Potentially Available Nitrogen and Phosphorus Along a Chaparral Fire Cycle Chronosequence

The objectives were to examine the effect of time, aspect, and soil horizon on N and P availabilities across a 4‐ to 85‐yr‐old post‐fire chaparral chronosequence. Nitrogen and P availabilites were measured with plant bioassays, soil incubations, soil chemical extractions, and field leaf analyses. The A horizons contained significantly higher levels of available N and P than B horizons. In general, N availability was higher in north aspect stands than in south aspects stands, while P availability was higher in south aspect stands than in north aspect stands. Nitrogen availability increased with increasing stand age up to 50 to 60 yr (aggradation phase); beyond 60 yr, N availability declined (degradation phase). Phosphorus availability decreased logarithmically with increasing age. Declining N availability in the older stands was due to declining total soil N and a decline in the fraction available; the declining P availability in the older stands was due to a decline in the fraction available. The lowering of both soil and plant N and P concentrations in older stands may contribute to stand senescence.