Responses of wetland graminoids to the relative supply of nitrogen and phosphorus

The biomass production of wetland vegetation can be limited by nitrogen or phosphorus. Some species are most abundant in N-limited vegetation, and others in P-limited vegetation, possibly because growth-related traits of these species respond differently to N versus P supply. Two growth experiments were carried out to examine how various morphological and physiological traits respond to the relative supply of N and P, and whether species from sites with contrasting nutrient availability respond differently. In experiment 1, four Carex species were grown in nutrient solutions at five N:P supply ratios (1.7, 5, 15, 45, 135) combined with two levels of supply (geometric means of N and P supply). In experiment 2, two Carex and two grass species were grown in sand at the same .ve N:P supply ratios combined with three levels of supply and two light intensities (45% or 5% daylight). After 12-13 weeks of growth, plant biomass, allocation, leaf area, tissue nutrient concentrations and rates and nutrient uptake depended signi.cantly on the N:P supply ratio, but the type and strength of the responses differed among these traits. The P concentration and the N:P ratio of shoots and roots as well as the rates of N and P uptake were mainly determined by the N:P supply ratio; they showed little or no dependence on the supply level and relatively small interspeci.c variation. By contrast, the N concentration, root mass ratio, leaf dry matter content and speci.c leaf area were only weakly related to the N:P supply ratio; they mainly depended on plant species and light, and partly on overall nutrient supply. Plant biomass was determined by all factors together. Within a level of light and nutrient supply, biomass was generally maximal (i.e. co-limited by N and P) at a N:P supply ratio of 15 or 45. All species responded in a similar way to the N:P supply ratio. In particular, the grass species Phalaris arundinacea and Molinia caerulea showed no differences in response that could clearly explain why P. arundinacea tends to invade P-rich (N-limited) sites, and M. caerulea P-limited sites. This may be due to the short duration of the experiments, which investigated growth and nutrient acquisition but not nutrient con­servation.     

Effect of N:K supply ratios on the performance of three grass species from herbaceous wetlands

Shifts from N limitation to P or K limitation in wetlands (e.g. due to mowing and high atmospheric N deposition) are reflected by increased N:P and N:K ratios of plant biomass and changes in species composition. So far, the implications of increased N:K ratios for wetland vegetation have hardly been investigated. We examined how the supply of N and K influences the growth of three wetland grass species (Holcus lanatus, Anthoxanthum odoratum, Deschampsia caespitosa) to determine at what N:K ratios growth is likely to be K limited, how N:K ratios influence biomass allocation and nutrient uptake, and whether the responses to high N:K ratios vary among species. Plants were grown in sand at six N:K supply ratios ranging from 0.17 to 40.5 and combined with two levels of fertility in a factorial design. In 15 weeks of growth, plant biomass increased with increasing N:K supply ratios, indicating that growth was mostly limited by N and not by K across the entire range of N:K ratios. However, there were indications of K deficiency at the highest N:K supply ratio, such as increased leaf mortality and strong reduction of K concentrations during leaf senescence. The response of total plant biomass to nutrient treatments did not differ among the three species. However, other plant traits, such as biomass allocation to roots, leaf senescence and N concentrations, suggested that D. caespitosa is better adapted to extremely low K availability than the other two species.The short-term responses of the three wetland grass species to N and K supply were similar to those found in previous experiments testing different N:P supply ratios. In both cases, growth depended mainly on N supply, despite high N:K or N:P supply ratios and very low K or P concentrations in plant biomass. In those previous experiments, P supply became more important in the second year. There are indications that the same could also be true for K in a longer-term experiment. Hence, the so-called ‘critical’ values for N:K ratios of plant biomass in the field might be lower than indicated by our short-term experiment.     

