Watershed nitrogen input and riverine export on the west coast of the US

This study evaluated the sources, sinks, and factors controlling net export of nitrogen (N) from watersheds on the west coast of the US. We calculated input of new N to 22 watersheds for 1992 and 2002. 1992 inputs ranged from 541 to 11,644 kg N km⁻² year⁻¹, with an overall area-weighted average of 1,870 kg N km⁻² year⁻¹. In 2002, the range of inputs was 490–10,875 kg N km⁻² year⁻¹, averaging 2,158 kg N km⁻² year⁻¹. Fertilizer was the most important source of new N, averaging 956 (1992) and 1,073 kg N km⁻² year⁻¹ (2002). Atmospheric deposition was the next most important input, averaging 833 (1992) and 717 kg N km⁻² year⁻¹ (2002), followed by biological N fixation in agricultural lands. Riverine N export, calculated based on measurements taken at the furthest downstream USGS water quality monitoring station, averaged 165 (1992) and 196 kg N km⁻² year⁻¹ (2002), although data were available for only 7 watersheds at the latter time point. Downstream riverine N export was correlated with variations in streamflow (export = 0.94 × streamflow − 5.65, R ² = 0.66), with N inputs explaining an additional 16% of the variance (export = 1.06 × streamflow + 0.06 × input − 227.78, R ² = 0.82). The percentage of N input that is exported averaged 12%. Percent export was also related to streamflow (%export = 0.05 × streamflow − 2.61, R ² = 0.60). The correlations with streamflow are likely a result of its large dynamic range in these systems. However, the processes that control watershed N export are not yet completely understood.     

The influence of climate on average nitrogen export from large watersheds in the Northeastern United States

The flux of nitrogen in large rivers in North America and Europe is well explained as a function of the net anthropogenic inputs of nitrogen to the landscape, with on average 20 to 25% of these inputs exported in rivers and 75 to 80% of the nitrogen retained or denitrified in the landscape. Here, we use data for average riverine nitrogen fluxes and anthropogenic inputs of nitrogen over a 6-year period (1988–1993) for 16 major watersheds in the northeastern United States to examine if there is also a climatic influence on nitrogen fluxes in rivers. Previous studies have shown that for any given river, nitrogen fluxes are greater in years with higher discharge, but this can be interpreted as storage of nitrogen in the landscape during dry years and flushing of this stored nitrogen during wet years. Our analyses demonstrate that there is also a longer-term steady-state influence of climate on riverine nitrogen fluxes. Those watersheds that have higher precipitation and higher discharge export a greater fraction of the net anthropogenic inputs of nitrogen. This fractional export ranges from 10 to 15% of the nitrogen inputs in drier watersheds in the northeastern United States to over 35% in the wetter watersheds. We believe this is driven by lower rates of denitrification in the wetter watersheds, perhaps because shorter water residence times do not allow for as much denitrification in riparian wetlands and low-order streams. Using mean projections for the consequences of future climate change on precipitation and discharge, we estimate that nitrogen fluxes in the Susquehanna River to Chesapeake Bay may increase by 3 to 17% by 2030 and by 16 to 65% by 2095 due to greater fractional delivery of net anthropogenic nitrogen inputs as precipitation and discharge increase. Although these projections are highly uncertain, they suggest a need to better consider the influence of climate on riverine nitrogen fluxes as part of management efforts to control coastal nitrogen pollution.     

微生物在近海氮循环过程的贡献与驱动机制

人类活动导致海岸带氮超载而富营养化,进而引起更多的生态环境问题.在全球变化背景下,进一步揭示微生物驱动的氮循环过程的驱动机制及贡献,对评价与预测近海生态系统服务功能变化、管理决策等至关重要.本文介绍了固氮、氨化、硝化、反硝化、硝酸盐铵化、厌氧氨氧化过程在近海多种生境沉积物中的生物地球化学(速率、通量、贡献)与微生物生态学(功能类群丰度)特征及时空变化规律,阐述温度、溶氧、盐度、活性溶解有机碳、无机氮、沉水植物、底栖动物活动等因素对各过程速率的影响及对各竞争性类群或过程(氨氧化细菌/氨氧化古菌,反硝化/硝酸盐铵化/厌氧氨氧化)的调控机制,并简析了海岸带微生物氮循环研究所面临的机遇与挑战.     

