title: Microbial responses to nitrogen addition in three contrasting grassland ecosystems.
abstract: The effects of global N enrichment on soil processes in grassland ecosystems have received relatively little study. We assessed microbial community response to experimental increases in N availability by measuring extracellular enzyme activity (EEA) in soils from three grasslands with contrasting edaphic and climatic characteristics: a semiarid grassland at the Sevilleta National Wildlife Refuge, New Mexico, USA (SEV), and mesic grasslands at Konza Prairie, Kansas, USA (KNZ) and Ukulinga Research Farm, KwaZulu-Natal, South Africa (SAF). We hypothesized that, with N enrichment, soil microbial communities would increase C and P acquisition activity, decrease N acquisition activity, and reduce oxidative enzyme production (leading to recalcitrant soil organic matter [SOM] accumulation), and that the magnitude of response would decrease with soil age (due to higher stabilization of enzyme pools and P limitation of response). Cellulolytic activities followed the pattern predicted, increasing 35-52% in the youngest soil (SEV), 10-14% in the intermediate soil (KNZ) and remaining constant in the oldest soil (SAF). The magnitude of phosphatase response did not vary among sites. N acquisition activity response was driven by the enzyme closest to its pH optimum in each soil: i.e., leucine aminopeptidase in alkaline soil, beta-N-acetylglucosaminidase in acidic soil. Oxidative enzyme activity varied widely across ecosystems, but did not decrease with N amendment at any site. Likewise, SOM and %C pools did not respond to N enrichment. Between-site variation in both soil properties and EEA exceeded any treatment response, and a large portion of EEA variability (leucine aminopeptidase and oxidative enzymes), 68% as shown by principal components analysis, was strongly related to soil pH (r = 0.91, P < 0.001). In these grassland ecosystems, soil microbial responses appear constrained by a molecular-scale (pH) edaphic factor, making potential breakdown rates of SOM resistant to N enrichment.
authors.affiliation: Department of Biology, MSC03 2020, University of New Mexico, Albuquerque, NM 87131, USA. lzeglin@unm.edu
authors.forename: Lydia H
authors.initials: LH
authors.lastname: Zeglin
authors.forename: Martina
authors.initials: M
authors.lastname: Stursova
authors.forename: Robert L
authors.initials: RL
authors.lastname: Sinsabaugh
authors.forename: Scott L
authors.initials: SL
authors.lastname: Collins
mesh_terms: D000704:Analysis of Variance; D001419:Bacteria / Q000378:metabolism*; D018533:Biomass; D002244:Carbon / Q000378:metabolism; D017753:Ecosystem; D004798:Enzymes / Q000378:metabolism*; D007615:Kansas; D009516:New Mexico; D009584:Nitrogen / Q000378:metabolism*; D006109:Poaceae / Q000254:growth & development*; D012987:Soil / Q000032:analysis; D012988:Soil Microbiology; D013019:South Africa
chemical_list: D004798:Enzymes; D012987:Soil; D002244:Carbon; D009584:Nitrogen