Searching for an indicator of N evolution during organic matter decomposition based on amino acids behaviour: a study on litter layers of pine forests

Rovira P, Kurz-Besson C, Hernàndez P, Coûteaux M-M, Vallejo VR
Plant Soil (2008) 307: 149.

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Soil microflora can directly take up the amino acids (Aas) released by decomposing plants and use them, together with de novo synthesized Aas, as building blocks for their own structures, which are expected to have an Aa composition that differs from plant-derived structures. The degree of change in the Aa composition during OM decomposition should reflect the degree of N evolution. Here we apply this concept to the study of three litter decompositionn sequences in European forests: boreal (Pinus sylvestris L., Jädraås, Sweden), cool Atlantic (Pinus sylvestris L., La Gileppe, Belgium), and warm Atlantic (Pinus pinaster L., Montemor, Portugal). Litter was sorted into six decomposition stages according to morphological features: from intact, light-brown needles to dark needle fragments with faunal perforations. At each stage, the Aa composition of the litter was studied by acid hydrolysis plus liquid chromatography of the hydrolysates. Both the Aa content of the litter and the unhydrolyzable/total OC ratio increased with decomposition, but this was not the case with the unhydrolyzable/total N ratio. There is no sole pattern of Aa change with decomposition: in Jädraås and Montemor most of the changes occur in the initial and final steps, while in La Gileppe the changes seem quite evenly spread throughout the decomposition. The fact that the Aa composition of the litter did not converge with decomposition suggests the existence of site-specific biotas with contrasted Aa fingerprints. The abundance of a given Aa relative to the total differed among Aas and sites; but for some Aas it was possible to detect common patterns of decomposition behaviour (either a consistent increase or a consistent decrease). The aa/total ratio for Gly, Ala and Thr consistently increased with decomposition. These Aas were used to search for a numerical index, the Aa signature (AaSIG), which would reflect the changes in Aa composition from fresh debris to highly decomposed organic materials. The change in the proposed AaSIG was found to correlate well with the mathematical distance from the starting point (calculated as the euclidean distance); this suggests that it could be potentially useful as an indicator of N evolution during litter decomposition, at least in its first stages.