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Severe drought impacts soil organic carbon (SOC) cycling. Yet, there is limited understanding of drought effects on rhizosphere C allocation and its fate in the soil, since belowground C contributes to new SOC formation while fueling soil microbial communities and SOC mineralization. Here, we quantified rhizosphere C inputs and losses in a 17-year long irrigation experiment in a dry Scots pine forest using 13 C-enriched soil ingrowth bags with different mesh sizes. Fungal and bacterial communities inside the ingrowth bags and in adjacent soils were analyzed by Illumina MiSeq sequencing.

After two years, net new SOC accumulation was 5 times greater in root-accessible vs root-exclusion bags (1000-vs 20-μm mesh). Irrigation stimulated new SOC formation in the first year as compared to natural drought both in root-accessible (+ 26%) and root-exclusion bags (+ 47%). Losses of “old” 13 C-enriched SOC …