Elevated Nitrogen Research Articles

Elevated Blood Urea Nitrogen Is Associated With More Impaired Cardiac Performance and Poorer Long-Term Clinical Outcomes Independent of Glomerular Filtration Rate

Elevated Blood Urea Nitrogen Is Associated With More Impaired Cardiac Performance and Poorer Long-Term Clinical Outcomes Independent of Glomerular Filtration Rate

Clinical features of systemic lupus erythematosus with retinopathy

[Objective] To observe the clinical features of systemic lupus erythematosus (SLE) with retinopathy.[Methods] Ninety-seven SLE patients were enrolled in this comparative clinical study.The patients were divided into retinopathy group (positive group,32 eyes of 23 patients) and non-retinopathy group (negative group,148 eyes of 74 patients).The age,course of disease,clinical features,laboratory Results in these two groups were comparatively analyzed.[Results] The positive rate of retinopathy in all SLE patients was 23.7 ％.Seventeen patients (22 eyes,73.9 ％) of positive group had retinal cotton-wool spot,retinal hemorrhage,tortuous retinal vein,retinal arterial spasm,microaneurysm and hard exudates.The other six patients (10 eyes,26.1％) in this group showed retinal main vessel occlusion.The incidence rate of rash,cutaneous vasculitis,elevated erythrocyte sedimentation rate (ESR),decreased complement C3and positive anti-double-stranded-DNA (anti-ds-DNA) antibody in the positive group were higher than those in the negative group (x2 =9.206,6.987,7.824,8.581,6.599; P ＜ 0.05).There was no significant difference between these two groups in age,course of disease,mucosal ulcers,arthritis,fever,headache,neutropenia,thrombocytopenia,proteinuria,elevated blood urea nitrogen,increased creatinine,positive antinuclear (ANA) and anti-Sm antibodies (t =0.321,0.063 ; x2 =0.135,0.046,0.176,0.002,0.036,0.113,0.053,0.032,0.012,0.000,0.004; P＞0.05).[Conclusion]s Tortuous retinal veins,retinal cotton-wool spots and retinal main vessels occlusion are the three major fundus features of SLE patient with retinopathy.Rash,cutaneous vasculitis,increased ESR,decreased complement C3 and positive anti-dsDNA antibody are the five major systemic clinical features of SLE patient with retinopathy. Key words: Lupus erythematosus, systemic/complications; Retinal diseases/etiology; Disease attributes

Climate Change Disproportionately Increases Herbivore over Plant or Parasitoid Biomass

All living organisms are linked through trophic relationships with resources and consumers, the balance of which determines overall ecosystem stability and functioning. Ecological research has identified a multitude of mechanisms that contribute to this balance, but ecologists are now challenged with predicting responses to global environmental changes. Despite a wealth of studies highlighting likely outcomes for specific mechanisms and subsets of a system (e.g., plants, plant-herbivore or predator-prey interactions), studies comparing overall effects of changes at multiple trophic levels are rare. We used a combination of experiments in a grassland system to test how biomass at the plant, herbivore and natural enemy (parasitoid) levels responds to the interactive effects of two key global change drivers: warming and nitrogen deposition. We found that higher temperatures and elevated nitrogen generated a multitrophic community that was increasingly dominated by herbivores. Moreover, we found synergistic effects of the drivers on biomass, which differed across trophic levels. Both absolute and relative biomass of herbivores increased disproportionately to that of plants and, in particular, parasitoids, which did not show any significant response to the treatments. Reduced parasitism rates mirrored the profound biomass changes in the system. These findings carry important implications for the response of biota to environmental changes; reduced top-down regulation is likely to coincide with an increase in herbivory, which in turn is likely to cascade to other fundamental ecosystem processes. Our findings also provide multitrophic data to support the general concern of increasing herbivore pest outbreaks in a warmer world.

