Susceptibility of new soil organic carbon to mineralization during dry-wet cycling in soils from contrasting ends of a precipitation gradient

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 (‘high_rain’) had higher SOC and lower iron concentrations than soils exposed to less rainfall (‘low_rain’). 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) low_rain soil than the leached (high SOC) high_rain 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 low_rain soil where SO¹³C was less water soluble and coprecipitated with ferrihydrite, and used half as efficiently as cellulose in high_rain 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.