Time-dependent, species-specific effects of N:P stoichiometry on grassland plant growth

N and P have different eutrophication effects on grassland communities, yet the underlying mechanisms are poorly understood. To examine plant growth in response to the varying (relative) supply of N and P, we conducted a two-year greenhouse experiment. Five grasses and three herbs were grown with three N:P supply ratios at two overall nutrient supply levels. During the first year the plant growth was relatively low at both high and low N:P supply ratios, whereas during the second year the growth was especially low at a high N:P supply ratio. This second-year low growth was attributed to the high root death rate, which was influenced by a high N:P supply ratio rather than by the nutrient supply level. Species responded differently, especially in P uptake and loss at a high N:P supply ratio. Each species seemed to have a different strategy for P limitation, e.g. an efficient P uptake or a high P resorption rate. Species typical of P-limited grasslands had neither better P uptake nor better P retention at a high N:P supply ratio. This study quantitatively demonstrates an increased plant root death triggered by strong P limitation. This finding indicates a possible extra effect of N eutrophication on ecosystem functioning via changed N:P stoichiometry.     

Marine nitrogen: Phosphorus stoichiometry and the global N:P cycle

Nitrogen supply is often assumed to limitmarine primary production. A global analysis of totalnitrogen (N) to phosphorus (P) molar ratios shows thattotal N:P is low (<16:1) in some estuarine andcoastal ecosystems, but up to 100:1 in open oceans.This implies that elements other than N may limitmarine production, except in human impacted, estuarineor coastal ecosystems. This pattern may reconcileconflicting enrichment studies, because N additionfrequently increases phytoplankton growth where totalN:P is expected to be low, but P, Fe, or Si augmentphytoplankton growth in waters where total N:P ishigh. Comparison of total N:P stoichiometry betweenmarine and freshwaters yields a model of the form ofthe aquatic N:P cycle.     

Changes of plant N:P stoichiometry across a 3000-km aridity transect in grasslands of northern China

Plant and Soil - Rice cultivars bred for fertile soils may not be the best suited to nutrient limited environments. Therefore, rice cultivars suited to sustain productivity in low-fertile soils and...     

Site-level importance of broadleaf deciduous trees outweighs the legacy of high nitrogen (N) deposition on ecosystem N status of Central Appalachian red spruce forests

Plant and Soil - Atmospheric nitrogen (N) deposition can influence forest ecosystem N status, and the resilience of forests to the effects of N deposition depends on a number of co-occurring...     

Combined effects of nitrogen enrichment, sulphur pollution and climate change on fen meadow vegetation N:P stoichiometry and biomass

Nitrogen (N) and sulphur (S) deposition, as well as altered soil moisture dynamics due to climate change can have large effects on fen meadow biogeochemistry and vegetation. Their combined effects may differ strongly from their separate effects, since each process affects different nutrients through different mechanisms. However, the impacts of these environmental problems are rarely studied in combination. We therefore investigated the separate and interactive effects of current levels of N- and S-deposition and changes in soil moisture dynamics on fen meadow vegetation. We focused on vegetation biomass and N:P stoichiometry, including access to soil P through root surface phosphatase activity, in a 3-year factorial addition experiment in an N-limited rich fen meadow in the Biebrza valley in Poland. We applied 29.5 kg N ha^?1 year^?1 and 32.1 kg S ha^?1 year^?1, which correspond to current deposition levels in Western Europe. Changes in soil moisture dynamics due to climate change were mimicked by amplified drying of the soil in summer. This level of N-deposition had limited effects on plant biomass production in this rich fen, despite low foliar N:P ratios that suggest N limitation. This level of S-deposition, however, resulted in decreased vegetation P-uptake and biomass. We also showed that increased summer drought resulted in considerable increases in vegetation biomass. We found no interactive effects on vegetation biomass or N:P stoichiometry, possibly as a result of the limited main effects of the separate processes.     

