The initial rate of C substrate utilization and longer-term soil C storage

The initial reaction of microbial transformation and turnover of soil carbon inputs may influence the magnitude of longer-term net soil C storage. The objective of this study was to test the merit of the hypothesis that the more rapid substrates are initially utilized, the longer the residual products remain in the soil. We used simple model C compounds to determine their decomposition rates and persistence over time. Pure C-14 compounds of glucose, acetate, arginine, oxalate, phenylalanine, and urea were incubated in soil for 125 days at 24 degrees C. Total respired CO2 and (CO2)-C-14 was quantitatively measured every day for 15 days and residual soil C-14 after 125 days. The percent C-14 remaining in the soil after 125 days of incubation was positively and significantly correlated with the percent substrate utilized in the first day of incubation. The C-14 in the microbial biomass ranged from 4 - 15% after 15 days and declined through day 125, contributing significantly to the C-14 that evolved over the longer time period. Priming of C-12 soil organic matter (SOM) was negative at day 3 but became positive, reaching a maximum on day 12; the total increase in soil C from added substrates was greater than the primed C. The primed C came from C-12 SOM rather than the microbial biomass. This data supports the concept that the more rapidly a substrate is initially mineralized, the more persistent it will be in the soil over time.