15N Natural Abundance Research Articles

The \u03b415N value of N2 fixing actinorhizal plants and legumes grown with N2 as the only nitrogen source

The aim of this study was to investigate the effects of different plant parts and the age of plants at harvest as well as N2 fixing bacterial strains on the N concentration in symbiotic plant parts, especially on the δ15N signature of the actinorhizal plants and legumes. The 15N natural abundance method was used. Two actinorhizal plants were studied: Alnus incana (L.) infected with the Frankia strains ArI3 or “lsF” (local source of Frankia) and Hippophaë rhamnoides (L.) infected with the Frankia strains T1 or E15b. Two legume species were studied: Hedysarum coronarium (L.), infected with a soil suspension, and Robinia pseudoacacia (L.), infected with a crushed nodule suspension. It was particularly interesting that in A. incana, the two Frankia strains affected not only N concentration and δ15N signature of leaves and roots, but also had an impact on plant growth at first harvest. In Hippophaë rhamnoides plants inoculated with the Frankia strains T1 and E15b, N concentrations and δ15N values did not differ at any harvest time. However, plants nodulated by the Frankia strain T1 showed a higher nitrogen fixation rate and higher plant dry matter at all harvesting times. Based on our results for the quantification of N2 fixation with the “B” value, that is the δ15N value of the N2 fixing plants relying only on N2 fixation, plant parts, ages and strains should be carefully considered.

15N Natural Abundance, Nitrogen and Carbon Pools in Soil-Sorghum System Amended with Natural and NH4+-Enriched Zeolitites

The use of rocks containing high amounts of natural zeolites (zeolitites) as soil amendment has been found as a valuable method for increasing agriculture sustainability. However, the potentialities and the effects of zeolitites on the biogeochemical cycles of nitrogen (N) and carbon (C) have still not been clearly addressed in the literature. The objective of this study was therefore to investigate the N and C pools and 15N distribution in an agricultural soil amended with both natural and NH4+-enriched zeolitites with the aim of understanding their effects on the soil-plant system, during sorghum cultivation, under fertilization reductions. Zeolitites were applied to an agricultural soil both at natural state (5 and 15 kg m−2) and in an enriched state with NH4+ ions from pig slurry (7 kg m−2). Both zeolitites at natural and enriched state increased soil cation exchange capacity and affected microbial biomass, causing an initial decrease of microbial C and N and then a possible increase of fungal population. N-NO3− content was lower in natural zeolitite treatments, that lead to a lower NO3− availability for denitrifying bacteria. Zeolitites slightly affected the fixed N-NH4+ pool. δ15N turnover indicated that N from NH4+-enriched zeolitites remained in the soil until the growing season and that fertilizers partially substituted the fixed pool. Leaf δ15N content indicated that plants assimilated N from NH4+-enriched zeolitites and evidenced a higher fertilization recovery in natural zeolitite treatments. Organic C tended to be higher in all zeolitite treatment rhizospheres. In soils amended with zeolitites at natural state (at both application rates) sorghum yield was similar (+3.7%) to that obtained in the control while it was higher (+13.9%) in the plot amended with NH4+-enriched zeolitites.

Nitrogen Isotopes in Soils and Plants of Tundra Ecosystems in the Khibiny Mountains

The isotopic composition of nitrogen in soils and plants may be an indicator of transformation of its compounds and sources of N nutrition of plants. Natural 15N abundance (δ15N) was determined in soils (the total, ammonium, and nitrate nitrogen) and in plant leaves and roots of four tundra ecosystems in the Khibiny Mountains. The studied soils (Folic Leptic Entic Podzol and Leptosols) significantly differ in N availability, and plants are represented by the species, forming ectomycorrhiza, ericoid mycorrhiza, and arbuscular mycorrhiza, as well as by the species, which usually do not form a mycorrhiza. The range of δ15N in soil inorganic compounds is from –16.2 ‰ in nitrates to +6.4‰ in ammonium, which reflects the correlation between the activities of N-mineralization and nitrification and δ15N- $${\text{NH}}_{4}^{ + }$$ , as well as a potentially strong effect on the isotopic composition of nitrogen in plants. The value of δ15N in plant leaves and roots changes in a narrower range (from –7.3 to +2.4‰), which may be related to N uptake from different sources and to fractionation of N isotopes during N assimilation. Roots are 15N-enriched in comparison with leaves in most of the studied plant species, which corresponds to the concept of mycorrhiza participation in N nutrition of plants. Regardless of the type of mycorrhizal symbiosis, the difference in δ15N between roots and leaves of most plant species decreases contrary to N availability in soils.

