Co-elevated CO₂ and temperature and changed water availability do not change litter quantity and quality of pine and oak

Elevated CO₂ concentration ([CO₂]) and air temperature (T_air) as well as changed soil water availability (W_soil) may affect quantity, chemistry, and microbial decomposability of tree leaf litter. However, our understanding is limited mainly to the effect of elevated [CO₂]. This study investigated the effects of elevated [CO₂] and T_air in combination with two W_soil regimes on the quantity and chemistry including the ratio of lignin to nitrogen (lignin/N) of litter produced by Pinus densiflora and Quercus variabilis saplings, and microbial respiration of the soils amended with the litters. Either elevated [CO₂] or high W_soil alone increased litter production; meanwhile elevated T_air alone decreased litter production. However, co-elevation of [CO₂] and T_air did not change litter production regardless of W_soil regime for both species. Among litter chemistry, the lignin/N, which is a robust indicator of litter decomposability, of litter was changed in parallel with litter quantity (i.e., lignin/N ratio increased when litter quantity increased and vice versa) mainly due to dilution of N. Due to the opposite effect of warming and elevated [CO₂] on litter quantity, lignin/N was not changed under co-elevated [CO₂] and T_air at a given W_soil regime for both species. Other litter chemistry including non-structural carbohydrates and minerals was also affected by [CO₂], T_air, or W_soil. However, changed litter chemistry did not change the CO₂ emission from the soils amended with the litters; however, addition of litter with low lignin/N and high nutrients increased microbial biomass in the soil. This study enlarges our understanding of the effects of changed climatic variables on litter quantity, chemistry, and microbial decomposability and suggests that co-elevation of [CO₂] and T_air may not cause a significant change in the litter parameters regardless of W_soil. Study with mature trees at a natural forest should further improve our understanding.