Atmospheric Nitrate Deposition, Microbial Community Composition, and Enzyme Activity in Northern Hardwood Forests

duced by N deposition could indirectly influence the composition and function of microbial communities. AlOn a global scale, human activity has increased the atmospheric ternatively, anthropogenic N deposition could directly input of NO 3 to many terrestrial ecosystems. Anthropogenic NO 3 may be a potent modifier of ecosystem function, especially in tempermodify soil microbial communities by suppressing the ate forests that are sometimes N limited. However, the impact of activity of lignin-degrading fungi (Berg, 1986; Fog 1988; chronic N deposition on soil microorganisms is still poorly understood. Dix and Webster, 1995), thus potentially lowering their Nitrate entering Lake States forests is rapidly assimilated by the microabundance and diminishing the overall capacity of mibial community and it is subsequently released as NH 4 . Because high crobial communities to degrade lignin and other pollevels of NH 4 inhibit the activity of lignin-degrading soil fungi, we yphenols (Carreiro et al., 2000). Such a response could reasoned that chronic N additions could alter the composition and alter rates of soil organic matter formation and the refunction of heterotrophic microbial communities in soil, and hence lease of plant nutrients from litter. Relatively high levels the ecosystem-level processes they mediate. We tested our hypothesis of NH 4 in soil suppress lignin oxidation (Keyser et al., in four northern hardwood ecosystems in northern Michigan, which 1978), and the rapid microbial assimilation of anthroporeceived experimental N additions (30 kg NO 3 –N ha 1 yr 1) during the past 8 yr. We quantified microbial community function by measurgenic NO 3 and its subsequent release as NH 4 (Zogg et ing the activity of extracellular enzymes involved in plant litter degraal., 2000) indicates that anthropogenic NO 3 could podation and described microbial community composition using phostentially suppress the abundance and activity of ligninpholipid fatty acid (PLFA) analysis. Chronic N additions significantly degrading fungi. suppressed -glucosidase activity by 24% in mineral soil and supWe have experimentally manipulated NO 3 deposipressed phenol oxidase activity by 35% in surface litter. We found tion in four northern hardwood stands in Michigan’s no evidence that chronic N additions altered microbial community Lower and Upper Peninsula. During the past 8 yr, the composition; NO 3 addition did not alter the relative abundance of addition of 30 kg NO 3 –N ha 1 yr 1 has significantly bacterial, actinomycetal, fungal, or protozoan PLFAs. However, increased the export of dissolved organic C (DOC) and NO 3 additions significantly reduced microbial biomass by 18% reladissolved organic N (DON) from these forest stands tive to the control treatment. Results indicate that N additions broadly suppressed all microbial groups, not just the activity and abundance (Pregitzer et al., 2003), suggesting a change in litter of lignin-degrading fungi. chemistry, a change in microbial community function, or both. Here, we investigate the alternative that longterm NO 3 deposition has altered microbial community function and composition by suppressing fungal activity H activity has globally increased the amount and abundance. During the course of one growing seaof N entering terrestrial ecosystems from the atson, we measured extracellular enzyme activity to gain mosphere (Galloway, 1998). The northeastern and east insight into microbial community function, and we ascentral USA receives the greatest amounts of N deposisessed community composition using PLFA analysis. tion in North America (Fenn et al., 1998), ranging from Our primary objective was to determine if chronic N 2 to 16 times background levels (Galloway et al., 1984). additions have altered the composition and function of The impact of anthropogenic N deposition on temperate the heterotrophic microbial community. forests is a primary concern, because increases in soil N availability have the potential to alter species diversity, plant community composition, and ecosystem function MATERIALS AND METHODS (Vitousek et al., 1997; Aber et al., 1998; Boxman et al., Study Area 1998; Gundersen et al., 1998). There are reasons to expect that anthropogenic N Four study sites were established along a 500-km climatic deposition also could directly or indirectly alter the comand N deposition gradient extending from northwest Upper Michigan to central Lower Michigan (Fig. 1). The study sites position and function of soil microbial communities. encompass the geographic extent of sugar maple (Acer sacPlant litter provides the primary energy source for hetcharum Marsh.)-dominated northern hardwood forest in erotrophic microbial growth in soil, and changes in the Michigan, and they are similar in composition, history, strucamount and type of organic substrates entering soil inture, and soil development (Table 1; Burton et al., 1993). The soils are classified as sandy, mixed, frigid Typic and Alfic J.L. DeForest and D.R. Zak, School of Natural Resources & EnvironHaplorthods (MacDonald et al., 1995). At each of the four ment, 430 E. University, Univ. of Michigan, Ann Arbor, MI 48109study sites, we established six 30 by 30 m experimental plots. 1115; K.S. Pregitzer and A. J. Burton, School of Forest Resources Three plots served as controls receiving ambient levels of N and Environmental Science, Michigan Technological Univ., 1400 Townsend Dr., Houghton, MI 49931. Received 20 Nov. 2002. *Corresponddeposition, whereas the remaining three plots received ambiing author ([email protected]). Abbreviations: DOC, dissolved organic carbon; DON, dissolved orPublished in Soil Sci. Soc. Am. J. 68:132–138 (2004).  Soil Science Society of America ganic nitrogen; FAME, fatty acid methyl esters; PLFA, phospholipid fatty acid. 677 S. Segoe Rd., Madison, WI 53711 USA