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In light of ongoing climate change, it is increasingly important to know how nutritional requirements of ectotherms are affected by changing temperatures. Here, we analyse the wide thermal response of phosphorus (P) requirements via elemental gross growth efficiencies of Carbon (C) and P, and the Threshold Elemental Ratios in different aquatic invertebrate ectotherms: the freshwater model species Daphnia magna, the marine copepod Acartia tonsa, the marine heterotrophic dinoflagellate Oxyrrhis marina, and larvae of two populations of the marine crab Carcinus maenas. We show that they all share a non-linear cubic thermal response of nutrient requirements. Phosphorus requirements decrease from low to intermediate temperatures, increase at higher temperatures and decrease again when temperature is excessive. This common thermal response of nutrient requirements is of great importance if we aim to understand or even predict how ectotherm communities will react to global warming and nutrient-driven eutrophication.

The growth rate hypothesis (GRH) posits that variation in organismal stoichiometry (C:P and N:P ratios) is driven by growth-dependent allocation of P to ribosomal RNA. The GRH has found broad but not uniform support in studies across diverse biota and habitats. We synthesise information on how and why the tripartite growth-RNA-P relationship predicted by the GRH may be uncoupled and outline paths for both theoretical and empirical work needed to broaden the working domain of the GRH. We found strong support for growth to RNA (r

Considerable attention is given to absolute nutrient levels in lakes, rivers, and oceans, but less is paid to their relative concentrations, their nitrogen:phosphorus (N:P) stoichiometry, and the consequences of imbalanced stoichiometry. Here, we report 38 y of nutrient dynamics in Flathead Lake, a large oligotrophic lake in Montana, and its inflows. While nutrient levels were low, the lake had sustained high total N: total P ratios (TN:TP: 60 to 90:1 molar) throughout the observation period. N and P loading to the lake as well as loading N:P ratios varied considerably among years but showed no systematic long-term trend. Surprisingly, TN:TP ratios in river inflows were consistently lower than in the lake, suggesting that forms of P in riverine loading are removed preferentially to N. In-lake processes, such as differential sedimentation of P relative to N or accumulation of fixed N in excess of denitrification, likely also operate to maintain the lake’s high TN:TP ratios. Regardless of causes, the lake’s stoichiometric imbalance is manifested in P limitation of phytoplankton growth during early and midsummer, resulting in high C:P and N:P ratios in suspended particulate matter that propagate P limitation to zooplankton. Finally, the lake’s imbalanced N:P stoichiometry appears to raise the potential for aerobic methane production via metabolism of phosphonate compounds by P-limited microbes. These data highlight the importance of not only absolute N and P levels in aquatic ecosystems, but also their stoichiometric balance, and they call attention to potential management implications of high N:P ratios.

Eutrophication is the most widespread water quality issue globally. To date, most efforts to control eutrophication have focused on reductions of external nutrient inputs, yet importance of nutrient stoichiometry and subsequent shift in plankton composition in aquatic ecosystem has been largely neglected. To address eutrophication, improved sanitation is one of the United Nations Sustainable Development Goals, spurring the constructions of wastewater treatment facilities that have improved water quality in many lakes and rivers. However, control measures are often targeted at and effective in removing a single nutrient from sewage and thus are less effective in removing the others, resulting in the changes of nutrient stoichiometry. In general, more effective phosphorus removal relative to nitrogen has occurred in wastewater treatment leading to substantial increases in N/P ratios in effluent relative to the influent. Unfortunately, high N/P ratios in receiving waters can impose negative influences on ecosystems. Thus, long-term strategies for domestic wastewater management should not merely focus on the total reduction of nutrient discharge but also consider their stoichiometric balances in receiving waters.

