Differential responses of litter decomposition in the air and on the soil surface to shrub encroachment in a graminoid-dominated temperate wetland

Background and aims Graminoid-dominated wetlands have been subjected to widespread shrub encroachment, yet the effect of this shift in species composition on litter decomposition remains unclear, especially in the standing-dead stage. Methods We collected labile ( Deyeuxia angustifolia ) and recalcitrant ( Carex schmidtii ) graminoid leaf litter from a freshwater wetland in northeast China, and used the litterbag method to characterize litter ash-free dry mass (AFDM) loss and nitrogen (N) release in the air and on the soil surface in open wetlands and two shrub islands ( Salix floderusii producing phenol-poor litter and Betula fruticosa producing phenol-rich litter) over 360 days of decomposition. Results Litter decomposition in the air and on the soil surface responded differentially to shrub expansion. In the air, AFDM loss and N release for both labile and recalcitrant litter were often lower in shrub islands than in open wetlands. On the soil surface, labile litter decomposition was decelerated in the presence of shrubs with phenol-rich litter, but accelerated in the presence of shrubs with phenol-poor litter. Despite the absence of significant differences in N release, recalcitrant litter AFDM loss was lower in shrub islands than in open wetlands after 360 days of decomposition on the soil surface. Conclusions Shrub encroachment retards litter decomposition in the air, but the effects on litter decomposition on the soil surface vary with shrub type and litter degradability in graminoid-dominated wetlands. Moreover, these findings emphasize that standing litter decomposition should be considered to enhance our understanding of shrub encroachment effects on litter decomposition in these wetlands.