Effects Of Nutrient Addition Research Articles

A long‐term study of burning effects on a plant pathogen in tallgrass prairie

Tallgrass prairie species have evolved with regular exposure to fire. However, burning has been used as a management tool for reducing plant disease in agricultural systems, posing the question of how plant pathogens of tallgrass prairie would be affected by burning. The rust fungus Puccinia dioicae, infecting Erigeron strigosus (Asteraceae), was studied for 8 years in long‐term experiments to evaluate the effects of burning in native tallgrass prairie. This experiment also allowed evaluation of the effects of nutrient additions, although E. strigosus was rare in the plots with added nutrients in most years. Burning reduced rust severity in most years, but effects from additions of nutrients were rarely observed. There was high interannual variation in rust severity within a location, suggesting that weather may be the most important of these three abiotic factors in determining infection. An analysis of weather variables associated with disease severity found that solar radiation in the month prior to sampling was associated with severity in unburned plots; temperature approximately 2 months prior to sampling was also associated with severity in burned plots. High interannual variation also suggests that the effects of this pathogen on its host would be sporadic and difficult to study in short‐term experiments.

Carry over effects of nutrient addition on the recovery of an invasive seaweed from the winter die-back

Nutrient enrichment of coastal waters can enhance the invasibility and regrowth of non-native species. The invasive alga Caulerpa cylindracea has two distinct phases: a well-studied fast-growing summer phase, and a winter latent phase. To investigate the effects of nutrient enrichment on the regrowth of the seaweed after the winter resting-phase, a manipulative experiment was carried out in intertidal rockpools in the North-western Mediterranean. Nutrients were supplied under different temporal regimes: press (constant release from January to May), winter pulse (January to March) and spring pulse (March to May). Independently from the temporal characteristics of their addition, nutrients accelerated the re-growth of C. cylindracea after the winter die-back, resulting in increased percentage covers at the peak of the growing season. Nutrient addition did not influence the number and length of fronds and the biomass. Native components of the algal community did not respond to nutrient additions. Our results show that nutrient supply can favour the spread of C. cylindracea even when occurring at a time of the year at which the seaweed is not actively growing.

Microbial Nursery Production of High-Quality Biological Soil Crust Biomass for Restoration of Degraded Dryland Soils.

Biological soil crusts (biocrusts) are slow-growing, phototroph-based microbial assemblages that develop on the topsoils of drylands. Biocrusts help maintain soil fertility and reduce erosion. Because their loss through human activities has negative ecological and environmental health consequences, biocrust restoration is of interest. Active soil inoculation with biocrust microorganisms can be an important tool in this endeavor. We present a culture-independent, two-step process to grow multispecies biocrusts in open greenhouse nursery facilities, based on the inoculation of local soils with local biocrust remnants and incubation under seminatural conditions that maintain the essence of the habitat but lessen its harshness. In each of four U.S. Southwest sites, we tested and deployed combinations of factors that maximized growth (gauged as chlorophyll a content) while minimizing microbial community shifts (assessed by 16S rRNA sequencing and bioinformatics), particularly for crust-forming cyanobacteria. Generally, doubling the frequency of natural wetting events, a 60% reduction in sunlight, and inoculation by slurry were optimal. Nutrient addition effects were site specific. In 4 months, our approach yielded crusts of high inoculum quality reared on local soil exposed to locally matched climates, acclimated to desiccation, and containing communities minimally shifted in composition from local ones. Our inoculum contained abundant crust-forming cyanobacteria and no significant numbers of allochthonous phototrophs, and it was sufficient to treat ca. 6,000 m2 of degraded dryland soils at 1 to 5% of the typical crust biomass concentration, having started from a natural crust remnant as small as 6 to 30 cm2 IMPORTANCE: Soil surface crusts can protect dryland soils from erosion, but they are often negatively impacted by human activities. Their degradation causes a loss of fertility, increased production of fugitive dust and intensity of dust storms with associated traffic problems, and provokes general public health hazards. Our results constitute an advance in the quest to actively restore biological soil covers by providing a means to obtain high-quality inoculum within a reasonable time (a few months), thereby allowing land managers to recover essential, but damaged, ecosystem services in a sustainable, self-perpetuating way as provided by biocrust communities.

Joint effects of nutrient addition and enemy exclusion on exotic plant success.