Nitrogen (N) and phosphorus (P) are important elements for global food production, but these nutrients cause pollution in water bodies without proper management. Furthermore, P is a finite resource with geologic reserves that are geographically restricted. Thus, sustainable use of P in agriculture has been the subject of much research over the past decade. This study jointly examines two examples of potential sustainability measures to address nutrient challenges: improved crop varieties and use of recycled fertilizers. We tested the effectiveness of dried biomass of a freshwater alga (Scenedesmus acutus) as a fertilizer with conventional lettuce [Lactuca sativa cv. Conquistador; WT (wild-type)] and genetically modified lettuce of the same variety that was transformed for improved nutrient-use efficiency [(type I Arabidopsis Vacuolar Pyrophosphatase (AVP1)]. In greenhouse conditions, we measured yield, soil nutrient content and losses, and soil P fractions after application of dried S. acutus biomass at different rates, with and without supplemental additions of conventional fertilizer. Yield was higher with commercial fertilizer compared with algal fertilizer, and AVP1 lettuce consistently produced better yield than the conventional lettuce with both fertilizer types, although the effect was stronger with the algal treatments. Soil P was mostly sequestered in the pools of NaHCO3-extractable organic P, NaOH-extractable organic P, and HCl-extractable P, which are poorly available. Although the algal fertilizer was ineffective in supporting short-term growth, the P was retained in the soils, which may improve soil fertility in the long term.

Grasslands are experiencing severe degradation globally, impacting aboveground vegetation and soil properties. The influences of grassland degradation on bacterial communities in soil are not well-understood. The normalized difference vegetation index (NDVI) was calculated to represent grassland status and indicate grassland degradation (decreasing NDVI). Soil ph, bacterial communities, as well as nutrient and organic carbon concentrations were measured. Bacterial alpha diversity had negative relationships with soil moisture, soil organic carbon (SOC), total nitrogen (TN), and total phosphorus (TP). Bacterial community structure was significantly associated with NDVI, the change rate of NDVI, moisture, ph, SOC, TN, as well as soil C:N and C:P ratios. Bacterial phyla were differentially related with these environmental variables. Moreover, network analysis showed that the network of soil bacteria had strong cooperation relationships (positive correlations between taxa) and was grouped into three modules. According to modularity, 71 keystone taxa were detected as network connectors and module hubs. All the modules had close relationships with environmental variables, and many keystone taxa were negatively associated with soil moisture and SOC. These results suggest that grassland degradation might be responsible for shifts in soil bacterial communities in terms of alpha diversity and community structure through changes in soil moisture, SOC, nutrients, and C:nutrient ratios. Moreover, in the network analyses, the strong co-occurrence relationships between taxa as well as close relationships between environmental variables and module structures and keystone taxa suggest low stability and high vulnerability of bacterial communities to the influences of grassland degradation.

Grassland degradation, a process of retrogressive succession, is an extensive global threat, especially on the Qinghai-Tibet Plateau. However, the influences of grassland degradation on carbon (C): nitrogen (N): phosphorus (P) ratios of streamwater and biofilms, as well as their implications for nutrient limitation, are not well studied. Assessing nutrient limitation and stoichiometry could facilitate understanding and mitigating the effects of nutrient input variations on stream ecosystems. Using a “space for time” substitution, contemporary grassland status quantified by landscape-level normalized difference vegetation index (NDVI) were used to study potential impacts of grassland degradation on nutrient cycling in streams feeding Qinghai Lake (China). C, N, and P concentrations and ratios of streamwater and benthic biofilms were assessed. The results showed that grassland degradation was associated with lower DOC, TN, DOC:TP, and TN:TP of streamwater, especially in September. DOC:TN:TP ratios of streamwater demonstrated relatively high variability with an average of 387:169:1 (molar). C, N, P biomass per unit area of stream biofilm also demonstrated considerable variability but their ratios were relatively constrained, with an average of 471:32:1. Moreover, biofilm C, N, and P concentrations and ratios were not closely associated with streamwater nutrients or grassland degradation. The complex chemical and biological composition of stream biofilms may make their stoichiometry difficult to interpret. However, the non-isometric relationships between biofilm C and P, as well as the large imbalance between C:N:P ratios of streamwater and biofilms, suggest that stream biofilms in this catchment are sensitive to P variation and potentially limited by DOC and P. By differentially impacting DOC and nutrient supplies, grassland degradation may alleviate P limitation but aggravate carbon limitation in stream biofilms.