High nitrogen : phosphorus ratios reduce nutrient retention and second-year growth of wetland sedges

Shifts from nitrogen (N)- to phosphorus (P)-limited growth due to high N deposition may alter the functioning of wetland vegetation. This experiment tested how N vs P deficiency affects the growth and nutrient use of wetland sedges. Five wetland Carex species were grown at nine N : P supply ratios (0.6-405) with two absolute levels of N and P. Biomass and nutrient concentrations were determined after one and two growing seasons. Shoot biomass was maximal at N : P supply ratios of 15-26 after one season but 5-15 after two seasons. Photosynthesis after the first season, second-year growth, leaf longevity, and the fraction of nutrient supply retained by plants over two seasons were all negatively related to N : P supply ratios, with small effects of absolute supply. The five Carex species responded similarly to N : P ratios but differed in nutrient resorption efficiency and biomass allocation. Plants treated with high N : P ratios appeared to lose nutrients below ground. Such losses may reduce plant performance in P-limited wetlands affected by high N deposition.     

Effects of a nutrient pulse supply on nutrient status of the Mediterranean trees Quercus ilex subsp. ballota and Pinus halepensis on different soils and under different competitive pressure

Nutrient availability is a key factor in Mediterranean ecosystems that affects the primary productivity and the community structure. The great variability of its natural availability is now increasing due to frequent fires, pollution events and changes in rainfall regime associated to climate change. Quercus ilex ssp. ballota and Pinus halepensis are the most abundant tree species in the NW Mediterranean basin. They frequently compete in the early and middle successional stages. We investigated the effects of N and P pulse supplies on nutrient uptake capacity in these two species in an after-fire field area and in nursery conditions on different soil types and competing conditions. In the field, N fertilisation had weak effects on nutrient concentration and mineralomass likely as a consequence of this nutrient not being limiting in this field site whereas P fertilisation increased the P mineralomass and the Mg, S, Fe, K and Ca concentrations and mineralomass in the different biomass fractions of both species 1 and 3 years after fertilisation application. In the nursery experiments, P fertilisation increased the mineralomass and concentrations of P, Mg, S, Fe, K and Ca in all biomass fractions including the roots in both species and in different soils and competition conditions. The increment of nutrient mineralomass was due to both the increase of growth and of nutrient concentrations. Both species were able to absorb significant amounts of the P applied by fertilisation (between 5 and 20%) in short time (18 months). Competing vegetation decreased the positive effects of P fertilisation, and in many cases the negative effect of competing vegetation on nutrient mineralomass was stronger when P availability was increased by fertilisation. Q. ilex subsp ballota showed a greater competitive ability for P than the more pioneer species Pinus halepensis in the field but not in the nursery conditions. Pinus halepensis had greater nutrient mineralomass in calcareous than in siliceous soils. Q. ilex subsp. ballota had a higher root biomass allocation and root nutrient allocation than P. halepensis, but both species showed a high capacity to increase their nutrient uptake when its availability increased by fertilisation, thus assuring a great nutrient reserve for future growth periods and contributing to retain nutrients in the ecosystem.     

Plant responses to nutrient addition and predictive ability of vegetation N:P ratio in an austral fen

1. Previous studies of the N:P ratio in wetland plants have been carried out in northern hemisphere wetlands where atmospheric nitrogen deposition is higher. There is little research on foliar N:P ratio as a potential indicator of nutrient limitation in vegetation communities in southern hemisphere wetlands. This study aimed to redress this knowledge gap and answer the following questions: how well does the plant tissue nitrogen to phosphorus (N:P) ratio predict wetland plant community nutrient limitation, as indicated by vegetation standing stocks and below-ground biomass, in southern hemisphere fens? Secondly, what are the impacts of realistic upper levels of farm nutrient run-off on natural montane fen vegetation?
2. Low (35 kg ha⁻¹ year⁻¹) and high (70 kg ha⁻¹ year⁻¹) levels of nitrate-N or ammonium-N with and without P (20 kg ha⁻¹ year⁻¹) were added to 81 vegetation plots over a period of 2.75 years. Species composition, plant nutrient status, and above-ground live vegetation standing stocks were assessed after 3 years, and below-ground biomass after 2 years.
3. Plant tissue analysis suggested the community was N limited or N and P co-limited; we found greater standing stocks of vegetation in plots treated with 70 kg ha⁻¹ year⁻¹ ammonium-N, indicating N limitation. No difference between other treatments was found in above-ground standing stocks or below-ground biomass. Plant species cover increased in both high N treatments, consistent with N limitation. These changes in plant species cover were accompanied by significant decreases in species richness in both high N treatments. Native species dominated the vegetation and this was unaffected by nutrient addition (90% cover).
4. This is one of the first studies to test and find support for the N:P ratio in southern hemisphere wetlands. Observed declines in species richness after N fertilisation in an N-limited fen suggests increased N may pose risks to austral wetlands. Responses by plant communities (changes in composition, biomass) to lower levels of nutrient addition may require longer periods of fertilisation to be apparent in slow growing ecosystems.