Nitrogen budget and gaseous nitrogen loss in a tropical agricultural watershed

Although agricultural systems in tropical monsoon Asia play a central role in the global nitrogen (N) cycle, details of the N cycle in this region on a watershed scale remain unclear. This study quantified the N budget in a tropical watershed of 221 km² on Java Island, where paddy fields cover 28% of the land, by conducting field surveys. The amount of net biochemical gaseous N loss to the atmosphere (X _GB ), which is generally difficult to determine, was calculated as the residual of the N balance. Assuming that NH₃ volatilization balances deposition, and hence subtracting NH₄–N from the N import with atmospheric deposition, the average total import and export of N per year was found to be 46.5 kg ha⁻¹ year⁻¹ over the watershed. Of this, 71% was imported as fertilizer (M _F ) and 29% with atmospheric deposition (M _AD ). On the export side, 42% was lost as X _GB , 37% with incineration of rice residues and wood fuel (X _GI ), 13% with river discharge (X _D ) and 9% with rice surplus export (X _R ). A large portion of X _GB , and consequently, a small portion of X _D could be explained by the high rate of denitrification resulting from the high temperature and humid climate, and are thought to be common features of tropical watersheds where paddy fields are found.     

Temporal and spatial trends in nitrogen and phosphorus inputs to the watershed of the Altamaha River,Georgia, USA

The watershed of the Altamaha River, Georgia, is one of the largest in the southeastern U.S., draining 36,718 km² (including parts of metro Atlanta). We calculated both nitrogen (fertilizer, net food and feed import, atmospheric deposition, and biological N fixation in agricultural and forest lands) and phosphorus (fertilizer and net food and feed import) inputs to the watershed for 6 time points between 1954 and 2002. Total nitrogen inputs rose from 1,952 kg N km⁻² yr⁻¹ in 1954 to a peak of 3,593 kg N km⁻² yr⁻¹ in 1982 and then declined to 2,582 kg N km⁻² yr⁻¹ by 2002. Phosphorus inputs rose from 409 kg P km⁻² yr⁻¹ in 1954 to 532 kg P km⁻² yr⁻¹ in 1974 before declining to 412 kg P km⁻² yr⁻¹ in 2002. Fertilizer tended to be the most important input of both N and P to the watershed, although net food and feed import increased in importance over time and was the dominant source of N input by 2002. When considered on an individual basis, fertilizer input tended to be highest in the middle portions of the watershed (Little and Lower Ocmulgee and Lower Oconee sub-watersheds) whereas net food and feed imports were highest in the upper reaches (Upper Oconee and Upper Ocmulgee sub-watersheds). Although the overall trend in recent years has been towards decreases in both N and P inputs, these trends may be offset due to continuing increases in animal and human populations.     

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.     

Conservation of nitrogen increases with precipitation across a major grassland gradient in the Central Great Plains of North America

Regional analyses and biogeochemical models predict that ecosystem N pools and N cycling rates must increase from the semi-arid shortgrass steppe to the sub-humid tallgrass prairie of the Central Great Plains, yet few field data exist to evaluate these predictions. In this paper, we measured rates of net N mineralization, N in above- and belowground primary production, total soil organic matter N pools, soil inorganic N pools and capture in resin bags, decomposition rates, foliar ¹⁵N, and N use efficiency (NUE) across a precipitation gradient. We found that net N mineralization did not increase across the gradient, despite more N generally being found in plant production, suggesting higher N uptake, in the wetter areas. NUE of plants increased with precipitation, and δ¹⁵N foliar values and resin-captured N in soils decreased, all of which are consistent with the hypothesis that N cycling is tighter at the wet end of the gradient. Litter decomposition appeared to play a role in maintaining this regional N cycling trend: litter decomposed more slowly and released less N at the wet end of the gradient. These results suggest that immobilization of N within the plant–soil system increases from semi-arid shortgrass steppe to sub-humid tallgrass prairie. Despite the fact that N pools increase along a bio-climatic gradient from shortgrass steppe to mixed grass and tallgrass prairie, this element becomes relatively more limiting and is therefore more tightly conserved at the wettest end of the gradient. Similar to findings from forested systems, our results suggest that grassland N cycling becomes more open to N loss with increasing aridity.     

Epiphytic lichen indication of nitrogen deposition and climate in the northern rocky mountains,USA

Lichen bioindication can provide economical and spatially extensive monitoring of climate and pollution impacts on ecological communities. We used non-metric multidimensional scaling of lichen community composition and generalized additive models to analyze regional climate and pollution gradients in the northern Rocky Mountains, U.S. Temperature extremes, relative humidity, and N-deposition were strongly related to lichen community composition. Eutrophic species (genera Physcia, Xanthomendoza, and Xanthoria) were associated with high N deposition, low precipitation, and temperature extremes. Estimated N deposition in our study ranged from <0.5 to 4.26 kg N ha⁻¹ year⁻¹ with degradation to lichen communities observed at 4.0 kg N ha⁻¹ year⁻¹, the indicated critical load. The resulting model can track changes in climate and N pollution related to lichen communities over time, identify probable sensitive or impacted habitats, and provide key information for natural resource management and conservation. The approach is broadly applicable to temperate ecosystems worldwide.     

