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Abstract

Quantitative information about the amount and stability of organic carbon (OC) in different soil organic-matter (OM) fractions and in specific organic compounds and compound-classes is needed to improve our understanding of organic-matter sequestration in soils. In the present paper, we summarize and integrate results performed on two different arable soils with continuous maize cropping (a) Stagnic Luvisol with maize cropping for 24 y, b) Luvic Phaeozem with maize cropping for 39 y) to identify (1) the storage of OC in different soil organic-matter fractions, (2) the function of these fractions with respect to soil-OC stabilization, (3) the importance and partitioning of fossil-C deposits, and (4) the rates of soil-OC stabilization as assessed by compound-specific isotope analyses. The fractionation procedures included particle-size fractionation, density fractionation, aggregate fractionation, acid hydrolysis, different oxidation procedures, isolation of extractable lipids and phospholipid fatty acids, pyrolysis, and the determination of black C. Stability of OC was determined by C-13 and C-14 analyses. The main inputs of OC were plant litter (both sites) and deposition of fossil C likely from coal combustion and lignite dust (only Phaeozem). Total soil OC stocks down to a depth of 65cm (7.83kg m(-2) in the Luvisol and 9.66kg m(-2) in the Phaeozem) consisted mainly of mineral-bound OC (87% of total SOC in the Luvisol and 69% in the Phaeozem). In the Luvisol, free light particulate OM, OM associated with sand and coarse silt, and particulate OM occluded in macro-aggregates represented SOM fractions with mean turnover times shorter than that of the bulk soil OC (54 y). Additionally, the turnover of all individual compounds or compound classes (except for black carbon) was faster than that of bulk soil OC. These OM fractions that were less stable than the bulk soil OM made up 13% to 20% of the total OC. Organic matter in fine and medium silt and clay fractions, particulate OM occluded in micro-aggregates (53-250 mu m) and OM resistant to acid hydrolysis had intermediate turnover times of about 50-100 y. These fractions with intermediate turnover times contributed 70%-80% to total soil OC. Passive OM with turnover times >200 y was isolated from the mineral-bound OM by different oxidation procedures (H₂O₂, Na2S2O8) and made up <= 10% of the total OC. The isotopic signature of PLFAs suggests an efficient recycling of OC derived from C₃ substrate. In the Phaeozem, partitioning of maize-derived C exhibited a pattern similar to the Luvisol, but turnover rates of vegetation-derived soil OC were lower, probably because of the considerably smaller input of plant residues. Fossil C contributed approx. 50% to the total OC and accumulated preferentially in the particulate OM occluded in aggregates and in the fine-sand and coarse-silt fractions. It formed a large stock of passive soil OM but a minor part also entered the microbial C cycle. The results show that the partitioning of OC derived from vegetation and deposition of fossil compounds among soil fractions differed mainly because of their different bioavailability and recalcitrance. There was no evidence for a high recalcitrance of individual plant compounds. Mineral-bound OM resistant to oxidation by H₂O₂ and Na2S2O8 represented highly stable OC pools in both soils. [References: 106]