Effects of elevated nitrogen and exotic plant invasion on soil seed bank composition in Joshua Tree National Park

We investigated the effects of exotic species invasion and 3 years of nitrogen (N) fertilization on the soil seed bank in Joshua Tree National Park, California, USA at four sites along an N deposition gradient. We compared seed bank composition and density in control (no N added) and fertilized (30 kg N ha−1 year−1) plots to determine if the seed bank would reflect aboveground changes due to N fertilization. Soil samples were collected and germinated in a greenhouse over 2 years. In the field, invasive species cover responded positively to N fertilization. However, we did not observe increased seed density of exotic invasive species in fertilized plots. While no significant differences were detected between treatments within sites, exotic invasive grass seeds overwhelmed the seed bank at all sites. Significant differences between sites were found, which may be due to differences in level of invasion, historic N deposition, and soil surface roughness. Sites experiencing low N deposition had the highest seed bank species richness for both control and fertilized treatments. Aboveground plant density did not correlate well with seed bank density, possibly due to the inherent patchiness of soil seed banks and differential ability of species to form seed banks. This seed bank study provided insight into site-specific impacts on native versus invasive species composition of soil seed banks, as well as magnitude of invasion and restoration potential at invaded sites.

Nitrogen in SL/RN direct reduced iron: origin and effect on nitrogen control in EAF steelmaking

In the steel plant considered here, direct reduced iron (DRI), produced by the coal based Stelco–Lurgi/Republic–National (SL/RN) process, makes up 50% or more of the total iron charge. The SL/RN DRI samples from a kiln cooler had high nitrogen contents (50–250 ppm, depending on particle size), contributing to elevated nitrogen levels in liquid steel produced in the electric arc furnaces. The proposed mechanism of nitriding of SL/RN DRI involves gaseous nitrogen (present within the rotary cooler) diffusing into the solid bed and is supported by a simple diffusion model. A strong correlation was found between the melt-in carbon content of the liquid steel and the final tap nitrogen content, with melt-in carbon of 0·3%C or higher resulting in nitrogen levels below 50 ppm at tap, even when charging DRI material that is high in nitrogen.

Direct and indirect effects of allelopathy in the soil legacy of an exotic plant invasion

Invasive species may leave behind legacies that persist even after removal, inhibiting subsequent restoration efforts. We examined the soil legacy of Cytisus scoparius, a nitrogen-fixing, putatively allelopathic shrub invading the western US. We tested the hypothesis that allelopathy plays a critical role in the depressive effect of Cytisus on the key native Douglas-fir, both directly on tree growth and indirectly via effects on its ectomycorrhizal fungi (EMF). In a greenhouse factorial experiment, we used activated carbon to inhibit Cytisus-produced allelochemicals and sucrose to reduce elevated nitrogen (N). We found that: (1) Cytisus-invaded soils depressed Douglas-fir growth compared to uninvaded forest soils. The effect of adding Cytisus litter was positive (possibly reflecting an N fertilization effect) only in the presence of activated carbon, providing evidence for a role of allelopathic compounds. Activated carbon did not increase growth in the absence of Cytisus litter. Finally, sucrose addition provided weak support for a nitrogen effect of Cytisus litter. (2) Seedlings grown in Cytisus soils had lower EMF abundance compared to those in uninvaded forest soils. In forest soil from one site, adding Cytisus litter also decreased EMF abundance. Douglas-fir growth increased significantly with EMF across sites and soils suggesting that changes in EMF were linked to tree growth. The fungal taxon Cenococcum geophilum was significantly depressed in Cytisus soils compared to forest soils, while Rhizopogon rogersii abundance was similar across soil types. These results together suggest an overall negative effect of Cytisus on the growth of a dominant native tree and its fungal symbionts. Our study suggests how the role of allelopathy in ecological restoration may play out on two time scales: through immediate, direct impacts on native plants as well as through long-term, persistent impacts mediated by the collapse or transformation of microbial communities.