     

Changes in nutrient use efficiency, status and retranslocation in young post-fire regeneration Pinus halepensis in response to sudden N and P input, irrigation and removal of competing vegetation

The capacity of Mediterranean species to adapt to variable nutrient supply levels in a global change context can be a key factor to predict their future capacity to compete and survive in this new scenario. We aimed to investigate the capacity of a typical Mediterranean tree species, Pinus halepensis, to respond to sudden changes in N and P supply in different environmental conditions. We conducted a fertilisation, irrigation and removal of competing vegetation experiment in a calcareous post-fire shrubland with an homogeneous young (5 years old) population of P. halepensis in order to investigate the retranslocation and nutrient status for the principal nutrients (N, P, Mg, K, S, Ca and Fe), and the nutrient use efficiency (NUE) of the most important nutrients linked to photosynthetic capacity (N, P, Mg and K). P fertilisation increased P concentration in needles, P, N, Mg and K retranslocations, and NUE calculated as biomass production per unit of nutrient lost in the litterfall. The P fertilisation was able to increase the aboveground biomasses and P concentration 3 years after P fertiliser application. Those responses to P fertilisation were enhanced by the removal of competing vegetation. The N needle and litterfall concentration decreased after P fertilisation and this effect was greater when the P fertilisation was accompanied by removal of competing vegetation. The increase of P availability decreased the P-NUE and increased the N-NUE when these variables were calculated as aboveground biomass production per unit of P present in the biomass. Both P-NUE and N-NUE increased when calculated as total aboveground production per unit of nutrient loss. The results show that it is necessary to calculate NUE on a different basis to have a wider understanding of nutrient use. The irrigation did not change the needle nutrient concentrations and the litterfall production, but it significantly changed the nutrient litterfall concentrations and total aboveground contents (especially P and K). These results show a high capacity of P. halepensis to quickly respond to a limiting nutrient such as P in the critical phases of post-fire regeneration. The increase in P availability had a positive effect on growth and P concentrations and contents in aboveground biomass, thus increasing the capacity of growth in future periods and avoiding immediate runoff losses and leachate. This capacity also strongly depends on neighbour competition.     

Biomass production of the last remaining fen with Saxifraga hirculus in Switzerland is controlled by nitrogen availability

Olde Venterink H. and Vittoz P. 2008. Biomass production of the last remaining fen with Saxifraga hirculus in Switzerland is controlled by nitrogen availability. Bot. Helv. 118: 165 – 174. For conservation management of endangered plants it is important to know which nutrient(s) control growth of the vegetation, because maintenance of low nitrogen (N), phosphorus (P) or potassium (K) availability requires different management measures. The aim of this study was to determine the type of nutrient limitation for the vegetation in the last remaining site with Saxifraga hirculus in Switzerland, using nutrient ratios in the aboveground vegetation as an indicator. We made vegetation relevees, collected biomass of the vascular plants, and took soil samples in three plots at this site. The biomass was very low (152–231 g m ^-2), and all three plots were clearly N-limited with N:P ratios of 7– 8. Soil extractable N concentrations were generally low, and P and K concentrations were moderate to high, which was consistent with the indicated N limitation. Hence conservation management first of all needs to prevent N-enrichment, and needs to avoid increased mineralization rates through drainage, or the accumulation of N in the system from atmospheric deposition. Therefore N output seems required through for instance grazing or mowing. The current grazing management seems to function well, since total aboveground biomass is very low and S. hirculus has a high abundance in this last remnant. Submitted 5 June 2008; Accepted 14 October 2008 Subject editor: Sonja Wipf     

Flood events overrule fertiliser effects on biomass production and species richness in riverine grasslands

