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Nutrient availability and nutrient use efficiency in plants growing in the transition zone between land and water
Abstract
The transition zone between terrestrial and freshwater habitats is highly dynamic, with large variability in environmental characteristics. Here, we investigate how these characteristics influence the nutritional status and performance of plant life forms inhabiting this zone. Specifically, we hypothesised that: (i) tissue nutrient content differs among submerged, amphibious and terrestrial species, with higher content in submerged species; and (ii) PNUE gradually increases from submerged over amphibious to terrestrial species, reflecting differences in the availability of N and P relative to inorganic C across the land-water ecotone. We found that tissue nutrient content was generally higher in submerged species and C:N and C:P ratios indicated that content was limiting for growth for ca. 20% of plant individuals, particularly those belonging to amphibious and terrestrial species groups. As predicted, the PNUE increased from submerged over amphibious to terrestrial species. We suggest that this pattern reflects that amphibious and terrestrial species allocate proportionally more nutrients into processes of importance for photosynthesis at saturating CO2 availability, i.e. enzymes involved in substrate regeneration, compared to submerged species that are acclimated to lower availability of CO2 in the aquatic environment. Our results indicate that enhanced nutrient loading may affect relative abundance of the three species groups in the land-water ecotone of stream ecosystems. Thus, species of amphibious and terrestrial species groups are likely to benefit more from enhanced nutrient availability in terms of faster growth compared to aquatic species, and that this can be detrimental to aquatic species growing in the land-water ecotone, e.g. Ranunculus and Callitriche. © 2015 German Botanical Society and The Royal Botanical Society of the Netherlands.
... Thus, studies of plant-nutrient relations should consider these issues in experimental design. For example, measurement of tissue nutrients is a more direct indicator of the supply available for growth than external concentrations [30][31][32][33]. ...
... 31]. Internal tissue concentrations above 3% is considered high for P. crispus [42] though Cavalli et al. [32] reports concentrations as high as 4.7% N. Furthermore, N limitation thresholds can be approximated by plotting N content against C:N ratios, and estimating when % N starts changing more so than the C:N ratio [56]. Using this approach, Cavalli et al. [32] suggests a limitation threshold of <2% for tissue N which corresponds reasonably well with our data when we plot our C:N ratios against % N (Fig. 3a) and look at the correlation between RGR and % N overall (Fig. 3b). ...
... Internal tissue concentrations above 3% is considered high for P. crispus [42] though Cavalli et al. [32] reports concentrations as high as 4.7% N. Furthermore, N limitation thresholds can be approximated by plotting N content against C:N ratios, and estimating when % N starts changing more so than the C:N ratio [56]. Using this approach, Cavalli et al. [32] suggests a limitation threshold of <2% for tissue N which corresponds reasonably well with our data when we plot our C:N ratios against % N (Fig. 3a) and look at the correlation between RGR and % N overall (Fig. 3b). ...
Motivated by stream ecosystem degradation by eutrophication, we mimicked slow flowing lowland stream conditions with a novel experimental setup to further our understanding of aquatic plant responses to increases in nitrate and light. We conducted a mesocosm growth experiment of two species from the genus Potamogeton: P. crispus (alien) and P. ochreatus (native), grown at four nitrate and four light levels. We hypothesised that (i) internal nutrient status of the plants would scale with water column nutrient concentration, and that (ii) plant performance would reflect the nutrient status of the plant. Furthermore, we hypothesised that (iii) a low irradiance level would negate the effects of an increased nitrate level. In relation to (ii) we hypothesised that (iv) the traits of the alien species would enable it to outperform the native species where both the availability of light and nutrient resources was high. Internal tissue N content was broadly similar in the two higher (>250 μg NO3− L−1) and the two lower nutrient treatments (
... Wilson et al. (2000) assumed a logistic growth model for water hyacinth population dynamics in temperate regions. Macrophytes in temperate rivers follow a seasonal growth pattern, with high growth rates in spring and early summer, resulting in high biomass accumulation during mid and late summer (Cavalli et al. 2016), followed by a biomass die-back in autumn and winter (Riis et al. 2020). Water hyacinth population dynamics are temperature dependent, and their growth is determined by warm temperatures and lack of frost in the winter months (Nikkel et al. 2023). ...
Anthropogenic nutrient enrichment is one of the key global change pressures threatening the health of estuaries. This is evident at the persistently eutrophic Swartkops Estuary located along the warm temperate coast of South Africa. Augmented nutrient-rich baseflows (e.g., stormwater runoff, wastewater treatment work discharges) have resulted in the persistent growth of invasive alien aquatic plants (IAAPs), particularly water hyacinth (Pontederia crassipes), in the upper estuarine reaches. As such, the objective of this study was to investigate the temporal population dynamics of water hyacinth in the Swartkops Estuary to inform management options. Methods included five-weekly sampling campaigns in winter, spring, and summer, interspersed with monthly sampling over a year-long period. Physico-chemical variables and river inflow were measured in situ, while samples were collected for inorganic nutrients, phytoplankton biomass, and IAAP measurements (cover, biomass, and tissue nutrients). Model results indicated that increased water temperature and inorganic nitrogen (ammonium and NOx) levels promoted increased coverage of water hyacinth, while seasonal analysis highlighted reduced (p < 0.05) dissolved oxygen levels during the peak summer IAAP accumulation period. Notably, model results indicated declining IAAP tissue TN and TP concentrations with increasing water temperature, yet overall TN and TP storage was highest in summer due to the extensive water hyacinth cover observed during this period. Overall, the proportionally low and transient nature of nutrient storage by water hyacinth populations, together with the detrimental consequences they facilitate, highlights the need to integrate short-term control measures with catchment-scale management interventions geared towards mitigating the causative drivers.
• This paper synthesises insights and offers new quantitative analyses of data gathered during 40 years of stream research by Danish researchers and international associates. Lowland Danish streams mostly drain fertile cropland and contain high nutrient concentrations saturating maximum yield and growth rate of plants. Concentrations of carbon dioxide (CO2) are variable, although usually high, supporting a range of wetland and permanently submerged species.
• All terrestrial and most amphibious species are obligate CO2 users, while the majority of permanently submerged species can supplement their inorganic carbon demand with bicarbonate. In lake outlets, the average CO2 concentration was close to air saturation during summer, whereas sites with no lake influence were 9-fold supersaturated. The 20% increase of atmospheric CO2 concentrations over the past 40 years has marginal influence on CO2 concentrations in the streams.
• In lake outlets with low CO2 concentrations, calculations on 33 stream species showed essentially no underwater photosynthesis by temporarily submerged terrestrial species, low rates by amphibious species, and high rates by permanently submerged bicarbonate users. Underwater photosynthetic rates increased at sites with high CO2 concentrations (no lake influence): they were lowest for terrestrial wetland species (mean 1.8 mg O2 g dry weight⁻¹ hr⁻¹), followed by homophyllous (3.0) and heterophyllous amphibious species (5.6), and highest among permanently submerged species (15.0). Terrestrial and amphibious species grew very slowly when CO2 levels were low, but rapidly in CO2 rich water, or when in contact with air above the water's surface. Decreasing CO2 concentrations from upstream to downstream caused lower photosynthesis rates of amphibious species, while photosynthesis by bicarbonate users was consistently high.