NOx and N2O fluxes in a nitrogen-enriched European spruce forest soil under experimental long-term reduction of nitrogen depositions

European temperate forest soils have been exposed to elevated nitrogen (N) and acid depositions for decades. High nitrous oxide (N2O) and nitric oxide (NO) emissions have been reported from these forests. Since the 1980s, a decline in atmospheric deposition rates has been occurring. Our study addressed the question as to how N oxide fluxes and N turnover processes have changed in response to the declining N depositions in a N-enriched spruce stand (Picea abies (L.) Karst.). Studies were conducted at the Solling roof site under a control-roof with ambient depositions and under a reduced-N-input-roof where N and acid depositions have been reduced to a pre-industrial level for 16–17 years. Open dynamic and closed chamber methods were used to determine NOx and N2O fluxes, respectively, and in situ incubation studies were conducted to measure net N mineralisation. Under the reduced deposition roof, net nitrification and nitrate in soil solution were reduced to undetectable levels causing the soil to change from a net source for NOx (0.62 ± 0.24 kg N ha−1 yr−1) into a net sink (−0.33 ± 0.01 kg N ha−1 yr−1). The uptake of NOx was exclusively controlled by the NOx concentrations of the forest air. Reversal of N enrichment did not affect annual N2O fluxes (0.08 kg N ha−1 yr−1) due to restricted denitrification in the well-aerated organic layer, but the origin of nitrate for denitrification changed from mainly soil-borne N to exclusively deposited N. It was demonstrated that less than two decades of reduced N and acid depositions are sufficient to reduce the surplus N and NOx emissions of this soil.

The simultaneous effect of elevated co2-level and nitrogen-supply on the fruit components of tomato

A two-year open field experiment was carried out to study the effect of elevated carbon dioxide (CO2) and nitrogensupply on fruit components of tomato. Korall determinate growth type processing tomato cultivar plants were planted and cultivated for the entire growing season in open top chambers (OTC) in years 2007 and 2008. Compared with the control (350 ppm) CO2 enrichment (700 ppm) significantly decreased the lycopene content at all three harvest dates in both years, but higher supply of nitrogen and 700 ppm CO2 resulted in significantly higher lycopene values in second year. Elevated nitrogen concentration combined with 700 ppm CO2 significantly increased the ○Brix, sugar content, total phenolics, and total antioxidant status (TAS) of tomato fruits.

Different types of sub‐alpine grassland respond similarly to elevated nitrogen deposition in terms of productivity and sedge abundance

AbstractQuestionIncreasing emissions of reactive nitrogen (N) compounds threaten the composition of species‐rich communities even in remote alpine areas. In the few studies available from (sub‐)alpine grassland, N deposition altered species composition in favour of sedges, which are assumed to be less limited by phosphorus (P) compared to other species. Is the magnitude of the sedges' relative increase modified by availability of other nutrients, which is in turn influenced by site‐specific edaphic and climatic conditions?LocationSwiss Central Alps.MethodsWe selected ten sub‐alpine sites that covered a broad range of communities and soil conditions, including both calcareous Seslerietum as well as acidic Nardetum pastures. As a model species, Carex sempervirens Vill. was present at each site. For two consecutive years, in addition to a control, two treatments were applied to the vegetation: (1) 50 kg·N·ha−1·yr−1 as NH4NO3 and (2) compound NPK fertilizer comprising the same amount of N. Total above‐ground biomass, fraction of all sedges, leaf length and leaf N, P and potassium (K) content of C. sempervirens were recorded and analysed for their responses to treatments and interaction with variables linked to productivity and nutrient availability (including soil water potential, pH and soil P).ResultsBiomass production was stimulated across all sites on average by 30% with elevated N deposition, and by 60% with NPK fertilizer application, revealing the sites to be similarly co‐limited by N and by P and K. Also across all sites, the fraction of sedges was increased on average 33% by N addition, but remained constant with NPK application, suggesting stronger P and/or K co‐limitation in non‐sedge species. N treatment effects were consistent in their direction but varied between sites; nevertheless this variation could not be explained by the measured edaphic or climatic factors. However, by trend, sedges benefited more from N fertilization on sites with higher pH, but benefited less on very dry sites.ConclusionOur findings reveal that the observed increase of sedges is a general response of sub‐alpine grassland to elevated N deposition and thus can be regarded as universal both for Seslerietum as well as Nardetum pastures.

