Non-fixing Species Research Articles

Drought enhances symbiotic dinitrogen fixation and competitive ability of a temperate forest tree

General circulation models project more intense and frequent droughts over the next century, but many questions remain about how terrestrial ecosystems will respond. Of particular importance, is to understand how drought will alter the species composition of regenerating temperate forests wherein symbiotic dinitrogen (N2)-fixing plants play a critical role. In experimental mesocosms we manipulated soil moisture to study the effect of drought on the physiology, growth and competitive interactions of four co-occurring North American tree species, one of which (Robinia pseudoacacia) is a symbiotic N2-fixer. We hypothesized that drought would reduce growth by decreasing stomatal conductance, hydraulic conductance and increasing the water use efficiency of species with larger diameter xylem vessel elements (Quercus rubra, R. pseudoacacia) relative to those with smaller elements (Acer rubrum and Liriodendron tulipifera). We further hypothesized that N2 fixation by R. pseudoacacia would decline with drought, reducing its competitive ability. Under drought, growth declined across all species; but, growth and physiological responses did not correspond to species' hydraulic architecture. Drought triggered an 80% increase in nodule biomass and N accrual for R. pseudoacacia, improving its growth relative to other species. These results suggest that drought intensified soil N deficiency and that R. pseudoacacia's ability to fix N2 facilitated competition with non-fixing species when both water and N were limiting. Under scenarios of moderate drought, N2 fixation may alleviate the N constraints resulting from low soil moisture and improve competitive ability of N2-fixing species, and as a result, supply more new N to the ecosystem.

Higher relative performance at low soil nitrogen and moisture predicts field distribution of nitrogen-fixing plants

Background and Aims Symbiotic associations between vascular plants and nitrogen-fixing bacteria are expected to be costly except when N availability is low. We tested the prediction that in low-N soils in dry climates, plants with nitrogen-fixing symbioses (N-fixing species) have higher growth rates, and occur relatively more frequently, than non-fixing species,

Nitrogen Fixation of Sulla under Mediterranean Conditions

Sulla coronaria (L.) B.H. Choi & H. Ohashi (=Hedysarum coronarium L.) is a Mediterranean forage legume grown extensively in rainfed Italian environments. Here we report the field quantification of its seasonal and annual N fixation, using the 15N isotopic dilution (ID) method, the influence of various nonfixing reference species on these estimates, and the allocation of fixed N between the aerial and the belowground biomass. Parallel experiments were conducted over two biennial crop cycles at two Italian locations (Sassari and Ancona), characterized by sandy‐loam to heavy clay‐loam calcareous soils. The proportion of nitrogen derived from the atmosphere (%Ndfa) present in the aerial biomass reached 78.2% at Sassari and 82.7% at Ancona, with no significant effect of the identity of the nonfixing reference species. At Sassari, the amount of fixed N present in the aerial dry matter (DM) of sulla was 187 kg ha−1 in the first crop cycle, and 265 kg ha−1 in the second, while the equivalent N yields at Ancona were 350 and 170 kg ha−1. The relationship between the quantity of fixed N in the aerial biomass and the aerial DM yield of sulla was significant and not site‐specific, realizing an equivalence of about 1.8 kg ha−1 of fixed N per 100 kg ha−1 of aerial DM. The relative contribution of the root biomass to the amount of fixed N was on average 4%. The N balance of sulla at the end of the second crop cycle was negative, but the N sparing effect compared with the nonfixing species ranged from 65 to 175 kg ha−1

Natural abundances of 15N and 13C in leaves of some N2-fixing and non-N2-fixing trees and shrubs in Syria