• The relative abundance of terrestrial and amphibious species decreased significantly as CO2 resources decline from upstream to downstream, while the abundance of permanently submerged species increased as streams progressed. However, plant abundance as a function of CO2 concentrations did not differ markedly among the plant groups.
• We conclude that photosynthesis and growth of species of different plant types under controlled experimental conditions resembling high in situ nutrient availability are closely related to inorganic carbon supplies, while their representation in plant assemblages is influenced by the spatially and temporally highly diverse ecological conditions from upstream to downstream in the mostly CO2-rich lowland streams.
• Submerged terrestrial and amphibious species restricted to CO2 use for photosynthesis can partly escape the slow gas diffusion under water by growing in very CO2-rich streams and having apical leaves in contact with air. Thus, strong restrictions on their growth and existence should be identified at sites where CO2 concentrations are consistently low and no air contact is possible, while co-limitation by nutrients is likely to be marginal.
• Generally, rising atmospheric CO2 is irrelevant to photosynthesis of permanently submerged plants because of their ability to use bicarbonate and the high CO2 supersaturation in most stream sites. In contrast, photosynthesis and growth of amphibious and terrestrial plants under water are highly dependent on CO2 concentrations varying from close to air saturation in lake outlets to supersaturation in headwaters.
In low‐gradient, macrophyte‐rich rivers, we expect that the significant change in macrophyte biomass among seasons will strongly influence both biological activity and hydraulic conditions resulting in significant effects on nutrient dynamics. Understanding seasonal variation will improve modelling of nutrient transport in river networks, including annual estimations of export, which could optimise decision‐making and management outcomes.
We explored the relationships among seasonal differences in reach‐scale nutrient uptake, macrophyte abundance, solute transport and transient storage in the River Gudenå (Denmark), a large macrophyte‐rich river. We used the minimal pulse addition technique to measure uptake of ammonium, nitrate, soluble reactive phosphorus, as well as reach‐scale metabolism, and surface transient storage in spring, summer, and autumn.
We found that riverine uptake changed among seasons and was linked to macrophyte biomass via both biological activity, reflected in reach‐scale metabolism, and through physical processes, as solute transport was influenced by longitudinal dispersion. In this macrophyte‐rich river, seasonal changes in macrophyte biomass affected contact time between the water and biota, which influenced ammonium and soluble reactive phosphorus uptake. Using stoichiometric scaling of reach‐scale metabolism, we found that seasonal variation also influenced the relative contributions of autotrophic and heterotrophic biota in assimilatory uptake.
In summary, riverine nutrient uptake was not static, highlighting the importance of seasonality, with significant implications for modelling of nutrient export in river networks. Moreover, current management strategies that remove macrophyte biomass (i.e. weed cutting and dredging) will short‐circuit the positive effects of enhanced nutrient uptake resulting from abundant macrophytes in rivers.
The aim of this work was to study the acclimation of photosynthesis in a boreal grass (Phalaris arundinacea L.) grown in controlled environment chambers under elevated temperature (ambient + 3.5°C) and CO2 (700 μmol mol−1) with varying soil water regimes. More specifically, we studied, during two development stages (early: heading; late: florescence completed), how the temperature response of light-saturated net photosynthetic rate (P
sat), maximum rate of ribulose-1,5-bisphosphate carboxylase/oxygenase activity (V
cmax) and potential rate of electron transport (J
max) acclimatized to the changed environment. During the early growing period, we found a greater temperature-induced enhancement of P
sat at higher measurement temperatures, which disappeared during the late stage. Under elevated growth temperature, V
cmax and J
max at lower measurement temperatures (5–15°C) were lower than those under ambient growth temperature during the early period. When the measurements were done at 20–30°C, the situation was the opposite. During the late growing period, V
cmax and J
max under elevated growth temperature were consistently lower across measurement temperatures. CO2 enrichment significantly increased P
sat with higher intercellular CO2 compared to ambient CO2 treatment, however, elevated CO2 slightly decreased V
cmax and J
max across measurement temperatures, probably due to down-regulation acclimation. For two growing periods, soil water availability affected the variation in photosynthesis and biochemical parameters much more than climatic treatment did. Over two growing periods, V
cmax and J
max were on average 36.4 and 30.6%, respectively, lower with low water availability compared to high water availability across measurement temperatures. During the late growing period, elevated growth temperature further reduced the photosynthesis under low water availability. V
cmax and J
max declined along with the decrease in nitrogen content of leaves as growing period progressed, regardless of climatic treatment and water regime. We suggest that, for grass species, seasonal acclimation of the photosynthetic parameters under varying environmental conditions needed to be identified to fairly estimate the whole-life photosynthesis.
Data on nutrient contents of 27 seagrass species at 30 locations were compiled from the literature. Mean (f SE) concentrations of carbon, nitrogen and phosphorus in seagrass leaves were 33.6 2 0.31, 1.92 f 0.05, and 0.23 2 0.011 % dry wt, respectively. The median C:N:P ratio was 474 :24: 1, which represents a C:P ratio more than 4 times, and a N:P ratio more than 1.5 times that of oceanic seston. These ratios are, however, less than those previously reported for marine macrophytes (550 : 30 : 1) by Atkinson & Smith (1984). Nitrogen and phosphorus variability within species was large, but carbon contents exhibited little variability. Accordingly, carbon:nutrient (N and P) ratios were inversely related to changes in nutrient content, and the rate of change in C:N and C:P ratios with increasing nitrogen or phosphorus content in plant tissues should shift from high to small as nutrient supply meets the plant's demands. The median nitrogen and phosphorus contents reported here (1.8 % N and 0.20 % P as % DW) correctly discriminated between seagrass stands that did or did not respond to nutrient enrichment, thus offering a useful reference for comparisons of seagrass nutrient contents.
Riparian zones possess an unusually diverse array of species and environmental processes. The ecological diversity is related to variable flood regimes, geograph-ically unique channel processes, altitudinal climate shifts, and upland influences on the fluvial corridor. The resulting dynamic environment supports a variety of life-history strategies, biogeochemical cycles and rates, and organisms adapted to disturbance regimes over broad spatial and temporal scales. Innovations in riparian zone management have been effective in ameliorating many ecological issues related to land use and environmental quality. Riparian zones play essential roles in water and landscape planning, in restoration of aquatic systems, and in catalyzing institutional and societal cooperation for these efforts.
