Elevated Nitrogen Research Articles

Nitrogen mineralization in O horizon soils during 27years of nitrogen enrichment at the Bear Brook Watershed in Maine, USA.

Chronic elevated nitrogen (N) deposition has altered the N status of temperate forests, with significant implications for ecosystem function. The Bear Brook Watershed in Maine (BBWM) is a whole paired watershed manipulation experiment established to study the effects of N and sulfur (S) deposition on ecosystem function. N was added bimonthly as (NH4)2SO4 to one watershed from 1989 to 2016, and research at the site has studied the evolution of ecosystem response to the treatment through time. Here, we synthesize results from 27years of research at the site and describe the temporal trend of N availability and N mineralization at BBWM in response to chronic N deposition. Our findings suggest that there was a delayed response in soil N dynamics, since labile soil N concentrations did not show increases in the treated watershed (West Bear, WB) compared to the reference watershed (East Bear, EB) until after the first 4years of treatment. Labile N became increasingly available in WB through time, and after 25years of manipulations, treated soils had 10× more extractable ammonium than EB soils. The WB soils had 200× more extractable nitrate than EB soils, driven by both, high nitrate concentrations in WB and low nitrate concentrations in EB. Nitrification rates increased in WB soils and accounted for ~ 50% of net N mineralization, compared to ~ 5% in EB soils. The study provides evidence of the decadal evolution in soil function at BBWM and illustrates the importance of long-term data to capture ecosystem response to chronic disturbance.

Effects of elevated nitrogen application on nitrogen partitioning, plant growth, grain quality and key genes involved in glutamate biosynthesis among three rice genotypes

Effects of elevated nitrogen application on nitrogen partitioning, plant growth, grain quality and key genes involved in glutamate biosynthesis among three rice genotypes

Six underlying health conditions strongly influence mortality based on pneumonia severity in an ageing population of Japan: a prospective cohort study

BackgroundMortality prediction of pneumonia by severity scores in patients with multiple underlying health conditions has not fully been investigated. This prospective cohort study is to identify mortality-associated underlying health conditions and to analyse their influence on severity-based pneumonia mortality prediction.MethodsAdult patients with community-acquired pneumonia or healthcare-associated pneumonia (HCAP) who visited four community hospitals between September 2011 and January 2013 were enrolled. Candidate underlying health conditions, including demographic and clinical characteristics, were incorporated into the logistic regression models, along with CURB (confusion, elevated urea nitrogen, tachypnoea, and hypotension) score as a measure of disease severity. The areas under the receiver operating characteristic curves (AUROC) of the predictive index based on significant underlying health conditions was compared to that of CURB65 (CURB and age ≥ 65) score or Pneumonia severity index (PSI). Mortality association between disease severity and the number of underlying health conditions was analysed.ResultsIn total 1772 patients were eligible for analysis, of which 140 (7.9%) died within 30 days. Six underlying health conditions were independently associated: home care (adjusted odds ratio, 5.84; 95% confidence interval, CI, 2.28–14.99), recent hospitalization (2.21; 1.36–3.60), age ≥ 85 years (2.15; 1.08–4.28), low body mass index (1.99, 1.25–3.16), neoplastic disease (1.82; 1.17–2.85), and male gender (1.78; 1.16–2.75). The predictive index based on these conditions alone had a significantly or marginally higher AUROC than that based on CURB65 score (0.78 vs 0.66, p = 0.02) or PSI (0.78 vs 0.71, p = 0.05), respectively. Compared to this index, the AUROC of the total score consisting of six underlying health conditions and CURB score (range 0–10) did not improve mortality predictions (p = 0.3). In patients with one or less underlying health conditions, the mortality was discretely associated with severe pneumonia (CURB65 ≥ 3) (risk ratio: 7.24, 95%CI: 3.08–25.13), whereas in patients with 2 or more underlying health conditions, the mortality association with severe pneumonia was not detected (risk ratio: 1.53, 95% CI: 0.94–2.50).ConclusionsMortality prediction based on pneumonia severity scores is highly influenced by the accumulating number of underlying health conditions in an ageing society. The validation using a different cohort is necessary to generalise the conclusion.