The Effects of Nutrients and Secondary Compounds of <I>Coffea arabica</I> on the Behavior and Development of <I>Coccus viridis</I>

Phytochemicals may modify the food quality, reduce a plant's palatability to insects, or defend against pests. This work aimed to study 1) relationships between the nitrogen and potassium levels given to plants in nutritive solutions and the foliar phytochemical concentrations, 2) the effect of nutrients and secondary compounds of Coffea arabica on the behavior of Coccus viridis, and 3) tolerance of C. arabica to losses. Deficient, normal, and excessive nitrogen and potassium fertilization treatments were used. Each treatment had two plants (one infested and one noninfested plant). The contents of phytochemicals in the infested plants' leaves and their dry matter of roots, stems, and leaves as well as the total contents in noninfested plants, were determined. The adults and nymphs of C. viridis were counted for 60 d in all treatments. It was verified that elevated nitrogen and potassium levels in the nutritional solutions led to increased of nymphs and adults of C. viridis to the coffee plants over time. Potassium and nitrogen had both direct and indirect effects on C. viridis. The direct effect was because of the increase of the nitrogen content in the leaves. The indirect effect instead was because of reductions in the caffeine and chlorogenic acid contents in the leaves. This is the first study to show relationship nutrient levels of coffee phytochemicals in response to herbivory by scale insects. Caffeine and chlorogenic acid applied on coffee leaves stimulated the locomotory activity of the green scale, thus reducing their feeding compared with untreated leaves. The elevation of caffeine and chlorogenic acid levels in coffee leaves affect this generalist insect by stimulating the locomotion of crawlers.

Differences of tolerance to simulated leaf herbivory in native and invasive tall form Spartina alterniflora populations: Effects of nitrogen availability

Rapid evolutionary changes in invasion-related ecological traits of alien plants are receiving considerable attention in current ecological research. Tolerance to herbivory is of particular interest because of the potential for using biological agents to control the spread of invasive plants. In this study, we compared herbivory tolerance in six populations of tall form Spartina alterniflora: three from its introduced range in Jiangsu Province, China, and three from its native range in Georgia, USA. Plants were grown in a greenhouse and subjected to a full-factorial experimental design with three simulated leaf herbivory levels and two levels of nitrogen availability for 20weeks. We found that native and invasive tall form S. alterniflora grown under different nitrogen conditions responded to simulated leaf herbivory differently with regard to underground and aboveground biomass and total culm height. Invasive S. alterniflora populations had a greater capacity to compensate for leaf damage than native populations in terms of underground biomass and aboveground biomass, particularly with low nitrogen availability. In addition, invasive S. alterniflora also showed higher compensatory capacity than native plants with regard to total culm height in both nitrogen treatments and with regard to tiller number in the high nitrogen treatment. Elevated nitrogen availability significantly increased the compensatory capacity of underground biomass in native S. alterniflora populations but did not affect any factor in invasive populations. Our results suggested a rapid evolutionary change in herbivory tolerance of invasive tall form S. alterniflora after its introduction to China. Understanding such changes in the tolerance to herbivory between native and invasive populations of alien plants has important implications for improving management efficiency.

Nutrient Release from Disturbance of Infiltration System Soils during Construction

Subsurface infiltration and surface bioretention systems composed of engineered and/or native soils are preferred tools for stormwater management. However, the disturbance of native soils, especially during the process of adding amendments to improve infiltration rates and pollutant removal, may result in releases of nutrients in the early life of these systems. This project investigated the nutrient release from two soils, one disturbed and one undisturbed. The disturbed soil was collected intact, but had to be air-dried, and the columns repacked when soil shrinkage caused bypassing of water along the walls of the column. The undisturbed soil was collected and used intact, with no repacking. The disturbed soil showed elevated releases of nitrogen and phosphorus compared to the undisturbed soil for approximately 0.4 and 0.8 m of runoff loading, respectively. For the undisturbed soil, the nitrogen release was delayed, indicating that the soil disturbance accelerated the release of nitrogen into a very short time period. Leaving the soil undisturbed resulted in lower but still elevated effluent nitrogen concentrations over a longer period of time. For phosphorus, these results confirm prior research which demonstrated that the soil, if shown to be phosphorus-deficient during fertility testing, can remove phosphorus from runoff even when disturbed.

