Soils and vegetation were analyzed in 20 lodgepole pine (Pinus contorta) forest stands, varying in age from 50 to 350 years, that had initiated following stand-replacing fire. Our goal was to determine how nitrogen availability (NH4+-N) and microbial community composition varied with stand age-class and to determine whether differences could be explained by canopy, soil, or understory characteristics. Gross NH4+ mineralization was measured using laboratory isotopic pool dilution, and microbial community composition was evaluated using microbial membrane lipids. The microbial community composition of stands in the 300-350 age class was distinct from stands in younger age classes. Microbial community composition among sites varied with pH, % organic matter, and phosphorus. Gross NH 4+ mineralization rates averaged 1.45±0.07 mg NH4+ kg soil-1 d-1 while consumption averaged 1.37±0.20 mg NH4+ kg soil-1 d-1, resulting in low net NH4+ mineralization rates (0.08±0.18 mg NH4+ kg soil-1 d-1), but rates were not significantly different with stand age-class at p<0.05. At p<0.10, net NH4+ mineralization was significantly higher in the 300-350 age class compared to the 125-175 age class. None of the measured variables significantly explained NH4 + consumption and net mineralization patterns. However, gross NH 4+ mineralization rates were best explained by information on microbial community structure (i.e. lipids). Variation among stands within a given age-classes was high, indicating that patterns of N cycling across landscapes reflect substantial heterogeneity among mature stands.
All Science Journal Classification (ASJC) codes
- Soil Science