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Key message: The proportional contribution of endophytic and phyllospheric diazotrophs to tree N nutrition can be quantified by 15N techniques. Observations of vigorous growth of Pinus spp. on unreclaimed gravel mining sites, together with measured net nitrogen (N) gains of 50 kg N ha–1 y–1 in sand culture, indicate the importance of alternative N sources when soil organic matter (SOM) cannot meet plant demand. Non-nodulated trees can respond positively to inoculation with N2 fixing bacteria under conditions of low N supply, suggesting that non-nodular symbiotic N2 fixation can make a significant contribution to tree nutrition, a well-established pathway for cereals, forage grasses and some industrial crops in N-limited environments. We reviewed the literature where non-isotopic approaches and techniques based on stable (15N) and radioactive (13N) tracers were used to estimate the relative contribution of endophytic and phyllospheric N2 fixation to the N economies of non-nodulated trees. The principal avenue of enquiry has so far involved determination of the response of trees to inoculation with diazotrophic bacteria. There is a need to obtain in situ measurements of the reliance of non-leguminous trees on endophytic and phyllospheric N2 fixation under a range of environmental and edaphic conditions. Current methodological obstacles have to be overcome to meet this challenge. Future research directions are suggested. [ABSTRACT FROM AUTHOR]

Methodologies based on 13C‐enrichment (E), 13C‐depletion (D) and 13C‐natural abundance (NA) to estimate the stability of biochar in soil were critically examined. The stability of 13C‐enriched biochar can be estimated by the quantitative recovery of excess 13C, either in the soil or in evolved CO2. Both approaches have advantages and disadvantages. Recovery in the soil is a measure of both residual biochar 13C + 13C immobilised in soil organic matter during biochar decomposition. Variable proportions of organic‐ and inorganic‐C are present in alkaline biochars, and few data exist on the uniformity of labelling, which is a basic requirement of the respired 13CO2 and E methodology. The E technique has had limited application due to the cost and difficulty of obtaining a uniformly‐enriched feedstock through continuous labelling of plants with 13CO2 at a constant 13C enrichment. In contrast, the NA technique has been widely applied. The NA and D techniques are in situ methods that involve the addition of C4‐derived biochar to a C3‐soil or vice versa. Stability is estimated by a two‐end‐member mixing model that allows the proportion of evolved CO2 derived from the biochar (Cdfb) to be estimated. The mixing model has recently been misused to estimate the Cdfb of 13C‐enriched biochar, with 13C‐abundance expressed as erroneously large δ values. 13C‐based methods provide a yardstick against which rapid stability tests should be evaluated. While numerous laboratory incubation comparisons have been conducted, very few field‐based data have been published. Highlights: 13C methods for estimating biochar stability are based on uniform isotopic labelling.Organic and inorganic constituents of biochar may not be uniformly labelled.Expression of 13C enriched biochar as large δ values (>500 units) rather than atom fraction excess led to larger errors in stability estimation.Few 13C field‐based estimates of biochar stability exist. [ABSTRACT FROM AUTHOR]

For many years after the discovery of the two stable isotopes of B (10B,11B) in 1920 it was not used routinely as a tracer in the biological sciences until the development of inductively coupled plasma mass spectrometry in the early 1980s. This development provided an impetus to the study of the role of B in plant nutrition, although many seminal discoveries were made beforehand without the aid of isotopes. Thus, much information on the mobility and physiological function of B in plants is post-1985. Boron is unusual in plant nutrition in that it is taken up by plants through roots or foliage as undissociated boric acid, unlike other essential elements that are taken up in ionic form. The within-plant mobility of B in species important in agriculture and horticulture is examined in the present review including the role of sugar alcohols in xylem to phloem transfer. The molecular basis of its movement at the cellular level via transporters as revealed through the strategic application of enriched 11B and 10B is addressed. Few studies have been carried out on B use efficiency by crops and results show great variability. Suggestions for future applications of tracing with 10B and 11B are provided. [ABSTRACT FROM AUTHOR]