Among species, photosynthetic capacity (Amax) is usually related to leaf nitrogen content (N), but variation in the species-specific relationship is not well understood. To address this issue, we studied Amax-N relationships in 23 species in adjacent Amazonian communities differentially limited by nitrogen (N), phosphorus (P), and/or other mineral nutrients. Five species were studied in each of three late successional forest types (Tierra Firme, Caatinga and Bana) and eight species were studied on disturbed sites (cultivated and early secondary successional Tierra Firme plots). Amax expressed on a mass basis (Amass) was correlated (pmass in 17 of 23 species, and Amax on an area basis (Aarea) was correlated (parea in 21 of 23 species. The slopes of Amax-N relationships were greater and intercepts lower for disturbance adapted early successional species than for late successional species. On a mass basis, the Amax-N slope averaged 15 mol CO2 [g N]-1 s-1 for 7 early secondary successional species and 4 mol CO2 [g N]-1 s-1 for 15 late successional species, respectively. Species from disturbed sites had shorter leaf life-span and greater specific leaf area (SLA) than late successional species. Across all 23 species, the slope of the Amass-Nmass relationship was related (p2=0.70) and negatively to leaf life-span (r2=0.78) and temporal niche during secondary succession (years since cutting-and-burning, r2=0.90). Thus, disturbance adapted early successional species display a set of traits (short leaf life-span, high SLA and Amax and a steep slope of Amax-N) conducive to resource acquisition and rapid growth in their high resource regeneration niches. The significance and form of the Amax-N relationship were associated with the relative nutrient limitations in the three late successional communities. At species and community levels, Amax was more highly dependent on N in the N-limited Caatinga than in the P-and N-limited Bana and least in the P-and Ca-limited Tierra Firme on oxisol-and differences among these three communities in their massbased Amax-N slope reflects this pattern (6.0, 2.4, and 0.7 mol CO2 [g N]-1 s-1, respectively). Among all 23 species, the estimated leaf Nmass needed to reach compensation (net photosynthesis zero) was positively related to the Amass-Nmass slope and to dark respiration rates and negatively related to leaf life-span. Variation among species in the Amax-N slope was well correlated with potential photosynthetic N use efficiency, Amax per unit leaf N. The dependence of Amax on N and the form of the relationship vary among Amazonian species and communities, consistent with both relative availabilities of N, P, and other mineral nutrients, and with intrinsic ecophysiological characteristics of species adapted to habitats of varying resource availability.
In this study we examine the influence of temperature on growth, photosynthetic performance and acclimation of two submerged
macrophyte species, Elodea canadensis L.C. Rich and Ranunculus aquatilis (L.) Wimmer. The plants were grown at 5, 10 and 15°C and a photon flux density of 300 μmol m−2 s−1 (PAR) in a medium with an alkalinity of 0.85 meq l−1 bubbled with atmospheric air containing 400 ppm CO2. In general, growth rates of both species increased with temperature with a Q
10 varying from 2.3 to 3.5. An exception was Elodea at 5°C, where growth was nearly arrested. Temperature effects on ambient rates of net photosynthesis and photosynthetic capacity
followed the pattern observed for growth. Dark respiration was not suppressed for Elodea at 5°C and both species had a Q
10 of 2.3. The light-use efficiency (αI) for photosynthesis declined with increasing growth temperature for Ranunculus. For Elodea no difference in αI was observed between 10 and 15°C; at 5°C, however, αI was reduced by about 30%. Both species acclimated to temperature as shown in a series of experiments in which the plants
were exposed to a change in temperature. Acclimation was faster for shoots transferred from low to high temperature, where
growth rates stabilised after a few days; for shoots transferred to low temperature growth rates still changed after 22 days.
Although acclimation was evident, the changes in the metabolic apparatus were insufficient to balance effects of temperature.
It is suggested that temperature may affect local distribution of the two species and their ability to grow in turbid or deep
water.
Factors that contribute to interspecific variation in photosynthetic nitrogen-use efficiency (PNUE, the ratio of CO2 assimilation rate to leaf organic nitrogen content) were investigated, comparing ten dicotyledonous species that differ inherently
in specific leaf area (SLA, leaf area:leaf dry mass). Plants were grown hydroponically in controlled environment cabinets
at two irradiances (200 and 1000 μmol m–2 s–1). CO2 and irradiance response curves of photosynthesis were measured followed by analysis of the chlorophyll, Rubisco, nitrate
and total nitrogen contents of the leaves. At both irradiances, SLA ranged more than twofold across species. High-SLA species
had higher in situ rates of photosynthesis per unit leaf mass, but similar rates on an area basis. The organic N content per
unit leaf area was lower for the high-SLA species and consequently PNUE at ambient light conditions (PNUEamb) was higher in those plants. Differences were somewhat smaller, but still present, when PNUE was determined at saturating
irradiances (PNUEmax). An assessment was made of the relative importance of the various factors that underlay interspecific variation in PNUE.
For plants grown under low irradiance, PNUEamb of high-SLA species was higher primarily due to their lower N content per unit leaf area. Low-SLA species clearly had an
overinvestment in photosynthetic N under these conditions. In addition, high SLA-species allocated a larger fraction of organic
nitrogen to thylakoids and Rubisco, which further increased PNUEamb. High-SLA species grown under high irradiance showed higher PNUEamb mainly due to a higher Rubisco specific activity. Other factors that contributed were again their lower contents of Norg per unit leaf area and a higher fraction of photosynthetic N in electron transport and Rubisco. For PNUEmax, differences between species in organic leaf nitrogen content per se were no longer important and higher PNUEmax of the high SLA species was due to a higher fraction of N in␣photosynthetic compounds (for low-light plants) and a higher
Rubisco specific activity (for high-light grown plants).
Carbon dioxide concentrating mechanisms (also known as inorganic carbon concentrating mechanisms; both abbreviated as CCMs) presumably evolved under conditions of low CO(2) availability. However, the timing of their origin is unclear since there are no sound estimates from molecular clocks, and even if there were, there are no proxies for the functioning of CCMs. Accordingly, we cannot use previous episodes of high CO(2) (e.g. the Palaeocene-Eocene Thermal Maximum) to indicate how organisms with CCMs responded. Present and predicted environmental change in terms of increased CO(2) and temperature are leading to increased CO(2) and HCO(3)(-) and decreased CO(3)(2-) and pH in surface seawater, as well as decreasing the depth of the upper mixed layer and increasing the degree of isolation of this layer with respect to nutrient flux from deeper waters. The outcome of these forcing factors is to increase the availability of inorganic carbon, photosynthetic active radiation (PAR) and ultraviolet B radiation (UVB) to aquatic photolithotrophs and to decrease the supply of the nutrients (combined) nitrogen and phosphorus and of any non-aeolian iron. The influence of these variations on CCM expression has been examined to varying degrees as acclimation by extant organisms. Increased PAR increases CCM expression in terms of CO(2) affinity, whilst increased UVB has a range of effects in the organisms examined; little relevant information is available on increased temperature. Decreased combined nitrogen supply generally increases CO(2) affinity, decreased iron availability increases CO(2) affinity, and decreased phosphorus supply has varying effects on the organisms examined. There are few data sets showing interactions amongst the observed changes, and even less information on genetic (adaptation) changes in response to the forcing factors. In freshwaters, changes in phytoplankton species composition may alter with environmental change with consequences for frequency of species with or without CCMs. The information available permits less predictive power as to the effect of the forcing factors on CCM expression than for their overall effects on growth. CCMs are currently not part of models as to how global environmental change has altered, and is likely to further alter, algal and aquatic plant primary productivity.
