Effects Of Nutrient Addition Research Articles

Does spatiotemporal nutrient variation allow more species to coexist?

Temporal heterogeneity in nutrient availability is known to increase phytoplankton diversity by allowing more species to coexist under different resource niches. Spatial heterogeneity has also been positively correlated with species diversity. Here we investigated how temporal and spatial differences in nutrient addition together impact biodiversity in metacommunities varying in the degree of connectivity among the patches. We used a microcosm experimental design to test two spatiotemporal ways of supplying nutrients: synchronously (nutrients were added regionally—to all four patches at the same time) and asynchronously (nutrients were added locally—to a different patch each time), combined with two different degrees of connectivity among the patches (low or high connectivity). We used three species of algae and one species of cyanobacteria as the primary producers; and five ciliate and two rotifer species as the grazers. We expected higher diversity in metacommunities receiving an asynchronous nutrient supply, assuming stronger development of heterogeneous patches with this condition rather than with synchronous nutrient supply. This result was expected, however, to be dependent on the degree of connectivity among patches. We found significant effects of nutrient addition in both groups of organisms. Phytoplankton diversity increased until the fourth week (transiently) and zooplankton richness was persistently higher under asynchronous nutrient addition. Our results were consistent with our hypothesis that asynchronicity in nutrient supply would create a more favorable condition for species to co-occur. However, this effect was, in part, transient and was not influenced by the degree of connectivity.

Nutrients and fungal identity affect the outcome of symbiotic germination in Bipinnula fimbriata (Orchidaceae)

Orchids produce small seeds with no energy reserves, relying entirely on orchid mycorrhizal fungi (OMF) for germination. This process, known as symbiotic germination, can lead to different outcomes depending on abiotic factors, such as nutrient availability. Previous studies have shown that nutrient addition has a negative effect on the outcome of symbiotic germination. However, if this effect varies across OMF species, and if it is related to a fungal response to nutrients, remains unknown. This paper examines the effect of fungal identity and nutrient addition (nitrogen and phosphorus) on the germination of the orchid Bipinnula fimbriata using seven OTUs of mycorrhizal fungi from the families Tulasnellaceae and Ceratobasidiaceae. We also evaluated the effect of nutrient addition on mycorrhizal fungi growth rates. Results showed that the nutrient effect on symbiotic germination varied depending on fungal identity. While there was a strong negative effect on symbiotic germination with all Tulasnella OTUs and two Ceratobasidium OTUs, less or no effect was observed on the other two Ceratobasidium OTUs. Further studies are needed to understand the mechanism underlying this variation and how variable is the effect of nutrient addition on symbiotic germination in Orchidaceae and OMF species.

Relative strength of top-down effects of an invasive fish and bottom-up effects of nutrient addition in a simple aquatic food web.

Introduction of exotic predators or runoff of fertilizers can alter aquatic food webs, in particular zooplankton communities, through top-down and bottom-up effects. In a mesocosm experiment, we manipulated the density of Western Mosquitofish (Gambusia affinis) and nutrient levels (nitrate and phosphate independently) and observed effects on zooplankton and phytoplankton in a fall, temperate zone system. If top-down regulation were important, we expected mosquitofish predation to reduce zooplankton abundance, which would indirectly benefit phytoplankton. If bottom-up regulation were important, we expected nutrient addition to increase both primary producers and zooplankton. Western Mosquitofish predation significantly decreased the abundance of several zooplankton taxa, resulting in a trophic cascade with increased chlorophyll a (i.e., primary productivity). This effect did not differ between mesocosms with 5 or 10 fish. Nutrient addition had no significant effects on zooplankton; however, chlorophyll a was positively affected by both nitrogen addition and phosphorus addition. Our results suggest weak bottom-up regulation in our experimental community, but strong top-down regulation, emphasizing the potential consequences of introducing non-native Western Mosquitofish to native aquatic ecosystems.