A survey study was conducted on man-made plantations located at two different areas in the arid region of Syria to determine the variations in natural abundances of the 15N and 13C isotopes in leaves of several woody legume and non-legume species, and to better understand the consequence of such variations on nitrogen fixation and carbon assimilation. In the first study area (non-saline soil), the δ15N values in four legume species (Acacia cyanophylla,−1.73 ‰ Acacia farnesiana,−0.55 ‰ Prosopis juliflora,−1.64 ‰; and Medicago arborea,+1.6 \ extperthousand) and one actinorhizal plant (Elaeagnus angustifolia,−0.46 to−2.1 ‰) were found to be close to that of the atmospheric value pointing to a major contribution of N2 fixing in these species; whereas, δ15N values of the non-fixing plant species were highly positive. δ13C ‰; in leaves of the C3 plants were found to be affected by plant species, ranging from a minimum of−28.67 ‰; to a maximum of−23 ‰. However, they were relatively similar within each plant species although they were grown at different sites. In the second study area (salt affected soil), a higher carbon discrimination value (Δ13C ‰) was exhibited by P. juliflora, indicating that the latter is a salt tolerant species; however, its δ15N was highly positive (+7.03 ‰) suggesting a negligible contribution of the fixed N2. Hence, it was concluded that the enhancement of N2 fixation might be achieved by selection of salt-tolerant Rhizobium strains.

Bottom-up rather than top-down processes regulate the abundance and activity of nitrogen fixing plants in two Connecticut old-field ecosystems

The maintenance of nitrogen limitation in terrestrial ecosystems remains a central paradox in biogeochemistry. Although plants that form a symbiotic association with nitrogen fixing bacteria should be at a competitive advantage over non-fixing plant species in N limited environments, N2 fixing plants are uncommon in most mid- to high-latitude ecosystems. Theory and observation suggest that preferential grazing on N-rich tissues by herbivores, resource limitations to growth, reproduction and N2 fixation, and temperature limitations to the activity of the N2 fixing enzyme nitrogenase, explain the rarity of N2 fixing plants. These ideas, however, have never been confronted by multifactor experiments in the field. In a 3 year field experiment, we found that the abundance, growth, reproductive output and fraction of plant-N derived from N2 fixation in temperate, old-field ecosystems was constrained by the availability of phosphorus (P). Although the availability of light was crucial to the performance of old-field N2 fixing plants, the largest gains in biomass and the rate of N2 fixation were observed in the plots fertilized with P. By contrast, herbivory had no effect on the abundance, biomass and activity of N2 fixing plants and inconsistent effects on foliar nitrogen concentrations (opposing directions, depending upon year), suggesting that herbivores do not affect the ecology of N2 fixing plants in old field ecosystems, at least not over the course of 3 years. Together with a recent study demonstrating that C limitation explains the absence of N2 fixing trees in temperate forests our analysis suggests that stand replacing disturbances shift the limitation on the abundance and activity of N2 fixing plants from P early in secondary succession to light later in succession, as the forest canopy closes and incident light levels decline precipitously.

A meta-analysis comparing tree growth in monocultures and mixed plantations

Ample evidence has indicated that mixed tree plantations are the most appropriate option for providing a broad range of goods and environmental services, but still debate continues about whether mixed plantations can achieve greater productivity than can monocultures. In this study, a meta-analysis of published studies was performed to synthesize results of growth in mixed plantations and monocultures across differing regions and species. A total of 14 studies representing 46 tree species from tropical and temperate ecosystems were included to test three hypotheses: stand composition affects tree height growth rate; stand composition affects tree diameter growth rate; and presence of nitrogen-fixing tree species in mixed tree plantations affects the diameter growth rate of non-fixing species. It was found that mixed plantations did not have larger height growth rates, but that the diameter growth rate was higher in mixed plantations, with a moderate but statistically significant effect size. Nitrogen-fixing tree species had a positive effect on the diameter growth rate of non-fixing species, with a large and statistically significant effect size. This study suggests that mixing tree species generally increases plantation growth rate. Furthermore, mixed tree plantations can play an important role in satisfying economic needs by shortening rotations yet adding other ecological benefits. Silviculturalists should adopt mixed plantations more widely across degraded landscapes, as well as nitrogen-fixing tree species to maximize positive interactions in mixed plantations.

