N2-fixing Species Research Articles

A Slash-And-Mulch Improved-Fallow Agroforestry System: Growth and Nutrient Budgets over Two Rotations

Agroforestry systems are important, globally affecting 1.2 billion people and covering 0.6 billion hectares. They are often cited for providing ecosystem services, such as augmenting soil fertility via N accumulation and increasing soil C stocks. Improved-fallow slash-and-mulch systems have the potential to do both, while reducing nutrient losses associated with burning. In the absence of burning, these systems also have the potential to grow trees through multiple rotations. This project collected soil, mulch, and biomass data over the course of one 9-year crop-fallow rotation and the first two years of the second rotation. A split-plot design was used to assess the effects of P + K fertilization and inclusion of an N-fixing tree species, Inga edulis, on crop and tree biomass production. Fertilization increased growth and nutrient accumulation during Rotation 1 by an average of 36%, ranging from 11% in Parkia multijuga to 52% in Ceiba pentandra. Residual P + K fertilization improved tree and crop growth 20 months into Rotation 2 by an average of 50%, ranging from 15% in Cedrela odorata to 73% in Schizolobium amazonicum. The improved-fallow slash-and-mulch system increased the rates of secondary succession biomass accumulation (11−15 Mg ha−1 yr−1) by 41–64% compared to natural succession (7–8 Mg ha−1 yr−1). Furthermore, P + K fertilization increased secondary-succession biomass accumulation by 9–24%. Nutrient accumulation through biomass production was adequate to replace nutrients exported via crop root and timber stem harvests.

Patterns of foliar and soil nitrogen isotope composition of Caragana microphylla, a leguminous shrub species in the semi-arid regions of northern China

Variation in foliar and soil nitrogen (N) isotope composition (δ15N) across environmental gradients has been widely studied at various scales. However, much less is known about the patterns of N2-fixing species that are presumed to be independent of climatic factors due to their capacity to fix atmospheric N2. In this study, the foliar and soil δ15N in Caragana microphylla, a leguminous shrub important for its role in controlling soil erosion and land desertification in northern China, were determined from 23 sites along an east–west transect across the species distribution range. The soil δ15N tended to be significantly higher than the foliar δ15N and there was a positive correlation between them (r2 = 0.58, p < 0.0001), though substantial variability was found in foliar and soil δ15N across all sites. Both soil and foliar δ15N were negatively correlated with precipitation, temperature, dry N deposition (DND) and longitude, but positively correlated with foliar N, soil N and altitude. The structural equation modeling further suggested growing season precipitation indirectly influenced foliar δ15N by its direct impact on soil δ15N and by its indirect impact on soil δ15N via DND and total N deposition (TND), whereas growing season temperature affected foliar δ15N indirectly both by its effect on foliar N and by its effect on soil δ15N via WND, TND and foliar N. Altogether, our results indicated that foliar δ15N also shifted along environmental gradients in leguminous plant species and showed primary dependence on soil δ15N, confirming previous findings in non-leguminous plants. These results augment our common understanding of foliar and soil δ15N patterns across climatic gradients in plants.

Water source partitioning and nitrogen facilitation promote coexistence of nitrogen-fixing and neighbor species in mixed plantations in the semiarid Loess Plateau

Effects of water source relations on the facilitation of nitrogen (N) acquisition between N2-fixing species (NFS) and neighbor species require further investigation, especially in water- and N-limited conditions. Plant water sources and leaf physiological parameters of Hippophae rhamnoides and two neighbor species were investigated in pure and mixed plantations. Then, N stable isotope and N content of leaves were analyzed to examine the N facilitation of H. rhamnoides to neighbor species. Finally, interactions between plant water sources and N acquisition were detected using the dissimilarity analysis. H. rhamnoides and Pinus tabuliformis obtained water from similar soil depths, whereas, water sources were partitioned between H. rhamnoides and Ulmus pumila. Compared with pure plantations, P. tabuliformis and H. rhamnoides could not maintain a stable net photosynthetic rate (Pn) during the dry period in mixed plantation. However, H. rhamnoides and U. pumila in mixed plantation could alleviate drought effect on Pn through stomatal adjustments, comapred with pure plantations. Furthermore, H. rhamnoides facilitated N to neighbor species regardless of their water source relations, however, this facilitation only significantly improved the leaf N content of U. pumila. Water source partitioning, stomatal adjustment, and N facilitation promoted stable coexistence of NFS and neighbor species in water- and N-limited environments.