Nutrient addition effects on chlorophyll a, phytoplankton biomass, and heterocyte formation in Lake Erie’s central basin during 2014–2017: Insights into diazotrophic blooms in high nitrogen water

Abstract Phosphorus (P) usually is the primary limiting nutrient of phytoplankton biomass, but attention towards nitrogen (N) and trace nutrients, such as iron (Fe), has surfaced. Additionally, N‐fixing cyanobacterial blooms have been documented to occur in N‐rich, P‐poor waters, which is counterintuitive from the paradigm that low N and high P promotes blooms. For example, Lake Erie's central basin has Dolichospermum blooms when nitrate concentrations are high, which raises questions about which nutrient(s) are selecting for Dolichospermum over other phytoplankton and why an N‐fixer is present in high N waters? We conducted a 4‐year (2014–2017) study in Lake Erie's central basin to determine which nutrient (P, N, or trace nutrients such as Fe, molybdenum [Mo], and boron [B]) constrained chlorophyll concentration, phytoplankton biovolume, and nitrate assimilation using nutrient enrichment bioassays. The enriched lake water was incubated in 1‐L bottles in a growth chamber programmed at light and temperatures of in situ conditions for 4–7 days. We also quantified heterocytes when N‐fixing cyanobacteria were present. Compared to the non‐enriched control, the P‐enriched (+P) treatment had significantly higher chlorophyll and phytoplankton biovolume in c. 75% of experiments. Combination enrichments of P with ammonium‐N, nitrate‐N, Fe, Mo, and B were compared to the +P treatment to determine secondary limitations. +P and ammonium‐N and +P nitrate‐N resulted in higher chlorophyll in 50% of experiments but higher phytoplankton biovolume in only 25% of experiments. These results show that P was the primary limiting nutrient, but there were times when N was secondarily limiting. Chlorophyll concentration indicated N secondary limitation in half of the experiments, but biovolume indicated only N secondary limitation in 25% of the experiments. To make robust conclusions from nutrient enrichment bioassays, both chlorophyll and phytoplankton biovolume should be measured. The secondary effects of Fe, Mo, and B on chlorophyll were low (<26% of experiments), and no secondary effects were observed on phytoplankton biovolume and nitrate assimilation. However, +P and Fe resulted in more chlorophyll than +P in experiments conducted during Dolichospermum blooms, and +P and B significantly increased the number of heterocytes in Dolichospermum. These results indicate that low Fe availability might select for Dolichospermum, and low B constrains heterocyte formation in the central basin of Lake Erie. Furthermore, these results could apply to other lakes with high N and low P where diazotrophic cyanobacterial blooms occur.

Submergence, nutrient enrichment, and tropical storm impacts on Spartina alterniflora in the microtidal northern Gulf of Mexico

Salt marshes face chronic anthropogenic impacts such as relative sea level rise and eutrophication, as well as acute disturbances from tropical storms that can affect the productivity of these important communities. However, it is not well understood how marshes already subjected to eutrophication and sea level rise will respond to added effects of episodic storms such as hurricanes. We examined the interactive effects of nutrient addition, sea level rise, and a hurricane on the growth, biomass accumulation, and resilience of the saltmarsh cordgrass Spartina alterniflora in the Gulf of Mexico. In a microtidal marsh, we manipulated nutrient levels and submergence using marsh organs in which cordgrasses were planted at differing intertidal elevations and measured the impacts of Hurricane Isaac, which occurred during the experiment. Prior to the hurricane, grasses at intermediate and high elevations increased in abundance. After the hurricane, all treatments lost approximately 50% of their shoots, demonstrating that added nutrients and elevation did not provide resistance to hurricane disturbance. At the end of the experiment, only the highest elevations had been resilient to the hurricane, with increased above- and belowground growth. Added nutrients provided a modest increase in above- and belowground growth, but only at the highest elevations, suggesting that only elevation will enhance resilience to hurricane disturbance. These results empirically demonstrate that S. alterniflora in microtidal locations already subjected to submergence stress is less able to recover from storm disturbance and suggests we may be underestimating the loss of northern Gulf Coast marshes due to relative sea level rise.

