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The persistence of soil organic carbon (SOC) is influenced by soil physicochemical properties, organic matter quality, and climatic conditions that govern its vulnerability to microbial activity. We compared the susceptibility of newly formed SOC to mineralization in two soils (Andosols) that developed under contrasting precipitation regimes. Soil from the high rainfall region (‘highrain’) had higher SOC and lower iron concentrations than soils exposed to less rainfall (‘lowrain’). We amended soils with 13C-labeled carbohydrates and measured the amount of substrate-derived SO13C mineralized when exposed to dry-wet cycling following months-long incubations. We hypothesized that susceptibility would differ due to iron content and mineralogy, initial SOC, substrate solubility (cellulose versus glucose amendment), and microbial substrate use efficiency (SUE). We found that SO13C was less susceptible to dry-wet cycling when more 13C was assimilated into microbial biomass and co-localized with mineral surfaces than when co-localized with existing organo-mineral surfaces, according to microscale NanoSIMS imaging. Considerably less SO13C was susceptible to mineralization in the ferrihydrite-rich (low SOC) lowrain soil than the leached (high SOC) highrain soil when C was added as either glucose (7.3-fold less C mineralized) or cellulose (15.2-fold less). The SUE of glucose was comparable to cellulose in lowrain soil where SO13C was less water soluble and coprecipitated with ferrihydrite, and used half as efficiently as cellulose in highrain soil. Our results show that the susceptibility of newly formed SOC to mineralization is modified by the effects of bioavailability on microbial metabolism and the availability of mineral surfaces for forming new organo-mineral complexes.

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The high aggregate stability of Andosols and the direct effects of sample drying led to several inconsistencies during physical soil organic matter fractionation. We have determined that NaCl addition displayed little influence on clay dispersion. At the microscale, we observed the reaggregation of the clay fraction caused by freeze-drying. This issue was avoided by analyzing aliquots of soil suspension. Thus, we recommend reserving a small soil liquid aliquot to be subjected to microscopy analysis.