Fleshy fungi of grass fields HI. Reaction to different fertilizers and to age of grass turf. Periodicity of fruiting

The fungus flora of an experimental grass field was studied over a ten years period. 48 squares were fertilized with combinations of NPK fertilizers. The nitrogen source was varied over the years. Calcium nitrate was applied in four seasons, ammonium sulphate and calcium ammonium nitrate were each applied in three seasons. Two series of squares had grass turf of different age, in the period ranging from one year to 12 years of age. The 55 species of fungi recorded were clearly divided in a nitrophilous group and a nitrophobous group. In the three first seasons, when an ongoing nitrate programme was continued, the species frequencies in the +N parcels were much higher than in the—N parcels. Most species in the nitrophilous group showed preference for nitrate, and application of ammonium had an almost poisonous effect on a large number of nitrophilous species, including most of the fairy ring formers. Agaricus campestris and two other nitrophilous species reacted positively to potassium. Most of the nitrophobous species were ± attached to moss and to grass debris. A few species had preference for grass turf 1–2 years old while the vaste majority of the fungi were established after 3–5 years.
The fruiting periods of the more common species were found to have a range of several months.     

Fleshy fungi in grass fields. II. Precipitation and fructification

An experimental field with grass had 192 parcels treated with combinations of NKP fertilizers and sown with 4 species of grass. Species of fleshy fungi in the parcels were counted over 4 years with 17–24 observations per year. Precipitation was measured at the field. The occurrence of the species varied with the distribution of the precipitation through the season. In early summer and after prolonged desiccation the fungus flora developed 25–30 days after heavy rain, while after mowing and after shorter desiccation periods, a new crop of fruit bodies had developed after 5 days. These intervals correspond to the maturation period of the mycelia, and to the period of the development of primordia already formed. Differences in the yearly periodicity of the individual species are discussed.     

Vegetation establishment on chalk marl spoil: the role of nurse grass species and fertiliser application

Abstract. The Channel Tunnel workings on the UK side have yielded nearly 4 million m³ of chalk-marl spoil which now forms a 36 ha landscaped reclamation platform. To establish vegetation of amenity and conservation interest on the spoil, seed mixtures of native wild flowers and grasses were sown with Lolium perenne (perennial rye grass) as a nurse species. Potentially, L. perenne is a suitable nurse species for grassland creation on infertile substrates as it provides rapid initial cover and stability, but it is non-persistent and declines in vigour with time, allowing wild flower species sown alongside to expand their cover and spread in the longer term. On very low fertility substrates like chalk marl, an initial application of fertilizer is needed to encourage plant growth. Results are reported of a fertilizer experiment on Channel Tunnel spoil to determine appropriate levels of fertilizer for establishment of species-rich grassland vegetation. An area hydroseeded with L. perenne and wild flowers in autumn 1992 was subjected to factorial treatment of four levels each of N and P in spring 1993. The results the following summer showed significant positive effects of nitrogen and phosphorus on L. perenne biomass and a negative impact of nitrogen on densities of wild flower species, especially legumes, establishing in the L. perenne sward. In general, low fertilizer applications encouraged low productivity and maximal species richness in the vegetation. Conversely high applications encouraged high productivity and competitive exclusion of sown wild flower species. Fertilizer applications must therefore balance encouragement of the stabilising nurse grass sward, while preventing competitive exclusion of wild flowers by the nurse grass.     

Predation of annual grass weed seeds in arable field margins

Seeds of three species of annual grass weeds (Alopecurus myosuroides, Avena fatua and Bromus sterilis) were placed in field margins around arable fields in a randomised block split-plot design experiment. The field margin vegetation was either sown or naturally regenerated and either cut or uncut. The seeds were either placed in cages designed to exclude small mammals and birds or were uncaged. The proportion of seeds removed was monitored on 10 occasions and mean seed loss was analysed. In general, a greater proportion of weed seeds was removed from uncaged trays in uncut swards, suggesting predation by small mammals, which inhabit tall grass. This effect was mainly due to removal of seeds of the two large-seeded species (A. fatua and B. sterilis), with A. fatua being especially favoured. It is therefore likely that small mammals play a role in the population dynamics of major crop weeds by feeding on their seeds in field margins, especially when these are dense and uncut.     

