Land-use alters the temperature
response of microbial carbon-use
efficiency

T. BÖLSCHER¹*, G.I. ÅGREN¹, A.M. HERRMANN¹

¹Swedish University of Agricultural Sciences, 750 07
Uppsala, Sweden (*correspondence:
tobiasboelscher@gmail.com)

Decomposition of soil organic matter is a major flux in the global carbon (C) cycle and small changes in this vast flux may seriously modify global climate. A key property during decomposition is the microbial community's carbon-use efficiency (CUE). It is defined as the fraction of microbial utilized C that is allocated to biosynthesis, with the remaining being respired. Therefore, CUE regulates C sequestration and experimental evidence suggests that it decreases with warming [1]. Yet, most soil C models consider CUE as a constant property. We hypothesized that (i) the temperature response of CUE differs across land-uses and (ii) that such dissimilarities significantly affects projections of soil C stocks under future temperature conditions.

To test our hypotheses, we applied a microbial
thermodynamics approach [2] using isothermal calorimetry
and incorporated our findings in the Q-model [3]. Soils from
arable, grassland, ley-farming, and forest sites situated in a
boreal climate were amended with two C substrates differing
in molecular complexity. All soils were then incubated at
various temperatures ranging from 5 to 20 °C. CUEs were
calculated from cumulative heat production and residual
substrate [2] when 15 % of the added substrate was utilized.

In the arable soil, CUE remained constant over the
temperature range between 5 and 20 °C. In contrast, CUEs in
ley farming, grassland and forest soils were constant in the
temperature interval 5 to 12.5 °C, but decreased non-linear
beyond 12.5 °C. For both substrate amendments, the
decrease in CUE was most pronounced in forest soil. Such
land-use specific temperature response funcations have not
been observed previously. Implementing this hitherto
unrecognized phenomenon into the Q model revealed
considerably different responses of soil C stocks to changes
in climate conditions across land-uses. Our findings
emphasize the need to understand temperature responses of
microbial CUE for projecting soil C dynamics.

[1] Frey et al. (2013) *Nat. Clim. Change* **3**, 395-398. [2] Bölscher et al. (2016) *Biol. Fertil. Soils* **52**, 547-559. [3] Ågren & Bosatta (1998) *Theoretical Ecosystem Ecology – Understanding Element Cycling*. Cambridge Univ. Press.