Distribution characteristics of Cd in different types of leaves of Festuca arundinacea intercropped with Cicer arietinum L.: A new strategy to remove pollutants by harvesting senescent and dead leaves

Although cost-effective, phytoremediation is too expensive when considering the large-scale pollution. Relative to harvesting the whole plant, it is more practicable to remove and dispose of senescent and dead leaves after phytoremediation. The phytoremediation efficiency of Festuca arundinacea for Cd was evaluated in this study, because over about 7% of the land area in China was contaminated with Cd. The accumulation, redistribution, and extraction of Cd were evaluated in different leaves of F. arundinacea intercropped with N-fixing species at different densities (Cicer arietinum L). The results showed that coordinate and malposed intercropping systems increased the dry weight of the senescent and dead leaves of F. arundinacea by 30–41% and 103–168% compared to the monoculture system, respectively. More Cd was redistributed to the senescent and dead leaves of F. arundinacea under both intercropping systems. Occupying only 22–30% of the total leaf biomass, senescent and dead leaves accumulated 74–88% of leaf Cd under different cultivation conditions. Relative to the monoculture system, intercropping decreased the amount of time needed to reduce soil Cd by 44–53%. The biomass production and Cd accumulation of F. arundinacea were higher in the malposed intercropping system, and it had higher remediation efficiency than the coordinate intercropping system. This study demonstrated that intercropping, especially malposed intercropping of F. arundinacea and C. arietinum L., is a practicable technology for leaf harvesting phytoremediation.

Photosynthesis, growth, and biomass allocation responses of two Inga species to contrasting light

Inga marginata and Inga subnuda are evergreen N-fixing tree species that are frequently found in the initial stages of forest regeneration. I. marginata has a wide geographic distribution, encompassing the Brazilian Atlantic Rainforest and dry forests of central Brazil, whereas I. subnuda is endemic to the Brazilian Atlantic Rainforest. We conducted this study to compare the physiological, growth, and biomass allocation responses of I. marginata and I. subnuda to contrasting light environments. Considering that I. marginata and I. subnuda have a similar position in the forest succession and different geographic distributions, we tested a hypothesis that these two congeneric species have similar photosynthetic and growth responses to light availability but different biomass allocation. The plants were grown under three light conditions (36, 15, and 6 mol photons m−2 day−1) for 105 days. Growth, biomass allocation, light–response curves, and leaf pigments were compared among the light conditions and between species by using two-way ANOVA. Our hypothesis was partially supported because the two species had similar photosynthetic responses to changes in light availability, but differences in growth and biomass allocation. The higher relative growth rate in mass of I. subnuda is associated with its higher allocation of biomass to light capture, as shown by the higher values of leaf mass fraction (LMF) and leaf area ratio (LAR). Conversely, the higher values of root mass fraction for I. marginata were in contrast to the higher values of LMF, LAR, and specific leaf area for I. subnuda; this indicates that I. marginata should be better adapted to environments where water could be a limiting factor, which is consistent with its wide geographic distribution.

Nitrogen and phosphorus fertilizations alter nitrogen, phosphorus and potassium resorption of alfalfa in the Loess Plateau of China

Nutrient resorption from senescing leaves is a pivotal component of nutrient conservation strategy in a plant. Thus understanding the response of nutrient resorption to fertilization is of great help to minimize fertilizer use and further optimize fertilization management. However, little is known about how nutrient resorption responds to fertilization in N2-fixing species. Nitrogen (N) and phosphorus (P) fertilizers were applied at different rates to alfalfa stands in the Loess Plateau. N fertilization hardly affected leaf N and K resorptions, but tended to increase P resorption. P fertilization increased N and K resorptions but affected P resorption in various ways. However, effect of N or P fertilization was significantly interplayed by P or N rate. At N100P60, alfalfa had the maximum biomass accumulation and less leaf resorption. Therefore, alfalfa could be performed well with 100 kg N ha−1 and 60 kg P2O5 ha−1 in this region.