Different temperature sensitivity and kinetics of soil enzymes indicate seasonal shifts in C,N and P nutrient stoichiometry in acid forest soil

Acid forest soils in the Bohemian Forest in Central Europe are biogeochemically imbalanced in organic C, N and P processing. We hypothesized that these imbalances can be due to different temperature sensitivities of soil enzyme activities and their affinities to substrate in litter and organic soil horizons. We measured potential activities of five main soil enzymes (β-glucosidase, cellobiohydrolase, Leu-aminopeptidase, Ala-aminopeptidase, and phosphatase) responsible for organic carbon, nitrogen and phosphorus acquisition. We also modeled potential in situ enzyme activities and nutrient release based on continuous in situ temperature measurements. We determined basic kinetic parameters (K_m, V_max), enzyme efficiencies (k_cat) and temperature sensitivities (E_a and Q₁₀) according to Michaelis–Menten kinetic and modified Arrhenius models. Our results showed significant differences in substrate affinities between the litter and organic soil horizons. Higher aminopeptidase affinity (lower K_m) in the litter soil horizon can lead to leaching of peptidic compounds to lower soil horizons. β-Glucosidase and phosphatase showed high temperature response following the Arrhenius model. However, both aminopeptidases showed no or even decreased activity with increasing temperature. The aminopeptidase temperature insensitivity means that peptidic compounds are degraded at the same or even lower rate in warmer and colder periods of the year in acid forest soils. This imbalance results in different release of available nutrients from plant litter and soil organic matter which may affect bacterial and fungal community composition and nutrient leaching from these ecosystems.     

光照条件对土壤—植物系统氮素状况影响的研究

应用盆栽试验 ,通过调节不同光照强度并控制其它条件相互一致的条件下 ,研究了光照条件对土壤 植物系统N素状况以及作物 (莴笋 )产量的影响 .结果表明 ,光照强度的改变会引起作物生长状况的相应变化 ,同时也导致土壤N素 (NH 4 N、NO-3 N)状况、作物吸收N量以及作物对N素吸收速度等的改变 .在试验所处的光照强度范围内 ,光照较强时 ,则作物吸收N素的速度较快、吸收N量增加 ,且产量高 ,但土壤中相应的N素含量(NH 4 N、NO-3 N)则只能维持在相对较低的水平 ;光照较弱时 ,则出现与此相反的情况 .     

Soil retention of 15N in a simulated N deposition study: effects of live plant and soil organic matter content

Background and aims

The impacts of atmospheric nitrogen (N) deposition on terrestrial ecosystem processes remain controversial, mostly because of the uncertainty regarding the fates of deposited N. We conducted a 16-week simulated deposition study to experimentally trace N in a greenhouse plant-soil system.

Methods

Using a two-way factorial design, we added (¹⁵NH₄)₂SO₄ solution twice a week to pots containing different soil organic matter (SOM) content and with or without a live plant (Salix dasyclados). The recoveries of ¹⁵N in soil, plant biomass, and leaching solution were quantified.

Results

We found most ¹⁵N was retained in soil (18.0–59.2%), with significantly more ¹⁵N recovered from high-SOM soils than from low-SOM soils. Plant presence significantly increased ¹⁵N retention in soil. Plant biomass accounted for 10–20% of the ¹⁵N input, with proportionally more ¹⁵N assimilated when plants were grown in low-SOM soils. Leaching loss of ¹⁵N was relatively low (10–17%).

Conclusion

Our study suggests that SOM content and plant presence significantly affect the fates of deposited N. Indeed, N would be preferentially retained in soils with high SOM content and live plant, while plants would assimilate more deposited N when grown in low SOM soils. Global biogeochemical models thus need to incorporate such soil-specific N retention and plant N assimilation.     

Soil N/P and C/P ratio regulate the responses of soil microbial community composition and enzyme activities in a long-term nitrogen loaded Chinese fir forest

Plant and Soil - Phosphorus (P) is an essential mineral element required in large quantities by plants. Globally, the availability of P in many soils is poor. Breeding crops that can acquire and...     