Worldwide, ecosystems are increasingly dominated by exotic plant species, a shift hypothesized to result from numerous ecological factors. Two of these, increased resource availability and enemy release, may act in concert to increase exotic success in plant communities (Resource-Enemy Release Hypothesis, R-ERH). To test this, we manipulated the availability of soil nutrients and access of vertebrate herbivores, insect herbivores, and fungal pathogens to intact grassland communities containing both native and exotic species. Our results supported both conditions necessary for R-ERH. First, exotics were less damaged than natives, experiencing less foliar damage (insect herbivory and fungal disease) than native species, particularly in communities where soil nutrients were added. Second, fertilization increased foliar damage on native species, but not exotic species. As well as fulfilling both conditions for R-ERH, these results demonstrate the importance of considering the effects of resource availability when testing for enemy release. When both conditions are fulfilled, R-ERH predicts that increasing resource availability will increase exotic abundance only in the presence of enemies. Our results fully supported this prediction for vertebrate herbivores: fertilization increased exotic cover only in communities exposed to vertebrate herbivores. Additionally, the prediction was partially supported for insect herbivores and fungal pathogens, excluding these enemies reduced exotic cover as predicted, but inconsistent with R-ERH, this effect occurred only in unfertilized communities. These results highlight the need to consider the influence of multiple enemy guilds on community processes like exotic plant invasions. Moreover, this study experimentally demonstrates that resource availability and natural enemies can jointly influence exotic success in plant communities.

Effects of soil nitrogen availability and native grass diversity on exotic forb dominance.

Exotic plants are often most successful in high resource environments. By drawing down available resources, species-rich communities may be able to reduce exotic success when resource supply is elevated. We tested the prediction that exotic success would be greatest in species-poor communities when nitrogen availability is high. We also tested two underlying assumptions of this prediction: species-rich communities draw down soil nitrogen availability more than species-poor communities following fertilization and exotic success increases when soil nitrogen availability is high. In a restored grassland where native grass diversity was manipulated (one, three, or five-species) sevenyears earlier to form a gradient in species richness, we manipulated nitrogen availability directly via fertilization, and indirectly via burning. We then examined the success of the exotic forb Galium verum L. Contrary to our prediction, diversity and nutrient treatments did not jointly influence exotic success. Instead, one-time fertilization increased exotic biomass in the first year of the study. This likely occurred because the effect of nutrient treatments on nitrogen availability was independent of diversity treatment. Thus, we found no evidence that species-rich communities are better able to reduce exotic biomass when nitrogen is added than are species-poor communities. This suggests that in some systems, the effects of increasing species richness can be overwhelmed by the effects of nutrient addition that promote exotic success.

Effects of nutrient addition and soil drainage on germination of N-fixing and non-N-fixing tropical dry forest tree species

To develop generalised predictions regarding the effects of atmospheric nitrogen (N) and phosphorus (P) deposition on vegetation communities, it is necessary to account for the impacts of increased nutrient availability on the early life history stages of plants. Additionally, it is important to determine if these responses (a) differ between plant functional groups and (b) are modulated by soil drainage, which may affect the persistence of added nutrients. We experimentally assessed seed germination responses (germination proportion and germination energy, i.e. time to germination) of commonly occurring N-fixing and non-N-fixing tropical dry forest tree species found in India to simulated N and P deposition in well-drained soils, as well as soils with impeded drainage. When soils were not allowed to drain, germination proportion declined with nutrient addition, while germination energy remained unchanged. Stronger declines in germination proportion were observed for N-fixing species. In free-draining soils, nutrient addition did not affect germination proportion in either functional group. However, we detected a trend of delayed germination with nutrient addition, especially in N-fixers. Our results suggest that nutrient deposition can lead to potential shifts in functional dominance and tree community composition of tropical dry forests in the long term through its effects on early life stages of trees, although the mechanisms underlying the observed germination responses remain unclear. Further, such effects are likely to be spatially variable across the geographic range in which tropical dry forests occur depending on soil drainage properties.

