Nitrogen Cycling and Nitric Oxide Emissions in Oil-Impacted Prairie Soils

A remote site in the Tallgrass Prairie Preserve of Oklahoma (The Nature Conservancy) was contaminated with crude oil from a pipeline break and is being bioremediated using landfarming techniques. Landfarming is designed to stimulate microbial-based catabolism of petroleum through combined dilution/mixing and fertilization-based effects. To evaluate nitrogen-based effects during remediation, the site was sectioned and treated with urea, ammonium sulfate, or ammonium nitrate. Samples were obtained from prairie soil without chemical nitrogen addition and with or without hydrocarbon contamination. Nitrogen cycling dynamics were followed by measuring ammonium, nitrite, nitrate, and volatile nitric oxide (NO_x) levels. Nitrifying and denitrifying bacterial numbers were estimated and compared to soil oxygen, carbon dioxide, and methane levels as well as to overall total petroleum hydrocarbon (TPH) reduction. For a prairie ecosystem of this type, a high level of fertilization, particularly with nitrogen, can have ecological effects almost as profound as the petroleum contamination itself. Fertilization of the oil-contaminated soil with the reduced and/or oxidized forms of nitrogen quickly resulted in elevated steady-state levels of both ammonium and nitrate, and exceptionally high levels of NO_x released from soil. Although nitrogen fertilization increased microbial nitrogen metabolism and nitrogen cycling, it had minimal effects on the overall remediation efficiency.     

Rapid Cycling of Organic Nitrogen in Taiga Forest Ecosystems

ABSTRACT We examined the dynamics of organic nitrogen (N) turnover in situ across a primary successional sequence in interior Alaska, USA, in an attempt to understand the magnitude of these fluxes in cold, seasonally frozen soils. Through a combination of soil extraction procedures and measurements of ¹³C-enriched CO₂ efflux from soils amended in the field with ¹³C-labeled amino acids, we were able to trace the fate of this N form. Amino acid turnover in situ at soil temperatures of 10°C or below show that amino acids represent a highly dynamic soil N pool with turnover times of approximately 3–6 h. The rapid turnover of free amino acids is associated with high soil proteolytic activity, which in turn is tightly correlated with soil protein concentration. Moreover, these estimates of soil amino acid turnover in the field correspond well with measurements of amino acid turnover under equivalent temperatures in the laboratory. The gross flux of amino acid-N over the growing season greatly exceeded the annual vegetation N requirement, suggesting that microbial biomass represent a significant sink for this organic N. Depending on the strength of this sink, N flow via free soil amino acids can potentially account for the entire N demand of vegetation in the absence of net N mineralization. These relationships underscore the important biogeochemical role of labile DON fractions in high-latitude forest ecosystems.     

Seasonal and Spatial Controls over Nutrient Cycling in a Pacific Northwest Prairie

West Coast prairies in the US are an endangered ecosystem, and effective conservation will require an understanding of how changing climate will impact nutrient cycling and availability. We examined how seasonal patterns and micro-heterogeneity in edaphic conditions (% moisture, total organic carbon, % clay, pH, and inorganic nitrogen and phosphorus) control carbon, nitrogen, and phosphorus cycling in an upland prairie in western Oregon, USA. Across the prairie, we collected soils seasonally and measured microbial respiration, net nitrogen mineralization, net nitrification, and phosphorus availability under field conditions and under experimentally varied temperature and moisture treatments. The response variables differed in the degree of temperature and moisture limitation within seasons and how these factors varied across sampling sites. In general, we found that microbial respiration was limited by low soil moisture year-round and by low temperatures in the winter. Net nitrogen mineralization and net nitrification were never limited by temperature, but both were limited by excessive soil moisture in winter, and net nitrification was also inhibited by low soil moisture in the summer. Factors that enhanced microbial respiration tended to decrease soil phosphorus availability. Edaphic factors explained 76% of the seasonal and spatial variation in microbial respiration, 35% of the variation in phosphorus availability, and 29% of the variation in net nitrification. Much of the variation in net nitrogen mineralization remained unexplained (R ² = 0.19). This study, for the first time, demonstrates the complex seasonal controls over nutrient cycling in a Pacific Northwest prairie.     

