Interactive effects of light and nutrients on phytoplankton stoichiometry

The stoichiometric composition of autotrophs can vary greatly in response to variation in light and nutrient availability, and can mediate ecological processes such as C sequestration, growth of herbivores, and nutrient cycling. We investigated light and nutrient effects on phytoplankton stoichiometry, employing five experiments on intact phytoplankton assemblages from three lakes varying in productivity and species composition. Each experiment employed two nutrient and eight irradiance levels in a fully factorial design. Light and nutrients interactively affected phytoplankton stoichiometry. Thus, phytoplankton C:N, C:P, and N:P ratios increased with irradiance, and slopes of the stoichiometric ratio versus irradiance relationships were steeper with ambient nutrients than with nutrients added. Our results support the light–nutrient hypothesis, which predicts that phytoplankton C:nutrient ratios are functions of the ratio of available light and nutrients; however, we observed considerable variation among lakes in the expression of this relationship. Phytoplankton species diversity was positively correlated with the slopes of the C:N and C:P versus irradiance relationships, suggesting that diverse assemblages may exhibit greater flexibility in the response of phytoplankton nutrient stoichiometry to light and nutrients. The interactive nature of light and nutrient effects may render it difficult to generate predictive models of stoichiometric responses to these two factors. Our results point to the need for future studies that examine stoichiometric responses across a wide range of phytoplankton communities.     

Elemental stoichiometry of the key calcifying marine phytoplankton Emiliania huxleyi under ocean climate change: A meta-analysis

The elemental composition of marine microorganisms (their C:N:P ratio, or stoichiometry) is central to understanding the biotic and biogeochemical processes underlying key marine ecosystem functions. Phytoplankton C:N:P is species specific and flexible to changing environmental conditions. However, bulk or fixed phytoplankton stoichiometry is usually assumed in biogeochemical and ecological models because more realistic, environmentally responsive C:N:P ratios have yet to be defined for key functional groups. Here, a comprehensive meta-analysis of experimental laboratory data reveals the variable C:N:P stoichiometry of Emiliania huxleyi, a globally significant calcifying phytoplankton species. Mean C:N:P of E. huxleyi is 124C:16N:1P under control conditions (i.e. growth not limited by one or more environmental stressors) and shows a range of responses to changes in nutrient and light availability, temperature and pCO₂. Macronutrient limitation caused strong shifts in stoichiometry, increasing N:P and C:P under P deficiency (by 305% and 493% respectively) and doubling C:N under N deficiency. Responses to light, temperature and pCO₂ were mixed but typically shifted cellular elemental content and C:N:P stoichiometry by ca. 30% or less. Besides these independent effects, the interactive effects of multiple environmental changes on E. huxleyi stoichiometry under future ocean conditions could be additive, synergistic or antagonistic. To synthesise our meta-analysis results, we explored how the cellular elemental content and C:N:P stoichiometry of E. huxleyi may respond to two hypothetical future ocean scenarios (increased temperature, irradiance and pCO₂ combined with either N deficiency or P deficiency) if an additive effect is assumed. Both future scenarios indicate decreased calcification (which is predominantly sensitive to elevated pCO₂), increased C:N, and up to fourfold shifts in C:P and N:P. Our results strongly suggest that climate change will significantly alter the role of E. huxleyi (and potentially other calcifying phytoplankton species) in marine biogeochemical processes.     

The C:N:P ratio of phytoplankton determines the relative amounts of dissolved inorganic nitrogen and phosphorus released during aerobic decomposition

Three phytoplankton assemblages, with different C:N:P ratios of 314:55:1, 103:17:1, and 57:5.5:1 (by weight), were prepared by growing lake phytoplankton in artificial media with different N:P supply ratios and then decomposed under aerobic conditions for three months.There was no net release of dissolved inorganic phosphorus (DIP) from the phytoplankton assemblage with the highest C:N:P ratio, in contrast with an abundant liberation of DIP from that with the lowest C:N:P ratio. On the other hand, there was an abundant release of dissolved inorganic nitrogen (DIN) from the phytoplankton assemblage with the highest C:N:P ratio, in contrast with little or virtually no release of DIN from those with lower C:N:P ratios. Thus, it was concluded that the C:N:P ratio of phytoplankton is an important parameter to determine the relative amounts of DIP and DIN released, when they are decomposed under aerobic conditions.Contribution from Otsu Hydrobiological Station, Kyoto University, No. 316 (Foreign Language Series).Contribution from Otsu Hydrobiological Station, Kyoto University, No. 316 (Foreign Language Series).     