Bioassay analysis of nutrient and Artemia franciscana effects on trophic interactions in the Great Salt Lake, USA

14-day microcosm experiments demonstrated the strong interactions between bottom–up and top–down effects of nutrient addition (control, nitrogen, phosphorus, nitrogen + phosphorus) and Artemia franciscana grazing on algae in Great Salt Lake water from Gilbert Bay. Nitrogen addition increased phytoplankton chlorophyll concentrations, while phosphorus addition had no stimulatory effect. A combined N + P treatment was synergistic, increasing both phytoplankton and periphyton >10-fold above controls. Our results suggest that phytoplankton were primarily limited by nitrogen and secondarily limited by phosphorus and that periphyton was colimited by nitrogen and phosphorus. The grazing effect increased as A. franciscana grew from nauplii to adults and by the final day, A. franciscana had markedly reduced both phytoplankton and periphyton abundance in the Control, +N, and +P treatments. Grazing also significantly reduced periphyton in the N + P treatments. Due to high phytoplankton growth rates in the N + P treatment, A. franciscana grazing did not significantly reduce chlorophyll concentrations during the bioassay. However, A. franciscana in the N + P treatment was significantly larger and had greater reproductive output than in the controls, suggesting that the following generation might have exerted greater grazing pressure.

Effects of nutrient addition, recovery thereafter and the role of macrophytes in nutrient dynamics of a Mediterranean shallow lake: a mesocosm experiment

Eutrophication in coastal wetlands has increased greatly because of human impact, reducing water quality and affecting aquatic plants. The aim of the present study was to investigate the eutrophication of Mediterranean wetlands and their possible recovery. An in situ mesocosm nutrient enrichment experiment was performed in a coastal shallow lake from eastern Spain, holding dense Chara meadows. In this lake, 36 mesocosms were installed and fertilised weekly with four levels of N and P addition over an 8-week period from June to August. Mesocosms fertilised with the two higher levels (N:P ≥5:0.5mgL–1) shifted to turbid status with macrophyte loss before Week 4. Saturation concentrations were similar under these two conditions, with averages ~1.0mgL–1 total particulate P and 700µgL–1chlorophyll-a reached in the last 3 weeks. After 5 weeks of the experiment, macrophytes were removed from the mesocosms without nutrient addition and those fertilised with 1:0.1mgL–1 N:P. Macrophyte removal resulted in a release of phosphate and ammonium from sediments, enhancing eutrophication. Three months after stopping nutrient addition, high nutrient and chlorophyll concentrations persisted in the highly fertilised mesocosms, but decreased in the low fertilised mesocoms, which shows that restoration of eutrophic Mediterranean shallow lakes depends to a great extent on previous external loads.

Climate modifies response of non-native and native species richness to nutrient enrichment.

Ecosystem eutrophication often increases domination by non-natives and causes displacement of native taxa. However, variation in environmental conditions may affect the outcome of interactions between native and non-native taxa in environments where nutrient supply is elevated. We examined the interactive effects of eutrophication, climate variability and climate average conditions on the success of native and non-native plant species using experimental nutrient manipulations replicated at 32 grassland sites on four continents. We hypothesized that effects of nutrient addition would be greatest where climate was stable and benign, owing to reduced niche partitioning. We found that the abundance of non-native species increased with nutrient addition independent of climate; however, nutrient addition increased non-native species richness and decreased native species richness, with these effects dampened in warmer or wetter sites. Eutrophication also altered the time scale in which grassland invasion responded to climate, decreasing the importance of long-term climate and increasing that of annual climate. Thus, climatic conditions mediate the responses of native and non-native flora to nutrient enrichment. Our results suggest that the negative effect of nutrient addition on native abundance is decoupled from its effect on richness, and reduces the time scale of the links between climate and compositional change.

Network analysis reveals functional redundancy and keystone taxa amongst bacterial and fungal communities during organic matter decomposition in an arable soil

Organic matter (OM) decomposition and breakdown of crop residues are directly linked to carbon (C) sequestration in agricultural soils as a portion of the decomposed C becomes assimilated into stable microbial biomass. Microbial decomposition of OM may vary with quality of OM, addition of nutrients and functional types of microbes. While the role of fungi and bacteria in OM decomposition has received considerable attention, the succession and co-occurrence patterns of these communities during decomposition remain unexplored. Using 454 pyrosequencing and network analysis of bacterial 16S rRNA and fungal ITS genes in a time-course microcosm experiment, this study shows a positive effect of nutrient addition on overall microbial biomass and abundance of bacteria and fungi. Abundance of different bacterial and fungal groups changed up to 300-folds under straw- and nutrient amended treatments while the rate of decomposition remained similar, indicating a possible functional redundancy. Moreover, addition of nutrients significantly altered the co-occurrence patterns of fungal and bacterial communities, and these patterns were resource-driven and not phylogeny-driven. Richness, evenness and diversity decreased and were negatively associated with decomposition rate. Acidobacteria, Frateuria and Gemmatimonas in bacteria and Chaetomium, Cephalotheca and Fusarium in fungi were found as the keystone taxa. These taxa showed strong positive associations with decomposition, indicating their importance in C turnover. Overall, we show that addition of nutrients reduces diversity but favours the keystone taxa, and thereby increases microbial biomass.

