Journal
SOIL BIOLOGY & BIOCHEMISTRY
Volume 169, Issue -, Pages -Publisher
PERGAMON-ELSEVIER SCIENCE LTD
DOI: 10.1016/j.soilbio.2022.108654
Keywords
Biological nitrogen fixation; Diazotrophs; nifH gene; Amino acids; Peat; GC-C-IRMS; N-15 probing
Categories
Funding
- SPECTRA (Soil Processes and Ecological Services in the Karst Critical Zone of Southwest China)
- NERC
- Newton Foundation [NE/N007603/1, NE/N007530/1]
- National Natural Science Foundation of China [41571130043]
- HEFCE SRIF
- University of Bristol
- National Environmental Isotope Facility (NEIF) [NE/V003917/1]
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Biological nitrogen fixation is vital to ecosystem functions, but direct quantification of nitrogen fixation rates has remained challenging. This study developed a novel method using analysis of soil amino acids to quantify biological nitrogen fixation in soils, providing previously unobtainable quantitative information.
Biological nitrogen fixation (BNF) performed by diazotrophs is vital to our understanding of ecosystem functions, as plant nitrogen (N) is commonly a limiting nutrient for primary productivity. However, significant limitations have remained in our knowledge of the controls and rates of this process, due to technical difficulties in directly quantifying nitrogen (N-2) fixation rates. To address this, we developed a novel compound-specific N-15-stable isotope probing method involving analysis of acid hydrolysable soil amino acids (AAs) by gas chromatography combustion-isotope ratio mass spectrometry (GC-C-IRMS) for the quantification of BNF in soils. By analysing N-15 enriched AAs (as N-acetyl, O-isopropyl derivatives), this new approach aimed to provide greater specificity compared to existing methods, and to contribute previously unobtainable quantitative information on the capture and flow of N-2 fixed in soils. Laboratory incubations using N-15(2) gas were carried out on surface peat over 15 days to obtain quantitative measures of N-2 fixation by the microbial community. Longer incubations with the addition of a glucose energy source significantly increased the level of N-15 enrichment, i.e. N fixed. The enhanced detection limits of N-15-AAs by GC-C-IRMS, compared to bulk soil delta N-15 value determinations, was key to assessments of N-2 fixation. Valuable insights into the assimilation pathway of the applied N-15(2)-substrate were revealed; for peat soils, N-15 incorporation into glutamate dominated over other AAs.
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