Search

Carbon isotopic analysis is a useful tool for investigating paleoenvironments, as the pedogenic carbonate δ13C is related to δ13CSOM and to the proportions of C3/C4 plants. In this work we interpreted the paleoenvironmental conditions at the time of carbonate precipitation in soils formed under different climates and during different geological ages. Samples were taken from a Bk (PR1, Holocene) and from two Bkm horizons (PR2 and PR3, Pleistocene). When the mean δ13C plant values and the most plausible paleotemperatures were used in the evaluation, PR1 showed a lower percentage of C4 plants (48%) than Pleistocene soils (~53%), in agreement with paleoclimate changes. When instead the δ13C values of current plants were used for PR1, C4 plants ranged from 59 (12°C) to 66% (18°C), suggesting two possible interpretations: either plant species changed during the Holocene, or the plant mean values normally used in the literature are not suitable for Pleistocene reconstructions, L'analyse des isotopes du C peut aider à l’évaluation des paléo-environnements puisque les δ13C des carbonates pédogénétiques sont liés au δ13CSOM et au rapport entre plantes C3 et C4. Avec cette technique on a déterminé les conditions paléo-environnementales dans des sols d’âges et sous climats différents, en utilisant un horizon Bk (PR1, Holocène) et deux Bkm (PR2 et PR3, Pléistocène). En effectuant l’évaluation avec les δ13C moyens des plantes et les paléo-températures les plus plausibles, PR1 montrait une proportion des plantes C4 (48%) inférieure à celle des sols du Pléistocène (~ 53%), en accord avec les changements paléo-climatiques. Les plantes C4, cependant, variaient de 59 (12°C) à 66% (18°C) lorsqu'on utilisait pour PR1 le δ13C des plantes actuelles. Ces résultats conduisent à deux interprétations possibles, soit que les espèces ont changé au cours de l'Holocène, soit que les δ13C moyens normalement utilisés dans la littérature ne sont pas appropriés pour la reconstitution du Pléistocène., L’analisi degli isotopi del C è un utile strumento per la valutazione dei paleoambienti poiché i δ13C dei carbonati pedogenetici sono relazionati ai δ13CSOM e al rapporto tra piante C3 e C4. Con questa tecnica sono interpretate le condizioni paleoambieantali al tempo di precipitazione dei carbonati in suoli formati in climi ed ere geologiche differenti, utilizzando un orizzonte Bk (PR1, Olocene) e due Bkm (PR2 e PR3, Pleistocene). Considerando i δ13C medi delle piante e le più plausibili paleotemperature, PR1 mostrava una più bassa proporzione di piante C4 (48%) rispetto i suoli pleistocenici (~53%), in accordo con i cambiamenti paleoambientali. Le piante C4, invece, variavano da 59 (12°C) a 66% (, EQA - International Journal of Environmental Quality; Vol 7 (2011); 17-24

The image analysis of soil thin sections is a very useful technique in studying the natural complexity of soil organic matter (SOM) and SEM-EDS technique can be further applied in mapping the distribution of the SOM forms on the basis of their chemical composition. On soil thin sections, bleaching with NaClO has been used to distinguish organic forms, and in the disturbed fine-earth fraction it is now a well established oxidative method used to isolate chemically stable SOM without affecting inorganic soil constituents. To investigate the relationship between microscale distribution of SOM and its stability in undisturbed soil samples, we combined optical and SEM-EDS analysis to the NaClO oxidation of soil thin sections. Moreover, to gain a better understanding on the distribution of different forms of SOM, we also took into account the soil pore system. The study involved five soils from Harwood Forest in Northumberland (UK) differently affected by water (two Gleysols and three Stagnosols), which is expected to influence SOM transformation in different ways. The image analysis indicated differences both in pore system and in SOM between the two soil types. In fact, unimodal distribution of pores was more evident for soil profiles showing stagnic properties, with one 10-50 μm modal pore class; while in the gleyic soils bimodality was detected, with an additional modal class around 200-500 μm. In both soils plant organs at different stage of decomposition and amorphous organic compounds were found, but in Stagnosols amorphous organic matter reddish in color was more frequent. The SEM-EDS elemental analysis confirmed that the reddish compounds were Fe-rich (mean Fe content 2.7%) and their distribution was related to the presence of pores smaller than 50 μm (r=0.90; p