Effects Of Nitrogen A Ddition And Precipitation Reduction On Nitrogen Acquisition And Carbon And Nitrogen Cycle Of Main Tree Species In A Korean Pine Broadleaved Forest

Roots of nearly all plants have a symbioses relationship with mycorrhizal fungi.The root-mycorhizosphere serves important roles in nutrient acquisition required for the plant growth,but also in soil carbon and nitrogencycling of terrestrial ecosystems.The identification of root-mycorhizosphere traits that can be linked to nutrient acquisition is of wide interest,especially for understanding and predicting plants nutrient acquisition strategies and ecosystem processes under climate changes.Nonetheless,variations of tree root and mycorrhizal fungi traits are still rarely quantified synchronously in previous studies.Climate change may alter plant nutrient uptake strategies and preferences,thereby affecting carbon and nitrogen cycling in ecosystems.Therefor,we conducted a decreased precipitation and N addition experiment.Since 2009,four treatment methods have been established in Changbai Mountain Nature Reserve,including the control,N supply(+N,50 kg N hm-2·yr-1),decreased precipitation(-P,30%of throughfall,approximate 200 mm·yr-1)and N addition combined with decreased precipitation(+N-P).The effects of long-term nitrogen addition and precipitation decrease on nitrogen acquisition strategies and carbon and nitrogen cycling of three main tree species in temperate forests of northern China,Korean pine,Fraxinus mandshurica and Tilia amurensis,were studied by using stable isotope technique.The main results are as follows:1.N addition significantly increased individual root length,individual root surface area and mean root diameter in low order roots(the first root order or the first to third root orders),while decreased or unchanged these traits in high root orders(the fourth and fifth root orders).N addition tended to decrease branching intensity in all root orders across all species.Decreased precipitation significantly increased individual root length and individual root surface area of low order roots(the first and second root orders),while decreased or unchanged these traits in others root orders across all species.Specific root length was significantly increased in the first to fifth root orders with decreased precipitation and decreased in the second to fifth root orders with N addition(except Pinus koraiensis).These results indicated that N addition and decreased precipitation significantly changed fine root traits,which would alter the belowground foraging strategies.2.N addition significantly decreased total hyphae length,and significantly increased the mean hyphae diameter.While decreased precipitation significantly increased the total hyphae length and the total hyphae surface area.The impacts of N addition and decreased precipitation on mycorrhizal fungi traits also were mediated by tree species.Across all species,N addition,decreased precipitation and their interaction led to significantly decreased mycorrhizal colonization.3.All three tree species preferred ammonium(NH4+)over glycine and nitrate(NO3-),contributed 73%to the soil total N uptake.Uptake of glycine was higher than that of NO3-.Precipitation reduction,N supply,and their interaction increased NH4+ uptake rate compared with control.Precipitation reduction decreased glycine uptake rate compared with control treatments;precipitation reduction decreased NO3-uptake rate compared with N supply.N addition and decreased precipitation changed the root morphological traits and mycorrhizal colonization,but did not change the total N absorption amount among all tree species.4.?15N values of foliage,litterfall and surface soil layer(0-10cm)total N were significantly increased by N addition,while ?15N values of fine roots and coarse roots were significantly decreased.N addition also significantly increased ?13C value of fine roots and total N concentration of surface soil layer compared with the control.The C concentration,?15N and ?13C values of foliage and ?15N values of fine roots were significantly increased by precipitation reduction,while N concentration of foliage and litterfall significantly decreased.Moreover,combined effects of N addition and precipitation reduction significantly increased the ?15N and 813C values of foliage and increased ?15N values of fine roots and ?13C values of litterfall.Furthermore,foliar ?15N values were significantly correlated with foliage ?13C values,surface soil ?15N values,surface soil N concentration and C concentrations.Foliage N concentrations and ?13C values were significantly correlated with fine roots ?15N values and N concentrations.In summary,The morphological changes of roots in different root orders under nitrogen addition and precipitation reduction implied that roots in different branching positions had different morphological structures and physiological functions.Nitrogen addition and precipitation reduction significantly changed the morphological characteristics of mycorrhizal fungi,thus changing the strategy of underground nutrient acquisition.This means that under climate change conditions,tree species can improve resource acquisition by regulating the characteristics of root-mycorrhizal fungi.At the same time,in temperate forest,fine roots of three tree species have high plasticity in nitrogen uptake,and have a certain response to nitrogen addition and precipitation reduction.This indicates that dominant tree species have strategies to adapt to environmental changes from a plant nutritional aspect.Plants increasingly take up the heavier 15N under N addition and take up the heavier 13C and 15N under precipitation reduction,suggesting that N addition and precipitation reduction may affect plant nutrient acquisition strategies and ecosystem carbon and nitrogen cycle.