Long‐term nitrogen fertilization decreases bacterial diversity and favors the growth of Actinobacteria and Proteobacteria in agro‐ecosystems across the globe

Long-term elevated nitrogen (N) input from anthropogenic sources may cause soil acidification and decrease crop yield, yet the response of the belowground microbial community to long-term N input alone or in combination with phosphorus (P) and potassium (K) is poorly understood. We explored the effect of long-term N and NPK fertilization on soil bacterial diversity and community composition using meta-analysis of a global dataset. Nitrogen fertilization decreased soil pH, and increased soil organic carbon (C) and available N contents. Bacterial taxonomic diversity was decreased by N fertilization alone, but was increased by NPK fertilization. The effect of N fertilization on bacterial diversity varied with soil texture and water management, but was independent of crop type or N application rate. Changes in bacterial diversity were positively related to both soil pH and organic C content under N fertilization alone, but only to soil organic C under NPK fertilization. Microbial biomass C decreased with decreasing bacterial diversity under long-term N fertilization. Nitrogen fertilization increased the relative abundance of Proteobacteria and Actinobacteria, but reduced the abundance of Acidobacteria, consistent with the general life history strategy theory for bacteria. The positive correlation between N application rate and the relative abundance of Actinobacteria indicates that increased N availability favored the growth of Actinobacteria. This first global analysis of long-term N and NPK fertilization that differentially affects bacterial diversity and community composition provides a reference for nutrient management strategies for maintaining belowground microbial diversity in agro-ecosystems worldwide.

Nitrogen concentration in moss compared with N load in precipitation and with total N deposition in Switzerland

Intensification of farming and an increase in motorised traffic have led to elevated nitrogen (N) emissions and thus to eutrophication of the environment, which threatens the nutrient balance in ecosystems. Earlier studies have demonstrated the suitability of mosses as biomonitors for measuring N deposition by comparing the N concentration in moss with that in precipitation. In our study however, we extended the comparison to the dry deposition of gases (nitrogen dioxide, nitric acid, ammonia) and aerosols (nitrate, ammonium), which, together with the N in precipitation, represent the main contributions to total N deposition. The aim of including several N compounds was to see whether the correlation with the N concentration in moss could be improved.We determined total N input from the atmosphere to the ecosystem at 24 sites in Switzerland and compared this value to the N concentration in two moss species collected <1000 m from these sites. Including the gases and aerosols improved the correlation between the N concentration in moss and N deposition. Ammonia was found to be the most important of the additionally included compounds at these sites. Especially at sites with a relatively high ammonia concentration in the air, the inclusion of ammonia improved the correlation of the comparison. We also demonstrate that the particular moss species tested had no influence on the correlation between N in moss and total N deposition. Our data supports the suitability of mosses as biomonitors for estimating N input into ecosystems.

Millet manuring as a driving force for the Late Neolithic agricultural expansion of north China

Research in to the nature of Neolithic agriculture in China is often focused on topics such as the domestication and spread of cereal crops and the reconstruction of human and animal diets in the past. Field management practices, such as organic manuring, have not been systematically investigated in Chinese archaeology. Here we present an isotopic dataset for archaeological foxtail millet (Setaria italica) and common millet (Panicum miliaceum) grains as well as associated faunal remains (both domesticated and wild) from seven sites in the Baishui Valley of north China, in order to find direct evidence of organic manuring during the Late Neolithic period. The elevated nitrogen isotope values of the millet grains (5500-3500 cal BP) in comparison with the estimated local vegetation indicates that millets were organically manured by animal dung, mostly likely originating from domestic pigs. Considering the low nitrogen contents of loess soils and their unsuitability for intensive cultivation, this organic manuring by animal dung would have played a key role in maintaining soil productivity and crop yield, which was necessary to support the demands of agriculture and cultural expansion during the Late Neolithic on the Loess Plateau of China.