Nitrogen assimilation in picocyanobacteria inhabiting the oxygen‐deficient waters of the eastern tropical North and South Pacific

AbstractProchlorococcus and Synechococcus are the most abundant free‐living photosynthetic microorganisms in the ocean. Uncultivated lineages of these picocyanobacteria also thrive in the dimly illuminated upper part of oxygen‐deficient zones (ODZs), where an important portion of ocean nitrogen (N) loss takes place via denitrification and anaerobic ammonium oxidation. Recent metagenomic studies revealed that ODZ Prochlorococcus have the genetic potential for using different N forms, including nitrate and nitrite, uncommon N sources for Prochlorococcus, but common for Synechococcus. To determine which N sources ODZ picocyanobacteria are actually using in nature, the cellular 15N natural abundance (δ15N) and assimilation rates of different N compounds were determined using cell sorting by flow cytometry and mass spectrometry. The natural δ15N of the ODZ Prochlorococcus varied from −4.0‰ to 13.0‰ (n = 9), with 50% of the values in the range of −2.1–2.6‰. While the highest values suggest nitrate use, most observations indicate the use of nitrite, ammonium, or a mixture of N sources. Meanwhile, incubation experiments revealed potential assimilation rates of ammonium and urea in the same order of magnitude as that expected for total N in several environments including ODZs, whereas rates of nitrite and nitrate assimilation were very low. Our results thus indicate that reduced forms of N and nitrite are the dominant sources for ODZ picocyanobacteria, although nitrate might be important on some occasions. ODZ picocyanobacteria might thus represent potential competitors with anammox bacteria for ammonium and nitrite, with ammonia‐oxidizing archaea for ammonium, and with nitrite‐oxidizing bacteria for nitrite.

Nanoscale Surface Compositions and Structures Influence Boron Adsorption Properties of Anion Exchange Resins.

Boron adsorption properties of poly(styrene-co-divinylbenzene) (PSDVB)-based anion-exchange resins with surface-grafted N-methyl-d-glucamine (NMDG) depend strongly on their local surface compositions, structures, and interfacial interactions. Distinct boron adsorption sites have been identified and quantified, and interactions between borate anions and hydroxyl groups of NMDG surface moieties have been established. A combination of X-ray photoelectron spectroscopy (XPS), solid-state nuclear magnetic resonance (NMR), and Fourier-transform infrared (FT-IR) spectroscopy were used to characterize the atomic-level compositions and structures that directly influence the adsorption of borate anions on the NMDG-functionalized resin surface. Surface-enhanced dynamic-nuclear-polarization (DNP)-NMR enabled dilute (3 atom % N) tertiary alkyl amines and quaternary ammonium ions of the NMDG groups to be detected and distinguished with unprecedented sensitivity and resolution at natural abundance 15N (0.4%). Two-dimensional (2D) solid-state 11B{1H}, 13C{1H}, and 11B{11B} NMR analyses provide direct atomic-scale evidence for interactions of borate anions with the NMDG moieties on the resin surfaces, which form stable mono- and bischelate complexes. FT-IR spectra reveal displacements in the stretching vibrational frequencies associated with the O-H and N-H bonds of NMDG groups that corroborate the formation of chelate complexes on the resin surfaces. The atomic-level compositions and structures are related to boron adsorption properties of resin materials synthesized under different conditions, which have important remediation applications.

Root–microbial interaction accelerates soil nitrogen depletion but not soil carbon after increasing litter inputs to a coniferous forest

