Can the labile carbon contribute to carbon immobilization in semiarid soils? Priming effects and microbial community dynamics Felipe Bastida a, * , Irene F. Torres a , Teresa Hernández a , Petra Bombach b , Hans H. Richnow b , Carlos García a a Department of Soil and Water Conservation, CEBAS-CSIC, Campus Universitario de Espinardo, Murcia 30100, Spain b UFZ-Helmholtz Centre for Environmental Research, Department of Isotope Biogeochemistry, Permoserstr.15, 04318 Leipzig, Germany article info Article history: Received 2 August 2012 Received in revised form 26 October 2012 Accepted 29 October 2012 Available online 15 November 2012 Keywords: Semiarid soil Carbon cycling Glucose Priming effect Stable isotope probing Phospholipid fatty acids abstract Tracer experiments with isotopic-enriched carbon compounds can provide information regarding the carbon cycling in semiarid soils. We studied priming effects and microbial utilization of glucose as an example of bioavailable labile molecule in the carbon cycle of a semiarid soil. The soil, which has low content of total organic carbon (5.0 g kg 1 ), was amended with U 13 C-glucose (99 atom %) at concen- tration of 75 mgCg 1 soil (LD) or 300 mgCg 1 soil (HD). Glucose-derived carbon remained in soil after two months of incubation. The percentage of residual carbon stabilized was greater in LD with 40% of the initial 13 C added compared to 30% of the initial 13 C added in the HD. Comparison of 13 C content in water- and sodium-pyrophosphate extracts pointed to a significant humification of up to 2.4% of the initial 13 C- glucose. Glucose was subjected to an intense mineralization in the first 17-days of 22.8% and 40.94% for the LD and HD, respectively. The stable isotope probing (SIP) of phospholipid fatty acids (PLFAs) by gas- chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) showed that bacteria dominated glucose metabolism in comparison to fungi. Gram-negative populations were initially more involved in glucose assimilation than Gram-positive bacteria. In the fatty acids fraction, up to 95% of the 13 C was predominantly found in fatty acids typical for Gram-negative bacteria. However, after 4 and 17 days the 13 C-enrichment in Gram-positive biomarkers increased. The mineralization of soil organic matter triggered by glucose additions was more intense in HD (3.6% of soil TOC) than LD (1.0% of soil TOC) and reached the highest level after 4 days in HD. Priming was controlled by Gram-negative populations but fungi and, particularly actinobacteria played an important role in latter steps. Our data indicated that the intense metabolism of SOM due to priming phenomena compromises the potential carbon sequestration in this semiarid soil amended with glucose. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Soils developed in arid and semiarid conditions are character- ized by very low organic matter content (OM) from the scarce plant biomass (García et al., 1994) and it has been proposed that these soils have a big potential for carbon sequestration (Lal, 2004). However, such carbon sequestration would depend on the type of organic substrates, soil type and microbial community. Within organic carbon, labile compounds such glucose are quickly assim- ilated by microbial biomass (Perelo and Munch, 2005; Fischer et al., 2010). In boreal and temperate climates, isotopic evidences found in experiments with 13 C- or 14 C-labeled glucose indicate that carbon added to soils is not completely mineralized (Blagodatskaya et al., 2007; Schneckenberger et al., 2008; Fischer et al., 2010). However, the extension of glucose mineralization in semiarid soils has not been fully examined yet. While a major part of the released CO 2 after organic amend- ment is supposed to derive from the mineralization of the exog- enous organic matter added to soil, an additional CO 2 fraction may come from the mineralization of autochthonous soil organic matter as a consequence of increased microbial activity. This last mechanism, which is called “priming effect” (Kuzyakov, 2006), has been proved in temperate and boreal soils with high OM content and even in soils with low total carbon content (Hoyle et al., 2008), but not in soils from semiarid areas with a very low OM content in a pre-desertic state. In such environments a reduction of soil organic matter as a consequence of priming may have ecological significance. * Corresponding author. Tel.: þ34 968396106; fax: þ34 968396213. E-mail address: fbastida@cebas.csic.es (F. Bastida). Contents lists available at SciVerse ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soilbio.2012.10.037 Soil Biology & Biochemistry 57 (2013) 892e902