Effects of Roundup formulations, nutrient addition, and Western mosquitofish (Gambusia affinis) on aquatic communities.

Aquatic communities can be affected by herbicides, nutrient addition, and non-native fish species. We conducted a mesocosm experiment to examine the direct and interactive effects of three stressors: (1) Roundup formulations (Roundup Weed and Grass Killer(®) and Roundup Poison Ivy and Tough Brush Killer Plus(®)), (2) nutrient addition, and (3) the presence of the non-native Western mosquitofish (Gambusia affinis), on experimental pond communities. Roundup formulations had the most widespread effects on the zooplankton community, but effects varied between formulations and among taxa. The only significant effect of nutrient addition was a lowering of Daphnia abundance in the nutrient addition treatments. The abundances of Daphnia, mid-sized cladocerans, and total zooplankton were lowered by mosquitofish, but no other taxa showed significant mosquitofish effects. We found several two-way and three-way interactions among the stressors, but these varied among zooplankton taxa. Chlorophyll a levels were higher with nutrient addition but were not significantly affected by Roundup formulation or mosquitofish. Our results suggest toxicity of Roundup formulations varies among taxa, and Roundup formulations differ in their toxicity to zooplankton, but with no cascading effects on primary producers. In addition, interactions among stressors affected the zooplankton community.

Absorptive roots trait plasticity explains the variation of root foraging strategies in Cunninghamia lanceolata

The architecture and morphology of absorptive roots show substantial plasticity in response to forest management practices. These traits are known to play important roles in the acquisition of soil resources by trees. However, the effects of nutrient addition, thinning and pruning on absorptive root traits and their feedback to root foraging strategies remain unclear.We investigated the values and plasticity of traits related to nutrient foraging (root architecture, growth and morphology) for first- and second-order roots (absorptive roots) following nitrogen (N) addition, phosphorous (P) addition, thinning and pruning treatments in a young Chinese fir (Cunninghamia lanceolata) plantation.We measured twelve traits of absorptive roots under the five treatments (control, N addition, P addition, thinning and pruning) and determined relationships between the values and plasticity of root traits and stem growth rate. We demonstrated clear patterns of root traits and their plasticity in response to the treatments. N and P addition increased root biomass (B) and root tissue density (RTD). Thinning and pruning led to larger specific root length (SRL) and root nitrogen concentrations (N), but resulted in lower root length (L) and root length density (RLD). Principal component analysis of the measured traits and plasticity provided evidence for two suites of traits related to resource acquisition and conservation strategies among treatments. The trait syndromes exhibiting resource acquisition strategy (SRL and N) is arrayed well along the positive part of the first axis, whereas the opposite trait syndromes with resource conservation strategy (root diameter and RTD) is along the negative part of the first axis. The first axis also separates nutrient-induced treatments (N and P addition) from light-mediated treatments (pruning and thinning). Furthermore, first-order root exhibited higher foraging sensitivity and precision (expressed as relative fine root length difference) in response to P addition than to the other treatments. The foraging sensitivity and precision, plus B, L and RLD showed that first-order roots are more responsive to environment than second-order roots across the treatments. Stem growth rate was correlated positively with absorptive root traits (biomass, root surface area index, root length density, and root tip number) after thinning, but negatively with these traits after N and P addition.These findings demonstrate that C. lanceolata finely tuned root foraging strategies between first- and second-order root traits and their plasticity at the intraspecific level in response to forest management practices. Further studies may explore nutrient-induced and light-mediated foraging strategies between absorptive roots across root branch orders in mature Chinese fir plantation.