Net primary productivity is expected to increase in many forests as Earth warms, which can increase litter inputs to soils and affect carbon (C) and nitrogen (N) dynamics. Understanding how increasing litter inputs affect soil C and N cycling in tropical and subtropical forests is important because they represent some of the most productive ecosystems on Earth, suggesting that small changes in these cycles can have large effects. To test the effects of increased litter inputs and the interactive effect between microbes and roots on soil C and N stocks and dynamics, we manipulated litter inputs and used trenching to exclude roots in a 40-year-old Cunninghamia lanceolata Lamb. (Chinese fir) plantation. At the site, we measured soil C and N pools, soil 13C and 15N natural abundance, and potential activities for C-, N-, and phosphorus-acquiring enzymes. After four years of experimental treatment, we found that increasing litter inputs reduced total soil N content by 26% relative to background litter inputs, but that increasing litter inputs did not affect soil C content in the plots with roots. In the plots without roots, both soil N and C did not change in response to litter inputs. In the plots with roots, soil δ15N increased with increasing litter inputs, but there was no effect in the plots without roots. We found a strong interactive effect between root and litter treatment on soil N pools and δ15N. The decline in soil N pools and increase in soil δ15N were associated with elevated potential enzyme activity for N-acquisition (N-acetyl glucosaminidase). Adding litter did not have a significant effect on soil C pools, likely because potential soil C losses were offset by increasing litter-derived C inputs. In contrast to C, adding litter decreased N availability, likely through multiple pathways including gaseous N losses, NO3− leaching, root N uptake, and interactions between saprotrophic microbes and roots during the four-year litter addition experiment. Global changes that increase litter production may lower N pools and imbalance C and N cycling in subtropical coniferous forest ecosystems.

Changes in soil organic carbon and nutrient stocks in conventional selective logging versus reduced-impact logging in rainforests on highly weathered soils in Southern Cameroon

Although disturbances associated with selective logging can cause pronounced changes in soil characteristics and nutrient stocks, such information is very limited for highly weathered soils in Africa. We assessed the effects of reduced impact logging (RIL, with a 30-year rotation management plan) and conventional logging (CL, without a management plan) on physical and biochemical characteristics of Ferralsol soils that developed on pre-Cambrian rocks in rainforests of Cameroon. Five to seven months after the logging operations were completed, we mapped the CL and RIL sites and quantified the disturbed areas: felling gaps, skidding trails, logging decks and roads. We selected four replicate plots at each site that encompassed these four disturbed strata and an adjacent undisturbed area as the reference. At each disturbed stratum and reference area per plot, we took soil samples down to 50 cm, and quantified soil physical and biochemical characteristics. Nutrient exports with timber harvest were also quantified.The logging intensity was very low with removals of 0.2 and 0.3 tree per hectare, and the ground area disturbed accounted only 5.2% and 4.0% of the total area in CL and RIL, respectively. In terms of area disturbance for each harvested tree, CL had 753 m2 tree−1 more affected ground area than RIL. Roads and logging decks were the most affected by logging operations, where effective cation exchange capacity, soil organic carbon (SOC), total nitrogen (N), Bray-extractable phosphorus (P) and exchangeable aluminum decreased whereas pH, 15N natural abundance and exchangeable manganese increased compared to the undisturbed reference area (P < 0.01–0.04). The disturbed area showed overall reductions of 21–29% in SOC, N and P stocks relative to the reference areas (P = 0.02–0.07). The amounts of C, N, P and base cations exported with harvested timber were only 0.4–5.9% of the changes in stocks of these elements in the disturbed strata. Nutrient reductions in the soil and exports through timber harvest were comparable between CL and RIL, after one logging event in this very low intensity logging systems. Our results suggest that unplanned operations together with frequent re-logging inherent to CL can increase area damage and enhance changes in SOC and nutrients as opposed to RIL, which may affect the recovery of the succeeding vegetation.

Changes in Acetylene Reduction Activities and nifH Genes Associated with Field-Grown Sweet Potatoes with Different Nursery Farmers and Cultivars

Sweet potato cultivars obtained from different nursery farmers were cultivated in an experimental field from seedling-stage to harvest, and the acetylene reduction activity (ARA) of different parts of the plant as well as the nifH genes associated with the sweet potatoes were examined. The relationship between these parameters and the plant weights, nitrogen contents, and natural abundance of 15N was also considered. The highest ARA was detected in the tubers and in September. Fragments of a single type of nitrogenase reductase gene (nifH) were amplified, and most of them had similarities with those of Enterobacteriaceae in γ-Proteobacteria. In sweet potatoes from one nursery farm, Dickeya nifH was predominantly detected in all of the cultivars throughout cultivation. In sweet potatoes from another farm, on the other hand, a transition to Klebsiella and Phytobacter nifH was observed after the seedling stage. The N2-fixing ability contributed to plant growth, and competition occurred between autochthonous and allochthonous bacterial communities in sweet potatoes.

