Shifts in the microbial community structure explain the response of soil respiration to land-use change but not to climate warming

Soil stores more carbon (C) than plants and atmosphere combined and it is vulnerable to increased microbial respiration under projected global changes including land-use change and future climate scenarios (mainly elevated temperature). Land-use change is known to have a direct impact on soil organic C and soil respiration (Rs) but the mechanisms that drive these changes remain debatable. Similarly, recent studies and simulation models predict that Rs will respond positively to projected climate warming. However, there are significant uncertainties in the magnitude and mechanisms of this feedback response of Rs to global change. To identify the mechanisms of Rs response to land-use change and climate warming, we first investigated Rs from different land use types. Soil respiration was estimated seasonally from four different Scottish land uses: moorland, birch woodland, grassland and pine forest (n = 24). Our results demonstrated that despite a dramatic loss of total C and nitrogen (N) in the soils under birch trees, the Rs in the birch woodland was similar to that of the moorland and pine forest, with Rs in the grassland being significantly higher. The microbial community structure, estimated by Multiplex Terminal-Restriction Fragment Length Polymorphism (MT-RFLP) and 454 pyrosequencing, was significantly different under each land use type. A strong correlation of Rs with soil properties (pH, inorganic N, C:N ratio and moisture) and with microbial community structure was identified.To test the impact of elevated temperature on Rs and to identify potential microbial mechanisms, we performed laboratory incubation studies. Soils from different land uses were incubated at 7 °C (mean annual temperature (MAT) in Scotland) and 10 °C (MAT + 3 °C) with and without the presence of a labile (13C-glucose) and recalcitrant (13C-lignin) form of C to identify the active groups of microbes and to determine the role of substrate availability on feedback response. The warming treatment induced an increase in Rs rates in all soils. The magnitude of the Rs response to warming was modulated by the land use types, and the Rs was more prominent in soils with high C contents. The addition of glucose substantially increased both total and rate of Rs compared to no substrate- and lignin-amended soils, providing evidence of labile C depletion as a mechanism for the thermal response of Rs. The warming treatment did not impact the composition of the active or total microbial community as revealed by phospholipid fatty acid-stable isotope probing (PLFA-SIP), MT-RFLP and 454 pyrosequencing. Our results showed that the microbial metabolic activity was higher under warming treatment suggesting that a positive feedback of Rs to increased temperature is mediated by changes in substrate availability and microbial metabolic rates