Saturday, 15 July 2006

Quantity and Chemical Composition of Dissolved Organic Carbon (Doc) in B and C Horizons of an Arable Soil.

A. Lavaud1, A. Chabbi Sr.2, C. Rumpel2, V. Jacquemet3, E. Gherman3, and J.-P. Croué Sr.4. (1) LCEE-CNRS-Univ de Poitiers, Poitiers, France, (2) INRA, Les Verrines, UEFE, Lusignan 86600, France, (3) Anjou Recherche-Véolia Water Maisons Laffitte, Paris, France, (4) LCEE-CNRS-Université de Poitiers, Poitiers, France

Land use and land management affect SOM dynamics and influence its stabilization. They are often considered a key mechanism for C sequestration in soil. In many ecosystems, dissolved organic carbon plays an important role in transport and stabilization of soil organic (SOM). Despite its importance for the microorganism activities and C losses from the system by lixiviation mechanism, DOC produced in agricultural soil is neither in the studies on OM stabilization nor in the models of global carbon cycle taken into account. Generally, DOC represents a small fraction of the total SOM, its movement however, through the soil profile can represent an important flux to the ground water. Information on the quantity and chemical composition of DOC in agricultural soils remains scarce because the DOC dynamic studies were conducted in forest ecosystems and not agroecosystems. In this study, we determine (i) the quantity of DOC in A, B and C-horizons of agricultural soil and (ii) the chemical composition of this DOC. The soil horizons were sampled from a cambisol profile at the long term observatory for environmental research, site of Lusignan, France. The top horizon is silty whereas B and C-horizons are clay dominated. The total organic carbon of the top horizon is around 10 mg g-1 and 3 to 2.5 mg g-1 respectively in B and C-horizons. To quantify the quantity of DOC from different soil horizons, 3 sequential extraction methods were applied: 1st cold water, 2nd hot water and 3rd ultrasonic dispersion using a probe-type ultrasonic disintegrator. The three extracts were centrifuged at 16000 rpm and the supernatant were filtrated through 0.45 µm before analysis. The filtered extract was then analyzed for its DOC content and chemical composition using UV254nm, fluorescence EEM and high performance size exclusion chromatography couple to a double UV and DOC detection (HPSEC/UV/COD). All the three extract, regardless of the extraction methods, showed that the DOC content decreased dramatically with soil depth. More DOC however, was extracted with the ultrasonic method. Spectrophotometry and spectroflurimetry analysis showed that DOC of the B and C horizons is less aromatic compared to DOC from the top horizon. The HPSEC/UV/COD showed that a large part of the DOC from B and C contained non-aromatic low molecular weight compounds. This corresponds well to previous observations, which suggest that SOM in subsoils is composed of small molecules, which are highly mobile.

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