Bradyrhizobial inoculation and P application effects on haricot and mung beans in the Ethiopian Rift Valley

To investigate factors limiting biological nitrogen fixation (BNF) by grain legumes in the Ethiopian Rift Valley in soils with different input legacies. Two field experiments were set up, at a high-input (Hawassa) and a low-input (Lokabaya) site, comparing each two varieties of haricot bean (Phaseolus vulgaris var. Hawassa dume and Awash1) and mung bean (Vigna radiata var. N26 and Sunaina) throughout two cropping seasons, with and without commercially available inoculants and/or phosphorous addition (2nd year only). Symbiotic performance was evaluated by estimating the proportion of N derived from the atmosphere (%Ndfa) using 15N natural abundance. Mean Ndfa of haricot bean ranged from 31 to 57% at Hawassa and 56 to 57% at Lokabaya with inconsistent differences between varieties and years. Mung beans had lower Ndfa values than haricot beans, but performed better at the low-input (47–53%) than the high-input (9–24%) sites. Overall, haricot bean fixed more N at Hawassa (71–99 kg ha−1) than at Lokabaya (17–36 kg ha−1), while BNF by mung bean was small at both sites (8–25 kg N ha−1). Both Ndfa and N yields responded positively to P addition in the second year, which roughly resulted in 50% increase in BNF by both beans at the low-input site Lokabaya but not at the high-input site Hawassa. Combining P addition with rhizobial inoculation increased BNF and N yields, but there was no significant difference to P addition alone. Low soil fertility typical for the Rift Valley smallholder farms strongly constrains grain legume yields despite good symbiotic performance. In marginal soils, inoculation together with P addition may increase Ndfa to high values (> 60%) in haricot bean, but N yields remain small relative to yields obtained in high-input soil. This indicates that the large yield gap commonly experienced by smallholder farmers cannot readily be overcome by the use of inoculants or P fertilization and that long-term soil fertility management is needed to increase haricot bean productivity in the region. By contrast, mung bean, which had similar yields with haricot bean in the marginal soil, is responsive to P addition and may thus be a valid alternative for small holders with marginal soils.

Potential nitrogen contribution from symbiotic fixation of dwarf pea (Pisum sativum) and clover (Trifolium resupinatum) in crop rotation and intercropping systems

Intercropping and rotation systems involving legumes can be useful means to achieve a sustainable agricultural production. Biological nitrogen fixation (BNF) is widespread among leguminous species thanks to their ability to establish symbiosis with nitrogen (N) fixing microorganisms. Moreover, the application of intercropped legumes increases the agricultural biodiversity and helps farmers to reduce N leaching preventing soil erosion and weed spreading. Little information is available regarding the contribution of N2 (atmospheric nitrogen) from symbiotic fixation of pea and clover when involved in intercropping or in succession systems with other vegetable crops. This study, carried out in a mountain agroecosystem (Venosta Valley, South Tyrol, Italy), has been conceived as part of a more complex research trial on N optimization of cauliflower intercropped and in succession with other vegetables. We have applied the 15N natural abundance technique to field grown clover and dwarf pea, with or without external N supplied, to assess the potential contribution of N2 for intercropping and crop rotation. The technique exploits naturally occurring differences in 15N abundance between plant available N sources in the soil (more enriched in the 15N isotope) and that of N2 in the atmosphere (δ15N=0‰). When a legume crop fixes N2, its 15N abundance should be lower than that of a no-N fixing crop, exploiting only the soil N pool. Reference plant N derived from atmosphere (%Ndfa) was the white cauliflower (Brassica oleracea var. botrytis), one of the most important vegetables on a world scale belonging to the Brassicaceae family and the first vegetable crop cultivated for fresh consumption in South Tyrol. In our study, clover and pea crop residues contained around 74 and 43 kg N ha‑1, respectively. The %Ndfa for the two legumes was similar and ranged from 20% (dwarf pea) to 22% (clover). In presence of external N fertilizers, values of δ15N of the species resulted lower in comparison with the unfertilized control, making the BNF estimation difficult. The study indicated clover and dwarf pea as a potential tool to reduce inorganic fertilizers supply when included in a rotation system. Further trials are needed to confirm our results on BNF of the analysed species in different agroecosystems.

