Differential responses of grass and a dwarf shrub to long‐term changes in soil microbial biomass C, N and P following factorial addition of NPK fertilizer, fungicide and labile carbon to a heath

Abstract

Microbial immobilization may decrease the inorganic nutrient concentrations of the soil to the extent of affecting plant nutrient uptake and growth. We have hypothesized that graminoids with opportunistic nutrient-acquisition strategies are strongly influenced by nutrient limitation imposed by microbes, whereas growth forms such as dwarf shrubs are less affected by the mobilization-immobilization cycles in microbes. By adding NPK fertilizer, labile C (sugar) and fungicide (benomyl) over a 5 yr period in a fully factorial design, we aimed to manipulate the sink-source potential for nutrients in a non-acidic heath tundra soil. After 2 yr, N and P accumulated in the microbial biomass after fertilization with no change in microbial C, which suggests that nutrients did not limit microbial biomass growth. After 5 yr, microbial C was enhanced by 60 % in plots with addition of labile C, which points to C-limitation of the microbial biomass. Microbial biomass N and P tended to increase following addition of labile C, by 10 and 25 %, respectively. This caused decreased availability of NH % + and P, showing close microbial control of nutrient availability. The most common graminoid, Festuca ovina, responded to fertilizer addition with a strong increase, and to labile C addition with a strong decrease in cover, providing the first direct field evidence that nutrient limitation imposed by immobilizing microbes can affect the growth of tundra plants. Also in support of our hypothesis, following addition of labile C the concentrations of N and K in leaves and that of N in roots of F. ovina decreased, whilst the demand of roots for P increased. In contrast, the most common dwarf shrub, Vaccinium uliginosum, was only slightly sensitive to changes in resource availability, showing no cover change after 4 yr addition of labile C and fertilizer, and little change in leaf nutrient concentrations. We suggest that the differential responses of the two growth forms are due to differences in storage and nutrient uptake pathways, with the dwarf shrub having large nutrient storage capacity and access to organic forms of N through its mycorrhizal association. While the fungicide had no effect on ericoid mycorrhizal colonization of roots or symbiotic function inferred from plant "&N natural abundance, it decreased microbial biomass C and N after 2 yr. Throughout the fifth season, the availability of soil NO $ − and inorganic P was decreased with no change in microbial biomass C, N or P, suggesting a negative impact of benomyl on N and P mineralization.

Soil Processes Affected by Sixteen Grassland Species Grown under Different Environmental Conditions

“…Much of the labile C was likely from rhizodeposition (e.g., exudates, mucilages, and lysates) rather than solely from leaching and decomposition of dead root cell soluble contents because root cell soluble concentration alone was not significantly related to microbial biomass. Since soil microbes are usually C limited (Michelsen et al, 1999;Smith and Paul, 1990;Van de Geijn and van Veen, 1993), plant C inputs, especially in the form of labile C such as root exudates, play an important role in supporting microbial biomass and activity (Kuzyakov, 2002), that could further obscure possible C litter chemistry effects on microbial biomass and nonlabile soil C decomposition. One plant species, the forb A. millefolium, showed very high labile C pools that did not correspond to similar increases in microbial biomass C and N, suggesting that this species produced labile C compounds that did not stimulate microbial growth.…”

Root growth and N dynamics in response to multi-year experimental warming, summer drought and elevated CO2 in a mixed heathland-grass ecosystem

“…Excess 15 N (atom %) was converted to absorption rate of N (in mg N g -1 root DW 2 h -1 ). The total N concentration was also calculated, subtracting the N that had been taken up during the bioassay (Michelsen et al 1999).…”

Root fungal symbionts interact with mammalian herbivory, soil nutrient availability and specific habitat conditions

“…In nutrient-rich conditions, the dependency of the host plant on mycorrhizal compartment may decrease because it is more cost-efficient to obtain nutrients directly from the soil than via carbon-demanding mycorrhizal fungi (Tuomi et al 2001). In contrast, a number of field studies have also reported fertilization to have no impact on mycorrhizal colonization (Heijne et al 1994;Caporn et al 1995;Michelsen et al 1999;Johansson 2000;Urcelay et al 2003). Soil nutrient availability is also closely associated with soil pH (Kinzel 1983;Eskelinen et al 2009), which, in turn, affects microbial communities (Madigan et al 2003;Högberg et al 2007;Eskelinen et al 2009).…”