Fire effects on stable isotopes in a Sierran forested watershed

This study tested the hypothesis that stable C and N isotope values in surface soil and litter would be increased by fire due to volatilization of lighter isotopes. The hypothesis was tested by: (1) performing experimental laboratory burns of organic and mineral soil materials from a watershed at combinations of temperature ranging 100 to 600 degrees C and duration ranging from 1 to 60 min; (2) testing field samples of upland soils before, shortly after, and 1 yr following a wildfire in the same watershed; and (3) testing field soil samples from a down-gradient ash/sediment depositional area in a riparian zone following a runoff event after the wildfire. Muffle furnace results indicated the most effective temperature range for using stable isotopes for tracing fire impacts is 200 to 400 degrees C because lower burn temperatures may not produce strong isotopic shifts, and at temperatures >= 600 degrees C, N and C content of residual material is too low. Analyses of field soil samples were inconclusive: there was a slightly significant effect of the wildfire on delta N-15 values in upland watershed analyses 1 yr postburn, while riparian zone analyses results indicated that delta C-13 values significantly decreased similar to 0.71 parts per thousand over a 9 mo post-fire period (p = 0.015), and ash/sediment layer delta C-13 values were similar to 0.65 parts per thousand higher than those in the A horizon. The lack of field confirmation may have been due to overall wildfire burn temperatures being < 200 degrees C and/or microbial recovery and vegetative growth in the field. Thus, the muffle furnace experiment supported the hypothesis, but it is as yet unconfirmed by actual wildfire field data.