Elevated CO2 improves yield and N2 fixation but not grain N concentration of faba bean (Vicia faba L.) subjected to terminal drought

Legumes grown in Mediterranean environments frequently experience terminal drought which reduces yield and N2 fixation processes. Decreased N2 fixation during reproductive phases may constrain seed nitrogen concentrations ([N]), reducing protein concentration of grain legumes. Plants grown under elevated atmospheric CO2 concentrations ([CO2]) have greater water use efficiency. This may result in reduced use of conserved/stored soil water, potentially helping to reduce soil water deficits later during grain filling. The extent that this process applies to drought sensitive grain legumes, which are extensively cultivated in Mediterranean environments is unclear. The objectives of this study were to investigate yield, N2 fixation and seed N response of faba bean (Vicia faba L. cv. ‘Fiesta’) grown in a dryland Mediterranean-type environment under elevated [CO2]. Plants were grown in soil columns under ambient [CO2] (˜400 ppm) or elevated [CO2] (e[CO2], ˜550 ppm) in a Free-Air CO2 Enrichment (FACE) facility in the field. One sub-group was continuously well-watered (80% field capacity, FC), whereas a second sub-group was exposed to a drought treatment (water was withheld until 30% FC was reached, which was then maintained during the reproductive phases). Biomass, gas exchange, 13C isotopic discrimination, N2 fixation by the natural abundance 15N method, nodulation and soil water content were assessed throughout the crop developmental stages.Initially, plants grown under elevated [CO2] depleted soil water more slowly in the drought treatment than those under ambient [CO2], but as plants grown under elevated [CO2] produced more biomass they used soil water more rapidly, especially towards the critical pod-filling phase. Water savings during the first phase of the drought treatment, through flowering up to the start of pod-filling, were associated with increased yield (+25%) and N2 fixation (+15%) under drought. Elevated [CO2]-induced stimulation of nodulation and nodule density helped maintain N2 fixation under drought, even though nodule activity decreased under the combined effect of e[CO2] and drought from pod-filling onwards. This later stage decrease was associated with decreased carbohydrate and increased amino acid concentrations in nodules, indicating a down-regulation of N2 fixation. Associated with the decrease of N2 fixation during pod-filling, seed N concentration was lower under the combination of e[CO2] and drought. We propose a conceptual model to explain the importance of N2 fixation during the grain filling stage to maintain seed N concentration under e[CO2]. These findings suggest that e[CO2]-induced savings in soil water may mitigate negative effects of drought on yield and N2 fixation of faba bean, without fully compensating the effect of prolonged drought on seed N concentration.

Labile C dynamics reflect soil organic carbon sequestration capacity: Understory plants drive topsoil C process in subtropical forests

AbstractThe huge background pool of soil organic carbon (SOC) is likely to impede the ready detection of SOC changes. We propose to explore SOC changes by monitoring the dynamics of soil labile organic carbon (LOC); namely if LOC could be largely retained in soils rather than respired rapidly, the SOC would be ready to be sequestered. The effects of the two major functional groups of plants, that is, canopy trees and understory plants, on SOC accumulation were then illustrated with this LOC‐based approach. The characteristics of LOC and SOC of topsoils (0–20 cm) in a field manipulation experiment with 5‐yr treatments of understory removal and tree girdling in both a young and a mature Eucalyptus plantations were examined. The concentration and potential turnover time of soil LOC were used to indicate the state of vegetation‐induced C accumulation in soils, which were estimated by a sequential fumigation‐incubation procedure. Soil natural abundances of 13C and 15N were measured to reflect the proportion of newly retained LOC in soils. We found that, in the young plantation, understory removal did not significantly affect both soil LOC and SOC concentrations, but significantly increased the potential turnover time of soil LOC. In contrast, in the mature plantation, understory removal significantly decreased soil LOC and SOC concentrations, but did not significantly alter the potential turnover time of soil LOC. However, tree girdling did not significantly affect SOC concentration, soil LOC concentration, or potential turnover time in either the young plantation or the mature plantation. These results demonstrated that understory plant‐derived C was one of the major components of LOC pool in topsoils, and it may be readily mineralized in the young plantation but accumulated as an important fraction of SOC in the mature plantation. This study suggests that the LOC‐based approach is potentially useful in monitoring SOC changes and improves our understanding of how plant functional groups and soil fertility status could jointly affect LOC and SOC dynamics. In considering the great contribution of understory plants to SOC processes, we propose that understory plants should be maintained in subtropical plantation ecosystems.

