Global patterns and drivers of initial plant litter ash concentration

Ash is a fundamental component of plant litter and plays a vital role in regulating litter decomposition. However, to date, global patterns and underlying mechanisms of initial litter ash concentrations remain unclear. Here, we used 570 observations collected from 104 independent publications to assess the global patterns of initial plant litter ash concentrations and evaluated the effects of mycorrhizal association [arbuscular mycorrhiza (AM) vs. ectomycorrhiza (ECM)], taxon group (gymnosperm vs. angiosperm), life form (tree vs. shrub vs. herb), leaf type (broadleaf vs. needle), and environmental factors such as climate and soil properties on initial litter ash concentration. The results showed that (1) global average ash concentrations varied significantly among different plant tissues and were 7.3, 4.5, 3.7, 3.5, 3.1, 2.4, and 1.5% in leaf, root, bark, reproductive tissue (flower and fruit), branch, stem, and wood litter, respectively; (2) in leaf litter, the initial ash concentrations of AM plants and species associated with both AM and ECM fungi were higher than those of ECM plants, and those of the tree species were lower than those of the herbs and shrubs; in root litter, the initial ash concentrations of the AM plants were lower than those of the species associated with both AM and ECM fungi but higher than those of the ECM plants; in both leaf and root litter, the initial ash concentrations of the angiosperms and broadleaf trees were higher than those of the gymnosperms and needle trees, respectively, while the effect of plant traits on branch litter was not obvious; and (3) the initial ash concentration of leaf litter was predominantly driven by mycorrhizal association and taxon group, while that of root litter tended to be driven by mycorrhizal association well as soil organic carbon. Our study clearly assessed the global patterns and underlying mechanisms of initial plant litter ash concentrations, which could help in better understanding the role of ash in litter decomposition and the related processes of carbon and nutrient cycling.