Journal
GLOBAL CHANGE BIOLOGY
Volume 9, Issue 11, Pages 1591-1601Publisher
WILEY
DOI: 10.1046/j.1365-2486.2003.00694.x
Keywords
atmospheric nitrogen deposition; nitrogen excess; nitrogen retention; northern hardwood forest; soil organic matter; soil sterilization
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Nitrogen retention in soil organic matter (SOM) is a key process influencing the accumulation and loss of N in forest ecosystems, but the rates and mechanisms of inorganic N retention in soils are not well understood. The primary objectives of this study were to compare ammonium (NH4+), nitrite (NO2-), and nitrate (NO3-) immobilization among soils developed under different tree species in the Catskill Mountains of New York State, and to determine the relative roles of biotic or abiotic processes in soil N retention. A laboratory experiment was performed, where N-15 was added as NH4+, NO2-, or NO3- to live and mercury-treated O horizon soils from three tree species (American beech, northern red oak, sugar maple), and N-15 recoveries were determined in the SOM pool. Mercuric chloride was used to treat soils as this chemical inhibits microbial metabolism without significantly altering the chemistry of SOM. The recovery of N-15 in SOM was almost always greater for NH4+ (mean 20%) and NO2- (47%) than for NO3- (10%). Ammonium immobilization occurred primarily by biotic processes, with mean recoveries in live soils increasing from 9% at 15 min to 53% after 28 days of incubation. The incorporation of NO2- into SOM occurred rapidly (<15 min) via abiotic processes. Abiotic immobilization of NO2- (mean recovery 58%) was significantly greater than abiotic immobilization of NH4+ (7%) or NO3- (7%). The incorporation of NO2- into SOM did not vary significantly among tree species, so this mechanism likely does not contribute to differences in soil NO3- dynamics among species. As over 30% of the (NO2-)-N-15 label was recovered in SOM within 15 min in live soils, and the products of NO2- incorporation into SOM remained relatively stable throughout the 28-day incubation, our results suggest that NO2- incorporation into SOM may be an important mechanism of N retention in forest soils. The importance of NO2- immobilization for N retention in field soils, however, will depend on the competition between incorporation into SOM and nitrification for transiently available NO2-. Further research is required to determine the importance of this process in field environments.
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