Black carbon contribution to soil organic matter composition in tropical sloping land under slash and bum agriculture

Tropical soils are generally depleted in organic carbon (OC) due to environmental conditions favouring decomposition and mineralisation of soil organic matter (SOM). In Northern Laos, sloping soils are subjected to slash and bum agriculture, which leads to production of black carbon (BC), a stable SOM fraction. BC may directly influence the quantity and quality of SOM sequestered in tropical soils. The aim of this study was to quantify BC content and evaluate its impact on the chemical and stable isotope composition of SOM along a catena composed of Dystrochrepts at the bottom of the slope, Alfisols (midslope) and Inceptisols at the top of the slope for different burning frequencies. Six soil profiles, situated on a slope ranging from a river bank to the summit of a hill, were sampled. The stable isotope compositions (C-13 and N-15) of samples from both organo-mineral A and mineral B and C horizons were determined. The chemical composition of SOM analysed by C-13 CPMAS NMR spectroscopy and the contribution of BC determined by dichromate oxidation were compared to OC and iron oxide content as well as land management including the burning cycle. The highest C contents were recorded a, midslope positions, At any position on the slope, delta(13)C and delta(15)N ratios showed an enrichment in 13 C and 15N with increasing soil depth. The OC content of soil horizons was related to their aryl C content, which is the component most likely driven by BC inputs. The BC contributions analysed by dichromate oxidation ranged from 3% to 7% of total OC. A positive correlation was obtained between aryl C and the BC content of SOM. Comparison of BC content and stable isotope composition of SOM showed that BC influenced the delta(13)C and the delta(15)N stable isotope ratios of these soils. BC was not associated with the mineral phase. The highest BC contents were measured under intensive slash and bum practice in the vicinity of the boundary of Alfisols at the top of the slope, where erosion was severe. Therefore, BC, a SOM component strongly influencing OC sequestration of these soils, is susceptible to translocation down the slope. (c) 2005 Elsevier B.V. All rights reserved

Carbon-13 natural abundance as a tool to study the dynamics of lignin monomers in soil: an appraisal at the Closeaux experimental field (France)

International audiencePlant residues incorporated into soils are subjected to contrasted stabilization and biodegradation processes and may contribute to pools of soil organic matter (SOM) displaying different turnover times. Little is known about the relationship between the chemical structure of plant macromolecules and their long-term turnover in soils. Our research objective was to quantify the in situ turnover of phenols derived from lignin, which is a major component of plant tissues often considered slowly biodegradable relative to total plant organic matter. In this study, we used natural 13C labeling of SOM generated by a 9-year chronosequence of maize C4 crop (δ13C around −12‰) replacing the previous wheat C3 crop (δ13C around −27‰) at the Closeaux experimental field, in France. Here we present the combined applications of CuO oxidation and gas chromatography coupled via a combustion interface to an isotope ratio mass spectrometer (GC/C-IRMS) to follow variations in the isotopic composition of lignin-derived monomers in soils of the wheat–maize transition chronosequence. This study aimed at evaluating: (1) the precision and repeatability of this molecular-level isotopic tracer technique, and (2) its potential for computing the proportion of newly derived C in each lignin phenol. Nine years of maize cropping influenced neither the total organic carbon (OC) content nor the lignin content and biodegradation degree in soil. The total SOM after 9 years of maize cropping was significantly enriched in 13C by 1.4‰ compared to the wheat soil. In the CuO-derived lignin phenols, the variations of 13C contents after 9 years of maize cropping ranged from 4.9‰ to 10.0‰, with an average value for total lignin of 7.3‰. These variations were well above the precision of the analytical method, calculated for maize plant and soils to be in the range 0.2–0.8‰. This study thus demonstrates that the CuO oxidation technique is applicable to the determination of the natural isotopic abundance variations in lignin monomers of soils of a C3/C4 chronosequence. Finally, we could calculate the proportion of newly derived OC after 9 years of maize cropping. This proportion was 9% for total SOM and 47% for lignin, which displayed faster dynamics in this soil than total OC. This study confirms, using in situ labeling technique in combination with lignin monomers analysis after CuO oxidation, that lignin macromolecules are not stabilized as such in these soils

Frações lábeis e recalcitrantes da matéria orgânica em solos sob integração lavoura-pecuária

O objetivo deste trabalho foi avaliar o estoque de carbono orgânico total (COT) e nitrogênio total (NT), C e N nas frações lábeis e recalcitrantes da matéria orgânica do solo (MOS) e o índice de manejo de C, em solos sob sistema de integração lavoura-pecuária (ILP), após quatro (ILP4) e oito anos (ILP8) de implantação. Para comparação, foram utilizados outros sistemas de manejo: pastagem e lavoura em plantio direto (PD), além de vegetação nativa. O experimento foi realizado em delineamento inteiramente casualizado, com cinco repetições. A integração lavoura-pecuária, estabelecida num período de oito anos, é capaz de alcançar um novo estado estável equivalente ao sistema sob plantio direto com 23 anos de implantação. Houve acúmulo no estoque de C total de 101,0 (ILP8) e 104,2 Mg ha-1 (PD) e N total de 5,5 (ILP8) e 5,8 Mg ha-1 (PD), na camada 0-30 cm, bem como aumento no estoque de C e N nas frações lábeis e recalcitrantes da MOS. Para o ILP com oito anos, o índice de manejo de C de 88 foi superior ao dos demais sistemas de manejo e não diferiu da vegetação nativa na camada 0-10 cm, todavia, foi similar ao PD na análise de todo o perfil (0-30 cm)