Fires can alter the elemental stoichiometry of ecosystems, reflecting altered patterns of biogeochemical cycling in the post-fire environment. However, elements other than carbon (C), nitrogen (N), and phosphorus (P) have rarely been studied in this context. Thus, we aimed to expand the understanding of fire’s stoichiometric and biogeochemical effects to encompass a broader suite of biogenic elements. We compared the stoichiometric ratios of C, N, P, potassium (K), sodium (Na), magnesium (Mg), and sulfur (S) in soil, plant litter, and beetles (Thalycrodes pulchrum) between forest plots that have been burned biennially at low intensity since 1972 and plots that have remained unburned. Multi-element stoichiometry differed strongly between the fire regimes. Low intensity biennial burning was associated with depletion of C, N, and S relative to P, K, and to a small extent Mg and Na, in soil and litter. The stoichiometry of T. pulchrum biomass was not significantly affected by fire regime, but fire regime-associated variation in the stoichiometry of T. pulchrum biomass was positively correlated with the fire-induced stoichiometric shifts in soil. The effects of low intensity prescribed fire on ecological stoichiometry extend to Na, K, Mg, and S in ways consistent with the respective potentials of these elements to volatilize during fire or accumulate in the absence of fire. These effects vary among ecosystem components in a manner that reveals the importance of biological processes and constraints as factors that determine the nature, magnitude, and potential consequences of the stoichiometric signatures of fire regimes. We conclude that long-term changes in fire regime can disrupt or even decouple the biogeochemical cycles of numerous biogenic elements in the soil–plant system.

Significance Invasive species are a leading cause of biodiversity loss, yet the mechanisms by which invaders progressively disrupt ecosystems and native species remain unclear. Using an extensive isotope dataset across multiple ecosystems and stages of invasion, we show that an invasive fish predator (lake trout) disrupted food webs by forcing native fishes to feed on suboptimal food sources in different habitats, resulting in the functional extirpation of the native predator, threatened bull trout. Our results provide insights into the magnitude, direction, and timing of food web disruption from invasive species and will be important for predicting ecosystem consequences of species invasions., Species invasions can have substantial impacts on native species and ecosystems, with important consequences for biodiversity. How these disturbances drive changes in the trophic structure of native food webs through time is poorly understood. Here, we quantify trophic disruption in freshwater food webs to invasion by an apex fish predator, lake trout, using an extensive stable isotope dataset across a natural gradient of uninvaded and invaded lakes in the northern Rocky Mountains, USA. Lake trout invasion increased fish diet variability (trophic dispersion), displaced native fishes from their reference diets (trophic displacement), and reorganized macroinvertebrate communities, indicating strong food web disruption. Trophic dispersion was greatest 25 to 50 y after colonization and dissipated as food webs stabilized in later stages of invasion (>50 y). For the native apex predator, bull trout, trophic dispersion preceded trophic displacement, leading to their functional loss in late-invasion food webs. Our results demonstrate how invasive species progressively disrupt native food webs via trophic dispersion and displacement, ultimately yielding biological communities strongly divergent from those in uninvaded ecosystems.

