Saturday, 15 July 2006

Study of the Stability of Organic Matter in Forest Soils from the Basque Country (N Spain).

Zuriņe Madinabeitia1, Marta Camps Arbestain2, and Felipe Macias1. (1) Univ de Santiago de Compostela, Dpto de Edafologia y Quimica Agricola, Facultad de Biologia, Santiago de Compostela, Spain, (2) NEIKER, Berreaga 1, Derio, Spain

The stability of soil organic matter is probably a more sensitive indicator than its concentration, of the decline in C stocks that may occur with changes in environmental conditions. The presence of refractory material in organic matter favours its persistence in soil, and its accumulation renders soils that are efficient reservoirs of organic C. The identification of the degree of stability of organic C forms in soil is fundamental as this will allow planning of future management activities. These activities should, whenever possible, favour the increment of organic C content in soils and also its stability. There are several methods of determining the stability of soil organic matter: (i) methods that identify the organic matter composition (carbohydrates, proteins, cellulosic polysaccharides, proteins, lignin), (ii) respirometric incubations, (iii) methods that identify the degree of humification of organic matter (e.g., pyrophosphate extraction, classical fractionation of organic matter), (iv) techniques that identify functional groups (e.g., FTIR, 13C-RMN), and (v) methods that determine the oxidability of organic matter (e.g., permanganate, dichromate, persulfate). In this study, the three latter methodologies were used to investigate the stability of organic matter in forest soils from the Basque Country (N Spain), and the results obtained are being related to other soil properties. Soil samples from the surface horizons of 44 soils under Fagus sylvatica L., and developed from a wide range of parent materials, were studied (limestones, sandstones, mudstones, marlstones, and basalt). The soils were air-dried and passed through a 2-mm sieve prior to analysis. The oxidability of soil organic C by KMnO4 (POC) was determined with 33 mM KMnO4 after different incubation times (1h, 3 h, 6 h, and 24 h). Analysis of soil organic C was also carried out using K2CrO7, with no external heating. Total organic C was determined by dry combustion, after elimination of carbonates, when present, by acid treatment. Organic C in sodium pyrophosphate was determined to estimate the humified C fraction. The Fourier transform infrared spectra of untreated soil samples were also obtained. For all soils, routine analyses (e.g., pH, total N, particle-size distribution) and other more specific analyses (e.g., acid ammonium oxalate extraction of Al, Fe and Si, dithionite-citrate extractable Fe, CuCl2-extractable Al and Fe) were carried out. The results obtained indicate that the cumulative amount of soil organic C oxidised by KMnO4 increased with time, following a logarithmic trend. The cumulative POC expressed as a fraction of soil organic C ranged from 3.7 to 12.5% after 1 h, from 3.9 to 17.5% after 3 h, from 6.7 to 22.5% after 6 h, and from 13.0 to 33.0% after 24 h. There was therefore a large fraction of soil organic C not oxidizable by KMnO4 after 24 h (>65%). In those beech soil samples with organic C contents >40 g kg-1, the mean ratio of POC/total organic C after 1 h incubation was ³8.5%, whereas that of soils with soil organic C contents <40 g kg-1 was <7.3%. Moreover, the results obtained to date suggest a relationship between the presence of short-range order Fe and Al oxy-hydroxides and aluminosilicate compounds and a low reactivity of organic C with permanganate, which may indicate the ability of this reagent to discriminate between the labile and non-labile organic carbon fractions in soils. We are still analysing some of the data obtained and once this is completed, we hope to be able to clarify the relationships between the low oxidability by KMnO4 of the soil organic C and the presence of short-range order Fe and Al oxy-hydroxides and aluminosilicate compounds.

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