The invasive grass,Melinis minutiflora,inhibits tree regeneration in a Neotropical savanna

Abstract Exotic grasses are becoming increasingly abundant in Neotropical savannas, with Melinis minutiflora Beauv. being particularly invasive. To better understand the consequences for the native flora, we performed a field study to test the effect of this species on the establishment, survival and growth of seedlings of seven tree species native to the savannas and forests of the Cerrado region of Brazil. Seeds of the tree species were sown in 40 study plots, of which 20 were sites dominated by M. minutiflora, and 20 were dominated by native grasses. The exotic grass had no discernable effect on initial seedling emergence, as defined by the number of seedlings present at the end of the first growing season. Subsequent seedling survival in plots dominated by M. minutiflora was less than half that of plots dominated by native species. Consequently, at the end of the third growing season, invaded plots had only 44% as many seedlings as plots with native grasses. Above‐ground grass biomass of invaded plots was more than twice that of uninvaded plots, while seedling survival was negatively correlated with grass biomass, suggesting that competition for light may explain the low seedling survival where M. minutiflora is dominant. Soils of invaded plots had higher mean Ca, Mg and Zn, but these variables did not account for the higher grass biomass or the lower seedling survival in invaded plots. The results indicate that this exotic grass is having substantial effects on the dynamics of the tree community, with likely consequences for ecosystem structure and function.     

Host-plant quality alters grass/forb consumption by a mixed-feeding insect herbivore, Melanoplus bivittatus (Orthoptera: Acrididae)

Abstract.  1. Factors affecting the nutritional ecology of mixed-feeding, polyphagous herbivores are poorly understood. Mixed-feeding herbivores do better when they consume both forb and grass species although they typically feed primarily on forbs, which are of relatively higher protein content than grasses.
2. In a field experiment, we examined the effects of nitrogen and phosphorus fertilization and associated changes in host-plant C:N:P on proportional grass consumption by a mixed-feeding insect herbivore, Melanoplus bivittatus , using natural abundance stable carbon isotope (¹²C^/13C) methods. We also examined a grass-feeding ( Phoetaliotes nebrascensis ) and forb-feeding ( Hesperotettix viridis ) species.
3. The C isotope signatures of M. bivittatus collected from plots fertilized with nitrogen (+N), phosphorus (+P), nitrogen and phosphorus (+N+P) and no fertilizer were compared with the C isotope signatures of plants in those plots to determine the proportion of assimilated C derived from C₄ grasses and C₃ forbs in each plot. We also examined the relationship between M. bivittatus diets and plant C:N:P stoichiometry.
4. The proportion of grass assimilated approximately doubled in N-fertilized treatments (39.1 ± 0.1%) compared with non-fertilized treatments (19 ± <0.1%), an increase associated with decreased C:N and increased N:P of grasses.
5. These results indicate that mixed-feeding M. bivittatus can selectively feed to balance C:N:P intake even when choosing between two structurally and chemically different groups of plants.
6. The strong relationship between diet selection and grass stoichiometry also suggests that plant nutrient composition may be more important than defensive chemistry in food choice.     