Above‐ground and below‐ground responses to long‐term nutrient addition across a retrogressive chronosequence

Summary There is much interest in understanding ecosystem responses to local‐scale soil fertility variation, which has often been studied using retrogressive chronosequences that span thousands of years and show declining fertility and plant productivity over time. There have been few attempts to experimentally test how plant nutrient limitation changes during retrogression. We studied a well‐characterized system of 30 forested lake islands in northern Sweden that collectively represent a 5350‐year post‐fire retrogressive chronosequence, with fertility and productivity decreasing as time since fire increases. For each island, we set up four plots on understorey vegetation, each subjected to a different fertilizer treatment over 6 years: no additions, nitrogen (N) only, phosphorus (P) only and N + P. We found that both N and P additions reduced feather moss and thus total plant biomass. Meanwhile, the three dominant vascular plant species showed contrasting biomass responses, but similar responses of foliar nutrient concentrations to nutrient additions. Fertilization reduced most microbial groups and altered CO2 fluxes, most likely through feather moss reduction. Against expectations, the majority of interactive effects of N and P were antagonistic. Changes in effects of nutrient additions during retrogression were usually modest. Empetrum hermaphroditum biomass was increasingly promoted by P and N + P addition, while vascular plant N‐to‐P ratios were increasingly reduced by P addition, indicating increasing plant limitation by nutrients (notably P) during retrogression. Below‐ground, positive effects of N addition on soil mineral N increased, while negative effects of N addition on soil fungi decreased during retrogression; no other below‐ground effects of fertilization changed along the gradient. Synthesis. Our results show that forest understorey communities on islands of different fire history and thus stages of retrogression show relatively modest differences in how they respond to nutrient addition despite large changes in ecosystem productivity and soil fertility, probably because of high species turnover and adaptation of communities to infertile conditions. While increased nutrient availability (as expected through global change) may have important ecological consequences, these effects are likely, especially below‐ground, to be rather similar across ecosystems that differ greatly in nutrient availability and productivity.

Effects of Soil Nitrogen and Atmospheric Carbon Dioxide on Wheat streak mosaic virus and Its Vector (Aceria tosichella Kiefer).

Management of vector-borne plant viruses requires understanding how abiotic (e.g., resource availability) and biotic (e.g., virus-vector interactions) factors affect disease via effects on epidemiological parameters that drive disease spread. We conducted two complementary experiments using Wheat streak mosaic virus (WSMV): (i) a field study to determine the effects of nitrogen (N) fertilization on winter wheat (Triticum aestivum L.) susceptibility to WSMV infection and (ii) a growth chamber study to evaluate the effects of N and carbon dioxide (CO2) enrichment on population growth rates of the wheat curl mite (WCM), the vector of WSMV, and whether the effects of nutrient addition on WCM reproduction were modified by WSMV infection. The relationship between N fertilization and plant susceptibility to WSMV infection was nonlinear, with infection rates increasing rapidly as soil nitrate increased from 0 to 20 ppm and more gradually at higher nitrate concentrations. In the growth chamber study, N fertilization increased WCM population growth rates when the vectors transmitted WSMV but had the opposite effect on nonviruliferous mites. CO2 enrichment had no observable effects on WCM populations. These results suggest that, whereas the spread of WSMV is facilitated by N addition, increases in atmospheric CO2 may not directly alter WCM populations and WSMV spread.

Effect of nutrient additions and site hydrology on belowground production and root nutrient contents in two wet grasslands

Belowground production can be a significant part of total plant biomass and represents a significant pool of carbon and other nutrients. Changes in root biomass and nutrient contents can influence root decomposition and nutrient turnover rates, thereby impacting ecosystem processes and functions. In-growth core bags were used in a long-term nutrient addition study to determine net belowground primary production (NBPP), root nutrient (C, N and P) percentages and stoichiometry under different nutrient treatments in two wet grasslands with either mineral or organic soil. It was hypothesized that fertilization will lead to reduced NBPP, but increased root nutrient contents, and that these differences will be greater over time. The fertilizer was added in two half doses in a growing season over seven years with NBPP and the nutrient measures sampled in years 2, 3, 5 and 7. Between-year differences in the measured parameters were tested by repeated measures ANOVA, while one-way ANOVAs were run to compare between-treatment differences in each year within each site. Linear regressions were run to relate NBPP, root nutrient content and the stoichiometric ratios to changes in site hydrology. NBPP and root nutrient contents changed over time, especially in the organic soil grassland, but were largely unaffected by the nutrient additions. Site hydrology, sometimes interacting with the nutrient treatments, appeared to be the more important factor. Prolonged flooding of the two grasslands in the latter two years of measurements led to significantly reduced NBPP, and N and P% especially in the organic soil site. The response of wet grassland belowground structures to changing hydrologic conditions, in tandem with nutrient addition, may be useful in developing management plans to deal with the expected effect of climate change on water availability at the local and regional scales.