Maternal obesity does not influence human milk protein 15N natural isotope abundance

ABSTRACTObesity increases protein metabolism with a potential effect on nitrogen isotope fractionation. The aim of this study was to test the influence of obesity on human milk extracted protein 15N natural isotope abundance (NIA) at one month post-partum and to compare human milk extracted protein 15N NIA and bulk infant hair 15N NIA. This cross-sectional observational study involved 16 obese mothers (body mass index (BMI) ≥ 30 kg m−2 before pregnancy) matched with 16 normal-weight mothers (18.5 kg m−2 ≤ BMI < 25 kg m−2) for age and pregnancy characteristics. Human milk extracted protein and bulk infant hair 15N NIA were determined by isotope ratio monitoring by mass spectrometry interfaced to an elemental analyser (IRM-EA/MS). No significant difference was found in human milk protein 15N NIA values between obese and normal-weight mothers (8.93 ± 0.48 ‰ vs. 8.95 ± 0.27 ‰). However, human milk protein 15N NIA was significantly lower than bulk infant hair 15N NIA: 8.94 ± 0.38 ‰ vs. 9.66 ± 0.69 ‰, respectively. On the basis of these results, it is concluded that human milk protein 15N NIA measured at one month post-partum is not influenced by maternal obesity. These findings suggest that 15N NIA may be exploited to study metabolism without considering maternal obesity as a confounder.

Ash application enhances decomposition of recalcitrant organic matter

Harvesting whole-tree biomass for biofuel combustion intensifies removal of nutrients from the ecosystem. This can be partly abated by applying ash from the combustion back to the system, as the ash is rich in nutrients. Ash is very alkaline and ash application raises soil pH, which in turn can stimulate microbial activity and thus decomposition and mineralization. Our aim was to test if ash induced decomposition activity was associated with enhanced turnover of recalcitrant, i.e. relatively old, organic pools. Two experiments were conducted in the same coniferous plantation after the application of 0, 3, 4.5 and 6 t ash ha−1, and 0, 3, 9, 15 and 30 t ash ha−1, respectively. We used natural abundance of 15N in mosses, mites and ectomycorrhizal fungi 26 months after ash application, as well as temporal variation in δ15N values of ectomycorrhizal fungi, as an indicator of decomposition of recalcitrant organic matter in the first experiment. Furthermore, in the second experiment we used measurements of extracellular manganese peroxidase activity almost 4 years after ash application as an indication of potential decomposition of lignin, an important component of recalcitrant organic matter.The δ15N signature increased significantly for ectomycorrhizal fungi, dead moss, Nothroid and Gamasida mites, and manganese peroxidase activity tended to increase, with increasing ash doses. This suggests that ash application stimulates turnover of recalcitrant organic matter, which can increase the available pool of nitrogen in the system. This will potentially enhance the fertilizer value of ash. However, the δ15N in ectomycorrhizal fungi tended to peak at 18 months after ash application, before decreasing, suggesting that the turnover of recalcitrant organic matter is reduced again with time.

Deepened winter snow significantly influences the availability and forms of nitrogen taken up by plants in High Arctic tundra

Climate change may alter nutrient cycling in Arctic soils and plants. Deeper snow during winter, as well as summer warming, could increase soil temperatures and thereby the availability of otherwise limiting nutrients such as nitrogen (N). We used fences to manipulate snow depths in Svalbard for 9 consecutive years, resulting in three snow regimes: 1) Ambient with a maximum snow depth of 35 cm, 2) Medium with a maximum of 100 cm and 3) Deep with a maximum of 150 cm. We increased temperatures during one growing season using Open Top Chambers (OTCs), and sampled soil and vascular plant leaves throughout summer 2015. Labile soil N, especially inorganic N, during the growing season was significantly greater in Deep than Ambient suggesting N supply in excess of plant and microbial demand. However, we found no effect of Medium snow depth or short-term summer temperature increase on soil N, presumably due to minor impacts on soil temperature and moisture. The temporal patterns of labile soil N were similar in all snow regimes with high concentrations of organic N immediately after snowmelt, thereafter dropping towards peak growing season. Concentrations of all N forms increased at the end of summer. Vascular plants had high N at the start of growing season, decreasing as summer progressed, and leaf N concentrations were highest in Deep, corresponding to the higher soil N availability. Short-term summer warming was associated with lower leaf N concentrations, presumably due to growth dilution. Deeper snow enhanced labile soil organic and inorganic N pools and plant N uptake. Leaf 15N natural abundance levels (δ15N) in Deep indicated a higher degree of utilization of inorganic than organic N, which was especially pronounced in mycorrhizal plants.