Introducing nitrogen-fixing tree species and mixing with Pinus massoniana alters and evenly distributes various chemical compositions of soil organic carbon in a planted forest in southern China

There are potential effects of converting a planted-forest type from conifer to broadleaf or mixed-species on soil organic carbon (SOC) chemical composition, and the quantity and distribution of various chemical compositions of SOC remain uncertain. An afforestation experiment with different tree species (Pinus massoniana, Erythrophleum fordii [N-fixing species], and mixed P. massoniana and E. fordii) was conducted in clear-cut sites of P. massoniana-planted forests in a subtropical region. Topsoil organic C quality and microbial diversity were assessed after eight years of afforestation. Referring to the species diversity index in the community, Pielou’s evenness index was used for estimating the evenness of distribution of various organic C compositions in the total organic C of soil, litter, and fine roots. The proportions of alkyl C and carbonyl C in SOC, as well as the alkyl C/O-alkyl C (A/O-A) ratios and the distributed evenness of the four main SOC chemical compositions were higher in the monospecific E. fordii and mixed-species-planted forests than in the monospecific P. massoniana-planted forest. Positive relationships of the A/O-A ratio and the evenness of distribution of various SOC chemical compositions in the litter as well as fine root C quality were observed. Microbial biomass C was positively correlated with the labile SOC (O-alkyl C). Multivariate analysis showed that the fine roots had a closer correlation with the chemical composition of SOC than the litter. These results highlight that introducing native N-fixing broadleaf tree species and mixing N-fixing species into conifer forests probably enhances the SOC chemical stability. It also minimizes the risk of C decomposition posed by any one soil organic matter (SOM) constituent having a huge impact during climate change through increasing the proportion of recalcitrant C components and even distribution of various SOM chemical compositions.

N2-fixing black locust intercropping improves ecosystem nutrition at the vulnerable semi-arid Loess Plateau region, China

The Loess Plateau in northwestern China constitutes one of the most vulnerable semi-arid regions in the world due to long-term decline in forest cover, soil nutrient depletion by agricultural use, and attendant soil erosion. Here, we characterize the significance of N2-fixing Robinia pseudoacacia L. and non-N2-fixing Juglans regia L. for improving nutrient availability and water retention in soil by comparing a range of biological and physicochemical features in monoculture and mixed plantations of both species. We found that N2-fixing Robinia facilitates the nitrogen and phosphorus composition of non-N2-fixing Juglans in the mixed stand as a consequence of improved soil nutrient availability, evident as higher levels of nitrogen and labile carbon compared to mono-specific stands. This demonstrates that intercropping N2-fixing Robinia with non-N2-fixing woody plants can greatly improve soil carbon and nitrogen bioavailability as well as whole-plant nutrition and can potentially mediate water retention with additional sequestration of soil organic carbon in the range of 1 t C ha−1 year−1. Thus, intercropping N2-fixing woody species (e.g. Robinia pseudoacacia or Hippophae rhamnoides L.) with locally important non-N2-fixing tree and shrub species should be considered in afforestation strategies for landscape restoration.

Production of biofuel from sugarcane molasses by diazotrophic Bacillus and recycle of spent bacterial biomass as biofertilizer inoculants for oil crops

Abstract This study was designed to identify the most potent N2-fixing and biofuel producing Bacillus species. Four isolates were selected as the most efficient N2-fixing organisms which confirmed by nifH gene expression. These isolates were identified genetically by 16S rRNA as Bacillus thuringensis, Bacillus subtilis, Bacillus pumilus and Bacillus licheniformis. The highest biohydrogen production was 2450 and 2300 ml/L from 6% sugarcane molasses by B. thuringensis and B. subtilis respectively. Nitrogenase activity of B. thuringensis and B. subtilis were 1.4 and 1.3 μmol C2H4 min−1 mg protein −1 at 6% molasses. Ethanol production was 1.55 and 1.03 g/L, while butyric acid was 10.39 and 5.9 g/L at 6% molasses by B. thuringensis and B. subtilis at 6% molasses, respectively. Acetic acid formation was 1.1 and 0.55, lactic acid was 0.07 and 0.05, while butyric acid was 10.39 and 5.9 g/L at 6% molasses by B. thuringensis and B. subtilis, respectively. Spent bacterial biomass of the two Bacillus species were reused as a biofertilizer for enhancing the growth of sunflower and corn plants. Inoculation of sunflower and corn seeds with B thuringensis and B subtilis significantly increased dry weight, total protein, total carbohydrates and pigment contents over control plants. This enhancement could be attributed to the efficiency of biological N2-fixation due to nitrogenase activity of the tested Bacillus species. These results suggest that the possibility of interlinking biofuel technology with biofertilizer production by reusing N2-fixing spent bacterial biomass of Bacillus could be increase the economic feasibility of the bioenergy production from molasses.