Question: Do severe winter flood events lift the nutrient limitation of biomass production in a river floodplain? How does this affect plant species richness? How long do the effects last? Location: Floodplain grassland on calcareous sandy loam near river Rhine in The Netherlands. Methods: Plots were fertilised with four treatments (control, N, P, N+P) for 21 years; plant species composition, vegetation biomass and tissue nutrient concentrations were determined every year between 1985 and 2005. Results: Fertilisation with N generally increased biomass production and reduced species richness, but these effects varied over time. During the first four years of the experiment, biomass production appeared to be co‐limited by N and P, while N fertilisation dramatically reduced plant species richness; these effects became weaker subsequently. Following two extreme winter floods in 1993–94 and 1994–95 and a drought in spring 1996, the effects of fertilisation disappeared between 1998 and 2001 and then appeared again. Flooding caused an overall reduction in species richness (from c. 24 to 15 species m^‐2) and an increase in biomass production, which were only partly reversed after ten years. Conclusions: Long time series are necessary to understand vegetation dynamics and nutrient limitation in river floodplains, since they are influenced by occasional flood and drought events, whose effects may persist for more than ten years. A future increase in flooding frequency might be detrimental to species richness in floodplain grasslands.     

Nitrogen, phosphorus and potassium nutritional status of semiarid steppe grassland in Inner Mongolia

In grazed semiarid steppe ecosystems, much attention has been paid to aspects of growth limitation by water. So far, potential limitation of primary production by plant nutrients was rarely considered. This knowledge is essential for identification of sustainable land-use practices in these large and important ecosystems on the background of over-exploitation and climate change. In the present study plant nutrient concentrations and ratios were investigated with factorial additions of water and N fertilizer at two sites with contrasting soil nutrient availability. Combined analysis of nutrient concentrations, contents, biomass production, and plant N:P ratios consistently confirmed primary growth limitation by water and a strong N limitation when sufficient amounts of water were supplied. P limitation only occurred at the site with low P availability when in addition to the natural supply, water and N fertilizer were given. According to reported thresholds of N:K and K:P ratios, K was not limiting in any plot. The observed nutritional patterns in the plant community were related to the dynamics of species composition and their specific nutrient status. Stipa grandis had the highest N:P ratio whereas Artemisia frigida showed lowest N:P. These nutrient characteristics were related to growth strategies of dominant species. Accordingly, the relative biomass contribution of S. grandis and A. frigida strongly affected the nutrient status of the plant community. Plant N:P ratios indicate the relative limitation by N or P in the semiarid grasslands under sufficient water supply, but other methods of nutritional diagnosis should be used when plant N:P ratios remain below critical values.     

Nutrient limitation in species‐rich Calthion grasslands in relation to opportunities for restoration in a peat meadow landscape

Questions: Which nutrient(s) limit(s) vegetation productivity in Calthion grasslands? Is phosphorus release a bottleneck for restoration of species‐rich Calthion grasslands on rewetted dairy meadows? Location: Three species‐rich Calthion grasslands in the Western Peat District in the Netherlands. Methods: We conducted a field fertilization experiment with nitrogen (N), phosphorus (P) and potassium (K) in three existing Calthion grasslands to evaluate the potential for restoration on rewetted dairy meadows. Responses of above‐ground biomass, tissue nutrient concentrations and nutrient ratios were determined after 2 yr of fertilization. Results: Biomass increased with fertilization with N‐only and K‐only but did not react to P‐only additions. Comparisons of tissue nutrient concentrations and nutrient ratios also gave indications of N and K limitation. Conclusions: The strong P release expected after rewetting should not necessarily interfere with restoration of Calthion communities on rewetted dairy meadows. It is concluded that for successful restoration management measures should focus on reducing N and/or K availability. Potassium might be an overlooked bottleneck in the restoration of species‐rich grasslands.     