Legume presence increases photosynthesis and N concentrations of co-occurring non-fixers but does not modulate their responsiveness to carbon dioxide enrichment.

Legumes, with the ability to fix atmospheric nitrogen (N), may help alleviate the N limitations thought to constrain plant community response to elevated concentrations of atmospheric carbon dioxide (CO(2)). To address this issue we assessed: (1) the effects of the presence of the perennial grassland N(2 )fixer, Lupinus perennis, on biomass accumulation and plant N concentrations of nine-species plots of differing plant composition; (2) leaf-level physiology of co-occurring non-fixing species (Achillea millefolium, Agropyron repens, Koeleria cristata) in these assemblages with and without Lupinus; (3) the effects of elevated CO(2) on Lupinus growth and symbiotic N(2) fixation in both monoculture and the nine-species assemblages; and (4) whether assemblages containing Lupinus exhibit larger physiological and growth responses to elevated CO(2 )than those without. This study was part of a long-term grassland field experiment (BioCON) that controls atmospheric CO(2) at current ambient and elevated (560 micromol mol(-1)) concentrations using free-air CO(2) enrichment. Nine-species plots with Lupinus had 32% higher whole plot plant N concentrations and 26% higher total plant N pools than those without Lupinus, based on both above and below ground measurements. Co-occurring non-fixer leaf N concentrations increased 22% and mass-based net photosynthetic rates increased 41% in plots containing Lupinus compared to those without. With CO(2) enrichment, Lupinus monocultures accumulated 32% more biomass and increased the proportion of N derived from fixation from 44% to 57%. In nine-species assemblages, Lupinus N derived from fixation increased similarly from 43% to 54%. Although Lupinus presence enhanced photosynthetic rates and leaf N concentrations of co-occurring non-fixers, and increased overall plant N pools, Lupinus presence did not facilitate stronger photosynthetic responses of non-fixing species or larger growth responses of overall plant communities to elevated CO(2). Non-fixer leaf N concentrations declined similarly in response to elevated CO(2) with and without Lupinus present and the relationship between net photosynthesis and leaf N was not affected by Lupinus presence. Regardless of the presence or absence of Lupinus, CO(2) enrichment resulted in reduced leaf N concentrations and rates of net photosynthesis.

Delta15N values of tropical savanna and monsoon forest species reflect root specialisations and soil nitrogen status.

A large number of herbaceous and woody plants from tropical woodland, savanna, and monsoon forest were analysed to determine the impact of environmental factors (nutrient and water availability, fire) and biological factors (microbial associations, systematics) on plant delta(15)N values. Foliar delta(15)N values of herbaceous and woody species were not related to growth form or phenology, but a strong relationship existed between mycorrhizal status and plant delta(15)N. In woodland and savanna, woody species with ectomycorrhizal (ECM) associations and putative N(2)-fixing species with ECM/arbuscular (AM) associations had lowest foliar delta(15)N values (1.0-0.6 per thousand ), AM species had mostly intermediate delta(15)N values (average +0.6 per thousand ), while non-mycorrhizal Proteaceae had highest delta(15)N values (+2.9 to +4.1 per thousand ). Similar differences in foliar delta(15)N were observed between AM (average 0.1 and 0.2 per thousand ) and non-mycorrhizal (average +0.8 and +0.3 per thousand ) herbaceous species in woodland and savanna. Leguminous savanna species had significantly higher leaf N contents (1.8-2.5% N) than non-fixing species (0.9-1.2% N) indicating substantial N acquisition via N(2) fixation. Monsoon forest species had similar leaf N contents (average 2.4% N) and positive delta(15)N values (+0.9 to +2.4 per thousand ). Soil nitrification and plant NO(3)(-) use was substantially higher in monsoon forest than in woodland or savanna. In the studied communities, higher soil N content and nitrification rates were associated with more positive soil delta(15)N and plant delta(15)N. In support of this notion, Ficus, a high NO(3)(-) using taxa associated with NO(3)(-) rich sites in the savanna, had the highest delta(15)N values of all AM species in the savanna. delta(15)N of xylem sap was examined as a tool for studying plant delta(15)N relations. delta(15)N of xylem sap varied seasonally and between differently aged Acacia and other savanna species. Plants from annually burnt savanna had significantly higher delta(15)N values compared to plants from less frequently burnt savanna, suggesting that foliar (15)N natural abundance could be used as marker for assessing historic fire regimes. Australian woodland and savanna species had low leaf delta(15)N and N content compared to species from equivalent African communities indicating that Australian biota are the more N depauperate. The largest differences in leaf delta(15)N occurred between the dominant ECM Australian and African savanna (miombo) species, which were depleted and enriched in (15)N, respectively. While the depleted delta(15)N of Australian ECM species are similar to those of previous reports on ECM species in natural plant communities, the (15)N-enriched delta(15)N of African ECM species represent an anomaly.

