Litter stoichiometric traits have stronger impact on humification than environment conditions in an alpine treeline ecotone

Abstract

Litter humification is vital for carbon sequestration in terrestrial ecosystems. Probing the litter humification of treeline ecotone will be helpful to understand soil carbon afflux in alpine regions under climate change. Foliar litter of six plant functional groups was chosen in an alpine treeline ecotone of the eastern Tibetan Plateau, and a field litterbag decomposition experiment (669 days) was conducted in an alpine shrubland (AS) and a coniferous forest (CF). Environmental factors, litter quality, humus concentrations (total humus, Huc; humic acid, HAc; and fulvic acid, FAc) and hue coefficient (ΔlogK and E4/E6) were measured to explore litter humification processes. Litter humification was controlled by both litter stoichiometric traits and local-environment conditions, while stoichiometric traits played a more obvious regulatory role. Significant discrepancies in litter humus were detected among six plant functional groups; more precisely, litter of evergreen conifer and shrubs showed a net accumulation of Huc and FAc during winter, whereas others experienced more mineralization than accumulation. Huc, HAc, and hue coefficient were mainly controlled by cellulose/N, cellulose/P, C/N, lignin/P, lignin/N, etc., yet FAc was more susceptible to local-environment conditions. Meanwhile, Huc, HAc and FAc, as well as humification degree and E4/E6 differed between AS and CF, with faster humification in AS. We suggest that litter stoichiometric traits are more responsible for regulating litter humification than environmental conditions in elevational gradients. Furthermore, potential upward shifts by plants may accelerate litter humification in alpine ecosystems.