Nitrogen Fluxes and Retention in Urban Watershed Ecosystems

Although the watershed approach has long been used to study whole-ecosystem function, it has seldom been applied to study human-dominated systems, especially those dominated by urban and suburban land uses. Here we present 3 years of data on nitrogen (N) losses from one completely forested, one agricultural, and six urban/suburban watersheds, and input–output N budgets for suburban, forested, and agricultural watersheds. The work is a product of the Baltimore Ecosystem Study, a long-term study of urban and suburban ecosystems, and a component of the US National Science Foundations long-term ecological research (LTER) network. As expected, urban and suburban watersheds had much higher N losses than did the completely forested watershed, with N yields ranging from 2.9 to 7.9 kg N ha^–1 y^–1 in the urban and suburban watersheds compared with less than 1 kg N ha^–1 y^–1 in the completely forested watershed. Yields from urban and suburban watersheds were lower than those from an agricultural watershed (13–19.8 kg N ha^–1 y^–1). Retention of N in the suburban watershed was surprisingly high, 75% of inputs, which were dominated by home lawn fertilizer (14.4 kg N ha^–1 y^–1) and atmospheric deposition (11.2 kg N ha^–1 y^–1). Detailed analysis of mechanisms of N retention, which must occur in the significant amounts of pervious surface present in urban and suburban watersheds, and which include storage in soils and vegetation and gaseous loss, is clearly warranted.     

Quantification of the Nitrogen Cycle in a Prairie Stream

Nitrogen (N) was added for 35 days in the form of ¹⁵NH₄Cl to Kings Creek on Konza Prairie, Kansas. Standing stocks of N in key compartments (that is, nutrients, detritus, organisms) were quantified, and the amount of labeled N entering the compartments was analyzed. These data were used to calculate turnover and flux rates of N cycling through the food web, as well as nutrient transformation rates. Inorganic N pools turned over much more rapidly in the water column of this stream than in pelagic systems where comparable measurements have been made. As with other systems, the mass of ammonium was low but it was the key compartment mediating nutrient flux through the ecosystem, whereas dissolved organic N, the primary component of N flux through the system, is not actively cycled. Nitrification was also a significant flux of N in the stream, with rates in the water column and surface of benthos accounting for approximately 10% of the total ammonium uptake. Primary consumers assimilated 67% of the inorganic N that entered benthic algae and microbes. Predators acquired 23% of the N that consumers obtained. Invertebrate collectors, omnivorous crayfish (Orconectes spp.), and invertebrate shredders dominated the N flux associated with primary consumers. Mass balance calculations indicated that at least 23% of the 309 mg of ¹⁵N added during the 35 days of release was retained within the 210-m stream reach during the release. Overall, the rates of turnover of N in organisms and organic substrata were significantly greater when C:N was low. This ratio may be a surrogate for biological activity with regard to N flux in streams. Received 2 August 1999; accepted 18 July 2000.     

Long-Term Nitrogen Storage and Soil Nitrogen Availability in Post-Fire Lodgepole Pine Ecosystems

