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).