Nutrient limitation of algal periphyton in streams along an acid mine drainage gradient.

Metal oxyhydroxide precipitates that form from acid mine drainage (AMD) may indirectly limit periphyton by sorbing nutrients, particularly P. We examined effects of nutrient addition on periphytic algal biomass (chl a), community structure, and carbon and nitrogen content along an AMD gradient. Nutrient diffusing substrata with treatments of +P, +NP and control were placed at seven stream sites. Conductivity and SO4 concentration ranged over an order of magnitude among sites and were used to define the AMD gradient, as they best indicate mine discharge sources of metals that create oxyhydroxide precipitates. Aqueous total phosphorous (TP) ranged from 2 to 23μg·L(-1) and significantly decreased with increasing SO4 . Mean chl a concentrations at sites ranged from 0.2 to 8.1μg·cm(-2) . Across all sites, algal biomass was significantly higher on +NP than control treatments (Co), and significantly increased with +NP. The degree of nutrient limitation was determined by the increase in chl a concentration on +NP relative to Co (response ratio), which ranged from 0.6 to 9.7. Response to nutrient addition significantly declined with increasing aqueous TP, and significantly increased with increasing SO4 . Thus, nutrient limitation of algal biomass increased with AMD impact, indicating metal oxyhydroxides associated with AMD likely decreased P availability. Algal species composition was significantly affected by site but not nutrient treatment. Percent carbon content of periphyton onthe Co significantly increased with AMD impact and corresponded to an increase in the relative abundance of Chlorophytes. Changes in periphyton biomass and cellular nutrient content associated with nutrient limitation in AMD streams may affect higher trophic levels.

Nutrient addition effects on tropical dry forests: a mini-review from microbial to ecosystem scales

Humans have more than doubled inputs of reactive nitrogen globally and greatly accelerated the biogeochemical cycles of phosphorus and metals. However, the impacts of increased element mobility on tropical ecosystems remain poorly quantified, particularly for the vast tropical dry forest biome. Tropical dry forests are characterized by marked seasonality, relatively little precipitation, and high heterogeneity in plant functional diversity and soil chemistry. For these reasons, increased nutrient deposition may affect tropical dry forests differently than wet tropical or temperate forests. Here we review studies that investigated how nutrient availability affects ecosystem and community processes from the microsite to ecosystem scales in tropical dry forests. The effects of N and P addition on ecosystem carbon cycling and plant and microbial dynamics depend on forest successional stage, soil parent material and rainfall regime. Responses may depend on whether overall productivity is N- versus P-limited, although data to test this hypothesis are limited. These results highlight the many important gaps in our understanding of tropical dry forest responses to global change. Large-scale experiments are required to resolve these uncertainties.

Synergistic effects between omnivorous filter-feeding fish and nutrient enrichment on algal biomass

Aim: The Nile tilapia - Oreochromis niloticus(Linnaeus 1758) - is an exotic omnivorous filter-feeding fish that has been stocked for three decades in man-made lakes of Northeastern Brazil. Most experiments manipulating omnivorous filter-feeding fish in eutrophic lakes and reservoirs shows that their presence tends to increase phytoplankton biomass and primary production. In this study, we tested the hypothesis that tilapia interact synergistically with nutrient enrichment so that the effects of omnivorous fish on phytoplankton biomass becomes more intense with increasing nutrient concentration.MethodsA field experiment with a 2×3 factorial design was performed during four weeks in twenty-four mesocosms (0.25 m3) to which six treatments were randomly allocated: fingerling addition (F), juvenile tilapia addition (J), nutrient addition (NP), nutrient and fingerling addition (NPF), nutrient and juvenile tilapia addition (NPJ) and a control treatment with no tilapia or nutrients addition (C). A two-way repeated measures ANOVA was done to test for time (t), tilapia and nutrients effects and their interaction on total phosphorus, total nitrogen and chlorophyll-a concentrations.ResultsThe results showed a positive effect of nutrient addition on total phosphorus, total nitrogen and chlorophyll-a concentrations and a positive effect of tilapia on the concentration of chlorophyll-a. As expected, we found a synergistic interaction between the positive effect of Nile tilapia and nutrient enrichment on phytoplankton biomass.ConclusionsThe above results suggest that controlling tilapia abundance through fisheries management is a potential tool to improve water quality and mitigate the effects of lake and reservoir eutrophication.