氮沉降对杉木人工林土壤有机碳矿化和土壤酶活性的影响

为探讨氮沉降对亚热带森林土壤有机碳矿化及土壤酶活性的影响规律,在杉木人工林中开展了野外模拟N沉降试验。试验设计为4种处理,分别为N0(对照)、N1(60 kg N·hm^-2·a^-1)、N2(120 kg N·hm^-2·a^-1)和N3(240 kg N·hm^-2·a^-1),每处理重复3次。通过28 d的培养后发现,各土层有机碳日均矿化量随培养时间的延长呈下降趋势,而有机碳累计矿化量则逐步增加。不同氮沉降处理下各土层有机碳累计矿化量总体趋势表现为:随着氮沉降量的增加而降低,日均矿化量降低幅度以N1最大,其次是N0和N2,N3降幅最小。相同N沉降处理下,参与土壤碳循环的6种主要酶(蔗糖酶、纤维素酶、淀粉酶、β-葡糖苷酶、多酚氧化酶、过氧化物酶)活性、土壤有机碳日均矿化量和有机碳累计矿化量均随土层加深而降低。氮沉降对6种土壤酶活性的影响存在差异,对纤维素酶和多酚氧化酶具有促进作用,而对淀粉酶和过氧化物酶表现出一定的抑制作用;中-低氮沉降(N1、N2)对蔗糖酶无影响,而对β-葡糖苷酶具有促进作用,高氮沉降(N3)促进了蔗糖酶活性,但抑制了β-葡糖苷酶活性。表层土壤中,土壤有机碳累积矿化量与土壤纤维素酶、β-葡糖苷酶、过氧化物酶活性呈显著正相关。因此,氮沉降促进了表层土壤纤维素酶、多酚氧化酶和蔗糖酶的活性,但在一定程度上抑制了淀粉酶和过氧化物酶,对土壤有机碳矿化也表现出明显的抑制作用。;Human activities have dramatically increased the quantity of nitrogen fixed in terrestrial ecosystems, due to fossil fuel combustion, production and use of chemical fertilizers, and livestock ranching. Increased N supply can have a fertilization effect on forest ecosystems, but in the long-term, excess N can negatively impact biogeochemical cycling, soil chemistry, and productivity. Most research to date has focused on aggregate effects of N deposition on soil chemistry and N cycling. However, very little is known about the response of the microbial activities that are responsible for soil nutrient cycling and decomposition to these environmental changes. To investigate the response of soil organic carbon mineralization and soil enzyme activities to elevated nitrogen deposition, a field experiment was conducted in a Chinese fir plantation at Shaxian State Forest Farm of Fujian Province, China. Nitrogen loadings were designed at 4 levels as N0 (control), N1, N2 and N3 at the doses of 0, 60, 120 and 240 kg·hm^-2·a^-1N, respectively. Each treatment comprised three replicate plots of 20m×20m which were sprayed with CO(NH₂)₂ solutions on the forest floor at the beginning of each month, lasting from January 2004 to the sampling time, March 2010. Soil samples were incubated in the laboratory at 28 °C for 28 days, and the alkali absorption method was applied to measure soil respiration. The carbon that mineralized as CO₂ evolved was measured on the third day and once every week thereafter, and determined on the 28th day for the last time. Soil invertase, cellulose and amylase were detected by 3, 5- dinitrosalicylic acid assay, β-glucosidase by nitrophenol colorimetric method, and polyphenol oxidase and peroxidase by iodometric titration. During the incubation period, the daily mineralization of soil organic carbon decreased, but the cumulative mineralization of soil organic carbon increased with increasing time. Generally, the cumulative mineralization of soil organic carbon decreased with increasing doses of nitrogen deposition. The daily mineralization of soil organic carbon decreased in the sequence of N1>N0>N2>N3. At the same level of nitrogen deposition, the activities of the six soil enzymes involving carbon cycle (invertase, cellulose, amylase, β-glucosidase, polyphenol oxidase and peroxidase),the daily mineralization of soil organic carbon and the cumulative mineralization of soil organic carbon all decreased with the increasing soil depth. The activities of six soil enzymes responded differently to nitrogen treatments. At the soil depth of 0-20 cm, nitrogen additions promoted cellulose and polyphenol oxidase activities, but inhibited amylase and peroxidase to some extents. High level of nitrogen loading (N3) showed significant positive impact on invertase, but negative on β-glucosidase. Significant correlation was established between some soil enzymes, with cellulose being positively related with invertase and amylase, polyphenol oxidase with amylase and cellulose, peroxidase with invertase, amylase, cellulose and polyphenol oxidase. Soil organic carbon mineralization was also found to be positively correlated with β-glucosidase and peroxidase, but negatively with invertase and cellulose at the surface soil (0-20 cm). Hence, nitrogen deposition in this experiment accelerated the activities of surface soil cellulose, polyphenol oxidase and invertase, but inhibited amylase and peroxide to some extents, and suppressed soil organic carbon mineralization.