Intercropping N-fixing shrubs in pine plantation forestry as an ecologically sustainable management option

We explored the ecological impact of a nitrogen-fixing shrub (Ulex europaeus, common gorse) cultivated as an intercrop species in the interlines of a young stand of maritime pine (Pinus pinaster). Nitrogen fixing species are well-known for their advantage of increasing the soil fertility in agriculture. Nevertheless, in forest ecosystems, their use is uncommon except in some associations in the tropics. The Landes de Gascogne forest range (SW France) has soil fertility issues and Ulex europaeus is a vigorous plant, native to this region. Therefore, an investigation of the association of this N-fixing species with trees is worthwhile.To this end, we compared a standard pine plantation management (control) with a treatment where gorse seeds were sown in the interlines at tree plantation (gorse sowing). This factor was combined with a phosphorus supply (control versus P-fertilisation), resulting in four treatments. The forestry practices evaluated here distinguish two main periods: a first period of six years after tree plantation when gorse individuals grew as an intercrop and formed dense vegetation, and a second period of three years after crushing this vegetation in the interlines.In the first period, pine mortality in the gorse sowing treatment was higher during the dry season than in the control treatment, suggesting competition for water. Pine diameter growth was reduced, but not height growth, indicating avoidance of light competition with gorse. In the presence of gorse, pine foliar-N was appreciably enhanced relative to the control, and natural pine pruning occurred, while pine browsing by animals was limited due to the protection of the thorny gorse thickets surrounding the trees. In the second period, soon after crushing the gorse vegetation in the interlines, enrichment of nitrogen in needles continued, suggesting a fertilising effect of the gorse remnants. Crushing also freed the pines from shrub competition and a “catch-up growth” was identified regarding diameter and stem volume growth. Two years after the gorse crushing, tree size and biomass were similar in the experimental treatments regardless of gorse sowing or not.Overall, during the first nine years after tree plantation, gorse introduction in the pine tree forests had various effects. On the one hand, gorse induced competition for light and water. On the other hand, it resulted in high inputs of nitrogen and organic matter into the ecosystem which could lead to sustained soil fertility and this might improve tree nutritional status. We recommend continuing the assessment of such pine-gorse forestry associations for longer observation periods and in very nutrient-poor sites in this forest range.

Nitrogen fixation by riparian plants belonging to Coriariaceae, Rhamnaceae, and Gunneraceae in Northwest Patagonia

Nitrogen fixation by symbiotic bacteria associated with different plant species is a key process of natural ecosystems. To better understand the role of native N2-fixing species in the N economy of riparian ecosystems in northwest Patagonia (Argentina), we evaluated: 1) foliar δ15N and N concentrations of actinorhizal Coriaria ruscifolia, Discaria chacaye, and Colletia hystrix, several non-actinorhizal plants (including Gunnera chilensis), and associated soils in riparian forest sites; 2) the proportion of N derived from the atmosphere (Ndfa) of the actinorhizal plant species from riparian forest and of D. chacaye and Ochetophila trinervis from steppe sites; and 3) trends of foliar and soil δ15N and N concentration with mean annual precipitation (MAP). Although soil N concentrations and δ15N did not vary among plant species within any of the sites, foliar N concentration and δ15N differed among species. In general, N2-fixing species had higher foliar N concentrations and δ15N values closer to 0, the atmospheric value. Both variables separated the groups of N2-fixing and non-N2-fixing plant species. Foliar and soil δ15N correlated positively for non-N2-fixing species but not for N2-fixing species. Across all sites, the Ndfa of C. ruscifolia and C. hystrix was ~100%, ~75% for D. chacaye, and ~50% for G. chilensis. For all species, foliar N concentration and soil δ15N was negatively correlated with MAP, but only non-N2-fixing species showed a significant correlation of foliar δ15N with MAP. These data suggest that plant available N decreases as MAP increases but with no effect on N2-fixing species.