The effects of nutrient availability and removal of competing vegetation on resprouter capacity and nutrient accumulation in the shrub Erica multiflora

Nutrient availability is increasing in the Mediterranean Basin due to the great number and intensity of fires and higher levels of anthropomorphic pollution. In the experiment described in this paper, we aimed to determine the effects of N and P availability and of the removal of competing vegetation on resprouter capacity, biomass, and nutrient accumulation in Erica multiflora. Plants of the resprouter species E. multiflora were clipped to 0% of aerial biomass in a post-fire Mediterranean shrubland and fertilisation experiments and removal of competing vegetation were established in a factorial design. The resprouting of clipped plants was monitored during the first year after clipping and at the end of the year, all plant resprout populations were harvested and their resprout structure, biomass and N and P content measured. N fertilisation had no significant effect on leaf biomass either at plant level or on the total aerial biomass per stump unit area; however N concentration in resprout biomass did increased. P fertilisation slightly increased resprouting vigour and had a significant effect on P content of the leaf biomass. The removal of competing vegetation increased the ratio between leaf biomass and stem biomass, the lateral expansion of resprout, the hierarchy of resprouts branching, and the P content of stems, above all when P fertilisation was applied. These results show that as a response to decreased competition E. multiflora has the capacity to modify the relative proportions of the nutrients in the aerial biomass. All these characteristics allow E. multiflora to persist in increasingly disturbed Mediterranean ecosystems and contribute to the retention of nutrients in the ecosystem during early resprouting phases.     

黄土丘陵区植被与地形特征对土壤和土壤微生物生物量生态化学计量特征的影响

研究黄土丘陵区植被与地形特征对土壤和土壤微生物生物量生态化学计量特征影响有助于深入理解黄土丘陵区不同植被带下土壤和土壤微生物相互作用及养分循环规律.选择黄土丘陵区延河流域3个植被区(森林区、森林草原区、草原区)和5种地形部位(阴/阳沟坡、阴/阳梁峁坡、峁顶)的土壤作为研究对象,利用生态化学计量学理论研究植被和地形对土壤和土壤微生物生物量生态化学计量特征的影响.结果表明: 土壤及土壤微生物生物量碳、氮、磷含量在不同地形之间的差别主要表现在沟坡位置和阴坡高于其他坡位和阳坡.植被类型的变化对两个土层(0～10、10～20 cm)土壤和土壤微生物生物量碳、氮、磷的影响均达到显著水平,坡向对表层(0～10 cm)土壤和土壤微生物生物量碳、氮、磷的影响强于坡位,而在10～20 cm土层,坡位对土壤和土壤微生物生物量碳、氮、磷影响更显著.植被类型显著影响土壤C∶N、C∶P、N∶P和土壤微生物生物量C∶N、C∶P,坡向和坡位仅影响土壤C∶P和N∶P,植被类型的变化是影响土壤C∶N的主要因素.同时,植被类型对土壤养分和微生物生物量碳、氮、磷含量及其生态化学计量特征的影响大于地形因子.标准化主轴分析结果表明,黄土丘陵区不同植被带土壤微生物具有内稳性,特别在草原带,土壤微生物生物量生态化学计量学特征具有更加严格的约束比例.在黄土丘陵区,土壤微生物生物量N∶P或许可以作为判断养分限制的另一个有力工具,若将土壤微生物生物量N∶P与植物叶片N∶P配合使用可能有助于我们更加精确地判断黄土丘陵区的土壤养分限制情况.     

Correlations between nutrient concentrations and zooplankton populations in a mesotrophic reservoir

1. A year-round study was conducted in a mesotrophic reservoir to determine the dynamics of zooplankton populations as a function of food availability (edible phytoplankton), nutrient concentration, temperature and hydraulic regime.
2. Rotifer biomass was correlated with soluble reactive phosphorus (SRP) concentration. The abundance of the rotifers Keratella cochlearis and Anuraeopsis fissa were not correlated with food availability (measured by chlorophyll and cell counts) but showed a strong dependence on P availability. Another rotifer, Synchaeta oblonga , and crustacean species were not related to nutrient availability but seemed to be dependent on food concentrations, especially of some phytoplankton taxa.
3. In this field study, rotifers seemed more susceptible than Daphnia or copepods to P-limitation. Among rotifer species, Keratella seemed to be more susceptible than Anuraeopsis to P limitation. Different susceptibilities of zooplankton species to nutrient limitation may be important in explaining the dynamics of these organisms in natural situations. Further analyses are warranted to clarify the interactions between nutrient limitation and energy limitation among zooplankton.     