Changes in species composition of European acid grasslands observed along a gradient of nitrogen deposition

Question: Which environmental variables affect floristic species composition of acid grasslands in the Atlantic biogeographic region of Europe along a gradient of atmospheric N deposition? Location: Transect across the Atlantic biogeographic region of Europe including Ireland, Great Britain, Isle of Man, France, Belgium, The Netherlands, Germany, Norway, Denmark and Sweden. Materials and Methods: In 153 acid grasslands we assessed plant and bryophyte species composition, soil chemistry (pH, base cations, metals, nitrate and ammonium concentrations, total C and N, and Olsen plant available phosphorus), climatic variables, N deposition and S deposition. Ordination and variation partitioning were used to determine the relative importance of different drivers on the species composition of the studied grasslands. Results: Climate, soil and deposition variables explained 24% of the total variation in species composition. Variance partitioning showed that soil variables explained the most variation in the data set and that climate and geographic variables accounted for slightly less variation. Deposition variables (N and S deposition) explained 9.8% of the variation in the ordination. Species positively associated with N deposition included Holcus mollis and Leontodon hispidus. Species negatively associated with N deposition included Agrostis curtisii, Leontodon autumnalis, Campanula rotundifolia and Hylocomium splendens. Conclusion: Although secondary to climate gradients and soil biogeochemistry, and not as strong as for species richness, the impact of N and S deposition on species composition can be detected in acid grasslands, influencing community composition both directly and indirectly, presumably through soil‐mediated effects.     

Responses of grassland ecosystems to precipitation and land use along the Northeast China Transect

Abstracts. The Northeast China Transect (NECT) has been used to study how water availability influences the composition of plant functional types, soil organic matter, net primary production, trace gas flux, and land‐use patterns. We discuss relations of plant species number, soil C and N and above‐ground biomass with a precipitation gradient and interactions with land‐use practices (grassland fencing, mowing and grazing), on the basis of data from the west part of NECT. The results indicate: 1. The above‐ground biomass of grassland communities has a linear relationship with precipitation under three land‐use practices, while plant species number, soil C, and total soil N have linear relationships with precipitation under fencing and mowing; under grazing the relationships are non‐linear. 2. Plant species number, soil C and total soil N have strong linear relationships with above‐ground biomass under both fencing and mowing, while they seem to have nonlinear relationships under grazing. 3. Land‐use practices along the precipitation gradient result not only in changes in grassland communities but also in qualitative changes of their structure and function. 4. Grasslands are more vulnerable to changes in climate under mowing than under fencing, and are more capable to store C in soil and plants. 5. At a given precipitation level, number of plant species, above‐ground biomass, and soil C are higher under low to medium intensity of human activities (mowing and grazing). A better understanding of how different intensities of human activities will affect the structure and function of grassland will require further research.     

DNA-probing indicates the occurrence of denitrification and nitrogen fixation genes in Hyphomicrobium. Distribution of denitrifying and nitrogen fixing isolates of Hyphomicrobium in a sewage treatment plant

Abstract: Twenty-six Hyphomicrobium isolates from the sewage treatment plant and its receiving water body in Plön (Schleswig-Holstein, Germany) and two culture collection strains were screened for the occurrence of genes coding for denitrification enzymes (dissimilatory nitrate, nitrite and nitrous oxide reductases), for dinitrogen fixation (nitrogenase reductase) and for nitrification (ammonia monooxygenase catalyzing the first stage of this process) by DNA-probing. More than one half of the isolates had genes coding for denitrification enzymes. The DNA-DNA hybridization signals obtained with the gene segments correlated with enzyme activity measurements. The DNA of some isolates distinctly hybridized with the nif H probe indicating the occurrence of nitrogenase in the genus Hyphomicrobium . No signal was detected with the gene probe for nitrification. The results show that probes consisting of gene segments can be employed successfully to monitor the occurrence of genes which can show complex expression and in bacteria growing at low rates. The distribution pattern of the denitrification genes indicates that methylotrophic prosthecate bacteria of the sewage treatment plant and its receiving water body occupy different ecological niches.     

