Nitrogen addition promoted litter decomposition in semi-arid grasslands.Decomposition of high-nitrogen litter was inhibited.N:P ratios in low-N litter remained stable throughout decomposition.Fungal/bacterial intra-kingdom interactions were mainly positive.These findings can improve predictive models of litter decomposition under changing nutrient inputs.The nitrogen (N) input can exert a dual effect on litter decomposition, depending on the litter quality. Additionally, increased N input alters plant nutrient composition, which directly impacts plant residue decomposition. However, this effect remains understudied, particularly in grassland ecosystems. We obtained two types of Leymus chinensis litter (low-N and high-N) from a long-term N addition experiment. A 730-day litter decomposition experiment was conducted to examine mass loss, nutrient release, stoichiometric changes, and microbial community dynamics. The results show that N addition increased litter mass loss by approximately 10.55%, and the mass loss of low-N litter increased by 10.14%. Furthermore, acid-unhydrolyzable residue accumulated over time, with greater accumulation in high-N litter, which may be a key factor underlying the slower decomposition of high-N litter. Another key factor may be the persistently high N:P ratio in high-N litter during decomposition, potentially making it more susceptible to P limitation. Our findings highlight that changes in litter quality under exogenous nutrient inputs play a key role in regulating decomposition, offering insights to improve predictive models of litter decomposition under changing nutrient inputs.