Carnivorous plants have access to several potential sources of nitrogen, including root uptake, predation, litterfall, atmospheric deposition and defecation by mutualistic animals. Our aim was to assess the relative importance of different N sources so as to better understand the ecology of these physiologically diverse plants that include many genera and species inhabiting terrestrial and aquatic environments worldwide. Plant physiology and habitat were the major determinants of the relative importance of N source. Our secondary aim was to examine protocarnivorous plants that do not fit the exact definition for carnivory. Several protocarnivorous plants were classified as carnivorous based on specialised trapping mechanisms, isotopic data and mixing models. Several carnivorous plants can transfer their functions of prey capture and digestion to mutualistic animal partners, which is termed ecological outsourcing. Outsourcing arthropod prey capture and digestion to mutualistic bats is a beneficial strategy for the carnivorous plant Nepenthes hemsleyana. Carnivorous plants acquire nutrients by several mechanisms, including insect predation, root uptake, litterfall, atmospheric deposition and defecation by mutualistic animals. The relative importance of these nutrient sources for a range of terrestrial and aquatic species with a wide geographical distribution was estimated by 15N stable-isotope tracing. [ABSTRACT FROM AUTHOR]

The relative uptake by plants of the two ionic nitrogen (N) forms, ammonium (NH4+) and nitrate (NO3−), has been the subject of much interest during the past 50 years, resulting in a considerable scientific literature. The general idea is that plants have choice, resulting in preference for either one mineral N form or the other. Unfortunately, there is no specific definition of preference or agreement on how it should be measured. In this review, we critically examine the alternative techniques that have been used to measure the relative uptake of NH4+ and NO3− by plants, including those based on unlabelled sources, 15N‐enriched mineral N forms and variations in the 15N natural abundance of mineral N sources. The main difficulty with using unlabelled N is the antecedent N in plant tissue prior to the imposition of treatments. Although 15N‐enrichment overcomes this obstacle, it is nevertheless difficult to separate uptake as 15NH4+, uptake as 15NO3− and uptake as 15NO3− derived from the nitrification of 15NH4+. With 15N natural abundance, isotopic fractionation during plant uptake complicates the interpretation of data. There is increasing evidence that plants exhibit flexibility or plasticity with respect to the use of mineral N forms rather than preference. Highlights: The concept of plant preference for NH4+ or NO3− forms of mineral N is examinedExperiments using 15N‐enrichment and 15N natural abundance are reviewedThe direct uptake of 15NO3− is confounded with the uptake of 15NO3− derived from nitrified 15NH4+Plants exhibit plasticity rather than preference in the acquisition of ammonium and nitrate [ABSTRACT FROM AUTHOR]

The dynamics of C in terrestrial ecosystems can be traced on the basis of the marked differences in the δ13C signatures of plants possessing the C3, C4 or CAM photosynthetic pathways. When two C sources differing in natural 13C abundance (δ13C) contribute to a mixture, the relative contribution of each can be quantitatively apportioned by the use of a two-end-member mixing model. This ability of δ13C to partition source is unique among stable isotopes important in the biosphere, where with few exceptions, natural abundance signatures are used as qualitative rather than quantitative tracers because of isotopic fractionation. The many and varied applications of δ13C in apportioning C source are examined herein. They include estimation of soil organic matter turnover due to either a known or presumed change in vegetation cover, the contribution of organic materials to the soil C pool, the contribution of C source to vertebrate and invertebrate diets, disentangling roots in mixed C3-C4 stands, and partitioning the sources of respired CO2 in various C3-C4 mixtures, i.e., between plants and soil, between plants, between animal excreta slurries and soil, within fungal media and within biochar-amended soil. [ABSTRACT FROM AUTHOR]

For many years after the discovery of the two stable isotopes of boron (10B,11B) in 1920 they were not used as tracers in the soil–plant system due to analytical constraints. However, with the advent of inductively coupled plasma mass spectrometry in the early 1980s, it became possible to measure precisely the natural variations of the B isotopes in biological systems. Nevertheless, up to the present time there is very little information in the literature on δ11B variations in soils, plants, fertilizers and foodstuffs. Therefore, information on the potential of δ11B as a tracer of B dynamics in natural- and agro-ecosystems remains limited. The relative abundance of the boron isotope (δ11B) in soils, plants and nutrient sources, and applications of variations in the natural abundance of 11B as a tracer of anthropogenic sources of nitrate contamination of water and the provenance of agricultural products are reviewed. [ABSTRACT FROM AUTHOR]