Resource Optimization and Symbiotic Nitrogen Fixation

In temperate forests, symbiotic nitrogen (N) fixation is restricted to the early phases of succession despite the persistence of N limitation on production late in succession. This paradox has yet to be explained adequately. We hypothesized that the restriction of N fixation to early stages of succession results from the optimization of resource allocation in the vegetation. Because of this optimization, N fixation should be restricted to periods when fixation is less costly than N uptake. Our analysis differs from others in the way we calculate the cost of N uptake; we assess the cost of N uptake as the amount of carbon (C) that could be assimilated if the resources necessary to acquire one gram of N from the soil were allocated instead to photosynthesis. We then simulate N fixation as an asymptotic function of the difference in cost between N uptake and N fixation and proportional to the abundance of host tissues for the N-fixing symbionts. The factors that contribute to conditions that favor N fixation are (a) elevated-carbon dioxide (CO2) concentrations, (b) an open canopy, (c) low available N in the soil, and (d) a soil volume already well exploited by roots. Our results indicate that changes in the relative cost of uptake vs fixation can explain most of the pattern in fixation through both primary and secondary succession, but that competitive interactions with nonfixing species play a role in the final exclusion of fixation in later stages of succession.

Species effects of Ceanothus velutinus versus Pseudotsuga menziesii, Douglas-fir, on soil phosphorus and nitrogen properties in the Oregon cascades

Abstract Many authors have hypothesized that nitrogen-fixing species, as a functional group, would express different controls on soil properties and ecosystem development than non-nitrogen-fixing species. Although nitrogen (N) accretion under nitrogen-fixing tree species has been well studied, the effect of nitrogen-fixing species on other soil nutrients, such as phosphorus (P), has received less attention. We studied differences in soil phosphorus and nitrogen properties beneath Ceanothus velutinus (Ceanothus), a nitrogen-fixing species, and Psuedotsuga menziesii (Douglas-fir), a non-fixing species, in a high elevation successional watershed in the H.J. Andrews Experimental Forest in Oregon. Total P was 20% greater in Douglas-fir soils than Ceanothus soils in surface horizons, but there was no significant difference in deeper soil horizons. Surface soils (5 and 15 cm) under Douglas-fir generally had higher concentrations of specific P fractions than surface soils under Ceanothus, but this difference either disappeared or was not as apparent at greater soil depths (30 and 60 cm). Total nitrogen, and extractable ammonium and nitrate were greater in surface soils under Ceanothus than under Douglas-fir. δ 15 N values of leaves and litter differed between Ceanothus and Douglas-fir ( p -value=0.0001 and 0.03, respectively), but the δ 15 N of bulk soil and KCl extracted nitrate and ammonium did not differ. Soil enzyme activities suggested greater mineralization of organic P (phosphatase activity) under Ceanothus in summer, but not in fall, while no significant differences in general decomposition (β-glucosidase activity) were found in soils between the two species.

Management of indigenous plant-microbe symbioses aids restoration of desertified ecosystems.