Field‐scale evaluation of effects of nitrogen deposition on the functioning of heathland ecosystems

Summary1. Plant communities that are adapted to low levels of nutrient availability are particularly sensitive to the effects of elevated nitrogen (N) deposition. Although site‐level manipulation experiments have been used widely to examine the effects of N deposition on (semi‐)natural ecosystems under controlled conditions, relatively few studies have investigated field‐scale impacts of elevated ambient N deposition on native plant communities and ecosystems.2. This study investigates the relationship between atmospheric N deposition, plant and soil nutrient status and microbial enzyme activities at 32 lowland heathland sites across the UK. Sites were chosen to cover a range of N deposition rates (13.3–30.8 kg N ha−1 year−1), geographical areas and different geologies.3. Significant relationships were found between rates of N deposition (total, reduced and oxidized) and concentrations of N and phosphorus (P) in Calluna shoots, litter and soil; relationships were generally stronger with total and reduced forms of N, compared with oxidized N. Litter and humus layers were deeper at sites receiving higher atmospheric N inputs, suggesting increased rates of soil carbon and N accumulation, despite higher levels of phenol‐oxidase activity (implying faster rates of organic matter decomposition) at these sites.4. The combination of elevated plant and soil P concentrations at sites receiving greater N inputs suggests strongly that N is increasing the availability and uptake rates of P, in addition to N. Furthermore, significant interactions between temperature and N deposition on indices relating to productivity and the turnover and uptake of nutrients highlight the influence of climate on ecosystem response to N deposition.5. Synthesis: Field‐scale evidence of changes in rates of nutrient cycling, organic matter accumulation and plant biochemistry suggests that ambient levels of N deposition are affecting the functioning of many heathland ecosystems and that the magnitude of these effects is also influenced by climate. Since such changes are known to be associated with reduced resistance to environmental stress and loss of plant diversity, current (and predicted future) levels of N deposition are likely to have important implications for the conservation and long‐term sustainability of nutrient‐poor ecosystems, particularly in the face of climate change.

Established native perennial grasses out-compete an invasive annual grass regardless of soil water and nutrient availability

Competition and resource availability influence invasions into native perennial grasslands by non-native annual grasses such as Bromus tectorum. In two greenhouse experiments we examined the influence of competition, water availability, and elevated nitrogen (N) and phosphorus (P) availability on growth and reproduction of the invasive annual grass B. tectorum and two native perennial grasses (Elymus elymoides, Pascopyrum smithii). Bromus tectorum aboveground biomass and seed production were significantly reduced when grown with one or more established native perennial grasses. Conversely, average seed weight and germination were significantly lower in the B. tectorum monoculture than in competition native perennial grasses. Intraspecific competition reduced per-plant production of both established native grasses, whereas interspecific competition from B. tectorum increased production. Established native perennial grasses were highly competitive against B. tectorum, regardless of water, N, or P availability. Bromus tectorum reproductive potential (viable seed production) was not significantly influenced by any experimental manipulation, except for competition with P. smithii. In all cases, B. tectorum per-plant production of viable seeds exceeded parental replacement. Our results show that established plants of Elymus elymoides and Pascopyrum smithii compete successfully against B. tectorum over a wide range of soil resource availability.