Ciliate community associated with aquatic macrophyte roots: effects of nutrient enrichment on the community composition and species richness

The objective of this study was to identify the impact of nutrient enrichment on the diversity of the ciliate community associated with the roots of the aquatic macrophyte Eichhornia crassipes. The experiment was performed in the Garças Lake, located in the Upper Paraná River floodplain, Brazil. We conducted two treatments (fertilized and control) with three replicates each. To increase the initial nutrient concentrations in each mesocosm of the fertilized treatment, we added 1000 μg L⁻¹ of KNO₃ and 200 μg L⁻¹ of KH₂PO₄ during each sampling date. We found a relative high number of ciliate species (85 species) and a predominance of hypotrichs. Among the recorded species, about 25% occurred exclusively in the fertilized treatment. Moreover, detrended correspondence analysis demonstrated that the ciliate community associated with E. crassipes roots changed significantly in response to the nutrient input in such a way that the species composition of the fertilized treatment was remarkably different from that of the control. In contrast to our expectations, species richness in the fertilized treatment was significantly higher than that in the control, refuting our hypothesis that species richness decreases under eutrophic conditions.     

Linking thaw depth with soil moisture and plant community composition: effects of permafrost degradation on alpine ecosystems on the Qinghai-Tibet Plateau

Background and aims

The warming of the planet in recent decades has caused rapid, widespread permafrost degradation on the Qinghai–Tibet Plateau. These changes may significantly affect soil moisture content and nutrient supply, thereby affecting ecosystem structure and function. This study aimed to describe the dynamic changes in thaw depth, assess the relationship between thaw depth and soil moisture content, and analyze the changes in species composition and water-use efficiency in response to permafrost degradation.

Methods

We surveyed species composition, thaw depth, ground temperature, soil moisture, nutrient content, and foliar stable carbon isotope compositions to gain insights into the response of alpine grassland ecosystems to permafrost degradation on the Qinghai-Tibet Plateau.

Results

Moisture content of the surface layer decreased with increasing thaw depth. The correlation between thaw depth and surface soil moisture content was strongest in June and decreased in July and August. The strongest correlation occurred at a depth of 20 cm to 30 cm. The dominant species shifted from Cyperaceae in alpine meadow to mesoxerophytes in alpine steppe before finally shifting to xerophytes in alpine desert steppe. Thaw depth correlation was significantly negative with organic C content (r?=??0.49, P?<?0.05) and with total N content (r?=??0.62, P?<?0.01). The leaf δ¹³C of Carex moorcroftii increased with increasing thaw depth and followed a linear relationship (R ²?=?0.85, P?=?0.008).

Conclusions

Permafrost degradation decreases surface soil moisture and soil nutrient supply capacity. Increasing permafrost degradation decreases the number of plant families and species, with hygrophytes and mesophytes gradually replaced by mesoxerophytes and xerophytes. The water-use efficiency of plants improved in response to increasing water stress as surface layers dried during permafrost degradation. Permafrost on the Qinghai–Tibetan Plateau is expected to further degrade as global warming worsens. Therefore, more attention should be dedicated to the response of alpine ecosystems during permafrost degradation.     

Potential effects of warming and drying on peatland plant community composition

Boreal peatlands may be particularly vulnerable to climate change, because temperature regimes that currently constrain biological activity in these regions are predicted to increase substantially within the next century. Changes in peatland plant community composition in response to climate change may alter nutrient availability, energy budgets, trace gas fluxes, and carbon storage. We investigated plant community response to warming and drying in a field mesocosm experiment in northern Minnesota, USA. Large intact soil monoliths removed from a bog and a fen received three infrared warming treatments crossed with three water‐table treatments (n = 3) for five years. Foliar cover of each species was estimated annually. In the bog, increases in soil temperature and decreases in water‐table elevation increased cover of shrubs by 50% and decreased cover of graminoids by 50%. The response of shrubs to warming was distinctly species‐specific, and ranged from increases (for Andromeda glaucophylla) to decreases (for Kalmia polifolia). In the fens, changes in plant cover were driven primarily by changes in water‐table elevation, and responses were species‐ and lifeform‐specific: increases in water‐table elevation increased cover of graminoids – in particular Carex lasiocarpa and Carex livida– as well as mosses. In contrast, decreases in water‐table elevation increased cover of shrubs, in particular A. glaucophylla and Chamaedaphne calyculata. The differential and sometimes opposite response of species and lifeforms to the treatments suggest that the structure and function of both bog and fen plant communities will change – in different directions or at different magnitudes – in response to warming and/or changes in water‐table elevation that may accompany regional or global climate change.     