This review aims to determine how environ- mental characteristics of aquatic habitats rule species occurrence, life-history traits and community dynamics among aquatic plants, and if these particular adaptations and responses fit in with general predictions relating to abiotic factors and plant communities. The way key abiotic factors in aquatic habitats affect (1) plant life (recruitment, growth, and reproduction) and dispersal, and (2) the dynamics of plant communities is discussed. Many factors related to plant nutrition are rather similar in both aquatic and terrestrial habitats (e.g. light, temperature, substrate nutrient content, CO2 availability) or differ markedly in intensity (e.g. light), variations (e.g. temperature) or in their effective importance for plant growth (e.g. nutrient content in substrate and water). Water movements (water- table fluctuations or flow velocity) have particularly drastic consequences on plants because of the density of water leading to strong mechanical strains on plant tissues, and because dewatering leads to catastrophic habitat modifi- cations for aquatic plants devoid of cuticle and support tissues. Several abiotic factors that affect aquatic plants, such as substrate anoxia, inorganic carbon availability or temperature, may be modified by global change. This in turn may amplify competitive processes, and lead ulti- mately to the dominance of phytoplankton and floating species. Conserving the diversity of aquatic plants will rely on their ability to adapt to new ecological conditions or escape through migration.
A summary is provided of the second in a series of Integrated Science Initiative workshops supported by the UNESCO International Hydrological Programme. The workshop brought together hydrologists, ecologists, biogeochemists, hydrogeologists and natural resource managers to discuss the processes that occur in hyporheic and riparian ecotones. The principal objectives were to share new ideas on the importance of biogeochemical processes that affect nutrients at the groundwater-surface water interface, to understand the impact of nutrient flux on stream (principally hyporheic) ecology, and to identify the management strategies for river corridors to mitigate the effects of nutrients applied to land and discharged via groundwater into rivers. The workshop concluded that: (1) more interdisciplinary research and environmental management practices are needed to better understand, predict and manage processes at the interface of environmental compartments; (2) the goal of environmental regulations to improve ecological health requires a holistic approach integrating our understanding of the ecological, hydrological, biogeochemical and physical processes; (3) upscaling spatially and temporally variable processes remains difficult and may hinder translation of research at micro-scales (molecular to grain size) into macro-scale (reach to catchment) decision-making; (4) scientists need to better communicate existing research to river managers, while smanagers must better communicate policy and regulatory-driven science requirements to researchers. Existing models, such as those that simulate stream-hyporheic exchange, are not widely known and rarely used by environmental managers.
Budget studies have shown that internal cycling may contribute a large proportion of the annual nutrient supply required to support new growth in trees. Use of budgets to quantify internal cycling only quantifies the net transfer of nutrients within the plant. Differential partitioning of remobilized nutrients and current nutrient uptake could lead to errors in the interpretation of results from these studies. We have quantified the dynamic relationships among tree growth, nutrient uptake and internal cycling by labeling the current uptake of N in trees that received contrasting amounts of nutrient.
Two-year-old seedlings of Sitka spruce (Picea sitchensis (Bong.) Carr.) were grown in sand culture in a greenhouse for one year. The trees received nutrients in a balanced solution at either a high (high-RAR) or a low (low-RAR) relative addition rate throughout the experiment. Current N uptake was labeled with ¹⁵N from April 13 to July 25. Thereafter, trees were re-potted in clean sand and unlabeled N applied until November 13.
Overall growth was sustained for approximately 10 weeks before treatment effects were observed. Initially, no differences in the partition of growth or remobilized N occurred, although partition of current uptake favored the roots of plants in the low-RAR treatment. After 6 weeks, the partition of both growth and remobilized N altered in favor of roots of plants in the low-RAR treatment. Nutrient supply had no effect on the amount or rate of N remobilization. No evidence was found to suggest that N taken up in the current season and partitioned to preexisting shoots or roots is remobilized late in the season to support growth of new shoots. However, some trees in the high-RAR treatment exhibited a second flush of growth later in the season that was partially sustained by remobilization of ¹⁵N from current shoots formed earlier in the season. Use of ¹⁵N demonstrated differential partitioning of current uptake and remobilized N. The results highlight the limitations of simple budget studies for quantifying internal cycling.
1. Many submerged aquatic macrophytes possess inducible biochemical or biophysical mechanisms that can enhance the internal inorganic carbon concentration above that which can be achieved by diffusive uptake of CO2 coupled with C-3 photosynthesis. 2. The possible interaction between nitrogen economy and use of a carbon concentrating mechanism was studied in Elodea canadensis, which has a mechanism based on bicarbonate usage. The nitrogen content of the plants was varied from approximately 800 to approximately 3500 mumol N g-1 DW by growing them in hydroponic cultures at a range of inorganic nitrogen concentrations. 3. The relative growth rate increased with increasing tissue-N from 0.020 day-1 at 800 mumol N g-1 DW to 0.045 day-1 at 1600 mumol N g-1 DW. The photosynthetic performance of the plants was also affected by tissue-N. The photosynthetic capacity increased from 172 to 465 mumol CO2 g-1 DW h-1 and the HCO3- uptake capacity from 63 to 310 mumol C g-1 DW h-1 with increasing tissue-N. The CO2 conductance, a measure of CO2 affinity, increased four times and the bicarbonate conductance 10 times. 4. High nitrogen concentrations are required in Elodea to maintain an efficient carbon-assimilation apparatus and an efficient carbon-concentrating mechanism. 5. The interdependence of nitrogen-use efficiency for photosynthesis (PNUE) and nitrogen content was dependent on the inorganic carbon concentration at which PNUE was measured. PNUE measured at saturating inorganic carbon concentrations declined with increasing tissue-N, presumably as a result of increased allocation of nitrogen into non-photosynthetic pools at high tissue-N. In contrast, PNUE increased with increasing tissue-N when measured at sub-saturating inorganic carbon concentrations. 6. The total Rubisco activity was positively correlated with tissue-N in Elodea and was substantially higher than the photosynthetic capacity indicating that Rubisco may not play a primary role in regulating the photosynthetic capacity of Elodea. Rather, it may have a role in regulating photosynthesis at low inorganic carbon concentrations, as suggested by the observed linear relationship between CO2 conductance and Rubisco.
A tissue analysis technique was used to evaluate nitrogen and phosphorus supplies in natural waters for the growth of angiosperm aquatic plants. Tissue content of nitrogen and phosphorus was employed as an index of element availability in lakes from which plants were collected. This required establishment in the laboratory of the critical level for each element, that is, the minimum tissue content associated with maximum growth.
To establish critical levels, a system was developed for culturing algae‐free plants in a synthetic nutrient medium. The critical nitrogen content for the several species studied was approximately 1.3%; the comparable phosphorus value was 0.13%.
The nitrogen and phosphorus contents of 13 species of aquatic plants collected during the summer from nine lakes were compared with the critical concentrations. In nine samples obtained mostly during periods of heaviest plant growth, the phosphorus content was at or below the critical level; in no sample was the nitrogen content less than the critical concentration. The results indicated that in all but one of the lakes, phosphorus supply was more likely to limit higher aquatic plant growth than was nitrogen.
The primary importance of this study is as an initial step in the development of a satisfactory technique for evaluating nutrient supplies in natural waters.
Rates of photosynthesis of four submerged stream macrophytes were examined under varying pH and composition of inorganic carbon
species. Callitriche stagnatis and Sparganium simplex used only CO2 for photosynthesis. Potamogeton crispus and P. pectinatus used HCO3 in addition to CO2, but with much lower efficiency. The photosynthetic rates at air equilibrium and a total inorganic carbon concentration of
5.0 mM were 2–3 times lower than maximum rates at CO2 saturation for the HCO3 users and 10–14 times lower for the CO2 users. The CO2 compensation point of entire plants of Callitriche (2.5 μM) and Sparganium (6.0μM) was well below the equilibrium concentration
(15 μM). and the low saturation points (250–500 μM) also pointed to efficient use of CO2. Callitriche and Sparganium compete successfully with HCO3− users in hardwater streams, which have a higher exchange and generation capacity of CO2 than stagnant and more soft waters.