Background On the front lines of climate change, glacier termini play crucial roles in linking glaciers and downstream ecosystems during glacier retreat. However, we lack a clear understanding of biological processes that occur in surface and basal ice at glacier termini. Methods Here, we studied the bacterial communities in surface ice and basal ice (the bottom layer) of a glacier terminus in the Yangtze River Source, Qinghai–Tibet Plateau. Results Surface and basal ice harbored distinct bacterial communities but shared some core taxa. Surface ice communities had a higher α-diversity than those in basal ice and were dominated by Proteobacteria, Firmicutes, Bacteroidetes, Actinobacteria, and Cyanobacteria while basal ice was dominated by Firmicutes and Proteobacteria. The bacterial communities were also substantially different in functional potential. Genes associated with functional categories of cellular processes and metabolism were significantly enriched in surface ice, while genes connected to environmental information processing were enriched in basal ice. In terms of biogeochemical cycles of carbon, nitrogen, phosphorus, and sulfur, bacterial communities in surface ice were enriched for genes connected to aerobic carbon fixation, aerobic respiration, denitrification, nitrogen assimilation, nitrogen mineralization, sulfur mineralization, alkaline phosphatase, and polyphosphate kinase. In contrast, bacterial communities in basal ice were enriched for genes involved in anaerobic carbon fixation, fermentation, nitrate reduction, 2-aminoethylphosphonic acid pathway, G3P transporter, glycerophosphodiester phosphodiesterase, and exopolyphosphatase. Structural equation modeling showed that total nitrogen and environmental carbon:phosphorus were positively while environmental nitrogen:phosphorus was negatively associated with taxonomic β-diversity which itself was strongly associated with functional β-diversity of bacterial communities. Conclusions This study furthers our understanding of biogeochemical cycling of the mountain cryosphere by revealing the genetic potential of the bacterial communities in surface and basal ice at the glacier terminus, providing new insights into glacial ecology as well as the influences of glacier retreat on downstream systems.

Fire regimes are shifting under climate change. Decadal-scale shifts in fire regime can disrupt the biogeochemical cycling of carbon (C), nitrogen (N), and phosphorus (P) within forest ecosystems, but the full extent of these disruptions is unknown. It is also unclear whether these disruptions have consequences for the ecological characteristics (e.g., biomass, abundance, and composition) of microbial and invertebrate communities, which together comprise the majority of terrestrial biodiversity and underpin many ecosystem processes. The theoretical framework of ecological stoichiometry has great potential in this context, but it has rarely been used to develop an integrated understanding of the biogeochemical and ecological effects of altered fire regime across trophic levels. Using one of the world’s longest-running fire experiments, located in Queensland, Australia, we carried out a comprehensive investigation into the stoichiometric consequences of a decadal-scale divergence in prescribed fire frequency and their links to coinciding changes in various structural characteristics of forest floor microbial and invertebrate communities. Forty-three years of biennial burning led to significantly N-depleted and/or P-enriched stoichiometry in soil, leaf litter, leaf litter-associated microbial biomass, and certain groups of invertebrates (significant effect sizes ranging 27–36%), although total invertebrate community stoichiometry was unaffected. Microbial biomass was 42% lower in biennially burned soils. Invertebrate community composition differed between fire regime treatments on some sampling dates, but fire regime did not have consistent effects on invertebrate biomass of abundance. Microbial biomass and the abundances of some invertebrate taxa were depressed at particularly low and/or high resource N:P, consistent with a coupling of these variables to the stoichiometric effects of decadal-scale fire regime. Litter transplants likewise indicated that some invertebrate abundances were sensitive to litter properties over twelve months. Together, our results indicate that long-term changes in fire regime can decouple the within-ecosystem cycling of N and P, with N and P cycling growing more and less conservative, respectively, under high-frequency fire in a way that propagates throughout forest floor food webs. Our study provides new insights into the coupled biogeochemical and ecological responses of forest ecosystems to novel fire regimes and establishes a basis for a stoichiometric framework for fire ecology.