Feeding behaviour of yearling and older hybrid grass carp

Hybrid grass carp resulting from the cross of a female grass carp, Ctenopharyngodon idella (Val.), and a male bighead carp, Hypophthalmichthys ( Aristichthys ) nobilis Rich., 12–18 months old ( c . 300 mm T.L.) were studied in a two-part experiment to determine feeding preference and total daily consumption fish^-1 on selected species of aquatic plants. Fish were maintained in circular pools with 6840·8 1 of water inside a temperature-controlled greenhouse. Preference tests were conducted at three temperature ranges; 25–28° C, 17–20° C and 12–15° C. Based on the time to complete consumption or the relative quantity consumed, the most preferred plant was Lemna gibba when in combination with six other species. Chara sp., Najas guadalupensis and Potamogeton peciinatus were readily consumed and considered to be of about equal preference. Ceratophyllum demersum and Myriophyllum brasiliense were least preferred. Hybrid grass carp generally consumed as much plant material species^-1 and in the same order of preference at the 12–15°C range as they did at 25–28° C. In the second part, mean daily consumption (g) fish^-1 at 25·7–31·0° C for five plant species tested separately was as follows: Chara sp. 369·8; Lemna gibba 178·2; Najas guadalupensis 172·6; Hydrilla verticillata 106·4 and Ceratophyllum demersum 8·8.     

New tools to boost butterfly habitat quality in existing grass buffer strips

There are approximately 29,000 ha of grass buffer strips in the UK under Agri-Environment Schemes; however, typically they are floristically poor and as such are of limited biodiversity value. Introducing a sown wildflower component has the potential to increase dramatically the value of these buffer strips for a suite of native species, including butterflies. This study investigates management practices aiming to promote the establishment and maintenance of wildflowers in existing buffer strips. The effectiveness of two methods used to increase the establishment of wildflowers for the benefit of native butterfly species were tested, both individually and in combination. The management practices were: (1) the application of a selective graminicide (fluazifop-P-butyl) which reduces the dominance of competitive grasses; and (2) scarification of the soil which creates germination niches for sown wildflower seeds. A wildflower seed mix consisting of nine species was sown in conjunction with the scarification treatment. Responses of wildflowers and butterflies were monitored for two years after establishment. Results indicate that the combined scarification and graminicide treatment produced the greatest cover and species richness of sown wildflowers. Butterfly abundance, species richness and diversity were positively correlated with sown wildflower species richness, with the highest values in the combined scarification and graminicide treatment. These findings have confirmed the importance of both scarification as a means of introducing wildflower seed into existing buffer strips, and subsequent management using graminicides, for the benefit of butterflies. Application of this approach could provide tools to help butterfly conservation on farmland in the future.     

Low allelopathic potential of an invasive forage grass on native grassland plants: a cause for encouragement?

Tall fescue (Festuca arundinacea Schreb.), a highly competitive European grass that invades US grasslands, is reportedly allelopathic to many agronomic plants, but its ability to inhibit the germination or growth of native grassland plants is unknown. In three factorial glasshouse experiments, we tested the potential allelopathic effects of endophyte-infected (E+) and uninfected (E−) tall fescue on native grasses and forbs from Midwestern tallgrass prairies. Relative to a water control, at least one extract made from ground seed, or ground whole plant tissue of E+ or E− tall fescue reduced the germination of 10 of 11 species in petri dishes. In addition, the emergence of two native grasses in potting soil was lower when sown with E+ and E− tall fescue seedlings than when sown with seeds of conspecifics or tall fescue. However, when seeds of 13 prairie species were sown in sterilized, field-collected soil and given water or one of the four tall fescue extracts daily, seedling emergence was lower in one extract relative to water for only one species, and subsequent height growth did not differ among treatments for any species. We conclude that if tall fescue is allelopathic, its inhibitory effects on the germination and seedling growth of native prairie plants are limited, irrespective of endophyte infection. On the other hand, the apparent inability of these plants to detect tall fescue in field soil could hinder prairie restoration efforts if germination near this strong competitor confers fitness consequences. We propose that lack of chemical recognition may be common among resident and recently introduced non-indigenous plants because of temporally limited ecological interactions, and offer a view that challenges the existing allelopathy paradigm. Lastly, we suggest that tall fescue removal will have immediate benefits to the establishment of native grassland plants.     