Effects of nutrient addition on degraded alpine grasslands of the Qinghai-Tibetan Plateau: A meta-analysis

Climate warming and human disturbance are supposed to have severely affected the alpine grasslands on the Qinghai-Tibetan Plateau (QTP), a region where the extremely harsh and fragile ecological environment has attracted great attention because of its sensitivity to global change. However, there is still no unified understanding of the degree and magnitude of grassland degradation and the effectiveness of nutrient addition in this vast landscape, since most previous studies have focused on short-term observations at a single site. Here, we conducted a meta-analysis of 145 published studies on degraded alpine grassland along with 90 published studies, which concerning nutrient addition (nitrogen [N], phosphorus [P], and combined N and P [NP]) to quantitatively assess the responses of plant and soil characteristics to land degradation and restoration. Our results revealed that the response ratio (RR) of above-ground biomass (AGB), below-ground biomass (BGB), soil organic carbon (SOC), and soil total N (TN) decreased significantly (−47.23 %, −43.45 %, −32.35 %, and −37.97 %, respectively) in degraded grassland compared with non-degraded grassland. The RR of AGB was correlated with mean annual precipitation (MAP), while the RR of BGB was correlated with the interaction of MAP and mean annual temperature (MAT). Severely degraded grassland required additional nutrients to aid recovery. NP addition to severely degraded sites increased plant AGB (+32.44 %), TN (+10.99 %), soil total P (+32.25 %), and soil moisture (+9.21 %), but significantly decreased species richness (−45.46 %), diversity (−30.40 % for Shannon−Wiener index) and soil pH (−3.91 %). N addition increased the RR of AGB and grass biomass significantly by 28.77 % and 36.49 %, but had no significant effect on sedge and forb biomass. MAP influenced the RR of AGB, TN, TP under NP addition, the RR of BGB and the AGB of different function groups were significantly affected by MAT. We evidenced that the QTP has endured severe vegetation and soil degradation, which cannot be completely mitigated by supplementary fertilisation. Fertilisation could yield positive effects on plant performance and soil quality, but negative effects on biodiversity. Climate warming and associated precipitation change may regulate the effects of fertiliser on plant biomass and soil nutrients.

Differential effects of N and P additions on foliar stoichiometry between species and community levels in a subtropical forest in eastern China

Abstract The responses of foliar nitrogen (N) and phosphorus (P) stoichiometry to nutrient additions are important indicators of various ecological processes, especially under climate change. However, how nutrient additions affect foliar N and P stoichiometry from species to community levels remain unclear, especially in P-limited systems. In this study, we conducted a nutrient additions experiment with control, N addition, P addition, and NP addition treatments in a P-limited subtropical forest to examine the effects of nutrient additions on foliar N and P stoichiometry. Our results showed that nutrient additions did not significantly influence foliar N concentration in seven dominant plant species over a three-year experimental period but significantly increased foliar P concentration and decreased foliar N:P ratio in species with lower initial foliar P concentration. The interactive effects of N and P additions on foliar P concentration and N:P ratio were synergistic and antagonistic, respectively. The response ratio of foliar P concentration to nutrient addition had a significantly negative correlation with initial foliar P concentration (r = −0.644), but had a positive one with initial foliar N:P ratio (r = 0.619). These results suggest that the limiting P dominated the responses of foliar N and P stoichiometry to nutrient additions at species level. At the community level, foliar P concentration and N:P ratio for both community weighted means (CWM) and community non-weighted means (CM) showed weaker positive responses to N and P additions than those at species level and there was no differential response between CWM and CM, suggesting the control of varied species responses on community foliar P concentration and N:P ratio response. Taken together, our results highlight the differential impacts of nutrient additions on foliar N and P stoichiometry between species and community levels, which needs to be considered in models when assessing the responses of ecosystem function to nutrient enrichments.

