Soil fungal mycelia have unexpectedly flexible stoichiometric C:N and C:P ratios

Abstract

Soil ecological stoichiometry provides powerful theories to integrate the complex interplay of element cycling and microbial communities into biogeochemical models. One essential assumption is that microbes maintain stable C:N:P (carbon:nitrogen:phosphorus) ratios independent of resource supply, although such homeostatic regulations have rarely been assessed in individual microorganisms. Here, we report an unexpected high flexibility in C:N and C:P values of saprobic fungi along nutrient supply gradients, overall ranging between 7‐126 and 20‐1488, respectively, questioning microbial homeostasis. Fungal N:P varied comparatively less due to simultaneous reductions in mycelial N and P contents. As a mechanism, internal recycling processes during mycelial growth and an overall reduced N and P uptake appear more relevant than element storage. The relationships among fungal stoichiometry and growth disappeared in more complex media. These findings affect our interpretation of stoichiometric imbalances among microbes and soils and are highly relevant for developing microbial soil organic carbon and nitrogen models.

Modeling Microbial Adaptations to Nutrient Limitation During Litter Decomposition

“…Similarly, we could not exclude mode (iii), consistent with evidence of increased microbial C:N ratio of fungal isolates under nutrient limitation (Camenzind et al, 2021), or at the community level in the earlier phases of litter decomposition (Van Meeteren et al, 2007). Other reports, however, indicate stoichiometric homeostasis at the microbial community level, despite large variations in substrate C:N ratios (Fanin et al, 2013;Schleuss et al, 2019).…”

“…We describe nutrient retention relative to C by allowing N to be internally recycled at senescence. Consistent with this mode, N was preferentially recycled in growing fungal hyphae, leaving behind a C enriched less active biomass (Camenzind et al, 2021). The turnover rate of microbial phosphorus (P) was shown to be faster in fertilized soils, indicating a relatively higher P retention when this element was less available (Spohn and Widdig, 2017).…”

“…The potential consequence of decoupling of C and nutrient acquisition is that some compounds rich in the least limiting element (C in litter) can be left behind during decomposition and the overall rate of decomposition decreases under nutrient limitation (Boberg et al, 2008). Microbial cellular composition can be changed when facing nutrient limitation (Camenzind et al, 2021), but empirical evidence points to relatively homeostatic behavior (i.e., stable element ratios) at the microbial community level when considering nitrogen (N) as a limiting factor (Manzoni et al, 2010;Fanin et al, 2013). Finally, microorganisms can retain the most limiting resource more efficiently, and reduce losses of limiting resources in necromass or extracellular products (Spohn, 2016;Spohn and Widdig, 2017;Camenzind et al, 2021).…”

“…Microbial cellular composition can be changed when facing nutrient limitation (Camenzind et al, 2021), but empirical evidence points to relatively homeostatic behavior (i.e., stable element ratios) at the microbial community level when considering nitrogen (N) as a limiting factor (Manzoni et al, 2010;Fanin et al, 2013). Finally, microorganisms can retain the most limiting resource more efficiently, and reduce losses of limiting resources in necromass or extracellular products (Spohn, 2016;Spohn and Widdig, 2017;Camenzind et al, 2021). Therefore, depending on the net effect of these adaptations, litter with low nutrient contents may promote or reduce C stabilization.…”

Non-mycorrhizal root associated fungi of a tropical montane forest are relatively robust to the long-term addition of moderate rates of nitrogen and phosphorus

“…This is an important point, because it implies that a lack of change in fungal community attributes does not necessarily translate in functional stability. In addition to these possibilities, recent experimental evidence has shown that soil fungi exhibit extremely flexible stoichiometric homeostasis in relation to the stoichiometric ratio of culturing media (Camenzind et al, 2021). If this ability is confirmed and widespread across the fungal kingdom, it will mean that non-mycorrhizal fungi have yet another possibility to bridge resource gaps in time or space, thus allowing taxa to persist despite shifts in the nutrient supply.…”