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Human disturbance and climate change both influence the input quantity of plant litter, significantly affecting soil respiration (R-s). However, what leads to a change of relationship between R-s in on-litter input (R-s(NL)) and natural litter input (R-s(CL)) is not yet fully understood. This study selected a man-made Populus forest in a dune-meadow transitional region to observe R-s(NL) and R-s(CL), soil temperature and soil moisture at a depth of 0-10 cm, groundwater depth between April and October in 2014 and 2015, and diurnal variations were carried out during the early, middle, and late stages of the growing seasons. Complex R-s diurnal patterns were observed in different growth stages. The soil moisture beyond field capacity inhibited root respiration and soil microbial activities, thus making R-s(NL) smaller than R-s(CL). When soil moisture was below field capacity with the increase of groundwater depth, dry air and low litter moisture content resulted in the shielding effect of the litter on R-s beyond its decomposition, thus making R-s(NL) consistently larger than R-s(CL). The temperature sensitivity was significantly affected by soil moisture. These results indicate that interactions among the quantity of litter, soil temperature, soil moisture and groundwater, which cause diurnal variations in R-s were diverse. Negative litter respiration may not be a measurement error, but caused by soil moisture which changes the relationship between R-s(NL) and R-s(CL). Thus, this study shows that plant litter has a significant influence on R-s in the dune-meadow transitional region. The findings of this study will provide useful information to understand these fragile ecosystems and the terrestrial carbon cycle.