Chris M. Yeager,¹ Diana E. Northup,² Christy C. Grow,¹ Susan M. Barns,¹
and Cheryl R. Kuske¹*

M888 Bioscience Division, Los Alamos National Laboratory, Los Alamos, NM 87545.

Abstract

This study was undertaken to examine the effects of forest fire on two important groups of N-cycling bacteria in soil, the nitrogen-fixing and ammonia-oxidizing bacteria. Sequence and terminal restriction fragment length polymorphism (T-RFLP) analysis of nifH and amoA PCR amplicons was performed on DNA samples from unburned, moderately burned, and severely burned soils of a mixed conifer forest. PCR results indicated that the soil biomass and proportion of nitrogen-fixing and ammonia-oxidizing species was less in soil from the fire-impacted sites than from the unburned sites. The number of dominant nifH sequence types was greater in fire-impacted soils, and nifH sequences that were most closely related to those from the spore-forming taxa Clostridium and Paenibacillus were more abundant in the burned soils. In T-RFLP patterns of the ammoniaoxidizing community, terminal restriction fragments (TRFs) representing amoA cluster 1, 2, or 4 Nitrosospira spp. were dominant (80 to 90%) in unburned soils, while TRFs representing amoA cluster 3A Nitrosospira spp. dominated (65 to 95%) in fire-impacted soils. The dominance of amoA cluster 3A Nitrosospira spp. sequence types was positively correlated with soil pH (5.6 to 7.5) and NH₃-N levels (0.002 to 0.976 ppm), both of which were higher in burned soils. The decreased microbial biomass and shift in nitrogen-fixing and ammoniaoxidizing communities were still evident in fire-impacted soils collected 14 months after the fire.

Keywords:Nitrogen-Fixing,Ammonia-Oxidizing Bacterial Communities,T-RFLP,nifH,Nitrosospira spp.,Clostridium ,Paenibacillus, Organis farming.