Land-use alters the temperature response of microbial carbon-use efficiency
Résumé
Microbial soil respiration is a major flux in global carbon dioxide (CO2) emissions and small changes in the vast flux may seriously modify global climate. The amount of CO2 released during decomposition of soil organic carbon (C) is determined by the microbial community’s carbon-use efficiency (CUE). This property 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 and varies among land-use systems. Still, 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 using isothermal calorimetry and integrated our findings in a soil C model4. Soils were sampled from arable, grassland, ley-farming, and forest research sites exposed to a boreal climate (64°07´N, 19°27´E). The samples were amended with two carbon substrates of different molecular complexity and incubated at a set of temperatures ranging from 5 to 20 °C. CUEs were calculated from cumulative heat production and residual substrate when 15 % of the added substrate was utilized. Land-use specific temperature response functions were fitted to the CUE data and incorporated into the Q model using Swedish soils in Northern and Southern Sweden. In the arable soil system, CUE stayed constant over the range from 5 to 20 °C, while in ley farming, grassland and forest soils CUEs were constant from 5 to 12.5 °C, but decreased nonlinear beyond 12.5 °C. The decrease in CUE was most pronounced in forest soil, and our general findings were independent from the substrate amendment. Implementing this hitherto unrecognized phenomenon into the Q model revealed considerably different responses of soil C stocks across land-uses to changes in climate conditions. Our findings emphasize the need to understand temperature responses of microbial CUE. Further studies should establish reliable proxies for CUE which can be exploited in modelling approaches.
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