Drought-induced reductions in plant residue decomposition across organs consistently differ between two tree species (gymnosperm and angiosperm) in drylands

Abstract Drought strongly influences plant residue decomposition and forest carbon cycling, and these effects are mediated by tree species traits. Although angiosperm residues generally decompose faster than gymnosperm residues due to higher nutrient concentrations, it remains unclear whether drought effects differ consistently between tree species across different plant organs and how underlying traits drive these differences. Herein, we examined the decomposition of wood, fine roots and leaf litter from Pinus tabuliformis (gymnosperm) and Robinia pseudoacacia (angiosperm) under 0%, 40%, and 80% rainfall reduction on the Loess Plateau, China. Drought significantly decreased plant residue decomposition, with the magnitude of reduction varying with both drought intensity and tree species. Decomposition of all plant residues declined more under 80% rainfall reduction than under 40% rainfall reduction. Moreover, under 80% rainfall reduction, decomposition declined significantly more in R. pseudoacacia than in P. tabuliformis, whereas no significant difference was observed under 40% rainfall reduction. This is because R. pseudoacacia has lower carbon and lignin concentrations, lower Proteobacteria abundance, and higher phosphorus concentration and Actinobacteriota abundance, yielding greater moisture sensitivity of its residue decomposition. Furthermore, drought-induced reductions in decomposition were greater for wood and leaf litter than for fine roots. Variation partitioning analysis and structural equation modeling further demonstrated that interactions among residue moisture, traits, and microbes jointly governed decomposition. Our findings highlight that contrasting tree species traits underlie drought-induced reductions in residue decomposition in drylands, which have important implications for predicting dryland carbon fluxes under future climate change characterized by increasing drought intensity and forest mortality.