The processes of urbanization and industrialization within geological phosphorus-rich mountains (GPMn) have resulted in water degradation within southwest China. Lake Dianchi, one of the most eutrophicated lakes in China, has epitomized this issue. Clear understandings of phosphorus (P) mitigation efforts, the evolution of P budgets, and possible risks in the Dianchi system will benefit future eutrophication control, providing valuable lessons for other plateau freshwater lakes. In this study, we applied systematic review methodology to investigate the above questions, and then compared the results with other lakes worldwide. Generally, meta-analytical approaches have indicated P levels remain a key factor in causing algal blooms. Post-2015, the P budget of the Dianchi system, especially in Caohai section, was modified. However, it's still experiencing high pressures from P enrichment (Caohai: 0.4 mg·l−1; Waihai: 0.2 mg·l−1). The flux of P in Dianchi remains high, both through the external P load (556 ton·a−1), and an internal cycle (304 ton·a−1 associated with the absorption, deposition and removal of algae biomass; and 380 ton·a−1 associated with sediment exchange). Meanwhile, significant P retention has been observed in the lake, in particular within the Waihai section (211 ton·a−1). Currently, water diversion (from external watersheds), sewage diversion, and sediment-dredging projects have benefited Dianchi. However, continuous urbanization and GPMn ecological degradation could introduce hundreds of tons of additional P, leading to subsequent algal blooms. Furthermore, beyond Lake Dianchi, other lakes and reservoirs in southwest China are facing similar issues regarding P mitigation, especially in GPMn regions, though corresponding knowledge is still limited. Therefore, effective and flexible sub-regional protection strategies and research related to external and internal P mitigations have become key requirements for Lake Dianchi management. Meanwhile, ecologically sensitive approaches to GPMn regions, as well as city development within basin and market driven treatments, should be incorporated into regional water source protection for southwest China.

1. Ecological stoichiometry theory was applied to investigate how a consumer contends with an extreme elemental mismatch between its food and its body via symbiotic facilitation. 2. The beetle Dendroctonus brevicomis LeConte develops in bark, a substrate extremely low in nitrogen (N) and phosphorus (P). Its survival there depends on interactions with mutualist and antagonist fungi. 3. This study found that mutualists transfer N and P from sapwood and phloem into bark, where beetles feed, whereas the antagonist moves these elements only to phloem, resulting in starvation of the insect. However, even with mutualists, N and P concentrations remained low in bark, resulting in low N and extremely low P concentrations in the beetle. 4. The N:P ratios found in D. brevicomis larvae were the highest thus far reported for beetles and among the highest for insects and invertebrates. This suggests that the beetle has evolved additional, nutrient‐sparing adaptations.

Microplastics are contaminants that are closely associated with human activity and are often abundant even in remote areas. As the largest natural freshwater lake in the western USA, Flathead Lake is a suitable site to study microplastics in lakes in less-populated areas of North America. Our assessment of microplastics in lake surface water samples showed that microplastic densities and concentrations in Flathead Lake were similar to those in other lakes located in less-populated areas around the world, with densities ranging from 8.00 × 10

Combined effects of cumulative nutrient inputs and biogeochemical processes that occur in freshwater under anthropogenic eutrophication could lead to myriad shifts in nitrogen (N):phosphorus (P) stoichiometry in global freshwater ecosystems, but this is not yet well-assessed. Here we evaluated the characteristics of N and P stoichiometries in bodies of freshwater and their herbaceous macrophytes across human-impact levels, regions and periods. Freshwater and its macrophytes had higher N and P concentrations and lower N : P ratios in heavily than lightly human-impacted environments, further evidenced by spatiotemporal comparisons across eutrophication gradients. N and P concentrations in freshwater ecosystems were positively correlated and N : P was negatively correlated with population density in China. These results indicate a faster accumulation of P than N in human-impacted freshwater ecosystems, which could have large effects on the trophic webs and biogeochemical cycles of estuaries and coastal areas by freshwater loadings, and reinforce the importance of rehabilitating these ecosystems.