Long-term, landscape patterns in inorganic nitrogen (N) availability and N stocks following infrequent, stand-replacing fire are unknown but are important for interpreting the effect of disturbances on ecosystem function. Here, we present results from a replicated chronosequence study in the Greater Yellowstone Ecosystem (Wyoming, USA) directed at measuring inorganic N availability (ion-exchange resin bags) and ecosystem N pools among 77 lodgepole pine stands that varied in age and density. Inorganic N availability ranged from 0.07 to 3.20 μN bag⁻¹ d⁻¹ and nitrate (NO₃⁻) was, on average, 65% of total resin-sorbed N. Total ecosystem N stocks (live + detrital + soil) averaged 109.9 ± 3.0 g N m⁻² (range = 63.7–185.8 g N m⁻²). Live N was 14%, detrital N was 29%, and soil N was 57% of total stocks. Soil NO₃⁻, total ecosystem N, live N, and detrital N generally increased with stand age, but soil N stocks decreased. Models (AIC_c) to predict soil N availability and N stocks included soil P, soil Ca, bulk density, and pH in addition to age (adj R ² ranged from 0.18 to 0.53) and density was included only for live N stocks. Patterns of N stocks and N availability with density were strongest for young stands (<20 years) regenerating from extensive fire in 1988; for example, litterfall N stocks increased with density (adj R ² = 0.86, P < 0.001) but inorganic N availability declined (adj R ² = 0.47, P < 0.003). Across the complex Yellowstone landscape, we conclude that N stocks and N availability are best predicted by a combination of local soil characteristics in addition to factors that vary at landscape scales (stand density and age). Overall, total ecosystem N stocks were recovered quickly following stand-replacing fire, suggesting that moderate increases in fire frequency will not affect long-term landscape N storage in Greater Yellowstone. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users. Author contributions   EAHS, MGT, and MGR conceived the study; DMK performed field research; EAHS and DMK oversaw laboratory analyses and analyzed data; EAHS wrote the paper.     

Nitrogen States in Plant Ecosystems: A Viewpoint

Terrestrial ecosystems are considered to be in only two possiblestates: N unsaturated or N saturated. This view lacks rigourand has led to three differing concepts of N saturation: (1)a continuum of changes in N states and processes; (2) a thresholdpoint, when N output increases; and (3) any equilibrium state,when N output equals N input. A simple model of ecosystem Ncontent, input and output is used to point out that, strictly,there are four possible N states of ecosystems: (1) at equilibrium,but N unsaturated and N-limited, so that they will respond toadditional N input with increased N accumulation; (2) not atequilibrium and accumulating N, either in response to additionalN input, or more commonly in nature, when recovering from fire,other disturbance or unfavourable conditions; (3) at equilibriumbut truly N saturated and not N-limited, so that any additionalN input is matched by equal N output; and (4) not at equilibriumand losing N because of disturbance or soil changes inducedby N addition itself or other factors. Most natural ecosystemsare, for most of the time, in one of the two non-equilibriumstates, especially in short-term N-addition experiments. Itis not meaningful to regard them as being either N saturatedor N-limited. Copyright 2000 Annals of Botany Company Nitrogen saturation, deposition, leaching, nitrification, plant ecosystems, forest     

Nitrogen Retention, Removal, and Saturation in Lotic Ecosystems

Increased nitrogen (N) loading to lotic ecosystems may cause fundamental changes in the ability of streams and rivers to retain or remove N due to the potential for N saturation. Lotic ecosystems will saturate with sustained increases in the N load, but it is unclear at what point saturation will occur. Rates of N transformation in lotic ecosystems will vary depending on the total N load and whether it is an acute or chronic N load. Nitrogen saturation may not occur with only pulsed or short-term increases in N. Overall, saturation of microbial uptake will occur prior to saturation of denitrification of N and denitrification will become saturated prior to nitrification, exacerbating increases in nitrate concentrations and in N export downstream. The rate of N export to downstream ecosystems will increase proportionally to the N load once saturation occurs. Long term data sets showed that smaller lotic ecosystems have a greater capacity to remove in-stream N loads, relative to larger systems. Thus, denitrification is likely to become less important as a N loss mechanism as the stream size increases. There is a great need for long-term studies of N additions in lotic ecosystems and clear distinctions need to be made between ecosystem responses to short-term or periodic increases in N loading and alterations in ecosystem functions due to chronic N loading.     