Characterizing the phytoplankton and zooplankton communities in Kootenay Lake: a time series analysis of 24 years of nutrient addition

Large-scale seasonal addition of limiting nutrients has been used for restoration in Kootenay Lake, British Columbia, since 1992 to mitigate cultural oligotrophication resulting from upstream hydropower development, river channelization, mysid shrimp introduction, and extensive tributary floodplain loss. Historical (1949) in-lake nutrient levels were targeted to stimulate bottom-up carbon transfer from native plankton communities to planktivorous and piscivorous fish populations that support popular fisheries. Analysis of 24 years of monitoring data assessed the effects of nutrient addition on the phytoplankton and zooplankton communities. Assessment involved comparisons of plankton community metric data from North Arm (1992–2003) and North + South arms (2004–2015) nutrient addition periods. A before–after, control–impact (BACI) analysis adjusted for these effects involved a series of phytoplankton and zooplankton metrics across the two lake arms. Time series analyses revealed significant serial correlation structure, significant increases in phytoplankton and zooplankton abundance and biomass, and increased stability within the phytoplankton and zooplankton communities. Results confirmed that adaptively managed nutrient restoration can effectively restore biological productivity and community structure in a large culturally oligotrophic lake.

Response of Microphytobenthos and Benthic Bacteria Viability to Eutrophication in a Benthic–Pelagic Coupling Mesocosm Experiment

Excessive primary productivity due to nutrient inputs is a potential problem in coastal areas when resulting in high organic matter sedimentation rates. Microphytobenthos and heterotrophic bacteria are two components of the benthic ecosystem that contribute to nutrient cycling and decomposition of organic matter. In this context, the effects of nutrient addition and the associated in situ produced organic matter on microphytobenthos community composition and benthic bacterial viability were assessed in a mesocosm experiment for 58 days. The experimental setup included triplicate mesocosms filled with sediment and water under three levels of nutrient addition (‘control’, ‘low’ and ‘high’). Benthic algal community composition was assessed using chemotaxonomy and bacterial viability was estimated using flow cytometry and a double-staining protocol. Multivariate analysis detected a significant effect of treatment and time on microphytobenthic community composition indicating a difference between control and low mesocosms and also between low and high treatments at Day 12 and Day 24 of the experiment. Nonetheless, microphytobenthos implied high resistance and redundancy of benthic algae to disturbance as all three treatments showed no significant difference in community structure between Day 0 and Day 58. Bacterial viability responded quickly to the high nutrient addition and was significantly lower than in the ‘control’ and ‘low’ treatments at Days 6 and 12. Both pelagic and benthic environmental variables were correlated to these changes in benthic community.

The effect of biochar, lime and ash on maize yield in a long-term field trial in a Ultisol in the humid tropics

A multi-season field trial was carried out to investigate the effect of the amendment of biochar, lime, ash and washed biochar on the growth of maize. A degraded, strongly acidic Ultisol (pHKCl 3.60), with a relatively high exchangeable aluminium content (2.4 cmolc/kg) and a low exchangeable calcium content (0.99 cmolc/kg), was used. Soil was treated once at the beginning of the field trial and crop growth was monitored over seven planting seasons (PS). All treatments increased maize yield. The average increases were; seven times for biochar, five times for lime, five times for washed biochar and eight times for ash treatment, when compared to the control across all PS. The effect of biochar, lime and ash treatments on maize yield were sustained over the seven PS. Soil pHKCl was significantly increased (p < 0.05 level) following the addition of all of the amendment materials. All treatments significantly reduced the concentration of Al3+ when compared to the control (p < 0.05), with the lowest concentrations for the lime and ash treatments. The ash treatment also increased the concentration of macronutrients (K, P and Mg) to the greatest extent. Results showed that there was a clear liming effect at play. The better performance of biochar compared to lime, despite lime having the highest pH and the lowest Al3+ concentration, can be explained by the additional K, Mg and P the biochar adds to the soil. Results also showed a clear nutrient addition effect where ash added the most nutrients. Overall, this work supports the fact that small scale farmers in Indonesia should produce biochar from their waste agricultural materials. Doing so not only provides an increase in crop productivity, but also sequesters carbon resulting in the best overall environmental benefit.