Elevated nitrogen metabolism and nitric oxide production are involved in Arabidopsis resistance to acid rain

Acid rain (AR) can induce great damages to plants and could be classified into different types according to the different SO42-/NO3− ratio. However, the mechanism of plants’ responding to different types of AR has not been elucidated clearly. Here, we found that nitric-rich simulated AR (N-SiAR) induced less leaves injury as lower necrosis percentage, better physiological parameters and reduced oxidative damage in the leaves of N-SiAR treated Arabidopsis thaliana compared with sulfate and nitrate mixed (SN-SiAR) or sulfuric-rich (S-SiAR) simulated AR treated ones. Of these three types of SiAR, N-SiAR treated Arabidopsis maintained the highest of nitrogen (N) content, nitrate reductase (NR) and nitrite reductase (NiR) activity as well as N metabolism related genes expression level. Nitric oxide (NO) content showed that N-SiAR treated seedlings had a higher NO level compared to SN-SiAR or S-SiAR treated ones. A series of NO production and elimination related reagents and three NO production-related mutants were used to further confirm the role of NO in regulating acid rain resistance in N-SiAR treated Arabidopsis seedlings. Taken together, we concluded that an elevated N metabolism and enhanced NO production are involved in the tolerance to different types of AR in Arabidopsis.

Atmospheric nitrogen deposition in terrestrial ecosystems: Its impact on plant communities and consequences across trophic levels

Abstract The global nitrogen cycle has been greatly perturbed by human activities resulting in elevated nitrogen deposition in many parts of the world. The threat nitrogen deposition poses to ecosystem function and biodiversity is increasingly recognised. In terrestrial systems, impacts on the plant community are mainly through eutrophication and soil acidification. Interactions with secondary environmental drivers such as extreme weather and disease are also key mechanisms. Impacts on consumers can be caused by changes in the quality or quantity of food as a result of changes in food plant chemistry or species composition, changes in vegetation structure leading to a change in the availability of prey species, nesting sites or cooled microclimates or changes in the phenology of plants leading to causing phenological asynchrony. Primary consumers have received considerably less research attention than plants but negative impacts have been observed for both folivorous insects and pollinators. Mammal herbivores have received little research attention. New analysis of changes in plant traits along a gradient of nitrogen deposition in the UK shows that plants pollinated by large bees were negatively associated with N deposition whilst low pH was associated with lower nectar production, reduced occurrence of plants pollinated by long‐tongued insects and a reduction in plants with larger floral units. Very few studies have investigated the effects on secondary consumers, but those that have suggest that there are likely to be negative impacts. This review identifies considerable knowledge gaps in the impacts of N deposition on higher tropic levels and highlights that for many groups, knowledge of N deposition impacts is patchy at best. Evidence that has been collected suggests that there are likely to be impacts on primary and secondary consumers making this a priority area for investigation.

Trace organic contaminant (TOrC) mixtures in Minnesota littoral zones: Effects of on-site wastewater treatment system (OWTS) proximity and biological impact

On-site wastewater treatment systems (OWTSs) are an international wastewater management strategy for rural and semi-rural communities without access to centralized sewage treatment. These systems are a suspected source of trace organic contaminants (TOrCs) that may be responsible for endocrine disrupting effects to resident fish species in Minnesota Lakes. This study assessed localized porewater concentrations of TOrCs in near-shore environments across five Minnesota Lakes. Sampling sites were designated as either likely (HOME) or unlikely (REF) to receive OWTS discharges based on their proximity to shoreline households. Sampling sites also served as sunfish spawning habitats concurrently studied for biological impacts to resident adult males. Two-group hypothesis tests demonstrated significantly (p = .02) higher total TOrC concentrations in HOME (Mean = 841 ng/L) versus REF (Mean = 222 ng/L) sites. HOME sites also contained a wider suite of TOrC detections relative to REF sites. The distance to the nearest household (most proximal distance; MPD) negatively correlated (r = −0.62) with total TOrC concentrations. However, 2,4-D and DEET were major contributors to these total concentrations, suggesting that anthropogenic influence from households may not be exclusively attributed to OWTS discharges. Further, TOrC presence and elevated nitrogen concentrations in REF site porewater suggest additional, non-household TOrC discharges to these lakes. Significantly higher blood concentrations of vitellogenin (p = .03) and 11-ketotestosterone (p = .01) were observed in adult male sunfish captured from HOME versus REF sites. Comparisons between chemical and biological data indicate enhanced bioactive effects of co-contaminants. The findings from this study demonstrate multiple diffuse transport pathways contribute to the presence of biologically active TOrC mixtures in Minnesota Lakes, and mitigation efforts should consider minimizing residential inputs of chemicals associated with both outdoor and OWTS activity.

