Soil carbon dynamics inferred from carbon isotope compositions of soil organic matter and soil respiration

To better understand 14C cycling in terrestrial ecosystems, 14C abundances were evaluated for fractionated soil organic matter (SOM) and soil respiration in an urban forest. In 2001 soil profile, Δ 14C values of litter and bulk SOM increased rapidly from litter surface (62.7 per mille) to uppermost mineral soil layer (244.9 per mille), and then decreased sharply to 6 cm depth of mineral soil (125.0 per mille). Carbon enriched in 14C by atmospheric nuclear weapons testing had penetrated to at least 16 cm depth of mineral soil. The average Δ 14C in atmospheric CO2 was 58.8 per mille in August 2001, suggesting recent carbon input to the topmost litter layer. Although a similar depth distribution was observed for Δ 14C values of residual SOM after acid hydrolysis, the Δ 14C values were slightly lower than those in bulk SOM. This indicates input of 'bomb' C into this organic fraction and higher 14C abundance in acid-soluble SOM. The most of CO2 may be derived from the microbial decomposition of the acid-soluble, or labile, SOM. Therefore, the labile SOM may become most influential pool for soil carbon cycling. In contrast, carbon in base-insoluble SOM remained considerably low in 14C abundance at all depths, suggesting no or little incorporation of 'bomb' C to this fraction. Values of Δ 14C in soil respiration ranged from 91.9 to 146.4 per mille in August 2001, showing a significant contribution from decomposition of SOM fixed over past 2-40 years. These results indicate that the use of bulk SOM as a representative of soil carbon pool would lead to severe misunderstand of the soil C dynamics on decadal and shorter time scales. (author)