Clipping defoliation eliminates the stimulating effects of nitrogen enrichment on the aboveground productivity of an alpine meadow

To investigate how clipping (CL) regulates the effects of nutrient addition, an experiment, including CL and nitrogen (N) addition, was conducted in an alpine meadow. Nitrogen treatment increased community coverage (48–113% higher than the control) and aboveground biomass (29–117% higher than the control), which was mainly attributed to grass growth. Both N and N + CL treatments showed a tendency to reducing species richness, while significant reduction only occurred in 2016 and 2017 in CL treatment. Clipping showed a tendency to decrease community cover (3–37% lower than the control) and aboveground biomass (2–34% lower than the control), while N + CL treatment had no effect, indicating that clipping can eliminate the simulated effects of N addition. Nitrogen addition significantly increased soil inorganic N (SIN, 528–1230% higher than the control), while SIN in N + CL was 25–48% lower than N treatment. The decrease in stimulated effects in N + CL was attributed to SIN decrease, which resulted from the aboveground biomass removal by clipping. Our results show that clipping can take away aboveground biomass and cause soil nutrients to decrease, which slows down the degraded grassland recovery. This suggests that grazing exclusion may eliminate the effect of nitrogen deposition on aboveground production in alpine grasslands.

Changes in Primary Producer and Consumer Communities in Response to Upstream Nutrient Addition in the Kootenai River, Idaho

The purpose of this study was to assess potential effects of nutrient addition in the previously unassessed lower Kootenai River (Meander Reach), 44–155 km downstream from the nutrient dosing site. We hypothesized that downstream periphyton and plankton densities would increase due to nutrient spiraling and organismal drift following seasonal upstream nutrient addition. Liquid inorganic ammonium polyphosphate fertilizer (10:34:0, N:P:K) was added to the Kootenai River in Idaho immediately downstream from the Idaho-Montana border with an in-river target concentration of 3.0 µg L-1 total dissolved phosphorus for 16 weeks (01 June–01 October) from 2006 through 2012. As predicted, mean periphyton, phytoplankton, and zooplankton densities increased significantly (P < 0.05) post-nutrient addition. Periphyton density increased 5.1 times (pre: 685.1 mm-2, post: 3501.0 mm-2), phytoplankton densities increased 1.7 times (pre: 1775.9 L-1, post: 3005.4 L-1), and zooplankton densities increased by 49% (pre: 67.0 L-1, post: 99.7 L-1). While nutrient uptake, water chemistry, algal and chlorophyll accrual, periphyton, macroinvertebrate, and native fish community responses to nutrient addition have been previously published, including results from upstream reaches of the Kootenai River, this study provides the first published account of lower trophic level responses at considerable distances downstream from the dosing site in altered, low velocity habitats which are very different from the upstream gravel-bed reaches reported in other studies. Results of this study add to the growing body of empirical knowledge regarding the use of nutrient addition for restoring large culturally denutrified rivers in the Pacific Northwest and provide valuable new insights regarding the longitudinal patterns of response magnitude.

Effects of nutrient addition on the functional traits of &lt;i&gt;Melilotus officinalis&lt;/i&gt; growing in a degraded grassland

Effects of nutrient addition on the functional traits of &lt;i&gt;Melilotus officinalis&lt;/i&gt; growing in a degraded grassland

Species Interaction and Selective Carbon Addition During Antibiotic Exposure Enhances Bacterial Survival.

Biofilms are multifaceted and robust microbiological systems that enable microorganisms to withstand a multitude of environmental stresses and expand their habitat range. We have shown previously that nutritional status alters antibiotic susceptibility in a mixed-species biofilm. To further elucidate the effects of nutrient addition on inter-species dynamics and whole-biofilm susceptibility to high-dose streptomycin exposures, a CO2 Evolution Measurement System was used to monitor the metabolic activity of early steady state pure-culture and mixed-species biofilms containing Pseudomonas aeruginosa and Stenotrophomonas maltophilia, with and without added carbon. Carbon supplementation was needed for biofilm recovery from high-dose streptomycin exposures when P. aeruginosa was either the dominant community member in a mixed-species biofilm (containing predominantly P. aeruginosa and S. maltophilia) or as a pure culture. By contrast, S. maltophilia biofilms could recover from high-dose streptomycin exposures without the need for carbon addition during antibiotic exposure. Metagenomic analysis revealed that even when inocula were dominated by Pseudomonas, the relative abundance of Stenotrophomonas increased upon biofilm development to ultimately become the dominant species post-streptomycin exposure. The combined metabolic and metagenomic results demonstrated the relevance of inter-species influence on survival and that nutritional status has a strong influence on the survival of P. aeruginosa dominated biofilms.

