Organo-organic interactions dominantly drive soil organic carbon accrual

Organo-mineral interactions have been regarded as the primary mechanism
for the stabilization of soil organic carbon (SOC) over decadal to
millennial timescales, and the capacity for soil carbon (C) storage has
commonly been assessed based on soil mineralogical attributes,
particularly mineral surface availability. However, it remains contentious
whether soil C sequestration is exclusively governed by mineral vacancies,
making it challenging to accurately predict SOC dynamics. Here, through a
400-day incubation experiment using 13C-labeled organic materials in two
contrasting soils (i.e., Mollisol and Ultisol), we show that despite the
unsaturation of mineral surfaces in both soils, the newly incorporated C
predominantly adheres to "dirty" mineral surfaces coated with
native organic matter (OM), demonstrating the crucial role of
organo-organic interactions in exogenous C sequestration. Such
interactions lead to multilayered C accumulation that is not constrained
by mineral vacancies, a process distinct from direct organo-mineral
contacts. The coverage of native OM by new C, representing the degree of
organo-organic interactions, is noticeably larger in Ultisol (~14.2%) than
in Mollisol (~5.8%), amounting to the net retention of exogenous C in
Ultisol by 0.2–1.3 g kg−1 and in Mollisol by 0.1–1.0 g kg−1. Additionally,
organo-organic interactions are primarily mediated by polysaccharide-rich
microbial necromass. Further evidence indicates that iron oxides can
selectively preserve polysaccharide compounds, thereby promoting the
organo-organic interactions. Overall, our findings provide direct
empirical evidence for an overlooked but critically important pathway of C
accumulation, challenging the prevailing “C saturation” concept that
emphasizes the overriding role of mineral vacancies. It is estimated that,
through organo-organic interactions, global Mollisols and Ultisols might
sequester ~0.1–1.0 Pg C and ~0.3–1.7 Pg C per year, respectively,
corresponding to the neutralization of ca. 0.5%–3.0% of soil C emissions
or 5%–30% of fossil fuel combustion globally.