Increased nitrogen supply promoted the growth of non-N-fixing woody legume species but not the growth of N-fixing Robinia pseudoacacia

Nitrogen (N) is an essential macronutrient for plant development and growth, and the deposition of N has increased in recent decades. Legumes that fix N can also provide N for nearby species. However, N in soil inhibits N fixation. We tested the effects of N fertilisation on one N-fixing (Robinia pseudoacacia) and two non-N-fixing (Sophora japonica and Senna surattensis) woody legume species, which were subjected to five different N levels (0, 1.5, 2.9, 5.9 and 11.4 mg N per plant day−1) under greenhouse conditions. The growth of the two non-N-fixing species was promoted by N supply, while that of R. pseudoacacia was unaffected. Among the three species, R. pseudoacacia had the largest specific leaf area and chlorophyll concentration, S. japonica had the largest root-to-shoot ratio and main root-to-lateral root ratio, and S. surattensis had the largest leaf N and phosphorus concentrations. The N-fixing species was mostly unaffected by N supply. The growth, leaf chlorophyll concentration, and leaf number in the non-N-fixing species were promoted by N supply. The N-fixing species showed better growth in low-N environments, while under increased N deposition, its growth was similar to that of the non-N-fixing species.

Nitrogen fixation does not axiomatically lead to phosphorus limitation in aquatic ecosystems

A long‐standing debate in ecology deals with the role of nitrogen and phosphorus in management and restoration of aquatic ecosystems. It has been argued that nutrient reduction strategies to combat blooms of phytoplankton or floating plants should solely focus on phosphorus (P). The underlying argument is that reducing nitrogen (N) inputs is ineffective because N2‐fixing species will compensate for N deficits, thus perpetuating P limitation of primary production. A mechanistic understanding of this principle is, however, incomplete. Here, we use resource competition theory, a complex dynamic ecosystem model and a 32‐year field data set on eutrophic, floating‐plant dominated ecosystems to show that the growth of non‐N2‐fixing species can become N limited under high P and low N inputs, even in the presence of N2 fixing species. N2‐fixers typically require higher P concentrations than non‐N2‐fixers to persist. Hence, the N2 fixers cannot deplete the P concentration enough for the non‐N2‐fixing community to become P limited because they would be outcompeted. These findings provide a testable mechanistic basis for the need to consider the reduction of both N and P inputs to most effectively restore nutrient over‐enriched aquatic ecosystems.

Tree species could have substantial consequences on topsoil fauna: a feedback of land degradation/restoration

The characteristics of the faunal community in the soil are closely related to soil quality and function. Land degradation, which reduces vegetation cover, may affect the soil surface-active fauna because both the above ground and below ground invertebrates depend on complex plant communities. In this study, we evaluated the effect of land degradation/restoration and factors affecting soil fauna in northern Iran. The studied land uses were virgin natural forest (VNF), Alnus subcordata C.A.M. plantation (ASP), Quercus castaneifolia C.A. Mey plantation (QCP), Cupressus sempervirens var. horizontalis plantation (CSP) and degraded natural forest (DNF). VNF and ASP enhanced soil earthworm density (2.43 and 2.12 ind. m−2) and dry mass (27.44 and 23.39 mg m−2) with more ratio of epigeic. The activities of acarina (91,851.37 and 85,810.43 ind. m−2), collembola (83,009.50 and 74,996.18 ind. m−2) and protozoa [921.25 and 851.81 (× 102 g soil)] were increased under VNF ≈ ASP, respectively. Nematode population (650 in 100 g soil) significantly improved under VNF. In general, good quality forest floor, alkaline soil and accumulation of macro-element nutrients improved biological activities under the VNF and ASP sites, while low-quality forest floor, acidic soil, less macro-elements nutrients decreased biological activities imposed by the QCP, CSP and DNF sites. More activities of the studied soil fauna were found in autumn and spring. The findings of this study support the importance of preserving natural forests. In addition, employing N2-fixing and suitable native broadleaved species have been proposed in a bid to rehabilitate DNFs.