Omnivorous fishes are prevalent in warm waters and may have strong impacts on water quality by excreting nutrients and reducing periphyton biomass. However, most studies have focused on large-sized species and overlooked the role of small omnivores. Filter-feeding mussels may modulate the negative effects of small omnivorous fishes on water quality, and stocking of mussels has been frequently used in shallow eutrophic freshwaters in China to improve the water clarity. However, the mechanisms behind such management practices are poorly studied. We conducted a mesocosm experiment to examine the ecosystem effects of the bitterling Acheilognathus macropterus as modulated by the mussel Sinanodonta woodiana, one of the mussels upon which it relies for breeding. We hypothesized that bitterling would exert negative effects on the lake environment, specifically higher phytoplankton biomass and lower water clarity, but that these effects might be alleviated by the filter-feeding activities of S. woodiana. In a 56-d mesocosm experiment with and without bitterling in the presence and absence of mussels, we found interactive effects of bitterling and mussels. In mesocosms with bitterling, nutrient concentrations, phytoplankton biomass, and total suspended solids (TSS) increased, but there were no changes in periphyton biomass in the mussel-free treatments. In contrast to the effects of large-sized omnivorous fishes reported from the literature, bitterling mainly affected TSS levels by increasing organic suspended solids rather than inorganic solids, indicating weak effects on sediment resuspension. However, the presence of mussels alleviated the negative effects of bitterling by decreasing nutrient levels, phytoplankton biomass, and TSS concentrations. Mussels alone had no effects on periphyton biomass, but the mussel–bitterling interactions boosted the growth of periphyton. Our study suggests that the negative effects of bitterling on water quality (e.g., increased nutrient concentrations and phytoplankton biomass) are alleviated by the presence of filter-feeding mussels, but the stimulatory interactive effects of mussels and bitterling on periphyton may impair the recovery of submerged macrophytes.

One debate about lake restoration is whether there is overemphasis of a “Phosphorus (P)-only” paradigm, while overlooking the limiting effect of nitrogen (N). In particular, the roles of internal nutrient cycling which could act as drivers of algal blooms are not yet well assessed. However, it is hardly possible to identify the complex mechanisms of nutrient limitation patterns in lakes only by in-situ experiments or monitored data. Numerical modeling can serve as a complementary approach by providing quantitative internal variations with high spatial and temporal resolutions. A three-dimensional model-based N:P stoichiometric approach was developed to explore the storage pools and fluxes that affect the limiting nutrient in hyper-eutrophic Lake Dianchi, the most eutrophic large lake in China with seasonal N limitation. The results highlighted the role of benthic P fluxes, which fluctuated considerably within a year and could supply enough P for algae, contributing over 50% of P input and leading to relative N deficiency during algal bloom. Further insights into N cycling indicated that N deficiency could be attributed to low N fixation and extremely high N losses from denitrification (~ 50% of total loss). Considering the continuous sediment P release and N losses that promote N deficiency, controlling both N and P loadings will benefit lake restoration.

Large-scale and rapid improvement in wastewater treatment is common practice in developing countries, yet this influence on nutrient regimes in receiving waterbodies is rarely examined at broad spatial and temporal scales. Here, we present a study linking decadal nutrient monitoring data in lakes with the corresponding estimates of five major anthropogenic nutrient discharges in their surrounding watersheds over time. Within a continuous monitoring dataset covering the period 2008 to 2017, we find that due to different rates of change in TN and TP concentrations, 24 of 46 lakes, mostly located in China's populated regions, showed increasing TN/TP mass ratios; only 3 lakes showed a decrease. Quantitative relationships between in-lake nutrient concentrations (and their ratios) and anthropogenic nutrient discharges in the surrounding watersheds indicate that increase of lake TN/TP ratios is associated with the rapid improvement in municipal wastewater treatment. Due to the higher removal efficiency of TP compared with TN, TN/TP mass ratios in total municipal wastewater discharge have continued to increase from a median of 10.7 (95% confidence interval, 7.6 to 15.1) in 2008 to 17.7 (95% confidence interval, 13.2 to 27.2) in 2017. Improving municipal wastewater collection and treatment worldwide is an important target within the 17 sustainable development goals set by the United Nations. Given potential ecological impacts on biodiversity and ecosystem function of altered nutrient ratios in wastewater discharge, our results suggest that long-term strategies for domestic wastewater management should not merely focus on total reductions of nutrient discharges but also consider their stoichiometric balance.

