MICROBIAL N AND BIOMASS, RESPIRATION AND N-MINERALIZATION IN SOILS BENEATH 2 CHAPARRAL SPECIES ALONG A FIRE-INDUCED AGE GRADIENT

A decline in available nutrients may contribute to the loss of vigor observed in older chaparral stands. We examined N mineralization and the storage of C and N in the microbial biomass of soil along a fire-induced chaparral chronosequence in San Diego County, Calif. Soil was collected under chamise (Adenostoma fasciculatum H. & A.) and ceanothus [Ceanothus greggii Gray var. perplexans (Trel.) Jeps] shrubs in stands burned 0, 2, 4, 11, 20, 54 and 80 yr prior to the study. Soil collected from the top 5 cm beneath chamise and ceanothus had similar microbial biomass patterns with stand aging. The amount of microbial C in soil remained relatively constant across the age gradient. Concentrations of microbial N and the amount of N mineralized fluctuated, with no significant trend across the stand-age gradient. For both chaparral species, NH4+ concentrations in soil were significantly higher in the recently burned stand (age = 0) than in stands burned from 2 to 80 yr earlier. Ammonium and nitrate concentrations in soil were not significantly different in most instances among soils from 2 to 80 yr-old stands. Soil collected under A. fasciculatum had significantly higher respiration rates, and higher concentrations of NH4+ and NO3-compared to C. greggii, suggesting that these chaparral species have differential influences on microbial processes in the soil around them.