Elevated nitrogen allows the weak invasive plant Galinsoga quadriradiata to become more vigorous with respect to inter-specific competition

Elevated nitrogen associated with global change is believed to promote the invasion of many vigorous exotic plants. However, it is unclear how a weak exotic plant will respond to elevated nitrogen in the future. In this study, the competitive outcome of a weak invasive plant (Galinsoga quadriradiata) and two non-invasive plants was detected. The plants were subjected to 3 types of culture (mixed, monoculture or one-plant), 2 levels of nitrogen (ambient or elevated at a rate of 2 g m−2 yr−1) and 2 levels of light (65% shade or full sunlight). The results showed that elevated nitrogen significantly promoted the growth of both the weak invader and the non-invasive plants in one-plant pots; however, growth promotion was not observed for the non-invasive species in the mixed culture pots. The presence of G. quadriradiata significantly inhibited the growth of the non-invasive plants, and a decreased negative species interaction was detected as a result of elevated nitrogen. Our results suggest that competitive interactions between G. quadriradiata and the non-invasive plants were altered by elevated nitrogen. It provides exceptional evidence that an initially weak invasive plant can become an aggressive invader through elevated nitrogen deposition.

Responses of soil phosphorus availability to nitrogen addition in a legume and a non-legume plantation

Elevated nitrogen (N) deposition can alter the composition and availability of soil phosphorus (P) and thus affect long-term plant growth. However, it remains elusive whether this effect differs between legume and non-legume forest ecosystems, which have distinctly different abilities to use N and P. In this study, soil P fractions were measured in a legume (Acacia auriculiformis) and a non-legume (Eucalyptus urophylla) plantation in subtropical China, after four-years of N addition. Results showed that the concentrations of soil total P, total Po, and NaOH-Pi (Po and Pi are organic and inorganic P, respectively) were significantly higher, but soil NaHCO3-Po was significantly lower, in the legume plantation compared to the non-legume plantation. Nitrogen addition significantly decreased soil labile P fractions (i.e. NaHCO3-Pi and NaHCO3-Po) in the legume plantation, but they did not change in the non-legume plantation. In contrast, intermediate P fractions (i.e. NaOH-Pi and NaOH-Po) significantly increased with N addition in the legume plantation, but only NaOH-Po experienced a small increase in the non-legume plantation. The recalcitrant P fractions were not significantly influenced by N addition in either plantation. Due to the greater decrease in labile P in the legume plantation, our results suggest that N deposition may lead to greater P limitation in legume plantations compared to non-legume plantations.

Resource Availability Drives Responses of Soil Microbial Communities to Short-term Precipitation and Nitrogen Addition in a Desert Shrubland.

Desert microbes are expected to be substantially sensitive to global environmental changes, such as precipitation changes and elevated nitrogen deposition. However, the effects of precipitation changes and nitrogen enrichment on their diversity and community composition remain poorly understood. We conducted a field experiment over 2 years with multi-level precipitation and nitrogen addition in a desert shrubland of northern China, to examine the responses of soil bacteria and fungi in terms of diversity and community composition and to explore the roles of plant and soil factors in structuring microbial communities. Water addition significantly increased soil bacterial diversity and altered the community composition by increasing the relative abundances of stress-tolerant (dormant) taxa (e.g., Acidobacteria and Planctomycetes); however, nitrogen addition had no substantial effects. Increased precipitation and nitrogen did not impact soil fungal diversity, but significantly shifted the fungal community composition. Specifically, water addition reduced the relative abundances of drought-tolerant taxa (e.g., the orders Pezizales, Verrucariales, and Agaricales), whereas nitrogen enrichment decreased those of oligotrophic taxa (e.g., the orders Agaricales and Sordariales). Shifts in microbial community composition under water and nitrogen addition occurred primarily through changing resource availability rather than plant community. Our results suggest that water and nitrogen addition affected desert microbes in different ways, with watering shifting stress-tolerant traits and fertilization altering copiotrophic/oligotrophic traits of the microbial communities. These findings highlight the importance of resource availability in driving the desert microbial responses to short-term environmental changes.