Shade trees have higher impact on soil nutrient availability and food web in organic than conventional coffee agroforestry

Conventional, intensively managed coffee plantations are currently facing environmental challenges. The use of shade trees and the organic management of coffee crops are welcome alternatives, aiming to reduce synthetic inputs and restore soil biological balance. However, little is known about the impacts of the different types of shade tree species on soil functioning and fauna. In this paper, we assess soil nutrient availability and food web structure on a 17-year old experimental coffee plantation in Turrialba in Costa Rica. Three shade types (unshaded coffee, shaded with Terminalia amazonia, and shaded with Erythrina poepiggiana) combined with two management practices (organic and conventional) were evaluated. Total C and N, inorganic N and Olsen P content, soil pH, global soil fertility, and nematode and microarthropod communities were measured in the top 10 cm soil layer, with the objective of determining how shade tree species impact the soil food web and soil C, N and P cycling under different types of management.We noted a decrease in soil inorganic N content and nematode density under conventional management (respectively −47% and −91% compared to organic management), which suggested an important biological imbalance, possibly caused by the lack of organic amendment. Under conventional management, soil nutrient availability and fauna densities were higher under shade, regardless of the shade tree species. Under organic management, only soils under E. poeppigiana, a heavily pruned, N2-fixing species, had increased nutrient availability and fauna density, while T. amazonia shade had a null or negative impact. The effects of coffee management and shade type on soil nutrient availability were mirrored by changes in soil food web structure. Higher fertility was recorded in soil with balanced food webs. These results emphasize the importance of the choice of shade tree species for soil functions in low input systems, more so than in fertilized systems.

Changes in C:N:P stoichiometry modify N and P conservation strategies of a desert steppe species Glycyrrhiza uralensis

Numerous studies have concluded that carbon (C):nitrogen (N):phosphorus (P) stoichiometry in both soils and plants tends to be decoupled under global change. We consequently hypothesized that plants will adjust nutrient conservation strategies to balance the altered elemental stoichiometry accordingly. To test our hypothesis, we conducted two pot-cultured experiments (with 8-level water and 6-level N addition treatments) using N-fixing species Glycyrrhiza uralensis Fisch from a desert steppe in northwestern China. We observed that high water availability lowered total N content and the N:P ratio in soils, further promoting both N and P resorption from senescing leaves of G. uralensis. High N addition enhanced soil N availability and the N:P ratio, thereby reducing N resorption, but increasing P resorption of G. uralensis. Comparatively, there were also great changes in senescing leaf C:N:P stoichiometry while no clear changes were observed in either green leaf or root C:N:P stoichiometry of G. uralensis. As expected, the altered C:N:P stoichiometry may, in turn, modify N and P conservation strategies through their close linkages with N and P uptake in green leaves of G. uralensis. This modification may also further exert effects on N and P cycling of the desert steppe.

Anionic exchange membranes, a promising tool to measure distribution of soil nutrients in tropical multispecific plantations

Establishing highly productive forest plantations or crops on poor soils requires appropriate management to ensure sustainable production. The current development of various ecological intensification practices calls for efficient tools to monitor their effects on agro-ecosystems. Ecological intensification such as an association between a N2-fixing tree species and a highly productive species, e.g. an acacia and a eucalypt, is an agro-ecological plantation design that can enhance nutrient cycling and preserve soil fertility in tropical and sub-tropical areas. In this study, Anionic Exchange Membranes (AEM) were used to assess in situ the effect of tree species on the availability and spatial variability of nitrate (N) and phosphorus (P) in pure Acacia mangium (A), pure eucalypt (E) and mixed-species treatments (MA-ME) in a randomized complete block design on a ferralitic arenosol. The results showed that the AEM detected the specific influence of tree species on N and P availability at the stand level as well as interactions between trees in the mixed-species treatment. Moreover, nutrients trapped using AEM were significantly correlated with N and P immobilized in the tree biomass. In the mixed stand, AEM made it possible to understand the specific impact of each tree species on N and P availability reflecting the respective biogeochemical mechanisms at work. This preliminary study showed that AEM are a promising tool that can be used in situ for intensive sampling in multi-local comparisons to highlight the effect of management practices on soil fertility as well as the relationships between vegetation cover and soil.

