Does long-term soil warming affect microbial element limitation? A test by short-term assays of microbial growth responses to labile C, N and P additions

Increasing global temperatures have been reported to accelerate soil
carbon (C) cycling but also to promote nitrogen (N) and phosphorus (P)
dynamics in terrestrial ecosystems. However, warming can differentially
affect ecosystem C, N and P dynamics, potentially intensifying elemental
imbalances between soil resources, plants and soil microorganisms. Here we
investigated the effect of long-term soil warming on microbial resource
limitation, based on measurements of microbial growth (18O incorporation
into DNA) and respiration after C, N and P amendments. Soil samples were
taken from two soil depths (0–10, 10–20 cm) in control and warmed
(>14 yrs warming, +4°C) plots in the Achenkirch soil warming
experiment. Soils were amended with combinations of glucose-C,
inorganic/organic N and inorganic/organic P in a full factorial design,
followed by incubation at their respective mean field temperatures for 24
hours. Soil microbes were generally C-limited, exhibiting 1.8-fold to
8.8-fold increases in microbial growth upon C addition. Warming
consistently caused soil microorganisms to shift from being predominately
C limited to become C-P co-limited. This P limitation possibly was due to
increased abiotic P immobilization in warmed soils. Microbes further
showed stronger growth stimulation under combined glucose and inorganic
nutrient amendments compared to organic nutrient additions. This may be
related to a prolonged lag phase in organic N (glucosamine) mineralization
and utilization compared to glucose. Soil respiration strongly positively
responded to all kinds of glucose-C amendments, while responses of
microbial growth were less pronounced in many of these treatments. This
highlights that respiration – though easy and cheap to measure – is not a
good substitute of growth when assessing microbial element limitation.
Overall, we demonstrate a significant shift in microbial element
limitation in warmed soils, from C to C-P co-limitation, with strong
repercussions on the linkage between soil C, N and P cycles under
long-term warming.