Intensive culture of grass carp and hybrid grass carp larvae

Intensive culture of grass carp and hybrid grass carp (Ctenopharyngodon idella x Aristichthys nobilis ) larvae was conducted under Florida (U.S.A.) conditions. The influence of different rearing facilities (indoor tanks, outdoor tanks and cages) and different methods of fish feeding using live zooplankton and artificial food was tested. High survival (86–100%) and satisfactory grass carp growth (47–56 mg) were obtained in the outdoor tanks and cages during the 10–day experiment. It is believed, that the technique described can be used after some improvements for commercial–scale grass carp larvae rearing. Low survival (1–3%) was obtained in the hybrid culture experiment even though satisfactory growth rates (84–212 mg) were obtained after 13 days. High mortality was attributed to genetic abnormalities caused by hybridization. The hybrid does not appear to be a promising fish for culture.     

Microbes, models and management of the New Zealand grass grub,

[First paragraph]Improved grasslands pose particular problems in pest management. The areas are extensive and the returns per hectare from grassland agriculture are much less than those from intensive cropping or horticulture, but pastures are usually sown to last for a number of years and have a much more stable ecology than in cropping or horticulture. For these reasons, biological controls and plant resistance have long been the preferred options for managing pasture pests. Within this context, the role of diseases in pasture pest population dynamics has received increasing attention, especially their ability to control pest outbreaks. Diseases are common within our major pasture pest species but their role in population regulation is often difficult to define. One of New Zealand’s major pasture pest species, the grass grub Costelytra zealandica, is widespread and often damaging throughout the country. The insect is an endemic species that has adapted to an introduced pasture system, dominated by exotic plant species (Lolium perene/Trifolium repens), where it can reach populations ten-fold higher than in its native habitat. Such high densities favour disease transmission and it is not surprising that a wide array of pathogens have been recorded from this insect (Glare et al., 1993). But the insect can still be highly damaging and can cause total loss of sown species within 3–4 years from sowing in grass grub prone regions. Probably the most important of these diseases, found throughout New Zealand, is amber disease caused by strains of the bacteria Serratia entomophila and S. proteamaculans. This is an unusual disease, controlled by a bacterial plasmid which has only been found in New Zealand bacterial isolates (Jackson et al., 2001).     

Effect of the grass Leersia hexandra on the dispersal,seed germination,and establishment of Pachira aquatica seedlings

1. In wetlands, hydrochory is one of the main mechanisms of seed dispersal and there is often synchrony between propagule production and the flood season. Different sources of disturbance can prevent seed dispersal to suitable sites, and if environmental conditions are not adequate for germination and seedling establishment, recruitment will be limited, affecting succession.
2. We worked in a disturbed tropical freshwater swamp where the native grass Leersia hexandra has dominated open areas, creating a grass matrix that surrounds patches of swamp forest. Leersia grows vigorously, forming cushions of dry matter that cover the soil, forming a potential obstacle to seed dispersal. We asked whether the vegetative growth of this grass prevents the entry and dispersal of seeds of the tropical swamp tree Pachira aquatica, thwarting seed germination and seedling establishment, and arresting succession. We set up transects in the grass matrix in two zones: close to the river (R) and bordering the tree patches or fragments (F). We quantified tree seed and seedling presence, survival and growth in situ and experimentally introduced seeds and seedlings in the field and monitored seed germination and the survival and growth of their seedlings, as well as that of transplanted seedlings.
3. There was a negative relationship between the number of seeds and established seedlings, and the distance to river or fragment (r = −0.86, p < 0.001 for zone R; and r = −0.77, p < 0.001 for zone F) and with the grass cushion (r = −0.68, p = 0.005 for zone R; and r = −0.66, p = 0.007 for zone F); the grass creating a barrier to dispersal. When seeds were sown after clearing the grass cushion, germination success was high, so this stage is not limited. The transplanted seedlings had better survival and a greater final height than the seedlings of the sown seeds. Grass cover had a negative effect on both types of seedlings. Seedling survival rates were inversely related to grass cover, showing that seedlings overgrown by grass had low survival rates. Flooding is a stress factor for seedlings and produced mortality, in addition to the effects of the grass.
4. Together, the field survey and the experiment show that succession is being arrested in two ways: (1) by limiting seed dispersal because the grass cushion slows the dispersal and penetration of seeds into the vegetation; and (2) by limiting seedling establishment because the grass competes for space and light. Our results show that even where the grass is native, slower growing, seed-dependent species may struggle to compete and establish. If grass cover is increasing, these swamps are very vulnerable to a decrease in area because it is very difficult for them to regenerate naturally.

