Root and leaf litter diversity belowground shape soil carbon stabilization and priming through altered decomposition dynamics

Root and leaf turnover is a major contributor to soil organic carbon (SOC)
dynamics and storage. However, while roots of multiple species occur in
soils, and leaves are often incorporated to soil by bioturbation
processes, it remains unknown how litter mixing in soils influences litter
decomposition processes, and the associated carbon (C) fluxes controlling
the SOC balance. To address this issue, we studied the (almost complete)
decomposition of 13C labeled single leaf and root litters and 15 realistic
litter mixtures incorporated into soil in the laboratory. We studied
litter mass loss and litter nitrogen concentration as litters decomposed,
tracked litter 13C into the mineral-associated organic matter (MAOM) and
particulate organic matter (POM) fractions of the SOC, and quantified
litter influence on native SOC mineralization (i.e., priming). Litter
diversity increased litter decomposition rates, yet not due to nitrogen
transfer and immobilization in decomposing litters. Litter tannins
dissimilarity enhanced decomposition rates. Plant litter diversity
enhanced soil MAOM-C formation, but had a negligible impact on overall SOC
stocks. This occurred because SOC formation and priming mixture effects in
the MAOM fraction were counterbalanced by those in the POM fraction. In
mixed litters, the dissimilarity in litter manganese (Mn) was responsible
for non-additive MAOM-C gains and POM-C losses, suggesting Mn transfer
between litters that foster lignins degradation and incorporation into
soil MAOM. Litter dissimilarity in Mn also reduced MAOM-C and increased
POM-C priming. Overall, we demonstrate that diversity effects, potentially
implying nutrient transfer between litters, can occur belowground among
root litters, as well as biologically incorporated leaf litters. It
further shows that different nutrients are implicated in diversity effects
controlling litter decomposition rate and soil C storage.