Nutrient Inputs and Hydrology Interact with Plant Functional Type in Affecting Plant Production and Nutrient Contents in a Wet Grassland

We studied the effect of nutrient additions and hydrology on the above-ground production and shoot nutrient contents of representative species of the conservative (Carex acuta) and competitive (Glyceria maxima) plant functional types in a minerotrophic wet grassland. Above-ground samples were collected in May, late June (time of maximum biomass) and August (re-growth following cutting) from plots subjected to nutrient addition treatments in years with differing hydrology, ranging from drought to prolonged flooding. Net above-ground primary production (NAPP) and shoot nutrient contents (C, N, P) were determined separately for C. acuta and G. maxima. Between-year and species differences were analyzed by repeated measures ANOVA and generalized linear mixed models (GLMM). NAPP decreased with hydrologic stress with the proportion due to C. acuta increasing with flooding, while G. maxima responded positively to nutrient additions. As expected, C% was greater, but N and P% were lower, in C. acuta shoots compared to G. maxima. Hydrology affected above-ground production and shoot nutrient contents more than nutrients, but both interacted with plant functional type, with likely impacts on ecosystem processes. Future studies must consider multiple factors to predict the effect of climate and management changes on wetlands.

Effects of nutrient addition on polyphenol and nutrient concentrations in leaves of woody species of a savanna woodland in Central Brazil

AbstractWe investigated whether changes in nutrient availability affected N, P, S and polyphenol concentrations in different leaf-development stages of three brevideciduous and three evergreen dominant woody species in a nutrient-limited savanna woodland in Central Brazil. Treatments included eight years of annual fertilization with 100 kg ha−1of N, P, N plus P and control, each replicated in four randomized 15 × 15-m plots. All species increased S concentrations (minimum 28%) in young and mature leaves in fertilized plots.Dalbergia miscolobiumdecreased total phenol concentrations with P (−34.3%, −23.7%) and NP fertilization (−28.2%, −17.1%).Blepharocalyx salicifoliusincreased total phenol (27.6%, 18.8%) and tannin (46.3%; 43.5%) in P fertilized and increased total phenol (33.9%) and tannin (27.8%, 43.5%) in NP fertilized plots. Total phenol concentration decreased with leaf age inOuratea hexasperma,Styrax ferrugineusandBlepharocalyx salicifolius, which also decreased tannin concentration with leaf age. For all treatments, brevideciduous species had higher N, P, total phenols and tannin concentrations and lower S concentration than evergreens. These differences between phenological groups suggest that tropical ecosystems responses to environmental changes are more complex than anticipated by global vegetation models, with consequences for predictions in ecosystem functions and resilience.

Comparative performance of the highly invasive Mimosa invisa and its less invasive subspecies under different nutrient and light conditions

SummaryInvasive plants may be more plastic than non‐invasive plants and maintain high fitness under various environmental conditions. Previous studies mainly focused on the comparisons between invasive and native plants, and comparisons between highly invasive and less invasive exotic species are still relatively rare, especially for comparisons at the subspecies level. This study examined the effects of nutrient addition and shading on the performance of the highly invasive Mimosa invisa and its less invasive subspecies M. invisa var. inermis under either isolated or competitive conditions. Nutrient addition increased biomass and plant height and decreased root‐to‐shoot ratio (R/S). Shading decreased biomass and R/S and increased plant height. Under isolated conditions, the two invaders did not differ in R/S, plant height and plasticity of these traits in response to nutrient addition or shading, and the two invaders also did not differ in biomass production under each of the nutrient and light treatments. When the two invaders competed with each other, M. invisa outcompeted M. invisa var. inermis under high soil nutrient conditions, and the two invaders did not differ in performance under other growth conditions. Thus, only considering competition may we find out the difference between highly invasive species and their closely related, less invasive subspecies. Management of M. invisa should focus on habitats with high soil nutrient availability, in which M. invisa is more likely to dominate.