The sustainable management of phosphorus (P) includes recycling and enhancing P-use efficiency (PUE) in agriculture. In this study, we compared plant yield and the PUE of lettuce growing with soil amendments of biosolids from three wastewater treatment plants in comparison to commercial fertilizer. Furthermore, we used AVP1-transformed lettuce (Lactuca sativa cv. Conquistador), which is genetically improved to enhance its PUE, and compared its performance to non-transformed (wildtype, WT) lettuce in greenhouse conditions. AVP1 lettuce produced higher yield than WT lettuce only with commercial-fertilizer treatments; the yield with biosolid treatments did not vary between the two lettuce types. PUE did not differ between WT and AVP1 lettuce but was higher for commercial fertilizer than for biosolids. WT lettuce had higher P content in below-ground biomass than AVP1 lettuce when both were treated with biosolids. This suggests that capability of AVP1 lettuce to acidify the root zone may have mobilized heavy metals from biosolids and these toxins reduced the yield, P uptake, and PUE in AVP1 lettuce. In particular, Cd and As contents were high in lettuce biomass from biosolid treatments and exceeded recommendations for human daily oral dose.

Grasslands across the world are being degraded due to the impacts of overgrazing and climate change. However, the influences of grassland degradation on carbon (C), nitrogen (N), and phosphorus (P) dynamics and stoichiometry in soil ecosystems are not well studied, especially at high elevations where ongoing climate change is most pronounced. Ecological stoichiometry facilitates understanding the biogeochemical cycles of multiple elements by studying their balance in ecological systems. This study sought to assess the responses of these soil elements to grassland degradation in the Qinghai Lake watershed on the Qinghai-Tibet Plateau (QTP), which has an average elevation of >4000 m and is experiencing serious grassland degradation due to its sensitivity and vulnerability to external disturbances. Substituting space for time, we quantified normalized difference vegetation index to gauge grassland degradation. C, N, and P concentrations and their molar ratios in soil and in soil microbial biomass were also measured. The results showed that grassland degradation decreased the concentrations of C and N, as well as the ratios of C:P and N:P in soil. The soil became relatively more P rich and thus N limitation is anticipated to be more apparent with grassland degradation. Moreover, C, N, and P concentrations in soil microbial biomass decreased with increased grassland degradation. C:N:P ratios of soil microbial biomass were highly constrained, suggesting that soil microorganisms exhibited a strong homeostatic behavior, while the variations of microbial biomass C:N:P ratios suggest changes in microbial activities and community structure. Overall, our study revealed that grassland degradation differentially affects soil C, N, and P, leading to decreased C:N and N:P in soil, as well as decreased C, N, and P concentrations in soil microbial biomass. This study provides insights from a stoichiometric perspective into microbial and biogeochemical responses of grassland ecosystems as they undergo degradation on the QTP.

Based on the elemental composition of major biochemical molecules associated with different biological functions, the 'growth rate hypothesis' proposed that organisms with a higher growth rate would be coupled to lower C:N, especially lower C:P and N:P ratios. However, the applicability of the growth rate hypothesis for plants is unclear, especially for shrubs growing under different water supply. We performed an experiment with eight soil moisture levels (soil water content: 4%, 6%, 8%, 13%, 18%, 23%, 26% and 28%) to evaluate the effects of water availability on leaf C:N:P stoichiometry in the shrub Zygophyllum xanthoxylum. We found that leaves grew slowly and favored accumulation of P over C and N under both high and low water supply. Thus, leaf C:P and N:P ratios were unimodally related to soil water content, in parallel with individual leaf area and mass. As a result, there were significant positive correlations between leaf C:P and N:P with leaf growth (u). Our result that slower-growing leaves had lower C:P and N:P ratios does not support the growth rate hypothesis, which predicted a negative association of N:P ratio with growth rate, but it is consistent with recent theoretical derivations of growth-stoichiometry relations in plants, where N:P ratio is predicted to increase with increasing growth for very low growth rates, suggesting leaf growth limitation by C and N rather than P for drought and water saturation.