Nutrient stoichiometry of linked catchment‐lake systems along a gradient of land use

1. Catchments export nutrients to aquatic ecosystems at rates and ratios that are strongly influenced by land use practices, and within aquatic ecosystems nutrients can be processed, retained, lost to the atmosphere, or exported downstream. The stoichiometry of carbon and nutrients can influence ecosystem services such as water quality, nutrient limitation, biodiversity, eutrophication and the sequestration of nutrients and carbon in sediments. However, we know little about how nutrient stoichiometry varies along the pathway from terrestrial landscapes through aquatic systems. 2. We studied the stoichiometry of nitrogen and phosphorus exported by three catchments of contrasting land use (forest versus agriculture) and in the water column and sediments of downstream reservoirs. We also related stoichiometry to phytoplankton nutrient limitation and the abundance of heterocystous cyanobacteria. 3. The total N : P of stream exports varied greatly among catchments and was 18, 54 and 140 (molar) in the forested, mixed‐use and agricultural catchment, respectively. Total N : P in the mixed layers of the lakes was less variable but ordered similarly: 35, 52 132 in the forested, mixed‐use and agricultural lake, respectively. In contrast, there was little variation among systems in the C : N and C : P ratios of catchment exports or in reservoir seston. 4. Phytoplankton in the forested lake were consistently N limited, those in the agricultural lake were consistently P limited, and those in the mixed‐use lake shifted seasonally from P‐ to N limitation, reflecting N : P supply ratios. Total phytoplankton and cyanobacteria biomass were highest in the agricultural lake, but heterocystous (potentially N fixing) cyanobacteria were most abundant in the forested lake, corresponding to low N : P ratios. 5. Despite large differences in catchment export and water column N : P ratios, the N : P of sediment burial (integrated over several decades) was very low and remarkably similar (4.3–7.3) across reservoirs. N and P budgets constructed for the agricultural reservoir suggested that denitrification could be a major loss of N, and may help explain the relatively low N : P of buried sediment. 6. Our results show congruence between the catchment export N : P, reservoir N : P, phytoplankton N versus P limitation and the dominance of heterocystous cyanobacteria. However, the N : P stoichiometry of sediments retained in the lakes was relatively insensitive to catchment stoichiometry, suggesting that a common set of biogeochemical processes constrains sediment N : P across lakes of contrasting catchment land use.     

浙江天童常绿阔叶林、常绿针叶林与落叶阔叶林的C:N:P化学计量特征

以浙江天童常绿阔叶林、常绿针叶林和落叶阔叶林为对象, 通过对叶片和凋落物C:N:P比率与N、P重吸收的研究, 揭示3种植被类型N、P养分限制和N、P重吸收的内在联系。结果显示: 1)叶片C:N:P在常绿阔叶林为758:18:1, 在常绿针叶林为678:14:1, 在落叶阔叶林为338:11:1; 凋落物C:N:P在常绿阔叶林为777:13:1, 常绿针叶林为691:14:1, 落叶阔叶林为567:14:1; 2)常绿阔叶林和常绿针叶林叶片与凋落物C:N均显著高于落叶阔叶林; 叶片C:P在常绿阔叶林最高, 常绿针叶林中等, 落叶阔叶林最低, 常绿阔叶林和常绿针叶林凋落物C:P显著高于落叶阔叶林; 叶片N:P比也是常绿阔叶林最高、常绿针叶林次之, 落叶阔叶林最低, 但常绿阔叶林凋落物N:P最低; 3)植被叶片N、P含量间(N为x, P为y)的II类线性回归斜率显著大于1 (p < 0.05), 表明叶片P含量的增加可显著提高叶片N含量; 凋落物N、P含量的回归斜率约等于1, 反映了凋落物中单位P含量与单位N含量间的等速损耗关系; 4)常绿阔叶林N重吸收率显著高于常绿针叶林与落叶阔叶林, 落叶阔叶林P重吸收率显著高于常绿阔叶林和常绿针叶林。虽然植被的N:P指示常绿阔叶林受P限制, 落叶阔叶林受N限制, 常绿针叶林受N、P的共同限制, 但是N、P重吸收研究结果表明: 受N素限制的常绿阔叶林具有高的N重吸收率, 受P限制的落叶阔叶林并不具有高的P重吸收率。可见, 较高的N、P养分转移率可能不是植物对N、P养分胁迫的一种重要适应机制, 是物种固有的特征。     

