Drought reduces soil carbon inputs by roots and mycorrhizal fungi and alters soil microbial communities in a pine forest

Drought impacts soil organic carbon (SOC) cycling. Yet, there is limited understanding of how water limitation affects C inputs from rhizosphere, which contribute to new SOC formation while fueling soil microbial communities. We quantified C inputs and losses from roots and mycorrhizal fungi after two decades of irrigation in a dry Scots pine forest using ¹³C-enriched soil ingrowth bags. Fungal and bacterial communities in the ingrowth bags and in adjacent soils were analyzed by Illumina MiSeq sequencing.

In the first year, the new SOC formation was stimulated by water addition as compared to natural drought both in root-accessible (+25%) and mycorrhizal-accessible (+50%) bags. After two years, the overall new SOC formation was 5 times greater in root-accessible than in mycorrhizal-accessible bags. Although root ingrowth increased by 70% in root-accessible bags, the irrigation had a limited effect on the amount of new C accumulated in root-accessible and mycorrhizal-accessible bags. The lacking irrigation effect on net new SOC formation may relate to higher respiratory losses of new C, which agrees well with the observed increase by 55% in old C losses under irrigation. This suggests that enhanced C inputs by roots and mycorrhizal fungi were rapidly mineralized under irrigated conditions. Increased supply and turnover of rhizosphere C under irrigation were paralleled by shifts in fungal and bacterial communities in ingrowth bags as well as in adjacent soils. Accordingly, the presence of roots was a main driver of fungal and bacterial community structures in the ingrowth bags.

Overall, our results indicate that naturally dry conditions slow SOC cycling, suppressing rhizosphere C inputs as well as C losses. The reduced supply of belowground C leads to cascading effects on soil microbial community composition under drought.