| Tannins are secondary metabolites of higher plants,which bind proteins in litter or soil to form recalcitrant tannin-protein complexes that are resistant to most microorganisms,therefore slow down soil nitrogen（N）mineralization.In the ecosystem where the dominant species is tannin-rich plants,a majority of organic N in the form of recalcitrant tannin-protein complexes is present in the soil N pools.Microbial depolymerization of the complexes may be a rate-limiting step in soil organic N decomposition.However,due to the complexity of tannin-protein complexes,the microbial degradation mechanism of the complexes has not been elucidated to date.In Tibetan alpine meadow,tannin-rich Polygonum viviparum is the dominant species in weed meadows,tannins of its entered into the soil may affect the decomposition of soil N,therefore,tannins in P.viviparum were used as the object in this study,the following studies were carried out by combining the field and indoor experiments:（1）in the indoor short-term（28-day）culture experiment,the effects and mechanisms of different types of tannins on soil N mineralization in P.viviparum grassland were clarified;（2）tannin-15N protein complexes were injected into the in-situ soil of the P.viviparum（ectomycorrhiza）,Kobresia capillifolia（non-mycorrhiza）and Elymus nutans（arbuscular mycorrhiza）grasslands,after short-term（4-day,28-day）decomposition experiment,the soil microbial degradation mechanism of tannin-protein complexes under field conditions and its effect on the decomposition products of complexes used by different plants were revealed;（3）in the indoor short-term（14-day）culture experiment,the regulation mechanism of carbon（C）and N input on tannin-protein complexes decomposition in P.viviparum soil was elucidated.The results will help to reveal the microbial degradation mechanism of complex organic N in alpine meadow soils,and provide a theoretical basis and new perspective for understanding the mechanism of plant tannins affecting soil N decomposition process in alpine meadows.The main results of this study are as follows:（1）The light-fraction,heavy-fraction and unfractioned tannins extracted from the leaf litter of P.viviparum significantly reduced the net N mineralization in P.viviparum soil（P<0.05）,decreased by 140%,88%and 131%,respectively,and the reduction of net N mineralization by light-fraction tannins with low mean degree of polymerization and high ratio of protocyanidin:proanthocyanidin monomer was significantly higher than that of heavy-fraction and unfractioned tannins（P<0.05）.The three types of tannins can significantly inhibit soil laccase and peroxidase activities,while polyphenol oxidase activity was only significantly inhibited by heavy-fraction tannins（P<0.05）.The three types of tannins significantly promoted N-acetyl-β-D-glucosidase（NAG）activity（P<0.05）.The decrease of soil net N mineralization after tannin additions was significantly positively correlated with the decrease of oxidoreductase（laccase and peroxidase）activities,indicating that tannins mainly reduced soil net N mineralization by inhibiting oxidoreductase activities.（2）After 28 days of decomposition of tannin-15N protein complexes in situ soil at the P.viviparum,K.capillifolia and E.nutans grasslands,the increase in the relative abundance of soil-specific saprotrophic fungi（saprotrophic fungi that the relative abundance of the complex treatments was significantly higher than that of the controls）was significantly positively correlated with the activities of soil oxidoreductases（peroxidase）andN-acquiringenzymes（NAG,leucine-aminopeptidase（LAP））;the total decomposition rate of the complexes was significantly positively correlated with peroxidase activity.The results suggest that soil-specific saprophytic fungi may play a dominant role in the decomposition of tannin-protein complexes.With the increase of complex treatment time（4 to 28 days）,the 15N recovery in soil microbial biomass gradually decreased,while 15N recovery by plant gradually increased,indicating that 15N in the complex decomposition product gradually transferred from microbial N pools to plant N pools.15N recoveries of the three dominant plants,P.viviparum,K.capillifolia and E.nutans were 0.94%,1.32%and2.50%,respectively.73.6%of the variation in plant 15N recovery were explained by plant traits（root biomass,total plant N and C:N ratio）,inorganic N content and the relative abundance of soil-specific saprotrophic fungi,indicating that soil-specific saprophytic fungi were one of the important factors affecting the acquisition of nitrogenous decomposition products of protein-tannin complexes by plants.（3）After exogenous C and N were added to the soil of tannin-15N protein complex,the total decomposition rate of the complexes under high C and high N combination addition significantly increased（P<0.05）.The increase in the amount of C and N combination addition was significantly positively correlated with the increase in soil N-acquiring enzyme（NAG and LAP）activity,indicating that C and N combination addition promoted the secretion of N-acquiring enzymes by microorganisms and helped to accelerate the decomposition of tannin-protein complexes.The drivers of C addition and C and N addition affecting the decomposition process of tannin-protein complexes were inconsistent.The decrease in 15N recovery of total dissolved N(TDN-15N)was mainly explained by the decrease in NO3--N content under C addition alone,and its net effect was 42.0%;the increase in TDN-15N recovery was mainly explained by the increase in N-acquiring enzyme（NAG）activity under C and N combination addition,and its net effect was 14.0%.15N recovery of soil microbial biomass under low C addition only significantly increased（P<0.05）.The results show that the C input alone leads to microorganisms fixing NO3--N,resulting in a decrease in TDN-15N recovery;the C and N combination input promoted the decomposition of tannin-protein complexes by promoting the release of N-acquiring enzymes by soil microorganisms.In summary,tannins significantly reduced soil net N mineralization in alpine meadow,and formed recalcitrant tannin-protein complexes in soil organic N pools.The decomposition of recalcitrant tannin-protein complexes is mainly driven by soil-specific saprotrophic fungi,and this decomposition process is regulated by the input of exogenous C and N such as plant root exudates,therefore,tannins have an important ecological role in promoting N nutrient feedback between plants and soil microorganisms. |
