Introduction
Ecological stoichiometry represents an important field originally
developed in aquatic ecosystems, with its principles being now also
applied to terrestrial soil systems (Sterner & Elser 2002). Models use
stoichiometric theory for predicting soil nutrient cycles and carbon
storage (Sardans et al. 2012). Based on the general observation
that C:N:P (carbon:nitrogen:phosphorus) ratios of soil microbial
communities are more narrow than resource C:N:P, critical ratios of C:N
and C:P are defined to predict nutrient demands of soil microbes,
mineralization versus immobilization patterns and C sequestration versus
respiration (Manzoni et al. 2012; Zechmeister-Boltensternet al. 2015). Thus, ecological stoichiometry facilitates the
incorporation of extremely complex processes into global models and
predictions, especially in the context of global change (Hall et
al. 2011a).
These models make crucial assumptions that are critical for their
validity (Mooshammer et al. 2014; Spohn 2016), one of them that
heterotrophic soil microbes are homeostatic, i.e. maintain stable C:N:P
ratios independently of the soil nutrient status (Persson et al.2010). Even though some models allow microbial C:N:P to vary slightly
(McGill et al. 1981; Nicolardot et al. 2001), homeostatic
flexibility is currently only attributed to microbial community shifts,
not an actual stoichiometric flexibility in individuals (Buchkowskiet al. 2019). Fixed microbial stoichiometric ratios are
interpreted as an indicator of nutrient demands: If microbes need to
maintain their narrow C:N ratios, N will be in limiting supply in
substrates characterized by wider C:N ratios (Manzoni et al.2010). These assumptions of soil microbial homeostasis are supported by
analyses of entire soil communities, which indeed show relatively little
variation in microbial C:N:P ratios compared to soil resource
variability (Cleveland & Liptzin 2007; Hartman & Richardson 2013). By
contrast, we know astonishingly little about the stoichiometry of
individual soil microbial groups.
Indeed, flexibility of C:N:P ratios in individual microbial species has
rarely been analyzed, and the few studies available mainly use aquatic
isolates and partly result in contradictory results depending on the
methods applied (Danger et al. 2016). Surprisingly, some of these
studies suggest that microbial C:N:P ratios may be less homeostatic than
assumed for heterotrophic organisms. For aquatic bacteria Scott et al.
(2012) and Godwin and Cotner (2018) demonstrated high variation in C:P
ratios for some but not all isolates (see also Makino et al.2003; Danger et al. 2008). In case of saprobic fungi, aquatic
hyphomycetes responded to varying element supply with non-homeostatic
adjustments in C:P ratios, while C:N remained stable (Danger & Chauvet
2013; Gulis et al. 2017). In soils there are also indications
that fungal C:N:P may exceed common textbook assumptions of C:N
~10-20 and C:P ~100-300 (Jennings 1995;
Strickland & Rousk 2010). A recent meta-analysis demonstrated wide C:N
and C:P ratios in few fungal samples (Zhang & Elser 2017), as also
reported for C:N in wood decomposing fungi under low N conditions (Levi
& Cowling 1969), while other studies indicate again only little
flexibility (Heck 1928; Egli & Quayle 1986; Mouginot et al.2014).
Physiological mechanisms causing non-homeostasis in microbes are still
unknown. Most authors assume P storage, i.e. element uptake in excess as
the exclusive mechanism (Scott et al. 2012; Danger & Chauvet
2013; Mooshammer et al. 2014), though this only explains
stoichiometric shifts in response to high element supply. The
significance of such stoichiometric shifts for fungal growth and
activity also remain unresolved. Available data show that N or P uptake
can be independent of fungal growth, potentially related to storage
mechanisms or primary C limitations (Levi & Cowling 1969; Guliset al. 2017).
In order to understand stoichiometric adjustments in soil saprotrophic
fungi in detail, we analyzed fungal mycelial element concentrations in
response to varying N, P and C supply in different growth media
specifically developed for this question (varying from highly controlled
to natural substrates). Derived from common assumptions in soil
ecological stoichiometry, we tested the hypothesis that saprobic fungi
are homeostatic, especially regarding their C:N ratios. Our results not
only further question the general assumption of microbial homeostasis,
but also provide insights into the physiological mechanisms of element
allocation in mycelia, and the significance of stoichiometric shifts for
fungal growth and activity under varying conditions.