The relationship between relative growth rate and whole-plant C:N:P stoichiometry in plant seedlings grown under nutrient-enriched conditions

Aims Recent theories indicate that N is more in demand for plant growth than P; therefore, N concentration and N : C and N : P ratios are predicted to be positively correlated with relative growth rate (RGR) in plants under nutrient-enriched conditions. This prediction was tested in this study.Methods We examined the whole-plant concentrations of C, N and P and RGR, as well as the relationship between RGR and the concentrations and the ratios of N : C, P : C and N : P, for different harvest stages (the days after seed germination) of the seedlings of seven shrub species and four herbaceous species grown in N and P non-limiting conditions. The relationships among plant size, nutrient concentrations and ratios were subsequently determined.Important findings RGR was positively correlated with N concentration and the ratios of N : P and N : C when the data were pooled for all species and for each shrub species, but not for individual herbaceous species. However, the relationship between RGR and P concentration and P : C was not significantly correlated for either shrubs or herbs. The variation of N among harvest stages and species was much greater than that of P, and the variation in N : P ratio was determined primarily by changes in N concentration. The shrub species differed from the herbaceous species in their N and P concentrations, nutrient ratios and in intraspecific relationships between RGR and nutrient ratios. These differences possibly reflect differences in the capacity for P storage and biomass allocation patterns. In general, our data support recent theoretical predictions regarding the relationship between RGR and C : N : P stoichiometry, but they also show that species with different life forms differ in the relationships among RGR and C : N : P stoichimetries.     

Inter- and intra-annual variability in the phytoplankton community of a high mountain lake: the influence of external (atmospheric) and internal (recycled) sources of phosphorus

1. The inter‐ and intra‐annual changes in the biomass, elemental (carbon (C), nitrogen (N) and phosphorus (P)) and taxonomical composition of the phytoplankton in a high mountain lake in Spain were studied during 3 years with different physical (fluctuating hydrological regime) and chemical conditions. The importance of internal and external sources of P to the phytoplankton was estimated as the amount of P supplied via zooplankton recycling (internal) or through ice‐melting and atmospheric deposition (external). 2. Inter‐annual differences in phytoplankton biomass were associated with temperature and total dissolved phosphorus. In 1995, phytoplankton biomass was positively correlated with total dissolved phosphorus. In contrast, the negative relationship between zooplankton and seston biomass (direct predatory effects) and the positive relationship between zooplankton P excretion and phytoplankton biomass in 1997 (indirect P‐recycling effects), reinforces the primary role of zooplankton in regulating the total biomass of phytoplankton but, at the same time, encouraging its growth via P‐recycling. 3. Year‐to‐year variations in seston C : P and N : P ratios exceeded intra‐annual variations. The C : P and N : P ratios were high in 1995, indicating strong P limitation. In contrast, in 1996 and 1997, these ratios were low during ice‐out (C : P < 100 and N : P < 10) and increased markedly as the season progressed. Atmospheric P load to the lake was responsible for the decline in C : P and N : P ratios. 4. Intra‐annual variations in zooplankton stoichiometry were more pronounced than the overall differences between 1995 and 1996. Thus, the zooplankton N : P ratio ranged from 6.9 to 40.1 (mean 21.4) in 1995, and from 10.4 to 42.2 (mean 24.9) in 1996. The zooplankton N : P ratio tended to be low after ice‐out, when the zooplankton community was dominated by copepod nauplii, and high towards mid‐ and late‐season, when these were replaced by copepodites and adults. 5. In 1995, the minimum demands for P of phytoplankton were satisfied by ice‐melting, atmospheric loading and zooplankton recycling over 100%. In order of importance, atmospheric inputs (> 1000%), zooplankton recycling (9–542%), and ice‐melting processes (0.37–5.16%) satisfied the minimum demand for P of phytoplankton during 1996 and 1997. Although the effect of external forces was rather sporadic and unpredictable in comparison with biologically driven recycle processes, both may affect phytoplankton structure and elemental composition. 6. We identified three conceptual models representing the seasonal phosphorus flux among the major compartments of the pelagic zone. While ice‐melting processes dominated the nutrient flow at the thaw, biologically driven processes such as zooplankton recycling became relevant as the season and zooplankton ontogeny progressed. The stochastic nature of P inputs associated with atmospheric events can promote rapid transitional changes between a community limited by internal recycling and one regulated by external load. 7. The elemental composition of the zooplankton explains changes in phytoplankton taxonomic and elemental composition. The elemental negative balance (seston N : P < zooplankton N : P, low N : P recycled) during the thaw, would promote a community dominated by species with a high demand for P (Cryptophyceae). The shift to an elemental positive balance (seston N : P > zooplankton N : P, high N : P recycled) in mid‐season would skew the N : P ratio of the recycled nutrients, favouring dominance by chrysophytes. The return to negative balance, as a consequence of the ontogenetic increase in zooplankton N : P ratio and the external P inputs towards the end of the ice‐free season, could alleviate the limitation of P and account for the appearance of other phytoplankton classes (Chlorophyceae or Dinophyceae).     

