| Forests in the Pacific Northwest receive very little nitrogen through atmospheric deposition and thus studying the nitrogen cycle in this region can provide insights into how the unpolluted nitrogen cycle functions. I examined the fate of organic nitrogen versus inorganic nitrogen and the effect of tannins on N retention by tracing N from 15N-labeled ammonium, organic nitrogen, tannin-complexed organic nitrogen, and the N2-fixing lichen Lobaria oregana to in situ soil cores. The litter/organic horizon was the largest nitrogen retention pool for all forms of nitrogen added. Within the litter/organic horizon, the microbial biomass initially accounted for most of the added nitrogen from the ammonium additions. On a different time scale, microbial biomass also played a significant role in the retention of nitrogen from other N forms.; I also studied mass loss and nitrogen dynamics during the decomposition of Lobaria oregana using 15N. Lichens placed in the field during the spring had a smaller decay constant (k = 1.24 yr -1) than the lichens placed in the field during the fall (k = 3.1 yr-1). The spring rate is similar to some labile leaf litters, but the fall rate is among the fastest decomposition rates measured for complex organic matter. Lichen from both seasons took up N from the surrounding environment during decay, while simultaneously losing N.; In an additional project I ran several simple models to explore the effects that added N could have on carbon sequestration. If CO2 emissions can not be reduced globally, other methods of sequestering carbon need to be explored. Because N is usually the most limiting nutrient in temperate terrestrial ecosystems, I hypothesized that adding N to land plants should cause the plants to grow bigger and therefore sequester more carbon. Various model runs showed that adding N, especially to the most N limited forests, could sequester large amounts of carbon. Although excess N in the environment has a variety of deleterious effects, the wise use of N2-fixing plants and N fertilizer on the most N-limited sites, especially high C:N forests, would increase C sequestration, at least temporarily. |
