Abstract
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.
Highlights
Microbial stoichiometry has been a focal point in research in recent years
Griffiths et al (2012) could show that microbial biomass C:P ratios were subject to shifts in a long-term P fertilization experiment, as microbial biomass C:P ratios decreased with P fertilization relative to an unfertilized control; supported by substrate-induced respiration measurements, they concluded that microbial biomass C:P shifts indicate P limitation of soil microorganisms
The analysis further revealed that (i) potential enzyme activities are associated with the CN-treatment in both soils, (ii) microbial growth rates, respiration and microbial biomass measures generally point towards all treatments that received C, and towards the C, CP- and CNPtreatments in particular, (iii) MB-P points towards the P- and NP-treatments and (iv) MB-N in the grassland soil is associated with the N- and CN-treatments
Summary
Microbial stoichiometry has been a focal point in research in recent years. It is believed that stoichiometric imbalances between microbial communities and their resources have major implications for biogeochemical processes, in soil systems (Manzoni et al 2010; Mooshammer et al 2014). Global-scale analyses found soil microbial biomass C:N:P composition to be rather constrained around 60:7:1 (Cleveland and Liptzin 2007) or 42:6:1 (Xu et al 2013), indicating a relatively strict homeostasis of soil microorganisms (Sterner and Elser 2002). Changes in microbial biomass C, N and P contents (Allen and Schlesinger 2004; Turner and Wright 2014; Camenzind et al 2018) and shifts in microbial stoichiometry upon nutrient addition are expected to indicate nutrient limitation for microbial growth (Cleveland and Liptzin 2007). Soil microbes were found to sequester high rates of N and P into their microbial biomass during leaf litter decomposition, which subsequently resulted in rather plastic microbial C:P and N:P ratios of the decomposer community (Fanin et al 2013). The aim of this current study is to evaluate whether shifts in microbial
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