Root and leaf litter diversity below‐ground shape soil carbon stabilization and priming through altered decomposition dynamics

Abstract 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. Particularly, it is unknown how the soil may interact with (e.g. buffer or amplify) the expected redistribution of litter compounds between mixed litters (e.g. nitrogen N and tannins), with consequences for litter decomposition and the resulting carbon (C) fluxes controlling the SOC balance. To address this issue, we studied the (almost complete) decomposition of 13 C labelled single leaf and root litters and 15 realistic litter mixtures incorporated into soil in the laboratory. We studied litter mass loss and litter N concentration as litters decomposed, tracked litter 13 C 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). We expected nutrient (in particular N) transfer between litters with dissimilar initial nutrient content to (1) stimulate C loss in mixed litters, although this effect would be potentially hampered by litter tannins, to (2) enhance SOC formation, and (3) limit SOC mineralization. As hypothesized, litter diversity increased litter decomposition rates, yet not due to N transfer and immobilization in decomposing litters. Also unexpectedly, 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 lignin 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 below‐ground 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. Read the free Plain Language Summary for this article on the Journal blog.