3:15 PM - 3:30 PM
[MIS03-13] Organic matter stabilization in Andisol and Ultisol revealed by isotopic tracer experiment and density fractionation
Different types of soils were sampled from two agricultural lands; a volcanic-ash soil (Andisol) from Japan and a highly-weathered tropical soil (Ultisol) from Indonesia. The incubation experiments were carried out after addition of 13C-labelled glucose (99 13C atom%, 0.1915 mmol 13C g 1soil as solution) or 13C, 15N-labeled glutamic acid to the soils (2-mm sieved, 5 g dry weight). The soils were incubated for 276 d at 30oC and 50% water holding capacity. After the incubation, soil was separated into three fractions according to its density using sodium polytungstate as heavy liquid: low (<1.8 g cm-3), middle (1.8-2.25 g cm-3 for Andisol, 1.8-2.5 g cm-3 for Ultisol), high (>2.25 g cm-3 for Andisol, >2.5 g cm-3 for Ultisol) density fractions. We measured the mass, isotopic ratios (13C/12C, 15N/14N) and total C and N concentrations of the density fractions as well as the amount of CO2 respired during the incubation by alkali trap method. We also measured the specific surface areas (SSA) of soil minerals and the concentrations of Al, Fe oxides/hydroxides.
For both soils, ca. 70 to 80 % of added 13C were mineralized to CO2 within 1 month after substrate addition. The density fractionation showed that 13C recovery in the low-density fraction was low (0.5 - 3.8%) throughout the incubation period. The 13C recovery within the mid- and high-density fractions was greater than 20%. This indicates that labile substrates were immediately incorporated into the mid- and high-density fractions through microbial processing in the both soils. The highest 13C recovery was observed in the mid-density fraction of Andisol and in the high-density fraction of Ultisol, respectively. MRTs of 13C in the density fractions positively correlated with SSAs for respective soil types. This can be explained by differences in mineralogy which contribute to OM stabilization through sorption; short-range-order minerals (e.g., allophane and imogolite) in Andisol and iron oxides in Ultisol, respectively. Our results support the hypothesis that newly-added OM is stabilized through association of microbial metabolites with mineral particles. However, dominant density class and turnover of stabilized OM could be variable depending on soil types and clay mineralogy with high specific surface areas.