Land-use alters the temperature response of microbial carbon-use efficiency in soils – a consumption-based approach

Abstract

Soil organic carbon (SOC) is a substantial source of atmospheric CO2, but also a large cause of uncertainties in Earth-system models. A principal control on soil CO2 release is the carbon-use efficiency (CUE) of microbial communities, which partitions the carbon (C) allocation between biosynthetic stabilization and CO2 respiration during SOC decomposition. In Earth-system models, CUE is commonly considered as a constant, although it should be susceptible to environmental factors such as temperature. We explored CUE across a set of land-uses and temperatures, and we show the hitherto neglected phenomenon that land-use can alter the temperature response of CUE. In arable soils, CUE was constant over a temperature range between 5 and 20 °C, but it decreased with temperature in ley farming, grassland, and forest soils at temperatures above 12.5 °C. The decrease in CUE was strongest for forest soils. Implementing our findings into a soil-C model revealed substantial differences in projected SOC losses: Assuming an increase of mean annual temperature of 2 or 4 °C, soils were projected to lose up to 6 or 15% of their current SOC, respectively, until they reach a new steady-state. These projections varied among land-uses. Our findings confront the current representation of CUE in global C models and challenges C sequestration strategies based on land-use changes, because land-uses such as e.g. forest ecosystems with current high C storage may lose substantially more C than agricultural soils due to strong declines of CUE.