In a silvi-medicinal system, the plant secondary metabolites (PSMs) released from medicinal herbs could affect tree litter decomposition and nutrient release. However, the specific effects of PSMs on arboreous litter decomposition are still not well understood. In this study, the extracts of nine types of medicinal herbs were used to treat Pinus armandii Franch. and Larix gmelinii (Rupr.) Kuzen. litter during a simulated half-year decomposition. The effects of the extracts on the decomposition and the N and P release of the conifer litter were investigated. The results indicated that most of the medicinal herb extracts significantly inhibited the late decomposition of P. armandii litter, whereas only two of them accelerated the entire decomposition process. Only a few significantly affected the decomposition of the L. gmelinii litter. Four of the nine types of extract significantly inhibited the N and P release of the P. armandii litter, while 3/9 and 6/9 inhibited the N and P release of the L. gmelinii litter, respectively. The accelerating effects of the extracts on the cellulase activity and the inhibitory effects on the polyphenol oxidase activity might be responsible for the early acceleration and late inhibition of litter decomposition, while the effects of the extracts on the activities of protease and phosphatase might not be the main reason for the inhibitory or accelerating effects on the N and P release. In general, the inhibitory effects of medicinal herbs on the nutrient cycling of ecosystems should be taken into consideration when building silvi-medicinal systems, especially in P. armandii forests.

Secondary metabolites in the leachates of medicinal herbs inhibit forest litter decomposition, affecting interspecific relationships and nutrient cycling in the silvi-medicinal system. In this study, a typical forest species in the Qinling Mountains area, Catalpa fargesii Bur., and nine species of common understory medicinal herbs were used as example species. A litter decomposition experiment was carried out by treating the C. fargesii litter with stem and leaf leachates (simulated using water extracts) from the medicinal herbs to investigate the effects on forest litter decomposition, nutrient release and soil enzyme activity. Lachates from Mentha haplocalyx Briq., Houttuynia cordata Thunb., Gynostemma pentaphyllum (Thunb.) Makino and Prunella vulgaris L. exhibited significant inhibitory effects on the decomposition and nutrient release of C. fargesii litter and on the activities of related soil enzymes. Consequently, it is suggested that plantation of these four medicinal herb species should be avoided under C. fargesii forests or interplanting density must be low to reduce inhibitory effects on litter decomposition and nutrient release.

[Objective] The effects of leachates from understory medicinal plants on decomposition of Pinus tabuliformis litter and soil enzyme activities were investigated to provide scientific basis for selecting suitable understory species.[Method] In this paper,and the effects of 9 leachates (water-extraction solution) from the stems and leaves of common medicinal plants (Taraxacum mongolicum,Corydalis bunF eana,Mentha haplocalyx,Houttuynia cordata,Asarum sieboldii,Nepeta cataria,Lonicera japonica,Gynostemma pentaphyllum and Prunella vulgaris) in typical Pinus tabuliformis forest in Qinling Mountains on litter decomposition,nutrient releases (C,N,P,Cu,Zn and Mn) and soil enzyme activities were studied.[Result] The leachates from Taraxacum mongolicum,Corydalis bungeana,Mentha haplocalyx,Houttuynia cordata,Lonicera japonica and Nepeta cataria had significant inhibitory effects on litter decomposition rates and release of nutrients (except L.japonica had no significant effect of N and N.cataria had no significant effect on N and P).The leachates from T.mongolicum,C.bungeana,M.haplocalyx,H.cordata and L.japonica lowered the activity of polyphenol oxidase.The leachates from L.japonica lowered activities of carboxymethyl cellulose,and N.cataria lowered activities of carboxymethyl cellulose and beta-glucosidase.[Conclusion] A.sieboldii,G.pentaphyllum and P.vulgaris were suggested to plant under P.tabuliformis forests,where other 6 species were not suggested as understory species or their inter planting density should be diminished to avoid the inhibitory effects.