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<p>The deep soil, <span class="inline-formula">&gt;1</span> m, harbors a substantial share of the global microbial biomass. Currently, it is not known whether microbial activity several meters below the surface is fueled by recently fixed carbon or by old carbon that persisted in soil for several hundred years. Understanding the carbon source of microbial activity in deep soil is important to identify the drivers of biotic processes in the critical zone. Therefore, we explored carbon cycling in soils in three climate zones (arid, mediterranean, and humid) of the Coastal Cordillera of Chile down to a depth of 6 m, using carbon isotopes. Specifically, we determined the <span class="inline-formula">¹³</span>C : <span class="inline-formula">¹²</span>C ratio (<span class="inline-formula"><i>δ</i>¹³</span>C) of soil and roots and the <span class="inline-formula">¹⁴</span>C : <span class="inline-formula">¹²</span>C ratio (<span class="inline-formula">Δ¹⁴</span>C) of soil organic carbon and CO<span class="inline-formula">₂</span>–C respired by microorganisms. We found that the <span class="inline-formula">Δ¹⁴</span>C of the respired CO<span class="inline-formula">₂</span>–C was significantly higher than that of the soil organic carbon in all soils. Further, we found that the <span class="inline-formula"><i>δ</i>¹³</span>C of the soil organic carbon changed only in the upper decimeters (by less than 6 ‰). Our results show that microbial activity several meters below the soil surface is mostly fueled by recently fixed carbon that is on average much younger than the total soil organic carbon present in the respective soil depth increments, in all three climate zones. Further, our results indicate that most decomposition that leads to enrichment of <span class="inline-formula">¹³</span>C occurs in the upper decimeters of the soils, which is possibly due to stabilization of organic carbon in the deep soil. In conclusion, our study demonstrates that microbial processes in the deep soil several meters below the surface are closely tied to input of recently fixed carbon.</p>