Plant and Soil Nitrogen in an Ombrotrophic Peatland, Southern Canada

We examined the concentration of nitrogen (N) and δ15N in vegetation and peat in the Mer Bleue bog, Ontario, Canada. Compared with other ecosystems, N concentration in bog plant foliage is low (generally < 1.2%), with more N stored belowground than in foliar tissues. The natural abundance of 15N varies by plant functional type, indicating different strategies for nutrient acquisition. δ15N values of evergreen shrubs and trees are lower (~− 5 to ~− 9‰) than sedges and herbs (~+ 1 to ~− 1‰), likely the result of ericoid mycorrhizal fungi, which retain 15N in fungal biomass, with preferential transfer of 14N to the host plant. Results suggest that N in Sphagnum moss capitula is derived from uptake of N from decomposing peat and litter, with δ15N values of − 8 to − 2‰, rather than from N2 fixation. Soil δ15N varies with depth, related to degree of decomposition and dominant plant species litter. Foliar N plays a critical role in influencing the rate of photosynthesis and CO2 exchange. Despite paucity of available inorganic N in the system, most bog plants (that is, evergreen shrubs) have adapted with conservative growth strategies. Owing to slow decomposition, N is stored in this system tightly bound with organic matter, with C:N ratios in deep peat ranging from 33:1 to 23:1 and small fluxes into, out of and within the system. An increase in N availability, through atmospheric N deposition or water table lowering, could jeopardize the C sink function of bogs by stimulating microbial decomposition of organic matter. In the long term, an increase in N cycling would shift plant community composition from Sphagnum and evergreen dominated to deciduous species, fundamentally altering bog ecosystem function.

Selection of Competitive and Efficient Rhizobia Strains for White Clover.

The practice of inoculating forage legumes with rhizobia strains is widespread. It is assumed that the inoculated strain determines the performance of the symbiosis and nitrogen fixation rates. However, native-naturalized strains can be competitive, and actual nodule occupancy is often scarcely investigated. In consequence, failures in establishment, and low productivity attributed to poor performance of the inoculant may merely reflect the absence of the inoculated strain in the nodules. This study lays out a strategy followed for selecting a Rhizobium leguminosarum sv. trifolii strain for white clover (Trifolium repens) with competitive nodule occupancy. First, the competitiveness of native-naturalized rhizobia strains selected for their efficiency to fix N2 in clover and tagged with gusA was evaluated in controlled conditions with different soils. Second, three of these experimental strains with superior nodule occupancy plus the currently recommended commercial inoculant, an introduced strain, were tested in the field in 2 years and at two sites. Plant establishment, herbage productivity, fixation of atmospheric N2 (15N natural abundance), and nodule occupancy (ERIC-PCR genomic fingerprinting) were measured. In both years and sites, nodule occupancy of the native-naturalized experimental strains was either higher or similar to that of the commercial inoculant in both primary and secondary roots. The difference was even greater in stolon roots nodules, where nodule occupancy of the native-naturalized experimental strains was at least five times greater. The amount of N fixed per unit plant mass was consistently higher with native-naturalized experimental strains, although the proportion of N derived from atmospheric fixation was similar for all strains. Plant establishment and herbage production, as well as clover contribution in oversown native grasslands, were either similar or higher in white clover inoculated with the native-naturalized experimental strains. These results support the use of our implemented strategy for developing a competitive inoculant from native-naturalized strains.

Natural 13C and 15N abundance of moss-substrate systems on limestones and sandstones in a karst area of subtropical China

Epilithic mosses (EM) and organic substrates (OS) constitute a special ecosystem developing on rocks and a critical interface regulating rock-atmosphere interactions and biological weathering. Carbon (C) and nitrogen (N) are critical elements influencing biotic-abiotic interactions in EM-OS systems. However, the dynamics of C and N in EM-OS systems and their differences among rock types remain unclear. This study investigated contents ([C], [N]) of C and N and their isotope compositions (δ13C, δ15N) in EM and OS layers on limestones and sandstones in Huanjiang karst observatory of Guangxi, China. Significant higher [C]EM (by ca. 5.0%) and lower δ13CEM values (by ca. 3.0‰) on sandstones than on limestones might reflect a lower water use efficiency in mosses on sandstones. Differences in δ13C values between EM and OS correlated positively with those in C contents, elucidating increasing 13C discriminations with the C decomposition in substrates. The δ15NEM and δ15NOS values were correlated positively, indicating mosses as a major contributor of organic N in OS. The [N]EM did not differ between two rock types, but significant lower [N]OS and higher δ15NOS values (by ca. 0.2% and 1.5‰, respectively) occurred on limestones than on sandstones, suggesting larger N losses and 15N enrichments in OS on limestones. These results underscore that elemental and isotopic signatures can elucidate different C and N dynamics in the EM-OS ecosystems as a function of rock types.