Disturbance of natural plant communities is the first visible indication of a desertification process, but damage to physical, chemical, and biological soil properties is known to occur simultaneously. Such soil degradation limits reestablishment of the natural plant cover. In particular, desertification causes disturbance of plant-microbe symbioses which are a critical ecological factor in helping further plant growth in degraded ecosystems. Here we demonstrate, in two long-term experiments in a desertified Mediterranean ecosystem, that inoculation with indigenous arbuscular mycorrhizal fungi and with rhizobial nitrogen-fixing bacteria not only enhanced the establishment of key plant species but also increased soil fertility and quality. The dual symbiosis increased the soil nitrogen (N) content, organic matter, and hydrostable soil aggregates and enhanced N transfer from N-fixing to nonfixing species associated within the natural succession. We conclude that the introduction of target indigenous species of plants associated with a managed community of microbial symbionts is a successful biotechnological tool to aid the recovery of desertified ecosystems.

Application of biochemical studies to improving nitrogen fixation

Improvement of symbiotic nitrogen fixation requires a multidisciplinary approach with a comprehensive program ranging from microbial ecology to plant breeding and genomics. Achievement of symbiotic nitrogen fixation requires at least 100 genes from each partner interacting in a favorable environment. The more information that we obtain from applied and fundamental studies of Rhizobium–legume and Frankia–non-legume symbioses, the greater are our chances to extend nitrogen fixation to non-fixing species. Studies with alfalfa (Medicago sativa L.) aimed at improving symbiotic nitrogen fixation have resulted in significant advances in germplasm development, plant biochemistry, microbial ecology and the understanding of plant genes involved in nodule nitrogen and carbon metabolism. However, translation to field improvement of symbiotic nitrogen fixation has proven elusive.

Correlations between foliar δ15N and nitrogen concentrations may indicate plant-mycorrhizal interactions.

Nitrogen isotope measurements may provide insights into changing interactions among plants, mycorrhizal fungi, and soil processes across environmental gradients. Here, we report changes in δ15N signatures due to shifts in species composition and nitrogen (N) dynamics. These changes were assessed by measuring fine root biomass, net N mineralization, and N concentrations and δ15N of foliage, fine roots, soil, and mineral N across six sites representing different post-deglaciation ages at Glacier Bay, Alaska. Foliar δ15N varied widely, between 0 and -2‰ for nitrogen-fixing species, between 0 and -7‰ for deciduous non-fixing species, and between 0 and -11‰ for coniferous species. Relatively constant δ15N values for ammonium and generally low levels of soil nitrate suggested that differences in ammonium or nitrate use were not important influences on plant δ15N differences among species at individual sites. In fact, the largest variation among plant δ15N values were observed at the youngest and oldest sites, where soil nitrate concentrations were low. Low mineral N concentrations and low N mineralization at these sites indicated low N availability. The most plausible mechanism to explain low δ15N values in plant foliage was a large isotopic fractionation during transfer of nitrogen from mycorrhizal fungi to plants. Except for N-fixing plants, the foliar δ15N signatures of individual species were generally lower at sites of low N availability, suggesting either an increased fraction of N obtained from mycorrhizal uptake (f), or a reduced proportion of mycorrhizal N transferred to vegetation (T r). Foliar and fine root nitrogen concentrations were also lower at these sites. Foliar N concentrations were significantly correlated with δ15N in foliage of Populus, Salix, Picea, and Tsuga heterophylla, and also in fine roots. The correlation between δ15N and N concentration may reflect strong underlying relationships among N availability, the relative allocation of carbon to mycorrhizal fungi, and shifts in either f or T r.