Rates of photosynthesis of Potamogeton crispus and P. pectinatus decreased at high pH. Depending on the two alternative hypotheses
for HCO3use, this decline can be explained by CO3−− inhibition of HCO3− uptake or by increasing capacity to buffer H+efflux from the plant. Habitats subject to high pH, e. g. small ponds with dense
vegetation, may have a strong selection for efficient mechanisms of HCO3 use.
Aquatic plant tissue (nitrogen) N and phosphorus (P) concentrations, together with the N:P ratio in general, respond poorly to ambient nutrient supply. The variability in plant tissue nutrient concentrations appears to be highly conditioned by taxonomic (mostly species) effects. The significant role that other attributes of the surrounding environment may have in modifying the degree of taxonomic dependence on plant nutrient concentration is yet to be tested with a broader characterisation of plant life forms (bryophytes, helophytes, hydrophytes), so as to investigate implications for ecosystem processes. Plant samples (378) were collected across 65 sites spanning a wide range of aquatic habitats (lotic and lentic) and biophysical zones (floodplain, glacial terraces, mountains) located in NE Scotland. Significant differences in plant tissues N, P and the N:P were found after grouping sampling sites by either habitat or biophysical zones; the latter even after removing the largest taxonomic effect. The range in plant tissue N and P was relatively wide within habitats and zones, reflecting the high species diversity of the studied area. As a group, bryophytes had smaller N and P concentrations, and consequently wider biomass C:N and C:P ratios, a situation which may impose constraints on the flow of energy through food-webs of the mountain and bog areas where they dominate.
Restoration of river systems allowing the transformation of former drained and dry riparian areas into riparian wetlands will increase the overbank storage of sediment and sediment-associated phosphorus (P). Wetland restoration is therefore a cost-effective mitigation measure to reduce the sediment and nutrient transport to river systems. The studied floodplain of the River Odense was restored in 2003 by remeandering the river channel along a 6-km reach. The restoration project involved 78 ha of riparian areas that were transformed from mainly arable land to extensive grassland and wetlands. The aim of the study was to quantify and model sediment and particulate P deposition on restored river floodplains. The present study suggests that during a 47-day flooding period, the river floodplain is able to retain 914.8% of the sediment and 1.13.7% of the particulate P transported in the river. Incubation experiments further showed that a maximum of 1125% of the deposited phosphorus can be released as dissolved inorganic phosphorus following deposition. The results from the best deposition model (R2 ≤ 0.42 for sediment and R2 ≤ 0.44 for particulate P) show that work should be done to further improve the performance of these models.
We examine the photosynthetic response of submerged plants from streams and lakes with contrasting free‐ CO 2 and nitrogen ( N ) availability. We hypothesized that: the photosynthetic capacity of stream plants is higher because of higher N availability; the photosynthetic N‐use efficiency ( PNUE ) is also higher because stream plants are acclimated to higher free‐ CO 2 ; and PNUE is lower in aquatic compared to terrestrial plants.
We tested these hypotheses by measuring tissue‐ N , photosynthetic capacity and inorganic C extraction capacity in plants collected from streams and lakes and by comparing the PNUE of aquatic plants with previously published PNUE of terrestrial plants.
We found that the organic N content was consistently higher in stream (3.8–6.3% w/w) than in lake plants (1.2–4.3% w/w). The photosynthetic capacity correlated positively with tissue‐N. The relationships were similar for stream and lake plants, indicating that N allocation patterns were similar despite variability in free‐ CO 2 between the two habitats.
The slope of the relationship between photosynthetic capacity and tissue‐N was lower than found for terrestrial plants, whereas the compensatory N content for photosynthesis was similar. This suggests that PNUE is lower in aquatic plants, perhaps reflecting that the selection pressure for a high C fixation rate per unit N is reduced as a result of low inorganic C availability in the aquatic environment.
(gIucose-6- phosphate ) and similar abbreviations for specific esters, P-lipid (phospholipid), P-choline (phosphatyl choline). 2 Throughout this article, solution concentrations will be given as molarities. The tissue content will usually be expressed as miIIimoles (mmoles), micromoles (I'moles) or nano moles (nmoles) per gram fresh weight (g fr wt); or else in terms of molarities, where the tissue is taken to be an equivalent volume of water. This should help in comparing solu tion and tissue concentrations and the results of different studies. I have made the neces sary conversions from other authors' data. Where values have been quoted in terms of dry weight alone, I have made the conversions by assuming dry wt/fresh wt ratios of 0.12, 0.14, and 0.17 for fleshy, normal, and woody tissues. They are sufficiently accurate for my comparisons. Unfortunately, authors do not always make clear the basis of their calculations. Calculations in terms of moles and molarities are free from ambiguity.
1. Nutrient limitation (mostly N or P) is a driving force in ecosystem development. Current techniques to determine the nature of nutrient limitation use laborious fertilization experiments. 2. It was hypothesized that the N:P ratio of the vegetation directly indicates the nature of nutrient limitation on a community level (N vs. P limitation). This hypothesis was tested by reviewing data on fertilization studies in a variety of European freshwater wetland ecosystems (bogs, fens, wet heathlands, dune slacks, wet grasslands). In a subset of the data (dune slacks) between-site intraspecific variation and within-site interspecific variation in nutrient content and N:P ratio was studied in five plant species. 3. A review of 40 fertilization studies reveals that an N:P ratio >16 indicates P limitation on a community level, while an N:P ratio
Abstract Accumulation of nitrogen (N) by plants in response to N supply outstripping demand is contrasted with storage of N, which implies that N in one tissue can be reused for the growth or maintenance of another. Storage can, therefore, occur in N-deficient plants; accumulation can not. The consequence of accumulation and storage of N is considered, particularly in relation to the reproductive growth of annual plants, which can often use a great deal of stored N. Nitrate and proteins are the forms of N most often stored in vegetative tissues and, quantitatively, ribulose 1,5-bisphosphate carboxylase/oxygenase is often the most important protein store. While storing nitrate will be less costly to the plant in terms of energy, protein stores offer several possible advantages. These advantages are (i) maximizing the potential for carbon assimilation, (ii) avoiding problems with the regulation of leaf turgor and (iii) allowing the reduction on nitrate to occur in the young, fully illuminated leaf.
1. We evaluated restoration success on macrophyte species diversity and composition in lowland streams using communities in 30 naturally meandering stream reaches in the western part of Jutland, Denmark, as reference target communities. Fuzzy set clustering was used to examine the floristic and environmental similarity among reaches, whereas fuzzy set ordination was used to relate floristic patterns to environmental variables.
2. Two major groups of streams were identified based on their floristic composition. One group consisted of reference and restored reaches and the other of the majority of channelised reaches. We found that management exerted a strong influence on the macrophyte communities and that the identified groups were related to differences in management intensity.