Effects of phosphorus addition on soil microbial biomass and community composition in three forest types in tropical China

Elevated nitrogen (N) deposition in humid tropical regions may aggravate phosphorus (P) deficiency in forest on old weathered soil found in these regions. From January 2007 to August 2009, we studied the responses of soil microbial biomass and community composition to P addition (in two monthly portions at level of 15 g P m−2 yr−1) in three tropical forests in southern China. The forests were an old-growth forest and two disturbed forests (mixed species and pine dominated). The objective was to test the hypothesis that P addition would increase microbial biomass and change the composition of the microbial community, and that the old-growth forests would be more sensitive to P addition due to its higher soil N availability. Microbial biomass C (MBC) was estimated twice a year and the microbial community structure was quantified by phospholipid fatty acid (PLFA) analysis at the end of the experiment. Addition of P significantly increased the microbial biomass and altered the microbial community composition in the old-growth forest, suggesting that P availability is one of the limiting factors for microbial growth. This was also reflected by significant increases in soil respiration after P addition. In contrast, P addition had no effect on the microbial biomass and the microbial community composition in the pine forests. Also in the mixed forest, the microbial biomass did not significantly respond to P addition, but soil respiration and the ratio of fungal-to-bacteria was significantly increased.

Testing the Field of Dreams Hypothesis: functional responses to urbanization and restoration in stream ecosystems

As catchments become increasingly urban, the streams that drain them become increasingly degraded. Urban streams are typically characterized by high-magnitude storm flows, homogeneous habitats, disconnected riparian zones, and elevated nitrogen concentrations. To reverse the degradation of urban water quality, watershed managers and regulators are increasingly turning to stream restoration approaches. By reshaping the channel and reconnecting the surface waters with their riparian zone, practitioners intend to enhance the natural nutrient retention capacity of the restored stream ecosystem. Despite the exponential growth in stream restoration projects and expenditures, there has been no evaluation to date of the efficacy of urban stream restoration projects in enhancing nitrogen retention or in altering the underlying ecosystem metabolism that controls instream nitrogen consumption. In this study, we compared ecosystem metabolism and nitrate uptake kinetics in four stream restoration projects within urban watersheds to ecosystem functions measured in four unrestored urban stream segments and four streams draining minimally impacted forested watersheds in central North Carolina, U.S.A. All 12 sites were surveyed in June through August of 2006 and again in January through March of 2007. We anticipated that urban streams would have enhanced rates of ecosystem metabolism and nitrate uptake relative to forested streams due to the increases in nutrient loads and temperature associated with urbanization, and we predicted that restored streams would have further enhanced rates for these ecosystem functions by virtue of their increased habitat heterogeneity and water residence times. Contrary to our predictions we found that stream metabolism did not differ between stream types in either season and that nitrate uptake kinetics were not different between stream types in the winter. During the summer, restored stream reaches had substantially higher rates of nitrate uptake than unrestored or forested stream reaches; however, we found that variation in stream temperature and canopy cover explained 80% of the variation across streams in nitrate uptake. Because the riparian trees are removed during the first stage of natural channel design projects, the restored streams in this study had significantly less canopy cover and higher summer temperatures than the urban and forested streams with which they were compared.

