Physical protection enhances acclimation of microbial efficiency to chronic soil warming

Soils contain a large proportion of the carbon (C) stored in terrestrial ecosystems, and this C is sensitive to rising temperatures. The rate of soil C loss is mediated by microbial efficiency, itself controlled by the accessibility to substrates. To better understand the mechanism of warming-accelerated soil C loss, temperate forest soils were experimentally continuously heated (+5°C) for nearly three decades. To address how physical protection affects microbial C use efficiency (CUE) in response to climate warming, bulk soils were separated into macro- and micro-aggregates (250 - 2000 µm and < 250 µm) and incubated intact or crushed at 15°C and 25°C for 24 h, with CUE measured using a substrate-independent (H₂¹⁸O) method. We hypothesized 1) crushing would reduce CUE compared to intact soils if more complex C is released inside aggregates, 2) CUE under long-term warming would be lower relative to ambient soils because of more complex C, and 3) microaggregates would have lower CUE than macroaggregates because of more complex C. We found that for ambient soils, crushing reduced CUE at 15°C ; however, crushing did not affect CUE in heated soils, suggesting that long-term warming may have affected the SOM protected inside soil aggregates and microbial thermal responses. Crushing also increased the temperature sensitivity of CUE in macroaggregates of ambient soils. Long-term warming reduced CUE in intact soils at 15 °C, but did not affect CUE in crushed soils, suggesting that physical protection enhances CUE acclimation. Warming also increased the temperature sensitivity of CUE in intact soils. Microaggregates had lower CUE than macroaggregates at 15°C for intact soils, possibly due to more complex C and relatively lower microbial activities (i.e., growth). Modeling sensitivity analysis shows that microbial-mineral interactions and CUE weigh equally in mediating warming-destabilized soil C. Our findings suggest that physical protection of SOM promotes more efficient microbial communities in ambient soils, and maintains less efficient microbial communities over long-term soil warming.