Use of a latitudinal gradient in bald cypress (Taxodium distichum) production to examine physiological controls of biotic boundaries and potential responses to environmental change

Aim Predictions of vegetation change with global warming require models that accurately reflect physiological processes underlying growth limitations and species distributions. However, information about environmental controls on physiology and consequent effects on species boundaries and ecosystem functions such as production is limited, especially for forested wetlands that are potentially important carbon sinks. Location The bald cypress (Taxodium distichum) region of the south‐eastern United States was studied to examine how production of an important forested wetland varies with latitude and temperature as well as local hydrology. Methods We used published data to analyse litter production across a latitudinal gradient from 26.2 to 37.8° N to determine how bald cypress swamps might respond to alternate climate conditions and what changes might occur throughout the distributional range. Results Litterfall rates followed a bell shaped curve, indicating that production was more limited at the distributional boundaries (c. 225 g/m² year^?1) compared to the mid‐range (795–1126 g/m² year^?1). This pattern suggests that conditions are suboptimal near both boundaries and that the absence of populations outside this latitudinal range may be largely due to physiological constraints on the carbon balance of dominant species. While dispersal limitations cannot be totally discounted, competition with other wetland types at the extremes of the range does not seem likely to be important because the relative basal area of bald cypress does not decrease near the edges of the range. Impaired hydrology depressed production across the entire range, but more in the south than the north. Main conclusions Our findings suggest that (1) physiological limitations constrain biotic boundaries of bald cypress swamps; (2) future changes in global temperature would affect litter production in a nonlinear manner across the distributional range; (3) local changes in hydrology may interact with climate to further reduce litter production, particularly at lower latitudes; and (4) southernmost forests could be extirpated if environmental conditions compromise carbon balance and water‐use efficiency of trees.     

Spatial variation of the stable nitrogen isotope ratio of woody plants along a topoedaphic gradient in a subtropical savanna

Variation in the stable N isotope ratio (δ¹⁵N) of plants and soils often reflects the influence of environment on the N cycle. We measured leaf δ¹⁵N and N concentration ([N]) on all individuals of Prosopis glandulosa (deciduous tree legume), Condalia hookeri (evergreen shrub), and Zanthoxylum fagara (evergreen shrub) present within a belt transect 308 m long × 12 m wide in a subtropical savanna ecosystem in southern Texas, USA in April and August 2005. Soil texture, gravimetric water content (GWC), total N and δ¹⁵N were also measured along the transect. At the landscape scale, leaf δ¹⁵N was negatively related to elevation for all the three species along this topoedaphic sequence. Changes in soil δ¹⁵N, total N, and GWC appeared to contribute to this spatial pattern of leaf δ¹⁵N. In lower portions of the landscape, greater soil N availability and GWC are associated with relatively high rates of both N mineralization and nitrification. Both soil δ¹⁵N and leaf [N] were positively correlated with leaf δ¹⁵N of non-N₂ fixing plants. Leaf δ¹⁵N of P. glandulosa, an N₂-fixing legume, did not correlate with leaf [N]; the δ¹⁵N of P. glandulosa’s leaves were closer to atmospheric N₂ and significantly lower than those of C. hookeri and Z. fagara. Additionally, at smaller spatial scales, a proximity index (which reflected the density and distance of surrounding P. glandulosa trees) was negatively correlated with leaf δ¹⁵N of C. hookeri and Z. fagara, indicating the N₂-fixing P. glandulosa may be important to the N nutrition of nearby non-N₂-fixing species. Our results indicate plant ¹⁵N natural abundance can reflect the extent of N retention and help us better understand N dynamics and plant-soil interactions at ecosystem and landscape scales.     

Increased temperature causes protein limitation by reducing the efficiency of nitrogen digestion in the ectothermic herbivore Spodoptera exigua

For insect herbivores, rising temperatures lead to exponentially higher metabolic rates. As a result, basic nutritional demands for protein and carbohydrates can be altered at high temperatures. It is hypothesized that temperature‐driven increases in metabolic nitrogen turnover will exacerbate protein limitation by increasing metabolic nitrogen demand. To test this hypothesis, the present study examines whether metabolic nitrogen turnover at higher temperatures causes protein limitation of a generalist herbivore, the beet armyworm Spodoptera exigua Hübner (Lepidoptera : Noctuidae). Third‐instar S. exigua larvae were reared at 25 and 30 °C on three artificial diets of varying protein : carbohydrate ratios (23 : 26, 17 : 26 and 6 : 26 %P : %C, respectively) and their growth rates, metabolic nitrogen demand and consumption rates were measured. Warming was found to lead to temperature‐induced protein limitation of the S. exigua larvae by increasing metabolic nitrogen demand at the same time as reducing nitrogen digestion efficiency. Because climate change is increasing atmospheric temperatures rapidly worldwide, it is suggested that a better understanding of how temperature change can influence metabolic demands will provide key information for predicting herbivore growth rates and foraging strategies in the future.     