Productivity, nutrient cycling, and succession in single- and mixed-species plantations of Casuarina equisetifolia, Eucalyptus robusta, and Leucaena leucocephala in Puerto Rico

Tree growth, biomass productivity, litterfall mass and nutrient content, changes in soil chemical properties and understory forest succession were evaluated over a 8.5-year period in single- and mixed-species (50 : 50) plantations of two N 2-fixing species, Casuarina equisetifolia and Leucaena leucocephala, and a non-fixing species, Eucalyptus robusta. At the optimal harvest age for maximum biomass production (4 years), total aboveground biomass ranged from 63 Mg ha −1 in the Eucalyptus monoculture to 124 Mg ha −1 in the Casuarina/ Leucaena mixture, and was generally greater in the mixed-species than in single-species treatments due to increased productivity of the N-fixing species in the mixed stands. Total litterfall varied from 5.3 to 10.0 Mg ha −1 year −1 among treatments, or between 5.9% and 13.2% of net primary production. Litterfall production and rates of nutrient return for N, P, K, Ca and Mg were generally highest for Leucaena, intermediate for Casuarina and lowest for Eucalyptus. These rates were usually higher in the mixed-species than in monospecific stands due to differences in biomass productivity, but varied considerably depending on their species composition. Total system carbon and nutrient pools (in biomass plus soils to 40-cm depth) for N, P, K, Ca, Mg, Mn at four years were consistently greater in the plantation treatments than in the unplanted control plots. Relative to the single-species plantations, these system pools were generally larger in the mixed-species plantations for C (−10% to +10%), N (+17% to +50%), P (−1% to +63%), K (−19% to +46%), Ca (−10% to +48%), Mg (+5% to +57%) and Mn (+19% to +86%). Whole-tree harvests at four years would result in substantial system carbon and nutrient losses, although these estimated losses would not exceed the estimated gains realized during the four-year period of tree growth at this site. At 7.5 years, soil organic matter and effective cation exchange capacity were reduced in all plantation treatments relative to the control. Changes in soil nutrient content from 0 to 7.5 years were highly variable and not significantly different among treatments, although stands containing Leucaena generally showed higher rates of nitrogen and phosphorus accretion in soils than those with Eucalyptus and/or Casuarina. Natural regeneration of secondary forest tree and shrub species increased over time in all plantation treatments. A total of 24 native or naturalized forest species were recorded in the plantations at 8.5 years. Woody species abundance at this age was significantly greater beneath Casuarina than either Eucalyptus or the Eucalyptus/ Leucaena mixed stands. Species richness and diversity, however, were greatest beneath stands containing Eucalyptus and/or Leucaena than in stands with Casuarina.

Leaf natural 15N abundance and total N concentration as potential indicators of plant N nutrition in legumes and pioneer species in a rain forest of French Guiana.

The suitability of the natural 15N abundance and of total N concentration of leaves as indicators of the type of plant N nutrition in a rain forest of French Guiana were tested. Leaf samples from primary legume species, non-legumes (pioneer species) and from the non-N2-fixing species Dicorynia guianensis were analyzed. Both δ15N and total leaf N varied widely (-1 ?δ15N (‰) ? 7 and 1 ? leaf N(%) ? 3.2) suggesting possible distinctions between diazotrophic and non-fixing plants. The δ15N also revealed two statistically distinct groups of non-N2-fixing species (δ15N = 5.14 ± 0.3 vs δ15N = 1.65 ± 0.17) related to the different ecological behaviors of these species in the successional processes. We conclude that the δ15N signature of plant leaves combined with their total N concentration may be relevant indicators for identifying functional groups within the community of non-N2-fixing species, as well as for detecting diazotrophy. Despite the variability in the δ15N of the non-N2-fixing species, N2-fixing groups can still be identified, provided that plants are simultaneously classified taxonomically, by their leaf δ15N and total N concentration and by the presence or absence of nodules. The variability in the δ15N of the non-fixing species is discussed.

