Abstract: The shallow soil layer and pronounced nutrient limitations in subtropical karst ecosystems render litter decomposition and nutrient return crucial for maintaining ecological functions. Global changes involving continuous increase in nitrogen (N) and phosphorus (P) deposition may exacerbate nutrient limitations in karst regions, although the specific patterns of these impacts remain unclear. A one-year litter decomposition experiment under nutrient addition treatments was conducted to investigate the effects of N and P addition on litter decomposition and nutrient release in a karst forest. The results demonstrated that: (1) Sole N addition was found to significantly inhibit litter decomposition (-9.50%), particularly during the early decomposition phase (0-180 d), while N+P co-addition significantly enhanced decomposition (+10.46%) in the later phase (180-360 d). No significant effect was observed with P addition alone. (2) N addition was shown to significantly suppress cellulose decomposition (-21.04%) in the early phase and lignin decomposition (-19.57%) in the later phase. Conversely, N+P co-addition significantly promoted lignin decomposition (+26.46%) initially and cellulose decomposition (+20.76%) subsequently. (3) N addition inhibited the release of the majority of elements, although significant inhibition was only achieved for N, Fe, and Mn during the later decomposition stage. Combined N+P addition promoted the release of most elements, but significant promotion was only observed for K in the early stage and for C and N in the later stage. All nutrient addition treatments inhibited P release, but significant inhibition was only attained under sole P and combined N+P additions in the later stage. Sole P addition was observed to have no significant effect on the release of other nutrients. (4) Significant positive correlations were identified between decomposition rates and the release of C, N, trace elements, as well as C/P and N/P ratios, highlighting the co-limiting effects of N and P with particular emphasis on P stoichiometric regulation. The N-P co-limitation in karst ecosystems was revealed to induce suppression or null effects on litter decomposition and nutrient release under single nutrient additions, whereas synergistic enhancement was achieved through N-P co-addition. These findings emphasize the critical importance of considering N-P co-limitation in nutrient cycling studies of karst ecosystems.

Key words: litter decomposition, nitrogen and phosphorus addition, nutrient release, nitrogen and phosphorus co-limitation, karst forest