Different response of CO2 and N2O fluxes to N deposition with seasons in a temperate forest in northeastern China

Elevated nitrogen (N) deposition had changed terrestrial carbon (C) and N cycling during the last several decades. Greenhouse gas (GHG) emission, serving as an important process of global C and N cyclings, was susceptible to elevated N deposition. However, the lack of GHG measurements in the spring freezing-thawing cycles (FTC) period and inconstant results of GHG emissions in growing season result in the uncertainties of GHG emission responding to elevated N deposition. Static chambers were used to monitor CO2 and N2O fluxes in growing season and spring FTC period in a simulated N deposition forest in northeastern China, which had been fertilized with different N amounts for 6 years. The temperate forest soil that subjected to N fertilizer was a net source of CO2 and N2O. The intensely fluctuant fluxes during the spring FTC period were due to stimulation of microbial activities induced by sharply changed soil temperature and moisture. The physical release of trapped gases beneath ice layer during the whole winter also contributed to the drastic increase of fluxes. No effect of N addition on the CO2 efflux was detected in the growing season or spring FTC period, indicating that N was not limited for microbial respiration. In contrast, N addition decreased the N2O flux by 72.8~85.7% in the growing season whereas increased by 11.2~17.8 times in the spring FTC period, which could be explained by the shift in structure of microbial community. Over the entire dataset, soil temperature and moisture could explain about 60 and 30% of seasonal variations of CO2 or N2O flux, respectively. By fitting the RS–T5–W5 model to separate season, poorer fit was achieved in the spring FTC period compared with growing season, suggesting a weakening effect of soil microclimates on CO2 efflux. These results indicate that long-term elevated N deposition affected CO2 and N2O fluxes differently with seasons in a temperate forest soil, which will help to predict soil GHG flux under the conditions of global climate change.

Uraemic optic neuropathy – a rare presentation of uraemia

Uraemic optic neuropathy (UON) is an acute but reversible loss of vision in patients with end-stage renal failure. Hypoperfusion of the posterior ciliary arteries, the major blood supply to the optic nerve head, results in ischaemic injury to the nerve. Anaemia, hypertension and elevated nitrogen urea level have been identified as factors contributing to optic nerve neuropathy. Toxic uraemic metabolites affect nerve conduction and their removal with dialysis results in improved vision. Optic neuropathy is a rare complication of uraemia in children. This is the first case of optic neuropathy related to uraemia in a child with end-stage renal disease in Nigeria.

Threshold responses of soil organic carbon concentration and composition to multi-level nitrogen addition in a temperate needle-broadleaved forest

Responses of soil organic carbon (SOC) cycling and C budget in forest ecosystems to elevated nitrogen (N) deposition are divergent. Little is known about the N critical loads for the shift between gain and loss of SOC storage in the old-growth temperate forest of Northeast China. The objective of this study was to investigate the nonlinear responses of SOC concentration and composition to multiple rates of N addition, as well as the microbial mechanisms responsible for SOC alteration under N enrichment. Nine rates of urea addition (0, 10, 20, 40, 60, 80, 100, 120, 140 kg N ha−1 year−1) with 4 replicates for each treatment were conducted. Soil samples in the 0–10 cm mineral layer were taken after 3 years of N fertilization. Soil aggregate size distribution and SOC physical fractionation were performed to examine SOC dynamics. Phospholipid fatty acid (PLFA) technique was used to measure the abundance and structure of microbial community. Three years of N addition led to significant increases in the concentrations of soil particulate organic C and aggregate-associated organic C fractions only. The responses of total N and each labile SOC fraction to the rates of N addition followed Gaussian equations, with the N critical loads being estimated to be between 80 and 100 kg N ha−1 year−1. The change in SOC concentration (ΔSOC) was positively correlated with the changes in aggregate associated OC (r2 > 0.80) and POC concentrations (r2 > 0.50). Significant correlations among the concentrations of labile SOC fractions, the percentages of soil aggregates, and the abundances of microbial PLFAs were observed, which implies a close linkage between microbial community structure and SOC accumulation and stability. Our results suggest that increase in soil moisture and shift of microbial community structure could control the critical N load for the switch between C accumulation and loss. The current N deposition rate (~ 11 kg N ha−1 year−1) to the northeast China’s forests is favorable for soil C accumulation over the short term.