Organic Matter and Nitrogen Cycles in a Pine Afforested Catchment with a Shrub Layer of Adenocarpus decorticansand Cistus laurifoliusin South-eastern Spain

Organic matter and nitrogen cycles in two mediterranean woody shrubs ( Adenocarpus decorticansBoiss. and Cistus laurifoliusL.) and two pine species were investigated during 1987–1989 in a small, recently pine-afforested catchment in Sierra de los Filabres (Almerı́a Province, south-eastern Spain). Pine cover was sparse because canopy closure was not yet completed, allowing strong shrub colonization in the clearings. The aim of this study was to compare the contribution of different annual fluxes and the distribution of organic matter and nitrogen in both shrub and pine components for the entire catchment. Special emphasis was placed on the N use strategy shown by the two shrub species, a symbiotic N 2fixing species ( A. decorticans) and a non-fixing species ( C. laurifolius). Our results showed that the characteristics of C and N cycling in the Nacimiento catchment are mainly determined by the early stage of pine layer development and the significant contribution of the two shrub species differing with respect to C acquisition and N use. The pine layer was characterized by a low standing biomass, substantial percentages of needle biomass and high (net production/biomass) and (needle production/total net production) ratios. N uptake was mainly diverted to biomass increment, as shown by the substantial allocation found in this study. Overall, these results are consistent with the patterns described for even-aged stands during the early-growth phase. Retranslocation supplied 21% of the annual N requirements for pine above-ground biomass production. C. laurifoliusshowed low litter-fall return, strong retranslocation, efficient utilization of N in bulk deposition and slow decomposition, suggesting a closer, self-regulated organic matter and N-conserving behaviour than A. decorticans. On the contrary, A. decorticansshowed high leaf turnover, strong litter-fall return, low retranslocation and fast mineralization of organic matter and nitrogen, indicating that internal organic matter and N cycling is less efficient, and thus, that this species is more dependent on the soil to meet its N requirements.

Changes in dinitrogen fixation in maturing stands of Casuarinaequisetifolia and Leucaenaleucocephala

Biological dinitrogen fixation in Casuarinaequisetifolia J.R. & G. Forst. and Leucaenaleucocephala (Lam.) de Wit was evaluated using the 15N-enrichment technique under field conditions in single-species and mixed-species plantings (with a nonfixing reference species, Eucalyptus ×robusta J.E. Smith) between 1.0 and 3.5 years of age in Puerto Rico. Following periodic labelling of trenched and untrenched plantation quadrats with 15N-enriched ammonium sulfate, analyses of foliar and whole-tree (weighted average) N-isotopic ratios and total biomass N were used to estimate the proportion of nitrogen derived from biological dinitrogen fixation (PNDFA) and total nitrogen derived from fixation (TNDFA) in C. equisetifolia and L. leucocephala. The 15N-enrichment technique yielded accurate estimates of dinitrogen fixation in maturing stands of these two tree species provided the reference species (Eucalyptus) was grown in close proximity to the N-fixing species in trenched, mixed-species plots. Changes in the 15N/14N ratio of soil-available nitrogen in single-species plots of the N-fixing and reference were found to yield inaccurate estimates of dinitrogen fixation in the single-species plots of C. equisetifolia and L. leucocephala after 2 years of age. The results confirm earlier findings that foliar sampling is a useful nondestructive alternative to whole-tree biomass sampling for the 15N-enrichment protocol. Between 1.0 and 3.5 years after plantation establishment, PNDFA in C. equisetifolia remained relatively constant between 50 and 60%, while PNDFA in L. leucocephala declined from nearly 100% at 1 year to less than 40% at 3.5 years. The rate of dinitrogen fixation (kg•ha−1•year−1) did not decline as the stands matured. Cumulative dinitrogen fixation (TNDFA) estimates at 3.5 years were very similar between species: 73 in C. equisetifolia and 74 kg N•ha−1•year−1 in L. leucocephala.

Nitrogen fixation in grazed and ungrazed subterranean clover pasture in south-west Australia assessed by the 15N natural abundance technique