Vegetation in contrasting soil water sites of upland herbaceous grasslands and N:P ratios as indicators of nutrient limitation

Effects of differences in long-term water supply were examined on soil characteristics, primary production and species composition in a wet and a dry site of an upland herbaceous grassland. Also the responses of species to N and P enrichments were examined. N and P concentrations of non-legume species were positively related, indicating that biomass N:P ratios seem to be mainly determined by N:P supply ratios. Forbs had generally higher concentrations than graminoids. Intermittent water inundation of soil in the wet site resulted in greater soil N and P availability. The greater productivity of this site promoted the growth of forbs. A fertilizer experiment showed that biomass was limited by N only in the wet site, but by both nutrients in the dry one. The species with the higher N and P concentrations were favored more after N and P enrichment, respectively; however, species enhancement was not related to N:P ratios of species. This indicates that N and P concentrations of species, rather than N:P ratios of species, are better predictors of species responses to N and P enrichment. N:P ratios of whole communities were 8.73 for the wet and 11.36 for the dry site. These values in comparison with the responses of plant communities to N and P fertilization show that thresholds of N:P ratios indicative of N or P limitation are much lower than those found for European wetlands.     

Can we predict nutrient limitation in streams and rivers?

1. Anthropogenic impacts on the biogeochemical cycles of nitrogen (N) and phosphorus (P) affect natural ecosystems worldwide. Modelling is required to predict where and when these key nutrients limit primary production in freshwaters. 2. We reviewed 382 nutrient‐enrichment experiments to examine which factors promote limitation of microphytobenthos biomass by N or P in streams and rivers. Using regression models, we examined whether the response of microphytobenthos biomass to N and P additions could be predicted by the absolute N and P concentrations in the water, the water N:P ratio or a combination of the two. 3. The absolute N concentration in the water was the best predictor of the magnitude of the response of microphytobenthos biomass to N additions. In comparison, the N:P ratio was the best predictor of whether or not N was limiting. However, predictions were uncertain except at extreme N:P ratios <1 : 1 and >100 : 1. 4. The absolute P concentration in the water was the best predictor of the magnitude of the response of microphytobenthos biomass to P additions. Neither the absolute nor the relative N and P concentrations predicted whether or not P was limiting. 5. The absolute and the relative N and P water concentrations contribute significant and complementary insights into the responses of microphytobenthos biomass to nutrient enrichment in running waters. However, ability to predict nutrient limitation from these concentrations is constrained by substantial error in the models. In the future, the prediction ability of models of nutrient limitation might be improved by focussing on regional scales and accounting for additional factors such as light and disturbance.     

The influence of a dissolved inorganic nitrogen gradient on phytoplankton community dynamics in a chain of lakes

In a chain of lakes along which nutrient availability varies in a gradient, we performed factorial nutrient enrichment experiments to determine if nitrogen limitation was the principal factor controlling the differences in phytoplankton biomass, photosynthetic productivity, diversity, and species composition among two of the lakes in the chain. In the least productive lake, East Graham Lake, P and C enrichments (in the absence of N enrichment) had no effect on biomass and diversity, whereas within two weeks the N enrichments (alone or in any combination with P and/or C) increased the biomass and decreased the diversity of East Graham Lake phytoplankton to levels similar or identical to those in more productive Shoe Lake. Short-term ¹⁴C photosynthetic rates in East Graham Lake water also responded only to N in the third week. However, photosynthesis was stimulated by P in the first week, and a few species did increase in numbers with P enrichment, suggesting that some degree of P limitation remains in addition to the strong N limitation in East Graham Lake. A number of species responded individually to the enrichments in a manner similar to that of the overall community, and a strong overlapping of discriminant analysis scores for N-enriched East Graham Lake with those of Shoe Lake was consistent with our prediction that the community structure of N-enriched East Graham Lake water would shift toward that of Shoe Lake. However, many species did not respond consistently with these results, and the nutrients tested were clearly not a major factor in the differences in abundance of those species among the two lakes. The results support the argument that overall biomass production and diversity of the phytoplankton community in a lake can be a relatively simple function of a single most-limiting nutrient. However, many of the species responses also confirm that, while nutrient availability is an important factor in the control of the species composition of the community, other factors are likely to prevent reliable predictions of all species effects on the basis of nutrient availability alone.     

Nutrient limitation and botanical diversity in wetlands: can fertilisation raise species richness?