Effects of nitrogen addition on activities of soil nitrogen acquisition enzymes：A meta-analysis

It has been suggested that elevated nitrogen (N) deposition may increase soil N mineralization in N-limited ecosystems, but the underlying mechanisms have been not adequately explored. Soil N-acquisition enzymes play important roles on organic N mineralization. Thus, their responses to N deposition will be crucial for explaining the above phenomenon. Here, we conducted a meta-analysis from 64 studies to synthesize the responses of soil N-acquisition enzyme activities to N addition. Results showed that N addition significantly increased activities of N-acetylglucosaminidase and urease by 5.5% and 11.6%, respectively. However, N addition had negative or negligible effects on activities of protein-depolymerization enzymes, with no response for non-specific protease and leucine aminopeptidase but a significant decrease of 33.0% for glycine aminopeptidase. Because protein comprises more than 60% of the N in plant and microbial cells, and the protein depolymerization is an important rate-limiting step of organic N mineralization, the suppressed protein depolymerization indicates either that the changes to microbial activity may be not a dominant mechanism for the increased N mineralization in N-limited ecosystems with N addition, or that the increased N mineralization may be overvalued in the previous studies.

Decreased soil organic P fraction associated with ectomycorrhizal fungal activity to meet increased P demand under N application in a subtropical forest ecosystem

Phosphorus (P) is an essential but limited nutrient in highly weathered subtropical or tropical forest soils; however, increased P demands owing to elevated nitrogen (N) deposition might further exacerbate the P limitation in these regions. How soil P fractions respond to N enrichment and the underlying mechanisms are not yet understood completely. To address this issue, we conducted an experiment with 3 years of N addition to a Castanopsis carlesii natural forest in Fujian, China; the soil P fractions, microbial biomass, enzyme activities, and phospholipid fatty acid (PLFA) levels were determined. We found that N addition did not significantly alter the microbial biomass C (MBC) and microbial biomass N (MBN), but significantly decreased microbial biomass P (MBP); further, the ratios of MBC:MBP and MBN:MBP increased significantly after N application, indicating that N enrichment enhanced the P demands of plants and soil microorganisms. Moreover, neither resin-P nor NaHCO3-P (extracted by resin and NaHCO3, respectively) was decreased, whereas organic P extracted by NaOH (NaOH-Po) was decreased after N addition, implying that NaOH-Po is a potentially significant source of available P. In particular, appositive correlation between abundance of ectomycorrhizal fungi and NaOH-Po and significant relationships between acid phosphomonoesterase activity (ACP) and labile P contents were noted, suggesting that ectomycorrhizal fungi can play a major role in the depletion of soil organic P, and these enzyme activities can mobilize the unavailable P to labile P in the investigated forest. Taken together, our results revealed a novel mechanism for the maintenance of soil labile P in a subtropical natural forest under chronic stress of N deposition.

Global-scale impacts of nitrogen deposition on tree carbon sequestration in tropical, temperate, and boreal forests: A meta-analysis.

