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Potential of Curie-point pyrolisis-GC-MS to monitor nitrogen transformations in a soil amended with 15N-enriched compost

AuthorsGonzález-Vila, Francisco Javier ; Tinoco, Pilar; Almendros Martín, Gonzalo ; Verdejo Robles, Trinidad ; Martín Martínez, Francisco
Issue Date1999
CitationLibro de Resúmenes. 9as. Jornadas de Análisis Instrumental. Expoquimia: 21 (1999)
AbstractOne of the most obscure points in the biogeochemical cycle of productive systems is the structural transfomations of N-forms in organic fertilizers and soil humic substances (humic acids, fulvic acids and humins). In fact, wet chemical analyses leaves up to 50% of the total N in compost and soil organic matter unidentified. This important methodological gap greatly limits the accurate molecular characterization of the N balance which is required for an efficient use of N from organic wastes and soil organic matter. The use of '5N as tracer isotope is the only way to monitor that balance since it allows continuous monitoring of exogenous N-forms incorporated to native organic matter pre-existent in soil. In this work we have intended to identify the N-compounds in compost and soil through the following experimental approach: i) 15N labelling fresh organic matter by incubation of a '5N-enriched mixture of city refuse compost (30%), wheat straw (64%) and 15NO3K (6%) for 80 days. The starting C/N ratio of the mixture was set to 20, where just 50% of the N was as 15N, ii) the labelled compost (1.6 g) was added to a soil free of organic matter (50 g), the moisture was adjusted to 66% of the water holding capacity and the mixture was incubated at 25 °C during 80 days, iii) by using standard laboratory procedures, both compost and soil were divided into different organic fractions: (a) water soluble, (b) alkali-soluble, acid-insoluble (humic acids), (c) acid-and alkali-soluble (fulvic acids) and (d) insoluble extraction residue (humin). In the case of the soil, a light material (free organic matter (e), of a density <1 g.cm-3) and a particulate, low-density humin fraction (f) (floating on a 1.8 g-cm-3 CHBr3—MeOH mixture) were obtained from the amended soil and its final extraction residue, respectively. Finally, (iv) different physico-chemical characteristics of these fractions, including their 14N/15N ratios have been analysed. Their molecular characterization have been performed by using Curie-Point pyrolysis combined with gas chromatography and mass spectrometry (Py-GC-MS). The whole molecular assemblages of pyrolysis fragments yields a fingerprint, which is diagnostic of the original sample. The patterns in the Curie-point pyrograms at 510 °C are very different between the various soil and compost fractions and reflects their different origin and transformation extent. In particular, peaks arising from polysaccharides, proteins and lipids are predominant in the more labile fractions isolated from soil and compost (a,c,e). It is noteworthy the great differences between the pyrograms of the colloidal humic materials (b); those from the soil are dominated by aromatic compounds. The insoluble particulate organic matter (f) include a substantial polyalkyl domain. The most abundant N-bearing pyrolysis products were pyrroles, indoles, pyridin derivatives and nitriles, and were mainly identified in the more labile fractions (a,c,e). The progressive disappearance in the pyrolytic patterns of diagnostic structural units of plant biomacromolecules and pigments, in addition to the differences found in N-speciation, can be used as criteria to establish long-term residual effects of sustainable soil remediation practices based in periodic inputs of compost.
DescriptionComunicación oral. Jornadas celebradas del 10 al 12 de noviembre, 1999, Barcelona.
Appears in Collections:(IRNAS) Comunicaciones congresos
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