The Fate and Retention of Organic and Inorganic super(15)N-Nitrogen in an Old-Growth Forest Soil in Western Oregon

Forests in the American Pacific Northwest receive very little nitrogen (N) through atmospheric deposition; therefore, they can provide insights into how the N cycle functioned in other regions before heavy atmospheric deposition of inorganic N began. Our objectives were to determine (a) if the fate of organic N differed from the fate of inorganic N, (b) the effect that polyphenols have on the fate of organic N, and (c) the effect of season of addition on the fate of N inputs. We traced N added to in situ soil cores as ammonium, organic N, tannin-complexed organic N, and the N sub(2)-fixing lichen Lobaria oregana. Total super(15)N recovery was between 74% and 109% for all N additions. Total super(15)N recovery did not vary significantly from the first sampling date to the last date. The litter/organic horizon, as a bulk pool, was the largest N retention pool for all forms of N addition. Within the litter/organic horizon, the chloroform-extractable microbial biomass initially accounted for nearly all of the added N from the ammonium additions. On a different time scale, microbial biomass also played a noteworthy role in the retention of N from organic N, tannin-complexed organic N, and Lobaria. Complexing organic matter with tannin appeared to slow N cycling, but it did not significantly change the ultimate distribution of added organic N. Season of N addition had little effect on the retention of added N; however, where differences did occur, spring additions had lower recoveries than autumn additions.