Shifts in microbial stoichiometry upon nutrient addition do not capture growth-limiting nutrients for soil microorganisms in two subtropical soils

• Microbial stoichiometry has become a key aspect in ecological research as shifts in microbial C:N, C:P and N:P ratios upon nutrient addition are presumed to give insight into relative nutrient limitations for soil microorganisms–with far-reaching implications for biogeochemical processes. However, this expectation has never been tested against direct methods of microbial growth responses to nutrient addition. We therefore manipulated a subtropical grassland and forest soil with multifactorial C-, N- and P-additions during 30 days to induce changes in limiting resources and evaluated the resulting soil microbial growth rates, microbial biomass stoichiometry, potential enzyme activities and microbial community composition. Our results show that microbial stoichiometric shifts upon nutrient addition ambiguously predict growth-limiting nutrients for soil microbes. For example, P- and NP-addition to the grassland soil significantly shifted the microbial N:P ratio, which suggests increased N- relative to P-limitation. Microbial growth responses however indicated that soil microbes remained C limited. The same applies for the forest soil, where P-, CN-, NP- and CNP-additions shifted the microbial N:P ratio, yet microbial growth remained C limited. This indicates that microorganisms can immobilize N and P for storage when C is the main limiting nutrient, and that intracellular storage of N and P is responsible for the observed shifts in microbial stoichiometry. Moreover, our data imply that shifts in microbial C:N ratios do not necessarily indicate shifts in microbial community composition and suggest that soil microorganisms–when subject to resource pulses–are stoichiometrically quite plastic.

Ämnesord

LANTBRUKSVETENSKAPER  -- Lantbruksvetenskap, skogsbruk och fiske -- Markvetenskap (hsv//swe)

AGRICULTURAL SCIENCES  -- Agriculture, Forestry and Fisheries -- Soil Science (hsv//eng)

NATURVETENSKAP  -- Biologi -- Ekologi (hsv//swe)

NATURAL SCIENCES  -- Biological Sciences -- Ecology (hsv//eng)

Nyckelord

Limiting factors for growth

Microbial biomass

Microbial growth

Microbial stoichiometry

Nutrient limitation

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