Nitrogen Dynamics in Ice Storm-Damaged Forest Ecosystems: Implications for Nitrogen Limitation Theory

Despite the widely recognized importance of disturbance in accelerating the loss of elements from land, there have been few empirical studies of the effects of natural disturbances on nitrogen (N) dynamics in forest ecosystems. We were provided the unusual opportunity for such study, partly because the intensively monitored watersheds at the Hubbard Brook Experimental Forest (HBEF), New Hampshire, experienced severe canopy damage following an ice storm. Here we report the effects of this disturbance on internal N cycling and loss for watershed 1 (W1) and watershed 6 (W6) at the HBEF and patterns of N loss from nine other severely damaged watersheds across the southern White Mountains. This approach allowed us to test one component of N limitation theory, which suggests that N losses accompanying natural disturbances can lead to the maintenance of N limitation in temperate zone forest ecosystems. Prior to the ice storm, fluxes of nitrate (NO₃ ^–) at the base of W1 and W6 were similar and were much lower than N inputs in atmospheric deposition. Following the ice storm, drainage water NO₃ ^– concentrations increased to levels that were seven to ten times greater than predisturbance values. We observed no significant differences in N mineralization, nitrification, or denitrification between damaged and undamaged areas in the HBEF watersheds, however. This result suggests that elevated NO₃ ^- concentrations were not necessarily due to accelerated rates of N cycling by soil microbes but likely resulted from decreased plant uptake of NO₃ ^-. At the regional scale, we observed high variability in the magnitude of NO₃ ^- losses: while six of the surveyed watersheds showed accelerated rates of NO₃ ^– loss, three did not. Moreover, in contrast to the strong linear relationship between NO₃ ^– loss and crown damage within HBEF watersheds [r ²: (W1 = 0.91, W6 = 0.85)], stream water NO₃ ^– concentrations were weakly related to crown damage (r ² = 0.17) across our regional sites. The efflux of NO₃ ^– associated with the ice storm was slightly higher than values reported for soil freezing and insect defoliation episodes, but was approximately two to ten times lower than NO₃ ^– fluxes associated with forest harvesting. Because over one half of the entire years worth of N deposition was lost following the ice storm, we conclude that catastrophic disturbances contribute synergistically to the maintenance of N limitation and widely observed delays of N saturation in northern, temperate zone forest ecosystems. Present address: Department of Ecology and Evolutionary Biology, Princeton University, Guyot Hall, Princeton, New Jersey 08544, USA.     

Nitrogen and Phosphorus Limitation over Long-Term Ecosystem Development in Terrestrial Ecosystems

Nutrient limitation to net primary production (NPP) displays a diversity of patterns as ecosystems develop over a range of timescales. For example, some ecosystems transition from N limitation on young soils to P limitation on geologically old soils, whereas others appear to remain N limited. Under what conditions should N limitation and P limitation prevail? When do transitions between N and P limitation occur? We analyzed transient dynamics of multiple timescales in an ecosystem model to investigate these questions. Post-disturbance dynamics in our model are controlled by a cascade of rates, from plant uptake (very fast) to litter turnover (fast) to plant mortality (intermediate) to plant-unavailable nutrient loss (slow) to weathering (very slow). Young ecosystems are N limited when symbiotic N fixation (SNF) is constrained and P weathering inputs are high relative to atmospheric N deposition and plant N:P demand, but P limited under opposite conditions. In the absence of SNF, N limitation is likely to worsen through succession (decades to centuries) because P is mineralized faster than N. Over long timescales (centuries and longer) this preferential P mineralization increases the N:P ratio of soil organic matter, leading to greater losses of plant-unavailable N versus P relative to plant N:P demand. These loss dynamics favor N limitation on older soils despite the rising organic matter N:P ratio. However, weathering depletion favors P limitation on older soils when continual P inputs (e.g., dust deposition) are low, so nutrient limitation at the terminal equilibrium depends on the balance of these input and loss effects. If NPP switches from N to P limitation over long time periods, the transition time depends most strongly on the P weathering rate. At all timescales SNF has the capacity to overcome N limitation, so nutrient limitation depends critically on limits to SNF.     