3. Our results also indicate that bank morphology and bed level affected macrophyte communities in the streams, particularly the richness and abundance of terrestrial species. The analyses performed suggest that shallow and wide banks allow for a larger migration of species from the stream banks into the streams, thereby enhancing species diversity within the stream channel.
4. The results of this study suggest that macrophyte communities in channelised lowland streams can recover following restorative interventions given that stream management (i.e. weed cutting and dredging) is minimised and that stream banks are reprofiled to improve the lateral connectivity between the stream and its valley.
1. Nitrogen (N) and phosphorus (P) are essential nutrients for photosynthetic carbon assimilation and most frequently limit primary productivity in terrestrial ecosystems. Efficient use of those nutrients is important for plants growing in nutrient‐poor environments.
2. We investigated the pattern of photosynthetic phosphorus‐use efficiency (PPUE) in comparison with photosynthetic nitrogen‐use efficiency (PNUE) along gradients of P and N availability across biomes with 340 tree and shrub species. We used both total soil N and P concentration and foliar N/P ratios for indicating nutrient‐availability gradients.
3. Photosynthetic phosphorus‐use efficiency increased with greater leaf mass per area (LMA) toward decreasing P availability. By contrast, PNUE decreased with greater LMA towards decreasing N and P availability.
4. The increase in PPUE with decreasing P availability was caused by the net effects of a relatively greater reduction in foliar P concentration and a relatively constant photosynthetic carbon assimilation rate. The decrease in PNUE with decreasing N availability was caused by the effects of a reduction in photosynthetic carbon assimilation rate with greater LMA.
5. Synthesis . Our results suggest that higher PPUE may be an effective leaf‐level adaptation to P‐poor soils, especially in tropical tree species. Future research should focus on the difference between PPUE and PNUE in relation to leaf economics, physiology and strategy.
The theories of Grime and Tilman are ambitious attempts to unify disparate theories regarding the construction of plants, their interaction with the environment and the assembly of communities. After over two decades of parallel research, their ideas have not been reconciled, hindering progress in understanding the functioning of ecosystems.
Grime's theories do not adequately incorporate the importance of non‐heterogeneous supplies of nutrients and how these supplies are partitioned over long time scales, are inconsistent regarding the importance of disturbance in nutrient‐limited habitats and need to reconsider the carbon economy of shade‐tolerant plants.
Failure to account for differences between aquatic and terrestrial systems in how resource supplies are partitioned led Tilman to develop a shifting set of theories that have become reduced in mechanistic detail over time. The most recent highlighted the reduction of nutrient concentrations in soil solution, although it can no longer be derived from any viable mechanistic model. The slow diffusion of nutrients in soils means that the reduction of average soil solution nutrient concentrations cannot explain competitive exclusion.
Although neither theory, nor a union of the two, adequately characterizes the dynamics of terrestrial plant assemblages, the complementarity in their assumptions serve as an important foundation for future theory and research.
Reconciling the approaches of Grime and Tilman leads to six scenarios for competition for nutrients and light, with the outcome of each depending on the ability of plants to pre‐empt supplies. Under uniform supplies, pulses or patches, light competition requires leaf area dominance, while nutrient competition requires root length dominance. There are still important basic questions regarding the nature of nutrient supplies that will need to be answered, but recent research brings us closer to a unified set of theories on resource competition.
Both roots and leaves of free‐floating plants can potentially take up nutrients. In this study, the ability and relative contribution of roots and fronds for N uptake by the floating macrophyte Lemna minor was investigated.
The NH 4 ⁺ and NO 3 ⁻ uptake kinetics of roots and fronds were measured on plants acclimated to three different NH 4 NO 3 concentrations.
Lemna had the capacity to take up NH 4 ⁺ and NO 3 ⁻ through both roots and fronds; uptake kinetics for the two tissue types were comparable on an area basis. The overall contribution of root and frond to whole‐plant uptake, estimated from measured kinetic characteristics, varied depending on plant N status (the root contribution increased from 32 to 73% for N‐satiated and N‐depleted plants, respectively).
The shift in the balance between root and frond contribution to whole‐plant uptake resulted from a 1.5–38 times greater increase in the area‐specific uptake capacity and affinity of roots relative to fronds, combined with a larger decrease in the minimum concentration for uptake ( C min ) for roots than fronds. At the morphological level, root–frond surface area increased with declining N supply, which might be beneficial to the plants since the area return per unit biomass invested was nine times greater for roots than for fronds.
Nitrogen (N) is an essential limiting resource for plant growth, and its efficient use may increase fitness. We investigated photosynthetic N‐use efficiency (photosynthetic capacity per unit N) in relation to N allocation to Rubisco and to cell walls in Polygonum cuspidatum Sieb. et Zucc. which germinated in May (early germinators) and August (late germinators).
There was a significant difference between early and late germinators in photosynthetic capacity as a function of leaf N content per unit area. Higher photosynthetic N‐use efficiency in late germinators was caused primarily by a larger allocation of N to Rubisco.
Nitrogen allocation to cell walls was smaller in late germinators. The shorter growth period in late germinators was associated with higher photosynthetic capacity, which was achieved by allocating more N to photosynthetic proteins at the expense of cell walls.
The trade‐off between N allocation to photosynthesis and to structural tissues suggests that plants change N allocation to increase either the rate or duration of carbon assimilation. Such plastic change would help plants maintain themselves and cope with environmental changes.
1. The Weichsel glaciation has divided Denmark into two regions with different susceptibility to acidification. East of the Weichselian terminal moraine, soils are usually clayey and calcareous, and the streams are alkaline (mean alkalinity 2.24 mmol 1‐ ‐1 ) and resistant to inputs of acidifying substances.
2. Trend analysis of pH and alkalinity of water samples taken over 15 years in two streams with alkalinities above 1.5 mmol 1 ⁻¹ in eastern Jutland, showed no trends of acidification.
3. West of the terminal moraine the soils are sandy and leached and alkalinity is lower (mean 0.59 mmol 1 ⁻¹ ). Although such streams with medium alkalinity are believed not to be vulnerable to acidification, we have documented significant decreases in their pH and alkalinity over 12 years.
4. Trends of pH and alkalinity in four western streams with mean alkalinities between 0.05 and 0.79 mmol 1 ⁻¹ showed annual decreases of 0.027 pH units and 4.7 nmol 1 ⁻¹ in alkalinity.
5. Overall, Danish streams contain about 7.9 times more calculated free CO 2 ( p CO 2 =10 ⁻² . ⁶ atm) than water in equilibrium with air ( p CO 2 = 10 ⁻³ . ⁵ atm). The calculated free CO 2 content has increased significantly in western Danish streams over the study period (6.9 μmol 1 ⁻¹ yr ‐1 ). This increase cannot be explained by the prevailing global increase in atmospheric p CO 2 which only can account for 0.54 pmol 1 ⁻¹ yr ⁻¹ at maximum.
6. Reasons for the ongoing stream acidification in the western part of Denmark are discussed. We suggest that atmospheric deposition causes stream acidification in a heath‐covered catchment without agriculture. In heavily cultivated regions the main acidification factor is argued to be proton production in the soil through nitrification of ammonium‐containing fertilizers.
1. This review is presented as a broad synthesis of riverine landscape diversity, beginning with an account of the variety of landscape elements contained within river corridors. Landscape dynamics within river corridors are then examined in the context of landscape evolution, ecological succession and turnover rates of landscape elements. This is followed by an overview of the role of connectivity and ends with a riverine landscape perspective of biodiversity.
