title: Plants control the seasonal dynamics of microbial N cycling in a beech forest soil by belowground C allocation.
abstract: Soil microbes in temperate forest ecosystems are able to cycle several hundreds of kilograms of N per hectare per year and are therefore of paramount importance for N retention. Belowground C allocation by trees is an important driver of seasonal microbial dynamics and may thus directly affect N transformation processes over the course of the year. Our study aimed at unraveling plant controls on soil N cycling in a temperate beech forest at a high temporal resolution over a time period of two years, by investigating the effects of tree girdling on microbial N turnover. In both years of the experiment, we discovered (1) a summer N mineralization phase (between July and August) and (2) a winter N immobilization phase (November-February). The summer mineralization phase was characterized by a high N mineralization activity, low microbial N uptake, and a subsequent high N availability in the soil. During the autumn/winter N immobilization phase, gross N mineralization rates were low, and microbial N uptake exceeded microbial N mineralization, which led to high levels of N in the microbial biomass and low N availability in the soil. The observed immobilization phase during the winter may play a crucial role for ecosystem functioning, since it could protect dissolved N that is produced by autumn litter degradation from being lost from the ecosystem during the phase when plants are mostly inactive. The difference between microbial biomass N levels in winter and spring equals 38 kg N/ha and may thus account for almost one-third of the annual plant N demand. Tree girdling strongly affected annual N cycling: the winter N immobilization phase disappeared in girdled plots (microbial N uptake and microbial biomass N were significantly reduced, while the amount of available N in the soil solution was enhanced). This was correlated to a reduced fungal abundance in autumn in girdled plots. By releasing recently fixed photosynthates to the soil, plants may thus actively control the annual microbial N cycle. Tree belowground C allocation increases N accumulation in microorganisms during the winter which may ultimately feed back on plant N availability in the following growing season.
authors.affiliation: University of Vienna, Department of Chemical Ecology and Ecosystem Research, Althanstrasse 14, A-1090 Vienna, Austria. christina.kaiser@univie.ac.at
authors.forename: Christina
authors.initials: C
authors.lastname: Kaiser
authors.forename: Lucia
authors.initials: L
authors.lastname: Fuchslueger
authors.forename: Marianne
authors.initials: M
authors.lastname: Koranda
authors.forename: Markus
authors.initials: M
authors.lastname: Gorfer
authors.forename: Claus F
authors.initials: CF
authors.lastname: Stange
authors.forename: Barbara
authors.initials: B
authors.lastname: Kitzler
authors.forename: Frank
authors.initials: F
authors.lastname: Rasche
authors.forename: Joseph
authors.initials: J
authors.lastname: Strauss
authors.forename: Angela
authors.initials: A
authors.lastname: Sessitsch
authors.forename: Sophie
authors.initials: S
authors.lastname: Zechmeister-Boltenstern
authors.forename: Andreas
authors.initials: A
authors.lastname: Richter
mesh_terms: D001419:Bacteria / Q000378:metabolism*; D002244:Carbon / Q000378:metabolism*; D004271:DNA, Fungal / Q000235:genetics; D017753:Ecosystem; D029964:Fagus / Q000502:physiology*; D005308:Fertilizers; D005658:Fungi / Q000235:genetics / Q000502:physiology; D058458:Nitrogen Cycle / Q000502:physiology*; D012621:Seasons; D012988:Soil Microbiology; D013997:Time Factors; D014197:Trees
chemical_list: D004271:DNA, Fungal; D005308:Fertilizers; D002244:Carbon