The increased demand for organic farm products, and the attendant restrictions on the use of synthetic N fertilizers in both traditional and organic agriculture, has focused attention on the efficacy of organic sources of N, including animal excreta. However, questions arise in regard to the agronomic and environmental benefits of excreta as alternative sources of N. These questions are best addressed through the experimental use of 15N-enriched excreta, which enable the labeled source to be unequivocally traced through the soil-plant-atmosphere continuum, which N difference or N equivalence techniques cannot achieve. In contrast to organic and inorganic N compounds that can be purchased commercially with a specified 15N label on the component N moieties, 15N-enriched excreta must be prepared by the investigator. The methods of production and the uniformity of labeling of the components of 15N-enriched excreta (feces, urine) and their admixture (slurry), the N use efficiencies of excreta in crop production, and comparisons and interactions of excreta with synthetic N sources are reviewed. Losses of N from excreta-amended soils to the environment and the residual N value of excreta in crop sequences are also examined. It was concluded that while similar agronomic and environmental issues surround the use of both synthetic fertilizers and excreta as sources of N for plant nutrition, the processes differ in intensity and duration both spatially and temporally. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

After decades of searching for a rapid method to estimate the N mineralization capacity of soil, there is still no consistent recommendation. It is legitimate to examine the causes for the often-conflicting results in literature. The efficacy of various references that have been used as benchmarks for assessing chemical and physical indices in the literature is critically reviewed in this paper. Gross N mineralization and consumption during waterlogged and aerobic incubations were estimated in a wide range of soils. It was found that equivalent to 17 to 90 and 23 to 59% of the mineralized N was consumed during the waterlogged and aerobic incubations, respectively. As net N production rate represents the balance between N-producing and N-consuming processes, it appears difficult to find a simple method that could be used to predict the net effect of several concurrent processes. We used the gross N mineralization as a reference criterion for N mineralization ability. Total organic N, water-soluble organic N, alkali-hydrolyzable N, acid-hydrolyzable N, hot salt-hydrolyzable N and N in the light organic matter fraction were assessed against this reference criterion. All indices except light fraction N were significantly related to gross N mineralization. Water-soluble organic N had the highest correlation of all the indices tested. None of the chemically hydrolyzed N fractions consistently showed closer relationships with N mineralization than total organic N, suggesting that these chemical methods are ineffective in extracting a biologically labile fraction of soil organic N.

Quantitative estimates of BNF are needed to improve our understanding of the ecology of N in the environment and aid efforts to improve agricultural N management. Static models based on the principle of 15N isotope dilution have been proposed to estimate the proportion of N in a N2-fixing species that is derived from the atmosphere via biological N2 fixation. Furthermore, equations have been developed to quantify the movement of biologically fixed N between neighboring species or from legumes to cereals in crop rotations. The present paper is structured to provide a comprehensive overview of these methods in a logical and systematic manner. While the relevant literature is vast, some aspects have fortunately been covered by recent in-depth reviews which will be identified and briefly summarized. The overview will emphasize the more practical indirect methodologies based either on artificial 15N enrichment or 15N depletion, or alternatively on 15N natural abundance. In considering methods used to estimate symbiotic dependence, the major structural division is whether or not a non-N2-fixing reference plant is employed, and approaches taken to remove this source of error are described. Four examples are provided to illustrate the contemporary success of 15N-based methods, one in basic research involving endophytic BNF, and three in applied research involving legume breeding for enhanced BNF, the response of legumes to climate change and biotic and abiotic factors affecting legume symbiotic performance. [ABSTRACT FROM AUTHOR]

Variations in the relative isotopic abundance of C and N (δ13C and δ15N) were measured during the composting of different agricultural wastes using bench-scale bioreactors. Different mixtures of agricultural wastes (horse bedding manure + legume residues; dairy manure + jatropha mill cake; dairy manure + sugarcane residues; dairy manure alone) were used for aerobic-thermophilic composting. No significant differences were found between the δ13C values of the feedstock and the final compost, except for dairy manure + sugarcane residues (from initial ratio of −13.6 ± 0.2 ‰ to final ratio of −14.4 ± 0.2 ‰). δ15N values increased significantly in composts of horse bedding manure + legumes residues (from initial ratio of +5.9 ± 0.1 ‰ to final ratio of +8.2 ± 0.5 ‰) and dairy manure + jatropha mill cake (from initial ratio of +9.5 ± 0.2 ‰ to final ratio of +12.8 ± 0.7 ‰) and was related to the total N loss (mass balance). δ13C can be used to differentiate composts from different feedstock (e.g. C3 or C4 sources). The quantitative relationship between N loss and δ15N variation should be determined. [ABSTRACT FROM AUTHOR]