Species richness loss after nutrient addition as affected by N:C ratios and phytohormone GA3contents in an alpine meadow community

Aims From the light-competition hypothesis, competition for light is asymmetric and the observed increases in plant-size variability with increasingly denser canopies are primarily due to competition for light. Greater plant height provides pre-emptive access to light and produces increased height differences among species. The question is what produces these differences in plant height or height growth response among species in response to fertilization. Methods In 2009, a field experiment of N, P and N + P enrichments at three levels each was initiated in an alpine meadow on the northeast Qinghai- Tibet Plateau. Effects of fertilization on species richness, aboveground net primary production (ANPP), relative light intensity and plant height of different plant functional groups were determined. Festuca ovina (grass), Kobresia humilis (sedge), Oxytropis ochrocephala (legume), Taraxacum lugubre (rosette forb) and Geranium pylzowianum (upright forb) were selected as exemplars of each of the indicated functional groups. The N: C ratios in aboveground biomass, gibberellic acid (GA3) concentrations in leaves, plant heights and height relative growth rate (RGR) of these exemplar species were analyzed in detail. Important Findings Species richness of grasses significantly increased with increasing N + P levels. Species richness of legumes and upright forbs decreased after N and N + P additions. P addition had no significant effect on species richness. The effects of N + P addition on species richness and ANPP were consistently stronger than those of the single N or P fertilization. Reductions in species richness caused by nutrient addition paralleled the increases in ANPP and decreases in light intensity under the canopies, indicating indirect effect of nutrient addition on species richness via ANPP-induced light competition. The exemplar species that responded most positively to fertilization in height and RGR also displayed stronger increases in their GA3 content and N:C ratios. GA3 concentrations and N: C ratios were positively correlated with height RGR when the data were pooled for all species. The tallest and the fastest-growing grass, F. ovina, had the largest increase in N: C ratios and the highest leaf GA3 concentrations after nutrient addition. These results indicated that differential responses of GA3 concentrations and N: C ratios to fertilization were related to the inequality in plant heights among species.

Low‐to‐moderate nitrogen and phosphorus concentrations accelerate microbially driven litter breakdown rates

Particulate organic matter (POM) processing is an important driver of aquatic ecosystem productivity that is sensitive to nutrient enrichment and.drives ecosystem carbon (C) loss. Although studies of single concentrations of nitrogen (N) or phosphorus (P) have shown effects at relatively low concentrations, responses of litter breakdown rates along gradients of low-to-moderate N and P concentrations are needed to establish likely interdependent effects of dual N and P enrichment on baseline activity in stream ecosystems. We established 25 combinations of dissolved inorganic N (DIN; 55-545 µg/L) and soluble reactive P (SRP; 4-86 µg/L) concentrations with corresponding N:P molar ratios of 2-127 in experimental stream channels. We excluded macroinvertebrates, focusing on microbially driven breakdown of maple (Acer rubrum L.) and rhododendron (Rhododendron maximum L.) leaf litter. Breakdown rates, k, per day (d-1) and per degree-day (dd-l), increased by up to 6X for maple and 12× for rhododendron over our N and P enrichment gradient compared to rates at low ambient N and P concentrations. The best models of k (d- and dd-1) included litter species identity and N and P concentrations; there was evidence for both additive and interactive effects of N and P. Models explaining variation in k dd-1 were supported by N and P for both maple and rhododendron (R =0.67 and 0.33, respectively). Residuals in the relationship between k dd-1 and N concentration were largely explained by P, but residuals for k dd-1 and P. concentration were less adequately explained by N. Breakdown rates were more closely related to nutrient concentrations than variables associated with measurements of two mechanistic parameters associated with C loss (fungal biomass and microbial respiration rate). We also determined the effects of nutrient addition on litter C: nutrient stoichiometry and found reductions in litter C:N and C:P along our experimental nutrient gradient. Our results indicate that microbially driven litter processing rates increase across low-to-moderate nutrient gradients that are now common throughout human-modified landscapes.