Compound-specific amino acid (15) N stable isotope probing of nitrogen assimilation by the soil microbial biomass using gas chromatography/combustion/isotope ratio mass spectrometry

Rapid, sensitive analysis method for determining the nitrogen stable isotope ratio of total free amino acids in soil

RATIONALE

The amino acid-nitrogen (AA-N) isotope analysis of naturally abundant or isotope-labeled samples is indispensable for tracing nitrogen transfer in soil nitrogen biogeochemical cycling processes. Despite the usefulness of AA-N isotope analysis, the preparation methods are complex and time-consuming, and necessitate the use of toxic reagents.

METHODS

We present an improved, rapid method for AA-N isotope analysis with high precision. At a high pH, AA-N was released and oxidized to N₂ O using ClO^- under vacuum. Additionally, purge-and-trap isotope ratio mass spectrometry was used to analyze N₂ O. Moreover, we investigated the effect of various factors on the N₂ O conversion process with glycine and applied the results to seven representative single-N AAs (alanine, serine, cysteine, aspartic acid, glutamic acid, leucine, and phenylalanine) and five poly-N AAs (lysine, arginine, histidine, tryptophan, and asparagine), as well as side-chain analogs, blank reagent, and other N forms.

RESULTS

The concentration of ClO^- and the pH were determined to be crucial factors for achieving desirable AA-N to N₂ O conversion efficiencies. Glycine-N had the highest N₂ O yield of 70%, with isotopic results consistent with those of the reference values at a high precision (within 0.5‰ for natural abundance and 0.01 atom% for ¹⁵ N-enrichment) at the nanomolar N level. Additionally, the α-NH₂ AAs were labile, and the single-N AAs were more easily converted to N₂ O than poly-N AAs. With the exception of γ-aminobutyric acid, the N₂ O conversion efficiencies of the side-chain N analogs were very low (below 5%). This method was also applicable to the ¹⁵ N analysis of the total free AAs in complex soil samples without interference from analytical blanks and other forms of N.

CONCLUSIONS

Our method is highly selective for the α-NH₂ groups of an amino acid, and the oxidation of the side chain is difficult. In addition, the method is sensitive, rapid, and convenient, and does not require toxic reagents.

Mechanisms of nitrogen transfer in a model clover-ryegrass pasture: a 15N-tracer approach

PURPOSE

Nitrogen (N) transfer from white clover (Trifolium repens cv.) to ryegrass (Lolium perenne cv.) has the potential to meet ryegrass N requirements. This study aimed to quantify N transfer in a mixed pasture and investigate the influence of the microbial community and land management on N transfer.

METHODS

Split root ¹⁵N-labelling of clover quantified N transfer to ryegrass via exudation, microbial assimilation, decomposition, defoliation and soil biota. Incorporation into the microbial protein pool was determined using compound-specific ¹⁵N-stable isotope probing approaches.

RESULTS

N transfer to ryegrass and soil microbial protein in the model system was relatively small, with one-third arising from root exudation. N transfer to ryegrass increased with no microbial competition but soil microbes also increased N transfer via shoot decomposition. Addition of mycorrhizal fungi did not alter N transfer, due to the source-sink nature of this pathway, whilst weevil grazing on roots decreased microbial N transfer. N transfer was bidirectional, and comparable on a short-term scale.

CONCLUSIONS

N transfer was low in a model young pasture established from soil from a permanent grassland with long-term N fertilisation. Root exudation and decomposition were major N transfer pathways. N transfer was influenced by soil biota (weevils, mycorrhizae) and land management (e.g. grazing). Previous land management and the role of the microbial community in N transfer must be considered when determining the potential for N transfer to ryegrass.

SUPPLEMENTARY INFORMATION

The online version contains supplementary material available at 10.1007/s11104-022-05585-0.

Soil organic nitrogen: an overlooked but potentially significant contribution to crop nutrition

BACKGROUND

For more than a century, crop N nutrition research has primarily focused on inorganic N (IN) dynamics, building the traditional model that agricultural plants predominantly take up N in the form of NO₃ ^- and NH₄ ⁺. However, results reported in the ecological and agricultural literature suggest that the traditional model of plant N nutrition is oversimplified.

SCOPE

We examine the role of organic N (ON) in plant N nutrition, first by reviewing the historical discoveries by ecologists of plant ON uptake, then by discussing the advancements of key analytical techniques that have furthered the cause (stable isotope and microdialysis techniques). The current state of knowledge on soil ON dynamics is analyzed concurrently with recent developments that show ON uptake and assimilation by agricultural plant species. Lastly, we consider the relationship between ON uptake and nitrogen use efficiency (NUE) in an agricultural context.

CONCLUSIONS

We propose several mechanisms by which ON uptake and assimilation may increase crop NUE, such as by reducing N assimilation costs, promoting root biomass growth, shaping N cycling microbial communities, recapturing exuded N compounds, and aligning the root uptake capacity to the soil N supply in highly fertilized systems. These hypothetical mechanisms should direct future research on the topic. Although the quantitative role remains unknown, ON compounds should be considered as significant contributors to plant N nutrition.

Development of Alditol Acetate Derivatives for the Determination of 15N-Enriched Amino Sugars by Gas Chromatography-Combustion-Isotope Ratio Mass Spectrometry

Amino sugars can be used as indices to evaluate the role of soil microorganisms in active nitrogen (N) cycling in soil. This paper details the assessment of the suitability of gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) for the analysis of ¹⁵N-enriched amino sugars as alditol acetate derivatives prior to application of a novel ¹⁵N stable isotope probing (SIP) approach to amino sugars. The efficient derivatization and cleanup of alditol acetate derivatives for GC was achieved using commercially available amino sugars, including glucosamine, mannosamine, galactosamine, and muramic acid, as laboratory standards. A VF-23ms stationary phase was found to produce optimal separations of all four compounds. The structure of the alditol acetate derivatives was confirmed using gas chromatography/mass spectrometry (GC/MS). For GC-C-IRMS determinations, implementation of a two-point normalization confirmed the optimal carrier gas flow rate to be 1.7 mL min^-1. Linearity of δ¹⁵N value determinations up to δ¹⁵N_t of 469 ± 3.1‰ (where δ¹⁵N_t is the independently measured δ¹⁵N value) was confirmed when 30 nmol N was injected on-column, with the direction of deviation from δ¹⁵N_t at low sample amount dependent on the ¹⁵N abundance of the analyte. Observed between- and within-run memory effects were significant ( P < 0.007) when a highly enriched standard (469 ± 3.1‰) was run; therefore, analytical run order and variation in ¹⁵N enrichment of analytes within the same sample must be considered. The investigated parameters have confirmed the isotopic robustness of alditol acetate derivatives of amino sugars for the GC-C-IRMS analysis of ¹⁵N-enriched amino sugars in terms of linearity over an enrichment range (natural abundance to 469 ± 3.1‰) with on-column analyte amount over 30 nmol N.