Legume Intercropping With the Bioenergy Crop Sida hermaphrodita on Marginal Soil.

The cultivation of perennial biomass plants on marginal soils can serve as a sustainable alternative to conventional biomass production via annual cultures on fertile soils. Sida hermaphrodita is a promising species to be cultivated in an extensive cropping system on marginal soils in combination with organic fertilization using biogas digestates. In order to enrich this cropping system with nitrogen (N) and to increase overall soil fertility of the production system, we tested the potential of intercropping with leguminous species. In a 3-year outdoor mesocosm study, we intercropped established S. hermaphrodita plants with the perennial legume species Trifolium pratense, T. repens, Melilotus albus, and Medicago sativa individually to study their effects on plant biomass yields, soil N, and above ground biomass N. As a control for intercropping, we used a commercial grass mixture without N2-fixing species as well as a no-intercropping treatment. Results indicate that intercropping in all intercropping treatments increased the total biomass yield, however, grass species competed with S. hermaphrodita for N more strongly than legumes. Legumes enriched the cropping system with fixed atmospheric nitrogen (N2) and legume facilitation effects varied between the legume species. T. pratense increased the biomass yield of S. hermaphrodita and increased the total biomass yield per mesocosm by 300%. Further, the total above ground biomass of S. hermaphrodita and T. pratense contained seven times more N compared to the mono-cropped S. hermaphrodita. T. repens also contributed highly to N facilitation. We conclude that intercropping of legumes, especially T. pratense and T. repens can stimulate the yield of S. hermaphrodita on marginal soils for sustainable plant biomass production.

Precipitation is the main factor affecting the variation of foliar nitrogen isotope composition in two leguminous shrub species of northwestern China.

An increase in foliar nitrogen isotope composition (δ15N) with decreasing precipitation has been shown to occur widely in non-N2-fixing plant species. However, similar patterns have not been identified in N2-fixing species. Here, we tested the relationships of foliar δ15N with local environmental factors and leaf properties in two leguminous shrub species (Caragana korshinskii and Caragana liouana) sampled from 30 populations. Results indicated that the mean annual precipitation (MAP) primarily accounted for the variation of foliar δ15N in the two species. Further analysis revealed strong negative correlations between foliar δ15N and MAP within and across species. These results suggest that the foliar δ15N of leguminous shrub species also shift along precipitation gradients, which augments our understanding of the relationships between foliar δ15N and climatic factors.

The role of 15N-depleted fertilizers as tracers in N cycling studies in agroecosystems

A search of the literature (principally Google Scholar, using the keywords given below) revealed that 120 research papers have been published where 15N-depleted (14N-enriched) fertilizers, including ammonium sulfate, ammonium nitrate, urea and ammonia forms were used as tracers. The studies included annual cereals, vegetables, a grain legume and a fibre crop, and perennial crops including grasses, shrubs (berries), trees (fruits and nuts) and N2-fixing forage, grain and woody legumes and an actinorhizal species. Other minor examples include perennial ornamentals and trees harvested for wood. Applications included estimation of fertilizer recovery in crops, in post-harvest soil and estimation of N losses in the soil–plant system by mass balance. The residual value of 15N-depleted fertilizers and the recovery of 15N-depleted legume residues have also been reported. 15N depleted fertilizers have played an important role in differentiating the relative uptake of fertilizer N to N mobilized in storage tissues with respect to the N nutrition of deciduous horticultural trees, shrubs and vines in spring. The symbiotic dependence of N2-fixing species and transfer to companion non-legumes has been indirectly determined using 15N-depleted fertilizers. Thus 15N-depleted fertilizers have shared the same applications as their 15N-enriched counterparts. Because of their relative cheapness compared with 15N-enriched fertilizers, larger and more representative unconfined plots can be employed in field studies.