2. River corridors in the natural state are characterised by a diverse array of landscape elements, including surface waters (a gradient of lotic and lentic waterbodies), the fluvial stygoscape (alluvial aquifers), riparian systems (alluvial forests, marshes, meadows) and geomorphic features (bars and islands, ridges and swales, levees and terraces, fans and deltas, fringing floodplains, wood debris deposits and channel networks).
3. Fluvial action (erosion, transport, deposition) is the predominant agent of landscape evolution and also constitutes the natural disturbance regime primarily responsible for sustaining a high level of landscape diversity in river corridors. Although individual landscape features may exhibit high turnover, largely as a function of the interactions between fluvial dynamics and successional phenomena, their relative abundance in the river corridor tends to remain constant over ecological time.
4. Hydrological connectivity, the exchange of matter, energy and biota via the aqueous medium, plays a major though poorly understood role in sustaining riverine landscape diversity. Rigorous investigations of connectivity in diverse river systems should provide considerable insight into landscape-level functional processes.
5. The species pool in riverine landscapes is derived from terrestrial and aquatic communities inhabiting diverse lotic, lentic, riparian and groundwater habitats arrayed across spatio-temporal gradients. Natural disturbance regimes are responsible for both expanding the resource gradient in riverine landscapes as well as for constraining competitive exclusion.
6. Riverine landscapes provide an ideal setting for investigating how complex interactions between disturbance and productivity structure species diversity patterns.
Small unshaded streams in lowland regions receive drainage water with high concentrations of free␣CO2, and they support an abundant growth of amphibious and obligately submerged plants. Our first objective was to measure the
CO2 regime during summer in a wide range of small alkaline Danish streams subject to wide variation in temperature, O2 and CO2 during the day. The second objective was to determine the effect of these variations on daily changes in light-saturated
photosynthesis in water of a homophyllous and a heterophyllous amphibious species that only used CO2, and an obligately submerged species capable of using both HCO−
3 and CO2. We found that the median CO2 concentrations of the streams were 11 and 6 times above air saturation in the morning and the afternoon, respectively, but
stream sites with dense plant growth had CO2 concentrations approaching air saturation in the afternoon. In contrast, outlets from lakes had low CO2 concentrations close to, or below, air saturation. The amphibious species showed a reduction of photosynthesis in water from
morning to afternoon along with the decline in CO2 concentrations, while increasing temperature and O2 had little effect on photosynthesis. Photosynthesis of the obligately submerged species varied little with the change of
CO2 because of HCO3
−- use, and variations were mostly due to changes in O2 concentration. Independent measurements showed that changes in temperature, O2 and CO2 could account for the daily variability of photosynthesis of all three species in water. The results imply that CO2 supersaturation in small lowland streams is important for the rich representation of amphibious species and their contribution
to system photosynthesis.
We examined the dependence of photosynthetic capacity on leaf nitrogen content and light environment in leaf microsites of six rainforest species growing naturally in understory and clearing habitats. All nine descriptors of the light environment in leaf microsites, encompassing canopy openness, potential exposure to sunflecks, and photosynthetically active photon flux density (PFD) integrated over one day, three months, and an entire year, were highly correlated with each other. Among species, A
max was strongly dependent on leaf nitrogen and on all descriptors of the light environment in the leaf microsite. Leaf nitrogen was also dependent on all descriptors of the light environment. Simple regressions of A
max on all measures of the light environment were significant for plants in the clearing, but not for plants growing in the understory. Regressions of leaf nitrogen on at least one descriptor of the light environment were significant for both clearing and understory plants. Over all species, and for the clearing plants alone, leaf nitrogen and a single descriptor of the light environment together explained significantly more variance in A
max than either variable alone. Multiple regressions using leaf nitrogen and all nine descriptors of the light environment did not explain a significantly greater amount of variance in photosynthetic capacity than multiple regressions using leaf nitrogen and a single light descriptor. In most cases, A
max was predicted better by the most general descriptors of the light environment, such as canopy openness and long-term photon flux density, and less accurately by shorter-term sensor measurements or descriptors based on the duration of potential sunfleck exposure. The gest predictors of A
max were frequently different than the best predictors of leaf nitrogen. In every case examined, A
max was less sensitive to variation in the light environment in understory than in clearing plants.
Effects of inorganic carbon supply on nitrogen requirement and critical nitrogen concentration was examined for the submerged macrophytes E. canadensis and C. cophocarpa. The plants were grown in a factorial setup of four inorganic nitrogen (0.001–0.1 mM N as NH4 NO3) and two CO2 concentrations (17 and 430 μM) at an alkalinity of 0.85 meqv l−1. The two species are morphologically similar, but differ in their ability to use bicarbonate in photosynthesis. E. canadensis has a high affinity for bicarbonate whereas C. cophocarpa is restricted to use CO2. The growth rate was higher and the tissue-N concentration lower at high compared to low CO2 for both species. Further, the tissue-N concentration needed to saturate growth of E. canadensis was higher at low than at high CO2, 2.0 and 1.1 mmol N g−1 DW, respectively. For C. cophocarpa growth was saturated at 1.3 mmol N g−1 DW at high CO2. At low CO2, growth was suppressed by the low carbon availability but was independent of nitrogen availability. Growth rate and the efficiency of nitrogen use expressed as growth rate per unit tissue-N, was higher for E. canadensis than for C. cophocarpa at low CO2. In contrast, at high CO2, C. cophocarpa grew faster and had a higher growth rate per unit tissue-N than E. canadensis. This difference was greater at low nitrogen availability suggesting that C. cophocarpa may have a competitive advantage over E. canadensis when growing under conditions with limited nitrogen supply but ample CO2. Growth per unit tissue-N was higher at high than at low CO2 for both species. For E. canadensis the higher growth per unit N counterbalanced the lower tissue-N concentration at high CO2 and as a result the nitrogen requirement of E. canadensis was similar at high and low CO2. For C. cophocarpa the requirement was higher at high CO2 where growth was substantially higher than at low CO2. This suggests that depending on species and relative change in inorganic carbon concentration, higher growth rates can be expected in systems with higher inorganic carbon supply rates even without a concomitant increase in nitrogen load.
In submerged aquatic macrophytes the relationship between net photosynthesis and carbon concentration in the water often follows a less gradual pattern than anticipated assuming simple Michaelis-Menten kinetics. This indicates that other factors than the activity of the carboxylation enzymes are important in the regulation of photosynthesis in these plants. At low external concentrations of inorganic carbon, photosynthesis is restricted by the slow rate of diffusion from the bulk medium to the site of carboxylation, whereas the maximum photosynthetic capacity appears to be set by the enzyme activity or the turnover of intermediates in the carbon reduction cycle, including adenosine triphosphate (ATP) and reducing agents. The potential for active transport of inorganic carbon across the plasmalemma may also be of importance in some instances.