Herbivores, the Functional Diversity of Plants Species, and the Cycling of Nutrients in Ecosystems

Numerous investigators have suggested that herbivores almost always increase rates of nutrient and energy flow through terrestrial ecosystems by returning to the soil fecal material and urine with faster turnover rate than shed plant litter. These previous theories and models always treat the producer compartment as a homogenous pool. Essentially, they assume that consumers feed through a pureed cream of vegetable soup. However, many field observations and experiments have shown that consumers feed selectively (i.e., in a cafeteria) and that consumer choice is made on the same chemical basis that determines decomposition rates. Plants that are preferred food sources often have higher nutrient content, higher growth rates, and faster decomposition rates. As consumption reduces dominance of these species in favor of unpreferred species with slower decomposition, rates of nutrient cycling and energy flow should therefore decline. We analyze a model in which the consumer is given a choice among producers that vary in nutrient uptake rates, rates of nutrient return to decomposers, and consumer preference, and which is parameterized for plants and consumers characteristic of boreal regions. In this model, in an open, well-mixed system with one consumer and two such producers, the nutrient/energy flow will not exceed that of a system without the consumer. If the consumer has a choice between two such producers, it must choose one plant over the other at a greater ratio than that between the two plants in uptake and decay rates. In contrast, in a closed system the consumer must be less selective to coexist with the two plants. The system behavior is determined by the level of nutrient return through the consumer and the differences between the plants in nutrient uptake rates and consumer preference. Species richness affects properties of this model system to the extent that species are functionally distinct (i.e., have different rate constants) in a multivariate space of life history traits (i.e., nutrient uptake and palatability). We suggest that the biochemical variability of plant tissues that simultaneously determines both consumer preference and decomposition rates is an essential feature of food webs that cannot be ignored. Thus, ecosystem models should, at minimum, consider more than one producer type with consumer preference.     

Seasonal Changes in Fungal Production and Biomass on Standing Dead Scirpuslacustris Litter in a Northern Prairie Wetland

Decaying macrophytes are an important source of carbon and nutrients in fungal and bacterial communities of northern prairie wetlands. Dead macrophytes do not collapse into the water column immediately after death, and decomposition by fungi and bacteria begins while the plants are standing. The seasonal variations in fungal biomass and production on Scirpus lacustris stems, both above and below water, were measured to assess which environmental factors were dominant in affecting these variations in a typical prairie wetland. Fungal biomass and production were measured from early May to November, just prior to freeze-up. Fungal decomposition began and was greatest in the spring despite low water temperatures. The fungal production, as measured by the incorporation of [1-¹⁴C]acetate into ergosterol, ranged from 1.8 to 376 μg of C g of ash-free dry mass (AFDM)⁻¹ day⁻¹, and the biomass, as estimated by using ergosterol, ranged from nondetectable to 5.8 mg of C g of AFDM⁻¹. There was no significant difference in biomass or production between aerial and submerged portions of Scirpus stems. The water temperature was correlated with fungal production (r = 0.7, P < 0.005) for aerial stem pieces but not for submerged pieces. However, in laboratory experiments water temperature had a measurable effect on both biomass and production in submerged stem pieces. Changes in fungal biomass and productivity on freshly cut green Scirpus stems decaying in the water either exposed to natural solar radiation or protected from UV radiation were monitored over the summer. There was no significant difference in either fungal biomass (P = 0.76) or production (P = 0.96) between the two light treatments. The fungal biomass and rates of production were within the lower range of the values reported elsewhere, probably as a result of the colder climate and perhaps the lower lability of Scirpus stems compared to the labilities of the leaves and different macrophytes examined in other studies performed at lower latitudes.     

