Biotic and Abiotic Nitrogen Retention in a Variety of Forest Soils

T fact that most N in soils is associated with orNitrogen (N) immobilization in sterilized (abiotic) and non-steriganic matter has led many forest soil scientists and lized (biotic) O and A horizons was studied to determine the relative ecologists to assume that N retention in forest ecosysimportance of biotic and abiotic processes in N retention in forest tems is controlled almost exclusively by biological proecosystems. We collected samples from a variety of forest locations cesses. It is commonly assumed that competition among in Washington, Nevada, California, Tennessee, and North Carolina plant roots, heterotrophs, and nitrifiers for NH 4 domiwith differing soil types, vegetation, N status, and soil acidity. Included nates the fate of N in ecosystems (e.g., Vitousek et al., among these sites were adjacent stands of N2-fixing and non-N2-fixing 1979; Johnson and Edwards, 1979; Riha et al., 1986; species and sites of differing N status due to slope position at a given Johnson, 1992). Relatively little attention has been given location. We treated O and A horizon samples from each site with to abiotic processes controlling N retention in forest (NH4)SO4; sterilization was achieved by adding HgCl2, which proved to be highly effective. We found significant levels of both abiotic and soils (i.e., NH 4 retention by 2:1 clays and chemical reacbiotic N immobilization in all soils. Biotic N immobilization was much tions between NH 4 and organic matter), even though greater in the N-poor sites in California and Nevada than in the other the latter have been known to exist since the 1940s sites and was inversely related to N concentration overall. Biotic (Mattson and Koulter-Andersson, 1942, 1943; Mortland immobilization was directly related to pH and base saturation among and Wolcott, 1965; Nömmik, 1965; Nömmik and Vahall sites, but we hypothesize that these correlations resulted from a tras, 1982; Paul and Clark, 1989). Abiotic N retention correlation between those parameters and N concentration. Abiotic can result from physical condensation reactions of pheN immobilization varied less than biotic N immobilization across sites nols (originating from partially degraded lignin and and was unrelated to N concentration or pH. The percentage of total some fungal pigments) with either amino acids or NH3, N immobilization as abiotic N immobilization varied considerably resulting in the formation of “brown, nitrogenous hu(from 6–90%), and was positively correlated with N concentration. These results suggest that abiotic N immobilization can be a significant mates” (Mortland and Wolcott, 1965; Nömmik, 1965; process in a variety of soil types. Across soil types with increasing N Nömmik and Vahtras, 1982; Paul and Clark, 1989). saturation, biotic N immobilization decreases and abiotic N immobiliThese abiotic, autocatalytic reactions are important in zation accounts for a greater proportion of total N immobilization. the production of humus (Mortland and Wolcott, 1965; Paul and Clark, 1989). Abiotic incorporation of NH3 into humus is enhanced by high pH (because NH3 is the D.W. Johnson, Desert Research Institute, 2215 Raggio Parkway, and Environmental and Resource Sciences, Univ. of Nevada, Reno, NV reactive form of N) and high NH3 and/or NH 4 concen89512; W. Cheng, Dep. of Environmental Studies, Univ. of California, trations (Nömmik and Vahtras, 1982). Both of these Santa Cruz, CA 95064; I.C. Burke, Dep. of Forest Sciences and Natural Resource Ecology Lab., Colorado State Univ., Fort Collins, CO 80523. 1 The reader should recall that Oa horizons were either absent or *Corresponding author ([email protected]). too thin to be sampled in the ULVC and SB sites; thus, these sites are not included in any statistical analyses of patterns in Oa horizons. Published in Soil Sci. Soc. Am. J. 64:1503–1514 (2000). 1504 SOIL SCI. SOC. AM. J., VOL. 64, JULY–AUGUST 2000 Table 1. Vegetation, soils, soil parent material, elevation, mean annual temperature (MAT), mean annual precipitation (MAP, climatic, N deposition, N leaching, litter and soil C contents in the sites sampled. Code Major vegetation Soils and parent material Elev. MAP MAT N Dep. N Leach Litter N Soil N Reference m cm 8C kg ha21 yr21 Mg ha21