Biological fixation, transfer and balance of nitrogen in passion fruit (Passiflora edulis Sims) orchard intercropped with different green manure crops

Green manures can replace or supplement mineral fertilization and add organic matter to the soils, ensuring greater sustainability to fruit growing in semiarid regions. Biological fixation, transfer and balance of nitrogen were determined on an irrigated yellow passion fruit orchard (Passiflora edulis Sims) intercropped separately with three cover crops: sunn hemp, Crotalaria juncea (L.); pigeon pea, Cajanus cajan (L.) Mill; and jack bean, Canavalia ensiformis (L.) DC. In a fourth treatment, legumes were not planted, but spontaneous vegetation was left to grow freely. The legumes were croped for 90 days in three lines (0.5 m apart) inside the passion fruit plant lines (2.5 m apart). Fixation and transfers were determined by the 15N natural abundance technique, using sunflower as a reference plant. The three planted legumes nodulated abundantly and fixed nitrogen in high proportions (between 50 and 90% of their N), forming symbiosis with bacteria naturally established in the soil. Jack bean produced more biomass than sunn hemp and pigeon pea, and as much as the spontaneous plants, of which 23% were legumes. The amounts of fixed N (150, 43, 30 and 29 kg ha-1) were determined mainly by the biomass of legumes. More than 40% of the N of passion fruit plants came from the biological nitrogen fixation of the intercropped jack bean, which provided an amount of N higher than that exported in the fruits, generating a positive balance of more than 100 kg ha-1. Therefore, it is recommended to intercrop jack bean in irrigated passion fruit orchards.

Biomass Production and Potential Fixed Nitrogen Inputs from Leguminous Cover Crops in Subtropical Avocado Plantations

Nitrogen (N) fertiliser is applied to perennial horticultural crops to increase yields, but subsequent N losses in subtropical plantations may be high due to intense rainfall and warmer temperatures. While legume cover crops could potentially contribute N to the tree crops and reduce fertiliser-N requirements, few studies have quantified potential fixed-N inputs from cover crops legumes in tropical or subtropical tree crop systems. To address this, we investigated growth and N fixation of summer-growing Pinto peanut (Arachis pintoi Krapov. &amp; W. C. Greg cv. Amarillo) and winter/spring dominant white clover (Trifolium repens L. cv. Haifa) grown as a mixed species cover crop in two commercial subtropical avocado (Persea americana Mill. cv. Hass) plantations. Legume biomass was assessed prior to mowing of the inter-row (fortnightly in summer and every 6–8 weeks over winter) and N fixation was estimated using the 15N natural abundance technique. Biomass production was 7377 kg ha−1 (930 kg ha−1 for white clover and 6447 kg ha−1 for Pinto peanut) at the first site over the 14-month period from December 2014 to January 2016, and 4467 kg ha−1 (1114 kg ha−1 for white clover and 3353 kg ha−1 for Pinto peanut) at the second site over the same period. Estimation of N fixation was not possible at the first site, due to a lack of difference in isotopic discrimination between the legume shoots and the reference plant (kikuyu (Pennisetum clandestinum Chiov.)) material. While legume shoots accumulated 157 kg N ha−1 (38 kg ha−1 for white clover and 119 kg ha−1 for Pinto peanut) across the season at site 1, the % N derived from atmosphere (%Ndfa) in legumes was relatively low (50–60% in Pinto peanut during the warmer months and around 30% in autumn and early spring, and from 13 % in April to 69% in September for white clover). The low %Ndfa in the legumes may have been due to low rainfall or molybdenum (Mo) deficiency. Ultimately the legume cover crops contributed an estimated 50 kg fixed N ha−1, which could partially offset fertiliser N requirements of the tree crop. Our results demonstrate the need to quantify N fixation in legume cover crops to assess potential N benefits as opposed to relying on typical measurements of legume biomass and N accumulation.