Comparison of different soil organic matter fractionation methodologies: Evidences from ultrasensitive 14 C measurements Fabio Marzaioli a, * , Carmine Lubritto a , Ilaria Del Galdo a , Antonio D’Onofrio a , M. Francesca Cotrufo a,b , Filippo Terrasi a a CIRCE, Dipartimento di Scienze Ambientali, Seconda Università degli studi di Napoli and INNOVA, Via Vivaldi, 43, Caserta 81100, Italy b Department of Soil and Crop Sciences, Colorado State University, Fort Collins, Colorado, USA article info Article history: Available online xxxx PACS: 92.20.Xy 91.67.Qr 29.20.c Keywords: Soil organic carbon SOM turnover time Fractionation protocols Stabilization of soil organic matter Radiocarbon abstract Soils are studied with the aim to predict future climatic scenarios and find the best guidelines to manage terrestrial ecosystems for the mitigation of the atmospheric CO 2 rising. Carbon constituting soil organic matter (SOM) behaves as a cohort of different pools, characterized by a specific C turnover time. Both nat- ural and anthropogenic occurring 14 C reach the soil through plant littering, becoming a valid tool to trace SOM dynamics. In this study we present a series of Accelerator Mass Spectrometry (AMS) 14 C measurements on SOM samples obtained by means of different laboratory protocols used for the isolation of soil pools from bulk soil (fractionation protocols). Radiocarbon signature of SOM fractions is used as a keyhole to look at the more effective fractionation procedure and comparison among measured 14 C on SOM fractions revealed important indications for the proposal of a novel fractionation protocol. Our data put in evidence how particle size controls the recalcitrance of ancient SOM carbon pools. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Soil organic matter reservoir (1400–1600 PgC [1]) is determined by the net balance between inputs of C, mainly due to net primary production (56 PgC a 1 [2]), and outputs due to soil respiration (55 PgC a 1 [2]) and dissolved organic carbon export (0.4 PgC a 1 [2]). Experimental evidences indicate how terrestrial ecosystems, in re- sponse to the ongoing anthropogenic disturbances (i.e. fossil fuel burning, land use change), are increasing their C storing capacity. This net CO 2 sequestering flux (1.3 PgC a 1 [3]) make these ecosys- tems, and by consequence SOM (2/3 of the terrestrial C budget), cru- cial for the understanding of the earth feedbacks to the ongoing climate change. Even though look at soil reservoir is important, SOM dynamic parameters (i.e. carbon turnover time) are more indic- ative of the fate of this induced C sink. SOM can not be considered a homogeneous reservoir [4] and inventory measured C mean resi- dence times (32 years [5]) can lead to SOM dynamics misinterpreta- tions. SOM is a mixture of heterogeneous materials (i.e. plant, animal, and microbial) combined with a variety of decomposition products of different ages and complexity [6] interacting with soil minerals. One of the most used approach to study SOM dynamics consists of isolating soil pools (fractions) by means of laboratory pro- cedures (fractionation protocols). These protocols are mostly based on the main factors determining the SOM stabilization [7] during microbial processing of plant litter (roots and leaves) into the soil. Natural occurring carbon isotopes ( 13 C, 14 C) can reveal impor- tant clues about the dynamics of SOM fractions [8,9]. In particular both natural and anthropogenical produced radiocarbon can be used as a natural tracer of the rate C cycling in soils [10]. In this study we present main results arising from a series of 14 C measurements on bulk soils and SOM fractions achieved by means of two different conventional soil fractionation protocols relying on the density acid/base/acid [9] and aggregation [11] discrimination procedure. We use measured radiocarbon values on fractions to look at dis- persion around bulk soil radiocarbon signature with the aim of characterizing and comparing these procedures. Observed results drove us in the proposal and characterization of a novel fraction- ation procedure combining aggregation with density acid/base/ acid fractionation procedure. 2. Materials and methods Four and ten archived soils, sampled over the last 10 years, where fractionated according to the aggregation protocol and the 0168-583X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2009.10.098 * Corresponding author. Tel.: +390823274814; fax: +390823274605. E-mail address: fabio.marzaioli@unina2.it (F. Marzaioli). Nuclear Instruments and Methods in Physics Research B xxx (2009) xxx–xxx Contents lists available at ScienceDirect Nuclear Instruments and Methods in Physics Research B journal homepage: www.elsevier.com/locate/nimb ARTICLE IN PRESS Please cite this article in press as: F. Marzaioli et al., Comparison of different soil organic matter fractionation methodologies: Evidences from ultrasensitive 14 C measurements, Nucl. Instr. and Meth. B (2009), doi:10.1016/j.nimb.2009.10.098