Fire effects on ecosystem nitrogen cycling in a Californian bishop pine forest

Fire can cause severe nitrogen (N) losses from grassland, chaparral, and te mperate and boreal forest ecosystems. Paradoxically, soil ammonium levels a re markedly increased by fire, resulting in high rates of primary productio n in re-establishing plant communities. In a manipulative experiment, we ex amined the influence of wild-fire ash residues on soil, microbial and plant N pools in a recently burned Californian bishop pine (Pinus muricata D. Do n) forest. Ash stimulated post-fire primary production and ecosystem N rete ntion through direct N inputs from ash to soils, as well as indirect ash ef fects on soil N availability to plants. These results suggest that redistri bution of surface ash after fire by wind or water may cause substantial het erogeneity in soil N availability to plants? and could be an important mech anism contributing to vegetation patchiness in fire-prone ecosystems. In ad dition, we investigated the impact of fire on ecosystem N cycling by compar ing N-15 natural abundance values from recently burned and nearby unburned P. muricata forest communities. At the burned site, N-15 natural abundance in recolonising species was similar to that in bulk soil organic matter. By contrast, there was a marked N-15 depletion in the same species relative t o the total soil N pool at the unburned site. These results suggest that pl ant uptake of nitrate (which tends to be strongly depleted in N-15 because of fractionation during nitrification) is low in recently burned forest com munities but could be an important component of ecosystem N cycling in matu re conifer stands.