Elevated moisture stimulates carbon loss from mineral soils by releasing protected organic matter. Issue 1 (December 2017) - Huang, Wenjuan
Abstract Moisture response functions for soil microbial carbon (C) mineralization remain a critical uncertainty for predicting ecosystem-climate feedbacks. Theory and models posit that C mineralization declines under elevated moisture and associated anaerobic conditions, leading to soil C accumulation. Yet, iron (Fe) reduction potentially releases protected C, providing an under-appreciated mechanism for C destabilization under elevated moisture. Here we incubate Mollisols from ecosystems under C3 /C4 plant rotations at moisture levels at and above field capacity over 5 months. Increased moisture and anaerobiosis initially suppress soil C mineralization, consistent with theory. However, after 25 days, elevated moisture stimulates cumulative gaseous C-loss as CO2 and CH4 to >150% of the control. Stable C isotopes show that mineralization of older C3 -derived C released following Fe reduction dominates C losses. Counter to theory, elevated moisture may significantly accelerate C losses from mineral soils over weeks to months—a critical mechanistic deficiency of current Earth system models. The effect of soil moisture on microbial activity and soil carbon storage remains unclear. Here, via Mollisol incubation experiments, the authors show elevated soil moisture can accelerate total carbon loss by facilitating microbial access to previously protected carbon, released following iron reduction.