Can the labile carbon contribute to carbon immobilization in semiarid soils? Priming effects and microbial community dynamics

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 (UC)-C-13-glucose (99 atom %) at concentration of 75 mu g C g(-1) soil (LD) or 300 mu g C g(-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 C-13 added compared to 30% of the initial C-13 added in the HD. Comparison of C-13 content in water- and sodium-pyrophosphate extracts pointed to a significant humification of up to 2.4% of the initial C-13-glucose. Glucose was subjected to an intense mineralization in the first 17-days of 22.8% and 40.94% for the ID 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 C-13 was predominantly found in fatty acids typical for Gram-negative bacteria. However, after 4 and 17 days the C-13-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. (C) 2012 Elsevier Ltd. All rights reserved.