The 'resource balance hypothesis' proposes that the species richness of grassland vegetation is potentially highest when the N:P ratio of plant tissues is 10–15 (co-limitation), so that species richness could be raised by fertilisation with N or P at sites with lower or higher N:P ratios, respectively. Here we use data from field surveys in Swiss, Dutch and American fens or wet grasslands to analyse what changes in N:P ratios might produce noticeable changes in species richness. Plant species numbers, above-ground biomass, tissue N and P concentrations and soil pH were recorded in plots of 0.06–4 m². In each data set, plots with intermediate tissue N:P ratios (6–20) were on average most species-rich, but N:P ratios explained only 5–37% of the variation in species richness. Moreover, these effects were partially confounded with those of vegetation biomass and/or soil pH. The unique effects of N:P ratios (excluding those shared with biomass and pH) explained 11–17% of variation in species richness. The relationship between species richness and N:P ratios was asymmetric: plots with high N:P ratios were more species-poor than those with low N:P ratios. This was paralleled by a smaller species pool size at high N:P ratios (estimated from species numbers in multiple records), suggesting that fewer species are adapted to P-limited conditions than to N-limited conditions. According to these data, species richness in wetlands may possibly be raised by P-fertilisation when the initial N:P ratio of the vegetation is well above 20, but this option is not recommended for nature conservation as it might promote common species at the expense of rare ones.     

Stoichiometric and nutrient resorption characteristics of dominant tree species in subtropical Chinese forests

This study investigated seasonal patterns in stoichiometric ratios, nutrient resorption characteristics, and nutrient use strategies of dominant tree species at three successional stages in subtropical China, which have not been fully understood. Fresh leaf and leaf litterfall samples were collected in growing and nongrowing seasons for determining the concentrations of carbon (C), nitrogen (N), and phosphorus (P). Then, stoichiometric ratios (i.e., C:N, C:P, N:P, and C:N:P) and resorption parameters were calculated. Our results found that there was no consistent variation in leaf C:N and C:P ratios among different species. However, leaf N:P ratios in late‐successional species became significantly higher, indicating that P limitation increases during successional development. Due to the P limitation in this study area, P resorption efficiency and proficiency were higher than corresponding N resorption parameters. Dominant tree species at early‐successional stage adopted “conservative consumption” nutrient use strategy, whereas the species at late‐successional stage inclined to adopt “resource spending” strategy.     

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.     

内蒙古典型草原生态系统中N素添加对羊草和黄囊苔草N:P化学计量学特征的影响

过度放牧是中国典型草原生态系统日趋退化的主要原因.由于养分输入-输出的不均衡,将导致土壤养分库的耗竭,而确定退化草原受何种养分的制约是对其进行恢复的重要途径之一.应用N:P化学计量学的原理和方法,有望代替传统的野外养分添加实验,来研究不同草地受养分限制的状况.本文采用这两种方法在物种水平上研究限制性养分.此外,陆地植物器官中的N:P比相对恒定是植物在地球上生存的重要适应机制,养分添加为验证这一假说提供了一种有效手段.为此,我们采用野外N素添加的方法,研究了内蒙古典型草原两种演替系列样地中(围封22年的样地A和围封2年的样地B)羊草(Leymus chinensis(Ttin.)Tzvel.)和黄囊苔草(Carex korshinskyi Kom)生物量和N:P化学计量学特征的变异.N素添加梯度分别为0、5、15、30、50、80 gNH4NO3·m-2·a-1.研究结果表明,在施肥第一年,两个物种的地上生物量和P含量均不受N素添加的影响;相关分析结果表明,在施肥第二年两种植物的N:P比不受氮素添加的影响;施肥可以显著提高羊草和黄囊苔草地上器官的含N量,P含量只是在第二年有显著增大的趋势;2001年,两块样地中羊草和黄囊苔草的氮磷含量在不同施肥处理下均呈极显著地正相关.这表明,样地A中黄囊苔草缺乏P,样地B中羊草缺乏N,施肥两年后,两个物种器官中的N、P含量具有显著的协同关系,从物种水平上验证了我们提出的假说.同时,两年的实验结果还表明,生态系统中不同物种对添加N素的响应不同,笼统地界定一个生态系统受某种元素的制约是不恰当的.