Implications for soil carbon sequestration and nutrient cycling
What are the implications of these findings, and why do analyses of soil microbial communities primarily show homeostasis despite this apparent flexibility in individuals? Our results imply that most saprobic fungal species do adjust their stoichiometry and nutrient use efficiency to resource supplies, a finding not only giving interesting new insights into mycelial growth dynamics but also affecting our view of microbially driven element cycles. Since stoichiometric models in soil are based on whole microbial communities, our data on fungal individuals do not directly indicate flexible C:N:P ratios at the community level. Still, if many individuals in a microbial community were non-homeostatic, this would subsequently also allow for non-homeostatic adjustments in soil microbial communities. So far, occasionally observed C:N:P plasticity in soil microbes was only explained by community shifts (Fanin et al. 2013; Mooshammer et al. 2014). However, in case of the “true non-homeostasis” described here, interpretations of soil microbial C:N:P may be reevaluated: the commonly observed low microbial C:N:P would not be indicative of high N and P demands, but rather of C limitation but sufficient nutrient supply. Differences in stoichiometric flexibility among bacteria and fungi may also result in homeostasis at the soil community level, since bacteria are assumed to be homeostatic (even though aquatic studies also start to challenge this view (Scottet al. 2012; Godwin & Cotner 2018)), but are still relevant for models starting to recognize different decomposer groups (Waringet al. 2013; Riley et al. 2014). Regarding biogeochemical cycles, not only the lack of homeostasis but also the suggested differential use of C versus nutrients in fungi is relevant. High structural C demand for biomass buildup compared to an efficient use of nutrients by internal recycling not only shape fungal nutrient use efficiency, but especially subsequent C sequestration (Liang et al. 2019). It is likely that fungal necromass is enriched in C, as indicated by our stoichiometric analyses; which suggests that C sequestration by fungi may occur without large nutrient losses (van Groenigen et al. 2017).