     

Belowground competitive suppression of seedling growth by grass in an African savanna

Coexistence of N₂-fixing legumes and non-legume trees with grasses in African savannas results in intense competition between these life-forms. We hypothesised that belowground competition might induce different nutritional constraints in N₂- versus non-N₂-fixing species. A field (Hluhluwe-imFolozi nature reserve, South Africa) competition experiment with two N₂-fixing legume species (Acacia burkei and Acacia karroo) and two non-N₂-fixing species (Schotia brachypetala and Spirostachys africana) both with clipped grass and without grass was established. Plants were supplied with no fertilizer, or generous amounts of fertilizer (200?kg?N?ha^?1, 100?kg P₂O₅ ha^?1, 7.1?kg K₂O ha^?1) supplied as either 28?C10 (N?CK), P or a combination of these fertilizers (NPK). Regularly clipped grass suppressed growth (by more than 90?%) of both N₂- and non-N₂-fixing seedlings equally. Biomass accumulation of seedlings grown with grass and the grasses themselves responded positively to NK and/or NPK, but not P, although P-fertilization did have effects on foliar [N] and ??¹⁵N values of trees and grasses showing that plants accessed the fertilizer. Tree ??¹⁵N values and foliar [N] were also modified by NPK, demonstrating access to fertilizer. However, the ameliorative effects of NPK on grass competition-induced biomass suppression were only partial. This may be due to ??non-resource competition?? (i.e. root gaps) imposed by dense grass roots. The fact that nutrients were able to partially ameliorate the effects of grass competition, however, indicates that such ??non-resource competition?? may be partially overcome by even more generous supply of fertilizers.     

The effect of leachates from grass trash on establishment of ryegrass

These experiments have shown that whilst leachates from grass trash produced by killing grass with glyphosate can significantly affect germination and seedling growth of both perennial and Italian ryegrass on germination paper and on inert media, they had no effect when added to seeds on soil. This suggests that these leachates are unlikely to inhibit germination in direct drilled sowings in the field unless seeds are sown in direct contact with grass trash and in conditions of sufficient moisture availability to enable production of the leachates. Comparisons of the effect of leachates from grass killed with herbicides or by drying, both before and after passing through a soil column further confirmed this and suggested that the effect of these leachates could partly though not wholly be attributed to herbicide residues on the trash. Glyphosate was less toxic to germination than paraquat. However on sandy soils, glyphosate residues could still cause a reduction in germination. Surface trash produced by spraying with glyphosate had no effect when seeds were sown at a depth of 1 cm, again confirming that seeds must be in close proximity to grass trash before germination and establishment is reduced.     