Separating effects of soil microorganisms and nematodes on plant community dynamics

Plant and Soil - The incidence of extreme weather events, particularly drought is predicted to increase in the future and alter the ecosystem process. Despite that the interplay between plant...     

Changes in nutrient availability from calcareous to acid wetland habitats with closely related brown moss species: increase instead of decrease in N and P

To test whether shifts in nutrient availability from calcareous to mineral-poor habitats could be a driving force in the evolution of seven closely related wetland brown mosses, we measured soil and vascular plant nutrients and conducted a laboratory incubation experiment with Swedish and some Dutch samples, in which net N and P-mineralization, respiration and microbial characteristics were measured. In spite of high respiration and microbial N, net N-mineralization appeared to be low for the calcareous Palustriella falcata and Scorpidium spp. Net N-mineralization significantly increased (and respiration and microbial N decreased) for the mineral-poor Sarmentypnum exannulatum, Straminergon stramineum and Warnstorfia fluitans, probably due to a decrease in microbial N-demand. Even though values were mainly negative, net P-mineralization showed a similar increase from calcareous to mineral-poor fens, probably due to lower precipitation of calcium phosphate. The calcareous habitat of the early wetland mosses may thus have been nutrient-poor instead of nutrient-rich. Adaptation to mineral-poor habitats, probably driven by expansion of mineral-poor wetlands when the boreal zone became colder and wetter, may have been associated with higher availability of ammonium and phosphate. However, this may have stimulated Sphagnum more than brown mosses, which may have been restricted to particular niches with perhaps some nitrification.     

Role of plant species and soil phosphorus concentrations in determining phosphorus: nutrient stoichiometry in leaves and fine roots

Plant and Soil - Sustainable crop production is crucial to address global food security and requires a solid input of chemical fertilizers containing macro (e.g. nitrogen: N) and micro (e.g. zinc:...     

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

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

Direct and indirect effects of allelopathy in the soil legacy of an exotic plant invasion

Invasive species may leave behind legacies that persist even after removal, inhibiting subsequent restoration efforts. We examined the soil legacy of Cytisus scoparius, a nitrogen-fixing, putatively allelopathic shrub invading the western US. We tested the hypothesis that allelopathy plays a critical role in the depressive effect of Cytisus on the key native Douglas-fir, both directly on tree growth and indirectly via effects on its ectomycorrhizal fungi (EMF). In a greenhouse factorial experiment, we used activated carbon to inhibit Cytisus-produced allelochemicals and sucrose to reduce elevated nitrogen (N). We found that: (1) Cytisus-invaded soils depressed Douglas-fir growth compared to uninvaded forest soils. The effect of adding Cytisus litter was positive (possibly reflecting an N fertilization effect) only in the presence of activated carbon, providing evidence for a role of allelopathic compounds. Activated carbon did not increase growth in the absence of Cytisus litter. Finally, sucrose addition provided weak support for a nitrogen effect of Cytisus litter. (2) Seedlings grown in Cytisus soils had lower EMF abundance compared to those in uninvaded forest soils. In forest soil from one site, adding Cytisus litter also decreased EMF abundance. Douglas-fir growth increased significantly with EMF across sites and soils suggesting that changes in EMF were linked to tree growth. The fungal taxon Cenococcum geophilum was significantly depressed in Cytisus soils compared to forest soils, while Rhizopogon rogersii abundance was similar across soil types. These results together suggest an overall negative effect of Cytisus on the growth of a dominant native tree and its fungal symbionts. Our study suggests how the role of allelopathy in ecological restoration may play out on two time scales: through immediate, direct impacts on native plants as well as through long-term, persistent impacts mediated by the collapse or transformation of microbial communities.