevated levels of nitrogen in precipitation, with levels of inorganic N in wet deposition ranging from 5 to over 20 times preindustrial, background levels. This increase in N loading to the terrestrial system, coupled with changes in land use in coastal regions in particular, has dramatically increased the level of nutrient loading from watersheds to the point that coastal waters are today among the most intensely fertilized ecosystems on earth. Studies in upland, aggrading forests have generally found that precipitation N inputs are efficiently sequestered in forest biomass and soil organic matter. However, acidic soils, sandy, porous parent substrates, and chronic inputs of salt spray common to coastal watersheds may all reduce the potential for N sequestration by the terrestrial community.We assessed the role of coastal forests in the long-term storage and retention of atmospherically-derived N in the watersheds of Waquoit Bay, MA, an increasingly eutrophic estuary on Cape Cod, by measuring precipitation inputs, storage, and lysimeter outputs below the rooting zone in a chronosequence of sites released from agriculture at different times. Calculated annual retention efficiencies were relatively low for an N-limited, aggrading forest (40–62%), and leaching losses did not vary with site age from young pine stands to mature beech forests. Nearly all nitrogen input was retained during summer months except in months with very high rainfall events. Nitrogen was released during the dormant-season in proportion to water flux through the forest floor. The composition of lysimeter output was 76% DON, 11% NO ₃ ^– , and 13% NH ₄ ^– . Total water flux and infiltration appear to be more important determinants of N retention in this sandy, coastal site than in more upland forest ecosystems; sandy systems may inherently have a low N retention efficiency.     

The effects of quantity and duration of simulated pollutant nitrogen deposition on root-surface phosphatase activities in calcareous and acid grasslands: a bioassay approach

A field and laboratory based bioassay has been developed to investigate the effects of the quantity and duration of simulated pollutant nitrogen (N) deposition on root-surface phosphomonoesterase (PME) activities in calcareous and acid grasslands. Seedlings of Plantago lanceolata were transplanted to a calcareous grassland and Agrostis capillaris seedlings were grown in microcosms containing soil from an acid grassland that had received either 7 yr (long-term) N additions or 18 months (short-term) N and phosphorus (P) additions. The bioassay revealed that short-term N treatments had little effect on the enzyme activity, whereas long-term N additions significantly increased PME activity within 7 d of transplanting into the field plots. Root-surface PME activity of A. capillaris was significantly reduced in soil that received additions of P. In the plots receiving long-term additions of N, a strong relationship was observed between extractable soil ammonium and root-surface PME activity. Soil ammonium concentrations accounted for 67% of the variation in PME activity of P. lanceolata in the calcareous grassland, and 86% of the variation in PME activity of A. capillaris in the acid grassland. These results provide evidence that N deposition may have considerable effects on the demand and turnover of P in ecosystems that are approaching or have reached N saturation.     

大气有机氮沉降研究进展

大气氮素沉降是全球氮素生物地球化学循环的一个重要部分,包括干?湿沉降两种,以无机态和有机态形式发生沉降。长期以来由于受研究方法的限制,国际上对大气氮素沉降的研究多集中在无机态氮的沉降上,忽视了对有机态氮形式发生的沉降,因而造成了人们对大气氮素沉降总量的低估。在全面总结国内外文献的基础上,综述了大气有机态氮沉降的研究进展,具体包括大气有机氮的来源、种类?雨水有机氮的测定方法?有机氮沉降对大气氮沉降总量(氮沉降总量=无机氮沉降 有机氮沉降)的贡献,以及有机氮沉降可能的生态效应等。最后,指出了今后我国大气有机氮沉降研究需要加强的主要方面。