Elevated nitrogen (N) deposition may increase net primary productivity in N-limited terrestrial ecosystems and thus enhance the terrestrial carbon (C) sink. To assess the magnitude of this N-induced C sink, we performed a meta-analysis on data from forest fertilization experiments to estimate N-induced C sequestration in aboveground tree woody biomass, a stable C pool with long turnover times. Our results show that boreal and temperate forests responded strongly to N addition and sequestered on average an additional 14 and 13kg C per kg N in aboveground woody biomass, respectively. Tropical forests, however, did not respond significantly to N addition. The common hypothesis that tropical forests do not respond to N because they are phosphorus-limited could not be confirmed, as we found no significant response to phosphorus addition in tropical forests. Across climate zones, we found that young forests responded more strongly to N addition, which is important as many previous meta-analyses of N addition experiments rely heavily on data from experiments on seedlings and young trees. Furthermore, the C-N response (defined as additional mass unit of C sequestered per additional mass unit of N addition) was affected by forest productivity, experimental N addition rate, and rate of ambient N deposition. The estimated C-N responses from our meta-analysis were generally lower that those derived with stoichiometric scaling, dynamic global vegetation models, and forest growth inventories along N deposition gradients. We estimated N-induced global C sequestration in tree aboveground woody biomass by multiplying the C-N responses obtained from the meta-analysis with N deposition estimates per biome. We thus derived an N-induced global C sink of about 177 (112-243) Tg C/year in aboveground and belowground woody biomass, which would account for about 12% of the forest biomass C sink (1,400 Tg C/year).

Algal nitrogen and phosphorus content drive inter- and intraspecific differences in herbivore grazing on a Caribbean reef

Identifying how herbivorous reef fish select resources is an important aspect of understanding how coral reef communities are structured. Ecological stoichiometry, built around the mass balance of elements, necessitates that to build tissue, maintain growth, and reproduce, animals must maintain a homeostatic balance of elements within their body. Nitrogen (N) and phosphorus (P) are abundant in animal tissue relative to most plants and algae. Accordingly, N and P are often limiting for herbivores, which should therefore focus their foraging efforts on resources rich in these elements. Here, we explore how variation in N and P concentrations in algae impact herbivore resource selectivity on a coral reef. We conducted feeding assays using a brown alga from the genus Sargassum grown under elevated nitrogen (N), phosphorus (P) or both (N+P). Overall bite rates were higher on N enriched, but not P enriched, assays but individual foraging efforts varied among species and fish size. Individual Sparisoma aurofrenatum increased the number of bites taken per feeding foray on N enriched algae while Acanthurus coeruleus and Sparisoma rubripinne fed more on P enriched assays. Sparisoma chrysopterum increased their foraging effort on both N and P enriched assays. However, the influence of algal nutrient content on foraging effort declined with increasing fish size, such that full grown fishes did not respond to assay nutrient content. Overall, preference for enriched algae resulted in ~20% more consumption of N and N+P enriched algae relative to unenriched controls. Our study is the first to link the elemental nutrient content of algae to species-specific feeding decisions of reef herbivores and demonstrates the utility of ecological stoichiometry to explore herbivory on coral reefs.

Interplay of water quality and vegetation in restored wetland plant assemblages from an agricultural landscape

Water quality degradation from excessive fertilizer use and runoff is a worldwide problem. While this degradation impacts wetlands, these systems can also be a vehicle for water quality improvement. Restoration of wetlands in agricultural landscapes has recently increased, but little work has evaluated the relationship of plant assemblages and water quality parameters in restored, non-treatment wetlands. This study examines the impact of self-designed wetland plant assemblages on nitrogen and sediment dynamics. Thirty mesocosms were seeded with soil from restored wetlands and allowed to develop from the seed bank to emergent assemblages. During the 2015 growing season (seven to nine months after establishment), these assemblages were exposed to treatment loads of nitrogen and sediment, common stressors to wetlands in agricultural landscapes. Water samples were taken up to five days post-treatment in July and September to quantify interactions between the stressors and plant assemblages. Analyses showed plant assemblage identify was not structured by treatment, but by the site of soil origin. Treatment removal rates were influenced by total amount of the stressor present, with nitrogen removal rates being higher, in relative terms, in low nitrogen amended treatments. Additionally, plant quality, not quantity, was linked to nitrogen and sediment loss rates, and over time, elevated nitrogen and sediment loads were associated with decreased plant assemblage quality. This study demonstrates the ability of plants from restored wetlands to affect nutrient and sediment dynamics, with three significantly differing plant assemblages all exhibiting substantial nutrient and sediment reduction capacity. Nevertheless, we also found that in a relatively short time (seven to nine months) common stressors in agricultural settings can significantly impact wetland plant assemblage quality, and that this may be linked to a reduced capacity for nutrient and sediment removal.