Groundwater flow and nutrient transport were studied in a riparian meadow during a three-year period. The meadow is situated along a first order stream in the River Gjern catchment area, Jutland, Denmark. Field data included measurements of hydraulic head, hydraulic conductivity and soil characteristics. Groundwater sampled from piezometers was analysed for nitrate, ammonium and phosphate. Nitrogen and phosphorus contents in above-ground plant biomass were also measured. For the interpretation of our data we developed a one-dimensional hydraulic-transport model for the lateral groundwater flow, transport of nitrate, and nitrate removal in the meadow. The model is based on Darcy’s equation, and input data are horizontal and vertical distances, hydraulic heads, hydraulic conductivities, and nitrate concentrations. We also developed a scheme for evaluating uncertainties of the modeling results.Annual removal of nitrate in the saturated zone of the riparian meadow was 326, 340, and (59–68% of groundwater input) through the three-year period. The largest nitrate removal took place outside the growing seasons. Net loss of ammonium from the saturated zone was 0.4, 6.7, and . In two of the years this was counter-balanced by atmospheric nitrogen deposition. Phosphate was not retained during the first two years but lost at rates of 0.88 and 0.36 kg P ha−1 y−1. In year 3 phosphate retention was 0.47 kg P ha−1 y−1.These data show how a riparian ecotone along a first order stream can reduces nitrogen pollution from agricultural areas. Also, the pronounced year to year variations in our nutrient budgets show that shorter studies, for example based on one year of observations, should be interpreted cautiously as representing a general picture of nutrient pathways.
The use of nutrient concentrations in plant biomass as easily measured indicators of nutrient availability and limitation has been the subject of a controversial debate. In particular, it has been questioned whether nutrient concentrations are mainly species' traits or mainly determined by nutrient availability, and whether plant species have similar or different relative nutrient requirements. This review examines how nitrogen and phosphorus concentration and the N:P ratio in wetland plants vary among species and sites, and how they are related to nutrient availability and limitation. We analyse data from field studies in European non-forested wetlands, from fertilisation experiments in these communities and from growth experiments with wetland plants. Overall, the P concentration was more variable than the N concentration, while variation in N:P ratios was intermediate. Field data showed that the N concentration varies more among species than among sites, whereas the N:P ratio varies more among sites than among species, and the P concentration varies similarly among both. Similar patterns of variation were found in fertilisation experiments and in growth experiments under controlled nutrient supply. Nutrient concentrations and N:P ratios in the vegetation were poorly correlated with various measures of nutrient availability in soil, but they clearly responded to fertilisation in the field and to nutrient supply in growth experiments. In these experiments, biomass N:P ratios ranged from 3 to 40 and primarily reflected the relative availabilities of N and P, although N:P ratios of plants grown at the same nutrient supply could vary three-fold among species. The effects of fertilisation with N or P on the biomass production of wetland vegetation were well related to the N:P ratios of the vegetation in unfertilised plots, but not to N or P concentrations, which supports the idea that N:P ratios, rather than N or P concentrations, indicate the type of nutrient limitation. However, other limiting or stressing factors may influence N:P ratios, and the responses of individual plant species to fertilisation cannot be predicted from their N:P ratios. Therefore, N:P ratios should only be used to assess which nutrient limits the biomass production at the vegetation level and only when factors other than N or P are unlikely to be limiting.
Recent information on the relative roles of sediment and water as nutrient sources for rooted submersed freshwater macrophytes has facilitated the development of methods for culturing these plants. The use of natural sediments rather than culture solutions as the source of nitrogen, phosphorus and micronutrients largely prevents the occurrence of algal blooms and, for many purposes, obviates the need for axenic cultures.Growth requirements of submersed macrophytes are reviewed with regard to the provision of suitable culture conditions. Sediment substrate requirements are considered in relation to the role of sediment as a nutrient source. Two types of culture solution formulations are provided with procedures for establishing and maintaining submersed macrophyte cultures for experimental research.
Uncultivated riparian areas can play an important role in reducing nutrient loading to streams in agricultural watersheds. Groundwater flow and geochemistry were monitored in the riparian zone of a small agricultural watershed in southern Ontario. Hydraulic and geochemical measurements were taken along a transect of monitoring wells extending across the riparian area into an agricultural field. Chloride and nitrate concentrations in groundwater samples collected from the agricultural field were much higher than in samples from the riparian area. A sharp decline in both nitrate and chloride concentrations was observed near the field–riparian zone boundary. It appears that increased recharge within the riparian zone, as compared to the artificially drained field, caused nitrate-rich groundwater from the field to be diverted downward beneath the riparian zone, thus limiting the input of agrochemicals to the riparian area and consequently protecting the stream from potential contamination. Geochemical data also indicated that nitrate was attenuated in the downward moving groundwater. Patterns of dissolved oxygen concentrations and redox potential in the subsurface coincided with the pattern defined by groundwater nitrate. These patterns indicated that conditions within the riparian zone and at depth near the field–riparian zone boundary were conducive to denitrification. A linear relation between the and values of nitrate from the monitored transect also supported denitrification as the primary nitrate removal mechanism. This study provides a new conceptual model of how riparian zones may prevent nitrate contamination of streams, and highlights the need for a complete understanding of both groundwater flow and geochemistry in riparian environments.
Ribulose-1,5-bisphosphate (RuBP) carboxylase/oxygenase (Rubisco) catalyzes the first step in net photosynthetic CO2 assimilation and photorespiratory carbon oxidation. The enzyme is notoriously inefficient as a catalyst for the carboxylation of RuBP and is subject to competitive inhibition by O2, inactivation by loss of carbamylation, and dead-end inhibition by RuBP. These inadequacies make Rubisco rate limiting for photosynthesis and an obvious target for increasing agricultural productivity. Resolution of X-ray crystal structures and detailed analysis of divergent, mutant, and hybrid enzymes have increased our insight into the structure/function relationships of Rubisco. The interactions and associations relatively far from the Rubisco active site, including regulatory interactions with Rubisco activase, may present new approaches and strategies for understanding and ultimately improving this complex enzyme.
Ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the predominant protein in photosynthesizing plant parts and the most abundant protein on earth. Amino acids deriving from its net degradation during senescence are transported to sinks (e.g. developing leaves, fruits). Rubisco catabolism is not controlled only by the overall sink demand. An accumulation of carbohydrates may also accelerate senescence and Rubisco degradation under certain conditions. Amino acids produced by proteolysis are rapidly redistributed in plants with proper source-sink relationships. In leaves of wheat plants with reduced sink capacity (e.g. sink removal, phloem interruption by steam girdling at the leaf base), Rubisco is degraded and free amino acids accumulate. They may be washed out in the rain during late senescence. In leaves of depodded soybeans, Rubisco is degraded and amino acids can be reutilized in these leaves for the synthesis of special vacuolar proteins in the paraveinal mesophyll (vegetative storage proteins). Nitrogen deriving from Rubisco degradation in older (senescing) leaves of annual crops is integrated to some extent again in newly synthesized Rubisco in younger leaves or photosynthesizing tissues of fruits. Finally, a high percentage of this nitrogen is accumulated in protein bodies (storage proteins). At the subcellular level, Rubisco can be degraded in intact chloroplasts. Reactive oxygen species may directly cleave the large subunit or modify it to become more susceptible to proteolysis. A metalloendopeptidase may play an important role in Rubisco degradation within intact chloroplasts. Additionally, the involvement of vacuolar endopeptidase(s) in Rubisco catabolism (at least under certain conditions) was postulated by various laboratories.