Fungal C:N:P ratios in response to varying nutrient supply
Fungal C:N and C:P ratios were highly flexible for most isolates in all media types and nutrient manipulations, compared to more stable N:P ratios. Along a defined gradient of N and P supply using glucose as a C source, fungal C:nutrient ratios positively correlated with C:nutrient supply in the medium, with 1/HCN and 1/HCP values of isolates ranging from 0.16 to 0.65 (Fig. 1a,b). At high N supply (medium C:N = 5) fungal C:N only varied within 8 to 18, while reaching a maximum of 84 at low N supply (C:N = 200). Likewise, fungal C:P ranged from 23 to 199 at high P (C:P = 20), compared to values up to 1114 at low P supply (C:P = 3000) (Fig. 1). By contrast, shifts in N:P ratios resulted in lower 1/HNPthan 1/HCN and 1/HCP (P< 0.01), with most isolates remaining relatively homeostatic (Fig. 1c).
In similarly controlled N and P manipulations with cellulose as a more costly C source, the C:N ratio was likewise non-homeostatic in some isolates, overall even reaching higher C:N values up to 126 (Fig. S5a), while again N:P remained relatively stable (1/HNP 0.02 – 0.13; Fig. S5b). Also in complex soil-extract agar (SEA) varying N, P and C supply strongly affected fungal C:N:P ratios. In non-manipulated SEA fungal C:N:P was on average 103:7:1. N and P additions alone did not lower ratios significantly, whereas the addition of cellulose and glucose significantly increased C:N and C:P ratios four- to fivefold, respectively (Fig. 2). Increased C availability consistently led to higher fungal biomass (in parallel with C uptake) (Fig. S4), while N and P concentrations remained low. The supplementary addition of N and P again reduced C:N and C:P ratios to levels of the control, without affecting biomass production (Fig. 2, S4). Increased availability of N and P both affected C:P and C:N ratios to a similar extent. Consequently, N:P remained more stable than C:nutrient ratios, only varying up to twofold (Fig. 2c).