Microbial Nitrogen Cycling Processes in a Sulfidic Coastal Marsh

Sulfide distribution is a key controller of vegetation zonation in coastal ecosystems, but data are limited regarding its impact on the spatial distribution of important N cycling processes. We assessed vegetation distribution and density and, mineral N pool sizes, composition and transformations in a sulfidic coastal marsh in relation to distance from sulfur springs. We observed strong relationships between vegetation attributes (species and density) and mineral N status with greater total inorganic N, NO₃⁻ and denitrification enzyme activity (DEA) in sediment samples from areas populated by Crithmum maritimum (mid-way between S springs and sea shore) than in sediments from areas colonized by either Agropyron repens (closest to the S springs) or mangrove (Rhizophora mangleL., farthest from the springs). Our data also suggest close links between N cycling and SO₄⁻² reduction. The latter resulted in net release of NH₄⁺ ranging from 0.9 mg N kg⁻¹ in the low density C. maritimum to 3.2 mg N kg⁻¹ in the high-density A. repens, during a 3-day incubation. We also tested for microbial adaptation to long-term high sulfide exposure by measuring DEA using the C₂H₂ block method (which has been found to be strongly affected by the presence of sulfide) and amendment of marsh sediment samples with NaMoO₄ to suppress reduced S production. In sediments extracted from sites near the sulfur springs (A. repens and C. maritimum), the C₂H₂ blockage assay yielded similar results without and with NaMoO₄ addition. However, in samples from a mangrove located further downstream from the springs, DEA was substantially lower (2.3 vs. 6.8 mg N₂O-N kg⁻¹ sediment d⁻¹) when production of reduced S was not inhibited by NaMoO₄. These results suggest that denitrifying microbes in the high sulfide areas may have adapted to the presence of sulfide, allowing for high rates of N and S cycling to occur simultaneously in these marshes.     

Nitrogen Oxide Fluxes and Nitrogen Cycling during Postagricultural Succession and Forest Fertilization in the Humid Tropics

The effects of changes in tropical land use on soil emissions of nitrous oxide (N₂O) and nitric oxide (NO) are not well understood. We examined emissions of N₂O and NO and their relationships to land use and forest composition, litterfall, soil nitrogen (N) pools and turnover, soil moisture, and patterns of carbon (C) cycling in a lower montane, subtropical wet region of Puerto Rico. Fluxes of N₂O and NO were measured monthly for over 1 year in old (more than 60 years old) pastures, early- and mid-successional forests previously in pasture, and late-successional forests not known to have been in pasture within the tabonuco (Dacryodes excelsa) forest zone. Additional, though less frequent, measures were also made in an experimentally fertilized tabonuco forest. N₂O fluxes exceeded NO fluxes at all sites, reflecting the consistently wet environment. The fertilized forest had the highest N oxide emissions (22.0 kg N · ha⁻¹· y⁻¹). Among the unfertilized sites, the expected pattern of increasing emissions with stand age did not occur in all cases. The mid-successional forest most dominated by leguminous trees had the highest emissions (9.0 kg N · ha⁻¹· y⁻¹), whereas the mid-successional forest lacking legumes had the lowest emissions (0.09 kg N · ha⁻¹· y⁻¹). N oxide fluxes from late-successional forests were higher than fluxes from pastures. Annual N oxide fluxes correlated positively to leaf litter N, net nitrification, potential nitrification, soil nitrate, and net N mineralization and negatively to leaf litter C:N ratio. Soil ammonium was not related to N oxide emissions. Forests with lower fluxes of N oxides had higher rates of C mineralization than sites with higher N oxide emissions. We conclude that (a) N oxide fluxes were substantial where the availability of inorganic N exceeded the requirements of competing biota; (b) species composition resulting from historical land use or varying successional dynamics played an important role in determining N availability; and (c) the established ecosystem models that predict N oxide loss from positive relationships with soil ammonium may need to be modified. Received 22 February 2000; accepted 6 September 2000.     