Effects of nutrient addition on foliar phosphorus fractions and their resorption in different-aged leaves of Chinese fir in subtropical China

Anthropogenic activities have increased nitrogen (N) and phosphorus (P) inputs to terrestrial ecosystems, which may significantly alter P cycle through accumulation and resorption. We measured the concentrations of four different P fractions (inorganic, nucleic, sugar, and residual P) in both live leaves and senescent leaves in N and P additions in an evergreen plantation forest of subtropical China. Adding moderate N plus P increased total, inorganic, and sugar P concentrations, which sustained the leaf N/P balance and alleviated P limitation in Chinese fir (Cunninghamia lanceolata) plantation. Nitrogen and P (total, nucleic, and residue P) resorption proficiencies did but P (each of various fractions) resorption efficiency did not respond to nutrient addition. The concentrations and resorption efficiencies of most P fractions were lower in the old than young leaves, but their resorption proficiencies except for sugar P weren’t different. Internal P cycles of Chinese fir could be strongly altered through increasing accumulation of inorganic P and sugar P factions in respond to soil P enrichment when combined with suitable N addition. The easily degradable P (inorganic and sugar P) were preferentially resorbed regardless of nutrient addition. Leaf age was a key-factor influencing the resorption extent of P fractions in Chinese fir.

Nutrients and sediment modify the impacts of a neonicotinoid insecticide on freshwater community structure and ecosystemfunctioning.

Pesticides are important contributors to the global freshwater biodiversity crisis. Among pesticides, neonicotinoids are the best-selling class of agricultural insecticides and are suspected to represent significant risks to freshwater and terrestrial ecosystems worldwide. Despite growing recognition that neonicotinoid impacts may be modified by the presence of additional stressors, there is limited information about their interactions with other agricultural stressors in freshwater ecosystems. We conducted an outdoor pond-mesocosm experiment to investigate the individual and interactive effects of nutrients, fine sediment, and imidacloprid (a neonicotinoid insecticide) inputs on freshwater community structure (density, diversity, and composition of zooplankton and benthic invertebrates) and ecosystem functioning (ecosystem metabolism, primary production, and organic matter decomposition). We hypothesized antagonistic nutrient-imidacloprid, and synergistic sediment-imidacloprid interactions, affecting aquatic invertebrate communities. The three stressors had significant individual and interactive effects on pond ecosystems. The insecticide neutralized the positive effects of nutrient additions on benthic invertebrate richness and mitigated the negative effects of sediment on zooplankton communities (antagonistic interactions). Moreover, we observed compensatory responses of tolerant benthic invertebrates, which resulted in reversal interactions between sediment and imidacloprid. Furthermore, our observations suggest that imidacloprid has the potential to increase net ecosystem production at environmentally relevant concentrations. Our findings support the hypothesis that the impacts of imidacloprid may be modified by other agricultural stressors. This has important implications on a global scale, given the widespread use of these pesticides in intensive agricultural landscapes and the growing body of literature suggesting that traditional pesticide assessment frameworks, based on laboratory toxicity tests alone, may be insufficient to adequately predict effects to complex freshwater ecosystems.

The impacts of nutrient addition and livestock exclosure on the soil nematode community in a degraded grassland

AbstractNutrient addition and livestock exclosure are two strategies for restoring degraded grassland. Although their impacts on plant community properties are well understood, the effects of fertilization and livestock exclosure on belowground fauna community in degraded grassland remain largely unknown. We examined the main and interactive effects of nutrient addition and livestock exclosure on soil nematode community in a severely degraded grassland of northern China, by recording soil nematode community composition in 0‐ to 10‐cm soil layer in the summer of 2017. Neither nutrient addition nor livestock exclosure had significant effects on the biodiversity of nematode community in this degraded grassland. Livestock exclosure significantly increased the abundances of total nematodes and the maturity index of the nematode community, mainly due to its positive effect on omnivorous + carnivorous nematodes. Nutrient addition weakened the positive effects of livestock exclosure on soil nematode community by reducing soil pH and enhancing soil ammonium concentration. Our results highlight that the positive effects of livestock exclosure on soil nematode community would be reassessed under the scenarios of nutrient enrichment. Given that most grasslands are used for livestock production, the interactive effects of grazing and nutrient enrichment on soil nematode community deserve more attention in further research.