Effects of elevated nitrogen and temperature on carbon and nitrogen dynamics in Alaskan arctic and boreal soils

[1] Plant productivity in upland tundra and boreal forest is demonstrably limited by nitrogen (N) and indirect evidence from field studies suggests that decomposition by soil microbes may be similarly limited. As climate warms at high latitudes, understanding the response of soil organic matter (SOM) decomposition to increased soil temperature may be crucial for determining the net effect of warming on ecosystem carbon (C) balance because temperature directly affects decomposition but also because it has an indirect effect on C balance via nutrient mineralization. We incubated northern Alaskan soils at two temperatures (5°C and 15°C) and two levels of N addition (with and without) to directly test for N limitation of SOM decomposition and to explore the interaction between temperature and N limitation. Over the entire 924 day incubation of organic and mineral soils from two ecosystem types, we measured microbial respiration; over the initial 90 days of the incubation, we measured microbial biomass N, net N mineralization, and the isotopic signatures (δ13C and Δ14C) of microbial respiration. Across soil layers and ecosystem types, temperature always had a strong positive effect on SOM decomposition rates, whereas N addition had positive, negative, and neutral effects. When C respiration rates were high, the positive N response was generally most strongly expressed, for example, in the organic soils, in the warmer incubation, and at the outset of the experiment. Negative N responses often occurred when C respiration rates were lower, predominantly in mineral soils and at the middle or end of the experiment. In the subset of soil types where we measured the radiocarbon age of respired CO2, increased decomposition was related to increased use of older C. Net N mineralization and nitrification were not affected by temperature, but N addition increased net N immobilization in all soil layers and microbial biomass N in organic layers. Our data support the general idea that at least in these high-latitude organic soils, decomposition of labile carbon can be positively stimulated by added N, whereas decomposition of recalcitrant C is suppressed.

Soil purple phototrophic bacterial diversity under double cropping (rice-wheat) with free-air CO2 enrichment (FACE)

Soil purple phototrophic bacterial (PPB) communities and their responses to elevated atmospheric carbon dioxide (CO 2 ) concentration and nitrogen (N) fertilizer were investigated under a rotation of paddy rice (Oryza sativa L.) and winter wheat (Triticum aestivum L. cv. Yangmai 14) cultivation in a FACE (free-air CO 2 enrichment) system. Community structures and abundances of PPB were determined by denaturing gradient gel electrophoresis (DGGE) and real-time quantitative PCR respectively, targeting the pufM gene, which encodes a protein in the light reaction centre of PPB. Soil PPB communities were more diverse and larger under rice than under wheat cultivation, which may be attributed to the flooding of the paddy field and soil moisture changes. Elevated atmospheric CO 2 concentration significantly increased the abundance and biodiversity of PPB in soils under rice cultivation, while N fertilizer application rate had less effect on the abundance and diversity. Phylogenetic analysis showed that two common dominant DGGE bands belonged to Bradyrhizobium- and Rhodopseudomonas palustris-like PPB in both rice and wheat soils. The results demonstrated a significant shift in soil PPB communities during the rice-wheat rotation, and a strong positive response of PPB communities to an elevated atmospheric CO 2 concentration. Our results also indicated that a diverse and abundant soil PPB community could occur in upland crop fields as well as in aquatic environments and paddy-rice fields. These findings extend our understanding of the ecological significance of PPB in terrestrial soil environments and their responses to future climate change.

Exposure to aphids increases alder growth and nitrogen fixation

Plants can respond to herbivore damage by mounting a resistance response or by compensating for lost fitness. Both plant nutrition and interactions with soil microbes can affect these responses. It has been shown that resistance responses can occur before plants have been attacked by herbivores. Here we show that a tolerance type of response can occur when plants are exposed to, but not fed on by, herbivores. Alnus viridis (Chaix) DC. spp. crispa (Ait.) Turrill were grown in sealed containers under positive air pressure with either 0.5 mmol·L–1 or 2.0 mmol·L–1 nitrate and either inoculated or not inoculated with Frankia , their nitrogen-fixing symbiont. Plants were then exposed to the genus-specific aphid Prociphilus tessallatus Fitch, which failed to establish feeding colonies. Exposure to aphids, formation of nitrogen-fixing nodules, and elevated soil nitrogen levels all significantly increased plant yield with no interaction among these factors. A combination of high soil nitrogen, nodulation, and exposure to aphids resulted in the lowest plant root:shoot ratio. Plants that were grown with low nitrogen and were exposed to aphids showed increased nitrogen-fixing activity within a day of being exposed. These results provide further evidence to support the observation that plants can respond to cues from other organisms prior to receiving herbivore damage.