Elaeagnus angustifolia Elevates Soil Inorganic Nitrogen Pools in Riparian Ecosystems

Elaeagnus angustifolia L., a nonnative N₂-fixer, has established within riparian corridors of the interior western United States and is now the fourth most frequently occurring woody riparian plant in this region. We examined whether E. angustifolia alters pools and fluxes of soil inorganic N at eight sites dominated by Populus deltoides ssp. wislizeni along the Rio Grande in New Mexico over 2 years. E. angustifolia contributed a small fraction of total leaf fall (<5% across sites) but accounted for a disproportionately high amount of N (19%) that entered the system from P. deltoides and E. angustifolia leaf fall, due to the high N content (>2%) of E. angustifolia senesced leaves. Soil inorganic N concentrations and potential rates of nitrification and net N mineralization varied across sites. E. angustifolia leaf fall explained 59% of the variation in soil inorganic N concentrations across years. This relationship suggests that inputs of N-rich leaf litter from E. angustifolia may increase N availability in riparian soils. We detected no relationship between E. angustifolia leaf fall and fluxes of soil inorganic N, whereas others have measured both stimulation and inhibition of soil N cycling by E. angustifolia. Greater abundance of N₂-fixing species in riparian forests may augment growth of neighboring plants or increase N export to rivers. Given these possibilities, ecosystem studies and restoration projects should further examine the potential for E. angustifolia to affect N pools and fluxes along western North American rivers.     

Effects of Species Richness on Resident and Target Species Components in a Prairie Restoration

The ecological role of biodiversity in achieving successful restoration has been little explored in restoration ecology. We tested the prediction that we are more likely to create persistent, species‐rich plant communities by increasing the number of species sown, and, to some degree, by varying functional group representation, in experimental prairie plantings. There were 12 treatments consisting of 1‐, 2‐, 3‐, 4‐, 8‐, 12‐, and 16‐species mixtures of native perennials representing four functional groups (C₄ grasses, C₃ grasses, nitrogen‐fixing species, and late‐flowering composites) that predominate within Central Plains tallgrass prairies. In 2000, species were seeded into square plots (6 × 6 m), with five replicates per treatment, on former agricultural land. Annually, we measured total species richness and evenness, target species richness and cover, and richness and cover of resident species (i.e., those emerging from the seed bank). Both target species richness and rate of establishment of target communities were highest in the most species‐rich mixtures, but there was no additional benefit for treatments that contained more than eight species. Richness of resident species did not vary with target species richness; however, cover by resident species was lower in the higher target species treatments. Our results, indicating that establishment of species‐rich prairie mimics can be enhanced by starting with larger numbers of species at the outset, have implications for grassland restoration in which community biodiversity creation and maintenance are key goals.     

The Effects of Nitrogen Supply and Defoliation on the Seasonal Internal Cycling of Nitrogen in Molinia caerulea

Plants of Molinia caerulea were grown in pots for two seasonsat two levels of nitrogen (N) supply and two levels of defoliation.All N supplied was enriched with ¹⁵N in the first season andwas at natural abundance in the second season. This allowedthe contribution of remobilization from overwintering storesto be discriminated from current root uptake in supplying Nfor new shoot growth in the second season. The effects of Nsupply and defoliation upon the internal cycling of N in M.caerulea were quantified. N was remobilized from both roots and basal internodes to supportnew shoot, especially leaf, growth in spring. Roots suppliedmore N than basal internodes. Since the remobilization mainlyoccurred before the onset of root N uptake, internal cyclingwas important for the earliest period of shoot growth. An increasedN supply increased the amount of N remobilized to new shootgrowth, however, the proportion of N remobilized from overwinteringstores was independent of N supply. Defoliation increased theamount of N remobilized from the roots, and had no effect onthe ¹⁵N content of basal internodes of plants receiving a lowsupply of N. Remobilization of N from leaves of undefoliatedplants occurred later in the season. Remobilization from leavessupplied flowers in plants receiving a low N supply and bothflowers and new basal internodes in plants receiving a higherN supply. Key words: Molinia caerulea, internal cycling, nitrogen, defoliation