Plants control the seasonal dynamics of microbial N cycling in a beech forest soil by belowground allocation

• Published in: Ecology (Durham) 2011-05, Vol.92 (5), p.1036-1051
• Main Authors: Kaiser, Christina, Fuchslueger, Lucia, Koranda, Marianne, Gorfer, Markus, Stange, Claus F., Kitzler, Barbara, Rasche, Frank, Strauss, Joseph, Sessitsch, Angela, Zechmeister-Boltenstern, Sophie, Richter, Andreas
• Format: Article
• Language: English
• Subjects: Forest ecology; Forest ecosystems; Forest soils; Microbial biomass; Nitrogen; Plants; Soil ecology; Soil microorganisms; Trees; Winter
• ISSN: 0012-9658; 1939-9170

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 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 onethird 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 allocation increases N accumulation in microorganisms during the winter which may ultimately feed back on plant N availability in the following growing season.