Effects of leaf litter tannin on soil ammonium and nitrate content in two different forest soils of mount Wuyi, China

In forest ecosystems, plant secondary metabolites, such as tannin, affect soil net nitrogen (N) mineralisation and nitrification. However, the mechanisms underlying the effects of tannin in plant leaf litter on N transformation in subtropical acidic soils containing high levels of inorganic N in the form of NH4+-N occurs. An incubation experiment was carried out using two litter types and tannin acid addition into Hapludults (Hu) and Haplohumults (Ho) soils in order to determine if (i) addition of leaf litter with higher tannin content might decrease soil N content, and (ii) effect modes of litter or tannin addition on NH4+-N and NO3--N are different. The net production rate, relative change rate and relationship between effects of tannin and litters on NH4+-N or NO3-N were analyzed. Our results suggested that addition of each species litter into forest soil decreased soil inorganic N content, and the effects on NO3--N and NH4+-N dynamics were related to tannin content of the litter. Further, soil inorganic N content was reduced at a higher rate in the short term for NH4+-N than for NO3-N. Inorganic N loss was dependent on the form of carbon released at different stages of litter decomposition. Microbial immobilisation of N did not completely account for the marked fall in N. In forest soils, the influence of plant litter tannin content on N transformations including nitrification and immobilisation were related to soil types. Our study enhances our understanding of how plant litter quality influences N transformation in forest ecosystems.