Mineralization of15N-labelled legume residues in soils with different nitrogen contents and its uptake by Rhodes grass

Summary Soil was collected from pots that had grown 1,3 or 6 soybean (Glycine max) or Siratro (Macroptillium atropurpureum) crops that had received organic residue returns from each crop.¹⁵N-labelled residues were added to half the pots in the experiment and the other half left unamended. Half of each group was then sown to Rhodes grass (Chloris gayana) which was grown, under glasshouse conditions, for 12 weeks.Ten grams of organic matter residues were added to each pot (1.5 kg soil) and the pots subjected to two wetting and drying cycles. At the end of the second wet cycle, soil mineral N values ranged from 6 to 64 ppm in unamended soils and from 19 to 177 ppm in amended soils. These levels generally declined over a 12 week period both in the presence and absence of sown grass.Nitrogen uptake by the grass increased with the number of previous cycles and was higher in Siratro than soybean soils. Recovery of¹⁵N by plant growth from the incorporated soybean residues was little effected by previous crop history and averaged 15.4%. On the other hand, Siratro recoveries were 13.7, 42.4 and 55.5% from soils that had grown 1, 3 and 6 previous Siratro crops, respectively.The addition of organic residues stimulated the release of native organic N (positive priming effect) on all soils.These results show that the turnover rate of nitrogen from organic residues can be high and the net result of these additions depends on the nature of the organic residues and the soil system to which they are added. These data emphasise the need to consider the rate of nutrient turnover from organic sources rather than concentrate on the nature and size of the resident nutrient pools.     

Contribution of biological nitrogen fixation to Elephant grass (Pennisetum purpureum Schum.)

Aims

Because of its high dry matter (DM) productivity, elephant grass (Pennisetum purpureum) is an ideal candidate for biomass production for biofuel production if low N fertilizer rates are used to avoid high fossil fuel inputs. The objective of this study was to investigate the potential of different elephant grass genotypes to obtain contributions of plant-associated biological N₂ fixation (BNF).

Methods

Three field experiments with 4 or 5 different genotypes were conducted on low-fertility Acrisols, two in Rio de Janeiro State and one in Espirito Santo for the evaluation of DM and N accumulation and ¹⁵N abundance.

Results

DM and N accumulation rates of four genotypes in the two experiments in Rio State stabilized at high levels after 2?years of growth. In all experiments the spontaneously-occurring weeds in the plots were significantly higher in ¹⁵N abundance than the elephant grass genotypes. The lower ¹⁵N abundance of the elephant grass was shown not to be due to lower δ¹⁵N abundance at depth in the soil.

Conclusions

Four of the grass genotypes obtained between 18 and 70% of their N from BNF amounting to inputs of between 36 and 132?kg N ha^?1?yr^?1.     

Grassland plant species diversity decreases invasion by increasing resource use

Species richness of plant communities has been demonstrated to provide resistance to invasion by unsown species, though the relationship with resource availability varies between studies. The present work involved five grassland species grown in monocultures and in four-species mixtures sown in accordance with a simplex design. The species used represented different functional groups (i.e. grasses, legumes and non-N(2)-fixing species), each of which differed internally in terms of competitiveness. I hypothesized that sown diversity would negatively affect invader performance by decreasing the availability of light and soil nitrogen (N) for invading species, and that functional composition of the sown diversity would affect the functional composition of the invading flora. The experimental plots were harvested for two years, and were fertilized with 100 kg N ha(-1) each year. The number of unsown species (classified into four functional groups) invading each plot and their proportion of the biomass harvested were recorded. The penetration of incoming light through the canopy, the apparent N uptake by the sown species from the soil, and the mineral N content in the soil were measured. I found that diverse communities captured more resources both above- and belowground, and the number of invading species and their biomass production were smaller in mixed than in monoculture plots. However, the sampling effect of one grass was also strong. These results suggest that increased resource use in diverse communities can reduce invasion.     

