Contrasting N fertilization of swine lagoon effluent (SLE) versus ammonium nitrate (AN) modified the activities of soil enzymes involved in soil C, N and P cycling in dissimilar manners.  We assume these differences stemmed from shifts in soil microbial community composition and catabolic activities.  In this study, microbial community structure and catabolic function were characterized using phospholipid fatty acids (PLFAs) and community-level physiological profile (CLPP).  Soils (0 – 7.5 cm depth) were sampled in March 2004 from field plots planted with bermudagrass or tall fescue that received 0 to 600 kg plant available N of SLE or AN ha^-1 y^-1 during 1999 – 2001. We observed that PLFA profiles differed significantly (P < 0.05) between soils fertilized with SLE versus AN, specifically at the highest application rate.  The PLFA profiles were strongly correlated with fertilization-associated change in soil pH (r = 0.80, P < 0.01).  Microbial community in acidic soils (i.e. soils with high rate of N fertilization) contained relatively low concentrations of monounsaturated fatty acids but high ratios of saturated-to-unsaturated fatty acids and cyclopropyl fatty acids-to-their monoenoic precursors.  CLPP also exhibited significant differences (P < 0.05) between soils fertilized with SLE and AN, suggesting that changes in microbial community structure was manifested in microbial catabolic function.  Again, the variations in CLPP were highly correlated with soil pH (P < 0.01).   The significant correlation of PLFA or CLPP profile with soil pH denotes that fertilization-associated change in soil pH was the key factor regulating soil microbial community composition and catabolic function in this managed forage production system.