Effects of carbon concentration and carbon-to-nitrogen ratio on growth, conidiation, spore germination and efficacy of the potential bioherbicide Colletotrichum coccodes

The effect of carbon concentration and carbon-to-nitrogen ratio (C:N) as well as their interaction on Colletotrichum coccodes growth and sporulation in submerged flask culture were evaluated. When C:N ratios were held constant, both mycelial dry biomass and spore yield increased with increasing carbon concentration. The specific spore yields (spore yield g⁻¹ carbon), however, were not significantly different for the same C:N ratio in most cases. The highest spore yields (1.3 × 10⁸ spores per ml) were obtained from media containing 20 g per liter carbon with C:N ratios ranging from 5:1 to 10:1. When the C:N ratio was greater than 15:1, spore yields were significantly decreased with increasing C:N ratios. High carbon concentration (20 g L⁻¹) combined with high C:N ratios (above 15:1) reduced both mycelial growth and sporulation, and increased spore matrix production. Spores produced in medium containing 10 g L⁻¹ carbon with C:N ratios from 10:1 to 15:1 had 90% germination on potato dextrose agar after 12 h and caused extensive shoot dry weight reduction on the target weed, velvetleaf. These results suggest that C:N ratios from 10:1 to 15:1 are optimal for C. coccodes spore production. Received 9 December 1997/ Accepted in revised form 22 May 1998     

Effects of zooplankton on nutrient availability and seston C : N : P stoichiometry in inshore waters of Lake Biwa, Japan

Forty-eight-hour experimental manipulations of zooplankton biomass were performed to examine the potential effects of zooplankton on nutrient availability and phytoplankton biomass (as measured by seston concentration) and C : N : P stoichiometry in eutrophic nearshore waters of Lake Biwa, Japan. Increasing zooplankton, both mixed-species communities and Daphnia alone, consistently reduced seston concentration, indicating that nearshore phytoplankton were generally edible. The zooplankton clearance rates of inshore phytoplankton were similar to rates measured previously for offshore phytoplankton. Increased zooplankton biomass led to increased concentrations of nutrients (NH₄-N, soluble reactive phosphorus [SRP]). Net release rates were higher than those found in previous measurements made offshore, reflecting the nutrient-rich nature of inshore seston. Zooplankton nutrient recycling consistently decreased TIN : SRP ratios (TIN = NH₄ + NO₃ + NO₂). This effect probably resulted from the low N : P ratios of nearshore seston, which were lower than those commonly found in crustacean zooplankton and thus resulted in low retention efficiency of P (relative to N) by the zooplankton. Thus, zooplankton grazing inshore may ameliorate algal blooms due to direct consumption but tends to create nutrient supply conditions with low N : P, potentially favoring cyanobacteria. In comparison with previous findings for offshore, it appears that potential zooplankton effects on phytoplankton and nutrient dynamics differ qualitatively in inshore and offshore regions of Lake Biwa. Received: September 4, 2000 / Accepted: January 23, 2001     