The progress of N2 fixation by subterranean clover (Trifolium subterraneum L.) was followed throughout a growing season in adjacent grazed and ungrazed portions of a pasture at Mount Barker, W.A. Proportions of plant nitrogen derived from the atmosphere (%Ndfa) were determined at a sequence of sampling times using the 15N natural abundance technique with capeweed (Arctotheca calendula L.) as non-fixing reference species. Cumulative yields of fixed N by above ground biomass of clover were determined from %Ndfa values, concurrent estimates of dry matter (DM) yields, and percentage nitrogen in clover shoot DM. Seasonal DM yields of clover, capeweed and mixed grasses were in the approximate ratio 60 : 20: 20. Total herbage yields were 11.8 and 7.8 t ha-1 for the grazed and ungrazed swards respectively. Poorer performance of the latter was attributed to shading by taller grasses late in the season. Starting from a low value of 58%, Ndfa of the ungrazed sward became uniformly high (73-88%) for the rest of the season. Clover of the more productive grazed sward behaved similarly except for a significant mid winter depression to 55%Ndfa, probably caused by excessive defoliation through overgrazing. Fixed N recovered from clover shoot biomass was 103 and 188 kg N ha-1 for ungrazed and grazed pasture respectively. Mineral N under the grazed sward first consisted mostly of nitrate, and then predominantly of ammonium. Soil-derived N was utilized roughly equally by clover, grasses and capeweed and a field study of %Ndfa of subterranean clover grown in varying proportion with either the main pasture grass (Lolium rigidum Gaudin) or capeweed indicated the grass to be the more effective competitor for soil N against the clover. The data suggested that reliable estimates of seasonal accumulation of fixed N by pastures would be obtained from assessments of cumulative biomass yield of clover N with a single determination of %Ndfa at peak productivity in mid to late spring.

Nitrogen fixation capacity in the component species of contour hedgerows: how important?

The choice of an appropriate hedgerow species is one of the most critical decisions in exploiting the value of a contour hedgerow system. The implications of hedgerow species with nitrogen (N)-fixation capacity on hedgerow-crop competition and crop productivity have been widely debated. We examined the agronomic significance of N-fixation by comparing the performance of species representing three classes of hedgerow vegetation: A nitrogen-fixing tree legumeGliricidia sepium), a non-nitrogen fixing tree (Senna spectabilis syn.Cassia spectabilis), and a forage grass (Pennisetum purpureum). The 4-year study investigated the hedgerow biomass and nutrient yields, and their relative effects on the performance of two annual crops commonly grown in alley farming systems, with emphasis on hedgerow-crop interference. The work was done on an Ultic Haplorthox (pH 4.8, organic C 1.9%, total N 0.18%).Senna produced 46% more pruning biomass on an annual basis than didGliricidia; N supplied to the alley crops was similar toGliricidia in the first year of observation, but 20–30% higher in the succeeding years. Upland rice and maize grain yields and total dry matter were unaffected by tree species, but the nitrogen-fixing tree exerted less competitive effects on the annual crops growing in adjacent rows. Grass hedgerows reduced maize yields 86% by the second year, indicating an unsustainable drawdown of nutrients and water. We conclude that hedgerow systems composed of a nitrogen-fixing tree did not exert significant advantages compared to a non-fixing tree species, and that factors other than N-fixation were more important determinants for the choice of hedgerow species.

Influence of combined nitrogen level andEucalyptus competition on dinitrogen fixation in nodulatedCasuarina

Experiments were conducted to determine the effect on biological dinitrogen fixation byCasuarina of available nitrogen (N) in the substrate and competition by interplantedEucalyptus. In these experiments, combined N was applied to the plants after nodules were developed and functioning. Both environmental factors, nitrate and competition, were observed to influence biological dinitrogen fixation byCasuarina, but not yield (total dry weight). In one experiment, the proportion of nitrogen derived byCasuarina from atmospheric fixation (pNdfa) was observed to be inhibited by potassium nitrate in a linear fashion. However, substrate N did not significantly affect the weight of root nodules. Thus nodule dry weight was not highly correlated with the proportion of nitrogen fixed. In a second experiment, the presence of a non-fixing interplanted species,Eucalyptus, increased dinitrogen fixation inCasuarina.Casuarina interplanted withEucalyptus obtained a greater proportion of its nitrogen (94.75%) from fixation than didCasuarina grown alone (86.68%) suggesting that competition for substrate N influences the proportion of nitrogen fixed by this actinorhizal plant.