Mitochondrial phylogeography,contact zones and taxonomy of grass snakes (Natrix natrix,N. megalocephala)

Grass snakes (Natrix natrix) represent one of the most widely distributed snake species of the Palaearctic region, ranging from the North African Maghreb region and the Iberian Peninsula through most of Europe and western Asia eastward to the region of Lake Baikal in Central Asia. Within N. natrix, up to 14 distinct subspecies are regarded as valid. In addition, some authors recognize big‐headed grass snakes from western Transcaucasia as a distinct species, N. megalocephala. Based on phylogenetic analyses of a 1984‐bp‐long alignment of mtDNA sequences (ND4+tRNAs, cyt b) of 410 grass snakes, a nearly range‐wide phylogeography is presented for both species. Within N. natrix, 16 terminal mitochondrial clades were identified, most of which conflict with morphologically defined subspecies. These 16 clades correspond to three more inclusive clades from (i) the Iberian Peninsula plus North Africa, (ii) East Europe and Asia and (iii) West Europe including Corso‐Sardinia, the Apennine Peninsula and Sicily. Hypotheses regarding glacial refugia and postglacial range expansions are presented. Refugia were most likely located in each of the southern European peninsulas, Corso‐Sardinia, North Africa, Anatolia and the neighbouring Near and Middle East, where the greatest extant genetic diversity occurs. Multiple distinct microrefugia are inferred for continental Italy plus Sicily, the Balkan Peninsula, Anatolia and the Near and Middle East. Holocene range expansions led to the colonization of more northerly regions and the formation of secondary contact zones. Western Europe was invaded from a refuge within southern France, while Central Europe was reached by two distinct range expansions from the Balkan Peninsula. In Central Europe, there are two contact zones of three distinct mitochondrial clades, and one of these contact zones was theretofore completely unknown. Another contact zone is hypothesized for Eastern Europe, which was colonized, like north‐western Asia, from the Caucasus region. Further contact zones were identified for southern Italy, the Balkans and Transcaucasia. In agreement with previous studies using morphological characters and allozymes, there is no evidence for the distinctiveness of N. megalocephala. Therefore, N. megalocephala is synonymized with N. natrix.     

Effects of an African grass invasion on Hawaiian shrubland nitrogen biogeochemistry

African perennial C₄ grasses are highly successful invaders in Hawaiian ecosystems. We examined the effects of African molasses grass (Melinis minutiflora Beauv.) on Hawaiian shrubland nitrogen (N) dynamics without the influence of fire disturbance. Vegetation tissue carbon and nitrogen chemistry, soil inorganic N pools, net N mineralization rates, and total soil N were studied in three adjacent areas: a monospecificMelinis grassland, a mixed grass/shrubland mosaic, and an un-invaded shrubland.Melinis plots within the mosaic area exhibited the largest inorganic N pools and fastest net N mineralization rates, but were temporally variable with grass phenology. Un-invaded shrubland plots contained the smallest inorganic N pools and lowest net N mineralization rates. Grass foliar C:N and litter C:N were lower than those of common shrubland species, providing one possible link between species and ecosystem N dynamics at this site. The combined effects of N cycle modification, successful light competition, and fire-cycle enhancement make the invasion ofMelinis a significant perturbation to Hawaiian shrubland ecosystem function and successional dynamics. ei]Section editor H Lambers     

Nitrogen mineralization dynamics in grass monocultures

Although Wedin and Tilman (1990) observed large differences in in situ N mineralization among monocultures of five grass species, the mechanisms responsible were unclear. In this study, we found that the species did not change total soil C or N, and soil C: N ratio (range 12.9–14.1) was only slightly, but significantly, changed after four years. Nor did the species significantly affect the total amount of N mineralized (per g soil N) in year-long aerobic laboratory incubations. However, short-term N mineralization rates in the incubations (day 1–day 17) differed significantly among species and were significantly correlated with annual in situ mineralization. When pool sizes and turnover rates of potentially mineralizable N (N_o) were estimated, the best model treated N_o as two pools: a labile pool, which differed among species in size (N_l, range 2–3% of total N) and rate constant (h, range 0.04–0.26 wk^–1), and a larger recalcitrant pool with a constant mineralization rate across species. The rate constant of the labile pool (h) was highly correlated with annual in situ N mineralization (+0.96). Therefore, plant species need only change the dynamics of a small fraction of soil organic matter, in this case estimated to be less than 3%, to have large effects on overall system N dynamics.