Nutrient content of seagrasses and epiphytes in the northern Gulf of Mexico: Evidence of phosphorus and nitrogen limitation

We examined C:N:P ratios of seagrass leaves and epiphytic algae from the eastern shoreline of Grand Bay (Alabama, USA) and the entire shoreline of Big Lagoon (Florida, USA) during the summer of 2001 and March 2003, and used contour plotting of N:P ratios in both locations to examine spatial trends in our data. Results indicated phosphorus limitation for seagrass and epiphytes in each bay. In addition, C:N, C:P, and N:P ratios in both locations showed differences between summer and wintertime values for seagrasses; however, the only epiphytic elemental ratios to differ were C:P and N:P ratios in Grand Bay. Within Grand Bay, phosphorus limitation was stronger in epiphytes than seagrasses, with the largest amount of variation in N:P ratios occurring adjacent to the only developed land on the shoreline. In Big Lagoon, two distinct areas were present in N:P contour plots: the eastern end of the bay that was influenced by water from the Gulf of Mexico and Santa Rosa Sound, and the western end of the bay that was most influenced by Perdido Bay and a developed area along the northern shoreline. Detection of phosphorus limitation within Big Lagoon was not surprising, as both input sources to Big Lagoon are known to be low in phosphorus. However, phosphorus limitation in Grand Bay was unexpected, as both “feeder systems” (Mobile Bay and the Mississippi Sound) have high ambient phosphorus levels. As a result, C:N:P ratios from seagrasses and epiphytes may not accurately reflect ambient nutrient levels in Grand Bay due to decreased availability of some forms of phosphorus or increased competition for the uptake of phosphorus. Overall, our C:N:P analysis suggested that not only was P limitation greater than N limitation in Grand Bay and Big Lagoon, but patterns of nutrient limitation varied both temporally and geographically for inter- and intra-bay comparisons.     

C:N:P stoichiometry in soil: is there a “Redfield ratio” for the microbial biomass?

Well-constrained carbon:nitrogen:phosphorus (C:N:P) ratios in planktonic biomass, and their importance in advancing our understanding of biological processes and nutrient cycling in marine ecosystems, has motivated ecologists to search for similar patterns in terrestrial ecosystems. Recent analyses indicate the existence of “Redfield-like” ratios in plants, and such data may provide insight into the nature of nutrient limitation in terrestrial ecosystems. We searched for analogous patterns in the soil and the soil microbial biomass by conducting a review of the literature. Although soil is characterized by high biological diversity, structural complexity and spatial heterogeneity, we found remarkably consistent C:N:P ratios in both total soil pools and the soil microbial biomass. Our analysis indicates that, similar to marine phytoplankton, element concentrations of individual phylogenetic groups within the soil microbial community may vary, but on average, atomic C:N:P ratios in both the soil (186:13:1) and the soil microbial biomass (60:7:1) are well-constrained at the global scale. We did see significant variation in soil and microbial element ratios between vegetation types (i.e., forest versus grassland), but in most cases, the similarities in soil and microbial element ratios among sites and across large scales were more apparent than the differences. Consistent microbial biomass element ratios, combined with data linking specific patterns of microbial element stoichiometry with direct evidence of microbial nutrient limitation, suggest that measuring the proportions of C, N and P in the microbial biomass may represent another useful tool for assessing nutrient limitation of ecosystem processes in terrestrial ecosystems.     

Silicon application induces changes C:N:P stoichiometry and enhances stoichiometric homeostasis of sorghum and sunflower plants under salt stress