Temperature sensitivity of spring vegetation phenology correlates to within-spring warming speed over the Northern Hemisphere

The inter-annual shift of spring vegetation phenology relative to per unit change of preseason temperature, referred to as temperature sensitivity (days °C⁻¹), quantifies the response of spring phenology to temperature change. Temperature sensitivity was found to differ greatly among vegetation from different environmental conditions. Understanding the large-scale spatial pattern of temperature sensitivity and its underlying determinant will greatly improve our ability to predict spring phenology. In this study, we investigated the temperature sensitivity for natural ecosystems over the North Hemisphere (north of 30°N), based on the vegetation phenological date estimated from NDVI time-series data provided by the Advanced Very High Resolution Radiometer (AVHRR) and the corresponding climate dataset. We found a notable longitudinal change pattern with considerable increases of temperature sensitivity from inlands to most coastal areas and a less obvious latitudinal pattern with larger sensitivity in low latitude area. This general spatial variation in temperature sensitivity is most strongly associated with the within-spring warming speed (WWS; r = 0.35, p < 0.01), a variable describing the increase speed of daily mean temperature during spring within a year, compared with other factors including the mean spring temperature, spring precipitation and mean winter temperature. These findings suggest that the same magnitude of warming will less affect spring vegetation phenology in regions with higher WWS, which might partially reflect plants’ adaption to local climate that prevents plants from frost risk caused by the advance of spring phenology. WWS accounts for the spatial variation in temperature sensitivity and should be taken into account in forecasting spring phenology and in assessing carbon cycle under the projected climate warming.     

Response of the diatom flora in Jiaozhou Bay, China to environmental changes during the last century

The diatom flora in a 164 cm long sediment core obtained from Jiaozhou Bay (Yellow Sea, China) was analyzed in order to trace the response of diatoms to environmental changes over the past 100 years. The sediment core was dated by ²¹⁰Pb and ¹³⁷Cs and represented approximately 100 years (1899–2001 A.D.). The flora was mainly composed of centric diatoms (59–96%). The concentration of diatoms declined sharply above 30 cm (after ~ 1981 A.D.), while the dominant species changed from Thalassiosira anguste-lineatus, Thalassiosira eccentria, Coscinodiscus excentricus, Coscinodiscus concinnus and Diploneis gorjanovici to Cyclotella stylorum and Paralia sulcata. Species richness decreased slightly, and the cell abundance of warm-water species increased. We argue that these floral changes were probably caused by climate change in combination with eutrophication resulting from aquaculture and sewage discharge.     

Microbial responses to a changing environment: implications for the future functioning of terrestrial ecosystems

In this review, we present a conceptual model which links plant communities and saprotrophic microbial communities through the reciprocal exchange of growth-limiting resources. We discuss the numerous ways human-induced environmental change has directly and indirectly impacted this relationship, and review microbial responses that have occurred to date. We argue that compositional shifts in saprotrophic microbial communities underlie functional responses to environmental change that have ecosystem-level implications. Drawing on a long-term, large-scale, field experiment, we illustrate how and why chronic atmospheric N deposition can alter saprotrophic communities in the soil of a wide-spread sugar maple (Acer saccharum) ecosystem in northeastern North America, resulting in the slowing of plant litter decay, the rapid accumulation of soil organic matter, and the accelerated production and loss of dissolved organic carbon (DOC). Compositional shifts in soil microbial communities, mediated by ecological interactions among soil saprotrophs, appear to lie at the biogeochemical heart of ecosystem response to environmental change.     

The perils of climate change: In utero exposure to temperature variability and birth outcomes in the Andean region

The discussion on the effects of climate change on human activity has primarily focused on how increasing temperature levels can impair human health. However, less attention has been paid to the effect of increased climate variability on health. We investigate how in utero exposure to temperature variability, measured as the fluctuations relative to the historical local temperature mean, affects birth outcomes in the Andean region. Our results suggest that exposure to a temperate one standard deviation relative to the municipality’s long-term temperature mean during pregnancy reduces birth weight by 20 g. and increases the probability a child is born with low birth weight by a 0.7 percentage point. We also explore potential channels driving our results and find some evidence that increased temperature variability can lead to a decrease in health care and increased food insecurity during pregnancy.