Pairwise comparison of soil organic particle-size distributions in native savannas and Eucalyptus plantations in Congo Re ´mi d’Annunzio a,b, * , Sofian Conche c , Damien Landais d , Laurent Saint-Andre ´ b,e , Richard Joffre d , Bernard G. Barthe `s c a INRA, UMR 1092, LERFoB, Champenoux F-54280, France b CIRAD, UPR 80, Ecosyste `mes de Plantation, Montpellier F-34398, France c IRD, UR 179, SeqBio, Montpellier F-34060, France d CNRS, UMR 5175, CEFE, Montpellier F-34293, France e INRA, UR 1138, BEF, Nancy F-54280, France Received 21 May 2007; received in revised form 3 October 2007; accepted 7 October 2007 Abstract Conversion of native vegetation into fast-growing tree plantations is known to affect soil organic matter (SOM): soil carbon (C) and nitrogen (N) content and their distribution in particle-size fractions can be modified in various ways depending on numerous factors, such as soil properties, SOM levels prior to conversion, climatic conditions, silvicultural practices and fire occurrence. Since 1978, 43,000 ha of clonal eucalyptus plantations have been established on sandy coastal plains under savannas near Pointe-Noire, Congo. We investigated the effects of afforestation on topsoil (0–10 cm) C and N through the analysis of their distribution in particle-size fractions using a pairwise experimental design that compared adjacent savannas and plantations. The studied plantations were of different ages (2–30-year-old stands) and differently affected by accidental fires. No significant difference in total topsoil C, N or C/N was observed between young plantations and savanna. In old plantations that had not been affected by fire, total topsoil C content was twice as high as in savanna ( p ¼ 0:0016), on average, mostly involving fractions > 50 mm. By contrast, total topsoil N did not differ significantly at these sites. In old plantations affected by fire, total topsoil C content did not differ significantly from that in savanna, but total topsoil N was 26 % lower in plantations than in savanna ( p ¼ 0:0063), on average, and the decrease affected fractions < 200 mm especially. Whatever the fire occurrence, total topsoil C/N was higher in old plantations than in savanna, in fractions > 20 mm especially. # 2007 Elsevier B.V. All rights reserved. Keywords: Soil organic matter; Particle-size fractionation; Savanna; Eucalyptus; Fire 1. Introduction Soil organic matter (SOM) is an important determinant of soil fertility and productivity, and is a key factor in the global carbon cycle. Indeed, SOM affects soil chemical, physical, and biological processes, and thus fulfils a wide range of functions. It is an ion exchange material, it promotes the formation of soil aggregates and thereby influences water infiltration, water- holding capacity and root penetration, and it is a source of energy and nutrients for many soil organisms (Fernandes et al., 1997; Craswell and Lefroy, 2001). Moreover, SOM is an essential reservoir of carbon (C), including a larger C pool than the combined atmosphere and vegetation pools, and is thus a critical component of the global C balance (Lal et al., 1995). In fast-growing tree plantations prone to nutrient deficiencies (Bouillet et al., 2004), the role of SOM is even more important as it represents a major nitrogen (N) source through litterfall and decomposition cycles (Nzila et al., 2002). Particle-size fractionation has proven to be a valuable tool in studies about SOM dynamics and the effects of land use change (Christensen, 2001). It separates SOM pools that are in different states of decay and have different decomposition rates, fine fractions being generally more decomposed and more stable than coarser ones (Balesdent, 1996). Land use changes (e.g. conversion of native vegetation into cropland or plantations) strongly affect SOM. Studying conversion of pasture into forest, Paul et al. (2002) outlined www.elsevier.com/locate/foreco Available online at www.sciencedirect.com Forest Ecology and Management xxx (2007) xxx–xxx * Corresponding author at: INRA, UMR 1092 LERFoB, Route d’Amance, 54280 Champenoux, France. E-mail address: dannunzio@nancy.inra.fr (R. d’Annunzio). + Models FORECO-10707; No of Pages 7 0378-1127/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.foreco.2007.10.027 Please cite this article in press as: d’Annunzio, R., et al., Pairwise comparison of soil organic particle-size distributions in native savannas and Eucalyptus plantations in Congo, Forest Ecol. Manage. (2007), doi:10.1016/j.foreco.2007.10.027