Vegetation fire often raises levels of soil phosphorus (P) relative to those of nitrogen (N), while long-term fire exclusion is associated with accumulation of soil N and depletion of soil available P. Thus, high-frequency fire regimes might trigger N-limited plant growth in otherwise P-limited plant communities. We used soils from a long-term prescribed burning trial to grow Eucalyptus pilularis under several nutrient amendment conditions. We measured seedling growth, soil and plant chemistry, and root enzyme activities to assess nutrient status. Biennially-burned (2yB) soils had higher labile P concentrations than long-unburned (NB) soils, and lower total and labile N:P ratios. This did not correspond to N-limited growth or stimulate seedling N demand. Seedlings grown with addition of N, P and micro-nutrients in combination (μMax treatment) attained 68% more biomass than unfertilised seedlings. Addition of P resulted in higher total biomass than addition of N, and similar biomass to the μMax treatment, suggesting partially P-limited growth. Plants grown in 2yB soils tended to be enriched with P, K, Ca and Mg compared to those grown in NB or 4yB soils. High-frequency prescribed burning depletes soil N relative to P, but this does not trigger a shift toward N-limited growth of E. pilularis seedlings. Instead, E. pilularis seedlings appear to grow under partial P-limitation which persists regardless of their soil’s fire history.

Biological soil crusts (biocrusts) are a vital biotic component of dryland ecosystems that impact global nitrogen (N) and carbon (C) cycling. Water availability is the main controller of physiological function of biocrusts in these ecosystems, yet few studies have compared the carbon balance of different biocrusts responding to different water treatments. In this study, we explored the effects of water availability on carbon exchanges of different biocrusts by measuring in situ net CO2 fluxes in cyanobacteria and moss-dominated biocrusts under four water treatments (2 mm, 5 mm, 10 mm, and 20 mm) in a fenced grassland on the Loess Plateau, China. Our results indicate that water availability played a vital role in carbon balance of the biocrusts. Under higher water additions, both cyanobacteria and moss-dominated biocrusts fixed carbon but lower water additions did not always lead to net carbon gains, especially for the moss-dominated crusts. Importantly, our data indicate the existence of water thresholds for net carbon fixation of both cyanobacteria and moss-dominated biocrusts, below which carbon loss occurs. This implies that higher water availability would benefit moss growth, while lower water availability results in suitable circumstance for the growth of cyanobacteria.

It is vital to understand processes of microplastic ingestion and egestion by aquatic organisms in order to evaluate the potential effects and impacts of microplastics in aquatic ecosystems. In this study, goldfish (Carassius auratus) was used to investigate ingestion and egestion of polyethylene (PE) microplastics and how these processes were affected by size, color, and shape of microplastics. Results showed that goldfish ingested white PE microplastics only in the presence of fish feed and that microplastics larger than 2 mm were rejected even after being ingested. However, in the presence of food, more green and black microplastics were ingested compared with red, blue, and white microplastics while significantly higher amounts of microplastic films were ingested compared with fragments and filaments. Microplastics ingested by goldfish were egested within 72 h. However, the egestion rate of filaments was the lowest among all tested microplastic shapes. The presence of food appeared to reduce film and filament residues in fish after 72 h. Results of this study imply that different features of microplastics result in different exposure risks for fish. Thus, the specific features of microplastics (e.g. their shape, color, and size) should be considered in future ecotoxicological studies., Environmental chemistry; Environmental hazard; Environmental pollution; Environmental risk assessment; Environmental toxicology; Microplastics; Fish; Ingestion; Egestion; Environmental features.

The biogeochemical and stoichiometric signature of vegetation fire may influence post-fire ecosystem characteristics and the evolution of plant 'fire traits'. Phosphorus (P), a potentially limiting nutrient in many fire-prone environments, might be particularly important in this context; however, the effects of fire on P cycling often vary widely. We conducted a global-scale meta-analysis using data from 174 soil studies and 39 litter studies, and found that fire led to significantly higher concentrations of soil mineral P as well as significantly lower soil and litter carbon:P and nitrogen:P ratios. These results demonstrate that fire has a P-rich signature in the soil-plant system that varies with vegetation type. Further, they suggest that burning can ease P limitation and decouple the biogeochemical cycling of P, carbon and nitrogen. These effects resemble a transient reversion to an earlier stage of ecosystem development, and likely underpin at least some of fire's impacts on ecosystems and organisms. © 2018 John Wiley & Sons Ltd/CNRS.