Immobilization of ¹⁵N in forest litter studied by ¹⁵N CPMAS NMR spectroscopy

Abstract

Solutions labelled with I5N were applied as ('5NH4)2S04 or KI5NO3 to isolated microplots in the floor of mountain beech forest (Nothofagus solandri var. clifortioides) and incubated for 135 days under field conditions of moisture and temperature. Solid state 15N CPMAS NMR spectra of the forest litter layer showed that more than 80% of the total signal intensity was attributable to the secondary amide-peptide peak. The degree of 15N enrichment or form of N did not alter the relative intensity of signals attributable to 15N in peptides, nucleic acids and aliphatic amine groups (amino sugars and free NH2 on amino acids). Combinations of I3C and I5N-NMR spectra, edited by a process that exploited differences in proton spin properties between distinct categories of organic matter, indicated incorporation of 15N in humified organic matter rather than partly degraded plant material. This application demonstrated that solid state I5N CPMAS NMR has potential for use in studies of N immobilization under field conditions and with materials containing little N and small I5N enrichment.

Decomposition of 15N-labelled beech litter and fate of nitrogen derived from litter in a beech forest

“…As throughfall nitrate and ammonium can be incorporated at rates of approximately 1 kg N ha -1 year -1 in decomposing litter (Buchmann et al 1995;Downs et al 1996;Koopmans et al 1998), throughfall N could account for approximately 30-50% of the external N incorporation. 15 N-labelled nitrate or ammonium applied to beech litter is incorporated mostly via biological processes into peptides and other compounds, as detected by 15 N nuclear magnetic resonance (Clinton et al 1995). Altogether, N from fungi, bacteria and throughfall could account for most of the external N incorporated in the decomposing beech litter at Aubure.…”

Forest succession, soil carbon accumulation, and rapid nitrogen storage in poorly remineralized soil organic matter

“…We have no data on the molecular forms of the stable tracer N we observed, but some of it was perhaps stored in amides, functional groups that serve as peptide bonds. Tracer 15 N is transferred to amides from quite different soil amendments, including mineral N (Clinton et al 1995, Morier et al 2008), plant residue N (DiCosty et al 2003, and charred organic matter N (de la Rosa and Knicker 2011), so this transfer appears to be pervasive. Moreover, the amide signature appears stable, as it is present in the organic matter of all soil physical fractions, and remains constant through time even while the N concentrations of those fractions are changing (DiCosty et al 2003).…”

Determination of T1ρH Relaxation Rates in Charred and Uncharred Wood and Consequences for NMR Quantitation

“…T 1r H is also an informative parameter for chemical and structural characterization, since it is sensitive to both molecular mobility (see, for example, [7,[25][26][27][28][29][30][31]) and the presence of paramagnetic impurities (see, for example, [8,9,17,18,[32][33][34][35]). For samples that exhibit non-uniform T 1r H behaviour, proton spin relaxation editing (PSRE) can be used to generate subspectra of components with different T 1r H values [26,30,33,34]. PSRE based on differences in T 1 H rather than T 1r H has already proven successful in differentiating char and non-char components in de-ashed soils [36].…”