Beneficial effects of silicon (Si) on growth have been observed in some plant species, reportedly due to stoichiometric changes of C, N, and P. However, little is known about the effects on the stoichiometric relationships between C, N, and P when silicon is supplied via different modes in sorghum and sunflower plants under salt stress conditions. Therefore, the current study was performed to investigate the impact of differing modes of Si supply on shoot biomass production and C:N:P stoichiometry in sorghum and sunflower plants under salt stress. Two experiments were performed in a glass greenhouse using the strong Si-accumulator plant sorghum, as well as the intermediate type Si-accumulator sunflower, both of which were grown in pots filled with washed sand. Plant species were cultivated for 30 days in the absence or presence of salt stress (0 or 100 mM) and supplemented with one of four Si treatments: control plants (without Si), 28.6 mmol Si L⁻¹ via foliar application, 2.0 mmol Si L⁻¹ via nutrient solution, and combined application of foliar and nutrient solution, each group with five replications. The results revealed that supplied Si modified the C, N, and P concentrations, thereby enhancing the C:N:P stoichiometry and shoot dry matter of sorghum and sunflower plants under salt stress. Both application of Si via nutrient solution, as well as combined application via foliar and nutrient solution, increased the C:N ratio in both plant species under salt stress, but in sorghum plants decreased the C:P and N:P ratios and increased the shoot biomass production by 39%, while in sunflower plants increased the C:P and N:P ratios and increased the shoot biomass production by 24%. Our findings suggest that salt stress alleviation by Si impacts C:N:P stoichiometric relationships in a variable manner depending on the ability of the species to accumulate Si, as well as the route of Si administration.     

新疆67种荒漠植物叶碳氮磷计量特征及其与气候的关系

荒漠生态系统具有独特的耐旱植物种类和植物化学计量特征.本研究通过系统采集全疆63个荒漠地点的67种植物,探索荒漠植物叶碳、氮、磷(C、N、P)的计量特征及其与气候因子的关系.结果表明:荒漠植物叶C、N和P的平均含量分别为394、18.4和1.14 mg·g-1,C∶N、C∶P和N∶P平均值分别为28、419和18.灌木的N含量高于乔木和草本,灌木P含量比草本(乔木)低(高);C3植物叶的C、N、C∶P、N∶P高于C4植物.随年均降水量增加,叶C有先降低后升高的趋势,叶N、P先升高、后降低,叶C∶N、C∶P与叶N、P变化趋势相反,N∶P变化不显著;随年均温升高,叶C先降低后升高,叶N、P降低,C∶N变化不显著,C∶P、N∶P升高.年均降水量对叶片化学计量特征的影响大于年均温度和植物类型.本研究结果将有助于预测全球气候变化背景下的C、N、P元素循环规律的变化,并为干旱区生物地球化学建模提供参考和基础数据.     

Soil C:N:P stoichiometry responds to vegetation change from grassland to woodland

Woody encroachment has been a major land cover change in dryland ecosystems during the past century. While numerous studies have demonstrated strong effects of woody encroachment on soil carbon (C), nitrogen (N), and phosphorus (P) storage, far less is known about the plasticity of soil C:N:P stoichiometry in response to woody encroachment. We assessed landscape-scale patterns of spatial heterogeneity in soil C:N:P ratios throughout a 1.2 m soil profile in a region where grassland is being replaced by a diverse assemblage of subtropical woody plants dominated by Prosopis glandulosa, an N₂-fixing tree. Woody species had leaf and fine root C:N:P ratios significantly different from grasses. Variation in soil C:N ratios in both horizontal and vertical planes was remarkably smaller than that of soil N:P and C:P ratios. Spatial patterns of soil C:N ratio throughout the profile were not strongly related to vegetation cover. In contrast, spatial patterns of soil N:P and C:P ratios displayed a strong resemblance to that of vegetation cover throughout the soil profile. Within the uppermost soil layer (0–5 cm), soil N:P and C:P ratios were higher underneath woody patches while lower within the grassland; however, this pattern was reversed in subsurface soils (15–120 cm). These results indicate a complex response of soil C:N:P stoichiometry to vegetation change, which could have important implications for understanding C, N, and P interactions and nutrient limitations in dryland ecosystems.     

Strong seasonal patterns of DOC release by a temperate seaweed community: Implications for the coastal ocean carbon cycle

