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.

Effects of belowground litter addition, increased precipitation and clipping on soil carbon and nitrogen mineralization in a temperate steppe

“…Being a major component of soil microbial biomass (a second subset of fungi), AMF, also plays a distinct and unique role in soil C and N sequestration (Rillig et al, 2001;Kaiser et al, 2011). This group, which symbiotically colonizes plant roots, forms associations with 80 % of plant species (Smith and Read, 1997).…”

“…However, increasing evidence indicates that plants may exert much greater effects on soil C and N cycling than previously estimated by controlling belowground photosynthate allocation (Chapman et al, 2006;Kaiser et al, 2011). Högberg and Read (2006) reported that approximately half of the soil respiration is derived from belowground photosynthate allocation.…”

“…Högberg and Read (2006) reported that approximately half of the soil respiration is derived from belowground photosynthate allocation. This large photosynthate input to the soil microbial community may in turn significantly affect soil C and N cycling (Kaiser et al, 2011). For example, interrupting the photosynthates flow from the canopy leaves to the roots has led to an approximately 50 % reduction of mycorrhizal fungi in beech forest, and has strongly affected soil N cycling (Kaiser et al, 2010(Kaiser et al, , 2011.…”

“…This large photosynthate input to the soil microbial community may in turn significantly affect soil C and N cycling (Kaiser et al, 2011). For example, interrupting the photosynthates flow from the canopy leaves to the roots has led to an approximately 50 % reduction of mycorrhizal fungi in beech forest, and has strongly affected soil N cycling (Kaiser et al, 2010(Kaiser et al, , 2011. In temperate grasslands, land management practices such as heavy grazing and repeated harvesting (reducing belowground photosynthate allocation) probably significantly influences the direct impacts of belowground particulate litter (BPL) addition and increased precipitation on soil C and N mineralization.…”

“…In temperate grasslands, land management practices such as heavy grazing and repeated harvesting (reducing belowground photosynthate allocation) probably significantly influences the direct impacts of belowground particulate litter (BPL) addition and increased precipitation on soil C and N mineralization. For example, BPL addition (and increased precipitation) might accelerate the mineralization process, but this effect may be partly eliminated by reducing plant belowground photosynthate allocation due to the decreases in soil dissolved organic C levels and water fluxes (Zhang et al, 2005;Kaiser et al, 2011). Moreover, reducing belowground photosynthate allocation may alter the effects of BPL addition and increased precipitation on soil microbial communities (especially mycorrhizal fungi), thereby partly masking their effects on soil C and N mineralization.…”