Abstract: During the decomposition of plant litter mixture, interactions might cause significant nonadditive effects, which could lead to unpredictable variations in decomposition when compared to those predicted from single litter decomposition. Such variations would affect decomposition rate and nutrient release. This will also be relevant when considering  how increasing environmental nitrogen concentrations affect such interactions in the decomposition of litter mixture. In this study, the decomposition of litter from Pinus tabuliformis, Robinia pseudoacacia, and their mixture were examined in a 150day indoor experiment under 0, 4, 8, and 12 g N·m^-2·a^-1 nitrogen addition. At the end of the experiment, different litters were separated according to morphological differences. Then, their decomposition processes were compared with the corresponding monospecific decomposition processes, in order to investigate the effects of nitrogen deposition on the interaction of litter types during mixed decomposition. The results showed that (1) when there was no nitrogen deposition, P. tabuliformis litter significantly inhibited the decomposition and C release of R. pseudoacacia litter during mixed decomposition (P<0.05). Such inhibition effect was eliminated by 4 g·m^-2·a^-1 nitrogen deposition, but stimulated by 8 and 12 g·m^-2·a^-1 nitrogen deposition (P<0.05). (2) R. pseudoacacia litter significantly inhibited the N release of P. tabuliformis lit ter during the first 1-2 months of decomposition in all treatments, whereas N release of R. pseudoacacia litter was only inhibited by P. tabuliformis litter under 8 g·m^-2·a^-1 nitrogen deposition (P<0.05). (3) Under no nitrogen deposition, P. tabuliformis litter significantly inhibited P release of R. pseudoacacia litter (P<0.05), and such effects were eliminated by all nitrogen treatments. (4) In general, during early decomposition, slight nitrogen deposition tended to alleviate the inhibition of P. tabuliformis litter on the decomposition and nutrient release of R. pseudoacacia. However, moderate and heavy nitrogen deposition tended to intensify the inhibitory effects and thus hinder nutrient cycling in mixed forests.

Key words: nitrogen addition, non-additive effect, mutual effect, decomposition rate, nutrient release.