Release of dissolved organic carbon (DOC) by seaweed underpins the microbial food web and is crucial for the coastal ocean carbon cycle. However, we know relatively little of seasonal DOC release patterns in temperate regions of the southern hemisphere. Strong seasonal changes in inorganic nitrogen availability, irradiance, and temperature regulate the growth of seaweeds on temperate reefs and influence DOC release. We seasonally surveyed and sampled seaweed at Coal Point, Tasmania, over 1 year. Dominant species with or without carbon dioxide (CO₂) concentrating mechanisms (CCMs) were collected for laboratory experiments to determine seasonal rates of DOC release. During spring and summer, substantial DOC release (10.06–33.54 μmol C · g DW⁻¹ · h⁻¹) was observed for all species, between 3 and 27 times greater than during autumn and winter. Our results suggest that inorganic carbon (C_i) uptake strategy does not regulate DOC release. Seasonal patterns of DOC release were likely a result of photosynthetic overflow during periods of high gross photosynthesis indicated by variations in tissue C:N ratios. For each season, we calculated a reef-scale net DOC release for seaweed at Coal Point of 7.84–12.9 g C · m⁻² · d⁻¹ in spring and summer, which was ~16 times greater than in autumn and winter (0.2–1.0 g C · m⁻² · d⁻¹). Phyllospora comosa, which dominated the biomass, contributed the most DOC to the coastal ocean, up to ~14 times more than Ecklonia radiata and the understory assemblage combined. Reef-scale DOC release was driven by seasonal changes in seaweed physiology rather than seaweed biomass.     

Marine biodiversification in response to evolving phytoplankton stoichiometry

Diversification of the marine biosphere is intimately linked to the evolution of the biogeochemical cycles of carbon, nutrients, and primary productivity. A meta-analysis of the ratio of carbon-to-phosphorus buried in sedimentary rocks during the past 3 billion years indicates that both food quantity and, critically, food quality increased through time as a result of the evolving stoichiometry (nutrient content) of eukaryotic phytoplankton. Evolving food quantity and quality was primarily a function of broad tectonic cycles that influenced not just carbon burial, but also nutrient availability and primary productivity. Increasing nutrient availability during the middle-to-Late Proterozoic culminated in the production of food (phytoplankton biomass and fresh dead organic matter) with C:P Redfield ratios sufficient to finally promote geologically-rapid biodiversification during the Proterozoic–Phanerozoic transition. This resulted in further, massive nutrient sequestration into biomass that triggered positive feedback via nutrient recycling (bioturbation, mesozooplankton grazing) on phytoplankton productivity. Increasing rates and depths of bioturbation through the Phanerozoic suggest that nutrient recycling continued to increase. Increasing bioturbation and nutrient cycling appear to have been necessary to sustain the primary productivity and “energetics” (biomass, metabolic rates, and physical activity such as predation) of the marine biosphere because of the geologically-slow input of macronutrients like phosphorus from land and the continued sequestration of nutrients into marine and terrestrial biomass.     

Changes in soil-microbe-exoenzyme C:N:P stoichiometry along an altitudinal gradient in Mt. Datudingzi,Northeast China

海拔变化导致温度、水分、植被等条件的改变会显著影响土壤碳(C_soil)、氮(N_soil)、磷(P_soil)含量及其化学计量特征, 土壤微生物如何通过调整自身生物量和胞外酶化学计量特征进行适应仍不明确。为了研究海拔梯度变化对土壤微生物生物量和胞外酶活性的影响, 探索土壤-微生物-胞外酶C:N:P化学计量特征间的协变性, 该文以黑龙江省雪乡大秃顶子山800、1 100、1 600和1 700 m分布的典型生态系统(针阔混交林、针叶林、岳桦林和草地)为研究对象, 测定其C_soil、N_soil、P_soil含量, 微生物生物量C (C_mic)、N (N_mic)、P (P_mic)含量, 以及微生物获取C (β-1, 4-葡萄糖苷酶, BG), N (几丁质酶, NAG), P (酸性磷酸酶, AP)资源的相关胞外酶活性。结果表明: (1)海拔梯度变化对C_soil和C_mic含量没有显著影响; 不同海拔间土壤和微生物生物量N、P含量存在显著差异。(2) BG和NAG活性随着海拔的升高呈现显著降低趋势, 表明海拔升高导致的温度降低抑制了微生物的活性。(3)海拔对土壤C:N、微生物C:N:P以及胞外酶C:N:P均具有显著影响。胞外酶C:N:P随着微生物与土壤间C:N:P化学计量不平衡性(土壤C:N:P与微生物C:N:P的比值)的增加而逐渐降低。微生物可以通过调整自身生物量以及胞外酶C:N:P适应土壤化学计量特征的变异, 该结果支持了微生物的资源分配理论。     

N limitation increases along a temperate forest succession: evidences from leaf stoichiometry and nutrient resorption

温带森林演替加剧了氮限制：来自叶片化学计量和养分重吸收的证据 森林生产力和碳汇功能在很大程度上取决于土壤氮和磷的有效性。然而,迄今为止,养分限制随森林演替的时间变化仍存在争议。叶片化学计量和养分重吸收是预测植物生长养分限制的重要指标。基于此,本研究测定了温带森林4个演替阶段所有木本植物叶片和凋落叶中氮和磷的含量,并分析了演替过程中非生物因子和生物因子如何影响叶片化学计量和养分重吸收。研究结果表明,在个体尺度上,叶片氮磷含量在演替末期显著增加,而叶片氮磷比无显著变化;氮的重吸收效率随演替显著增加,然而磷的重吸收效率先增加后减少;氮重吸收效率与磷重吸收效率的比值仅在演替末期显著增加。此外,植物氮素循环对土壤养分的响应比磷素循环更弱。在群落尺度上,叶片氮磷含量随森林演替呈现先降低后升高的趋势,主要受香农-维纳多样性指数和物种丰富度的影响;叶片氮磷比随演替而显著变化,主要由胸径的群落加权平均值决定;氮的重吸收效率增加,主要受物种丰富度和胸径的影响,而磷的重吸收效率相对稳定。因此,氮重吸收效率与磷重吸收效率的比值显著增加,表明随着温带森林演替,氮限制加剧。这些结果可能反映了较高生物多样性群落中物种间对有限资源的激烈竞争,强调了生物因子在驱动森林生态系统养分循环中的重要性,为中国温带和北方森林可持续经营的施肥管理提供了参考。     

黄土高原子午岭林区典型树种叶片N、P再吸收特征

为揭示黄土高原子午岭林区不同演替阶段和植被类型主要树种养分再吸收特征,研究选取4种次生植被树种(白桦、山杨、辽东栎和油松)和2种人工植被树种(刺槐和侧柏),测定其成熟叶、凋落叶和林下土壤碳(C)、氮(N)、磷(P)含量,研究了叶片N、P再吸收率及其与养分指标的关系。结果表明:(1)不同树种叶片养分和林下土壤养分含量存在显著差异,土壤C、N含量和C∶N∶P计量比均表现为演替后期林地(辽东栎和油松)演替前期林地(山杨和白桦)人工林(侧柏和刺槐);(2)不同树种叶片N、P再吸收率分别为17.18%—43.34%和27.13%—58.12%,均表现为演替后期林地人工林演替前期林地,且P的再吸收率总体高于N的再吸收率;(3)不同树种叶片N、P再吸收率与叶片养分指标的关系强于土壤,与养分计量比的相关性大于养分含量的相关性。说明子午岭典型植被会通过叶片N、P再吸收来适应养分限制环境,尤其是演替后期植被再吸收能力更强,研究可为黄土高原植被恢复提供理论依据。     

PHOSPHORUS,NITROGEN AND THE GROWTH OF ALGAE IN LAKE KINNERET1

The intracellular concentrations of carbon, nitrogen, phosphorus and chlorophyll a of phytoplankton and zooplankton in Lake Kinneret were determined from 1969 to 1973. The ratios C:P, C:N, chlorophyll a:P, chlorophyll a:N of the algae showed fluctuations which could be related to the nutrient conditions that influence the annual pattern of phytoplankton development, especially in respect to the dinoflagellate bloom of Peridinium cinctum (OFM) Ehr. f. westii (Lemm.) Lef. Relatively high intracellurar P values at the start of the bloom indicated adequate availability of this nutrient and luxury consumption over a short period of time. Later, Peridinium continued to grow despite unusually high C:P ratios (> 300:1). In most years, phosphorus may have eventually limited growth, however, in 1970, the bloom censed despite comparatively high intracellular P concentrations. These observations, together with supplementary evidence from nutrient addition experiments and determinations of specific alkaline phosphatase levels, indicated that, for most of the growth phase of the bloom, Peridinium cells were not directly limited by P. The decline of the bloom usually, but not always (e.g., in 1970), was marked by very high C:P ratios. Thus, a shortage of P may often be a contributory factor to the cessation of the Peridinium bloom and may be limiting phytoplankton growth in the fall. Over the years 1969–73, possibly due to an overall drop in salinity, there appears to be a trend to lower levels of biologically bound phosphorus in Lake Kinneret, without a concomitant decrease in carbon biomass.