| As an important global change factor,atmospheric nitrogen deposition is becoming more and more serious due to the influence of human activities.The research shows that nitrogen deposition has an important effect on the material cycle and energy conversion of forest ecosystem,and the root and their symbiotic microorganisms are at the core of this ecological process.The change of soil physicochemical properties and plant root secretion characteristics caused by nitrogen deposition are bound to affect the diversity and community structure of soil microorganism,and directly affect the transformation and balance of rhizosphere nutrient.Arbuscular mycorrhizal fungi（AMF）,as an important functional group of soil microorganisms,can establish symbiotic relationship with plants and play an important role in promoting nutrient uptake by roots.Manganese is a necessary trace element for plant growth,and the soil acidification caused by nitrogen deposition can change the bioavailability of manganese,which may significantly regulate the decomposition rate of refractory components of litters（such as lignin）,in addition,excessive absorption of manganese may cause significant toxic effects on plants.In order to further clarify the influence of nitrogen deposition on underground ecological process in forest.11 a and 18 a Chinese fir plantation were chosen as study objects,three nitrogen addition levels were set(CK,LN,HN,namely,0,5,10 g N·m-2·a-1),nitrogen deposition has been simulated through continuous nitrogen（NH4NO3）addition since April 2019.In this paper,the effects of short-term simulation of nitrogen deposition on the physicochemical characteristics and microbial community structure of rhizosphere soil were studied,and focused on its impact on symbiotic AMF and root manganese content of Chinese fir.The main research results were as follow:Nitrogen addition had a significant effect on soil nutrient content and p H value,and increased the contents of ammonium nitrogen,nitrate nitrogen and total nitrogen in different seasons,while the total carbon increased and the p H value decreased in non-growing seasons only.The response of 11 a stand to nitrogen deposition was more sensitive than that of 18 a forest.Compared with the growth season,nitrate nitrogen content significantly increased and total phosphorus content significantly decreased under nitrogen treatment in the non-growing season,while there was no significant change under CK treatment.The content of ammonium nitrogen decreased significantly under CK treatment,LN and HN,but the decreases in non-growing season under nitrogen treatment was lower than that under the treatment of CK in 18 a stand,with the decreases of 62.54%,47.09%and 40.68%,respectively.p H value decreased significantly only under HN treatment.Nitrogen addition had no significant effect on the ratio of fungi/bacteria（F/B）and gram-positive/Gram-negative bacteria（G+/G-）.Nitrogen addition had a certain effect on soil microbial biomass,which was increased in the non-growing season.Season significantly affected soil microbial biomass and community structure.In 11 a stand,compared with the growing season,the biomass of all groups of soil microorganisms and F/B ratio,except gram-positive bacteria,increased significantly under CK,LN and HN treatments in non-growing season,with the increases of 209.52%,139.93%and 153.44%in fungi,85.17%,54.65%and 65.95%in bacteria;180.00%,125.97%and 96.65%in gram-negative bacteria;63.46%,52.14%and 51.58%in F/B,respectively;the G+/G-ratio decreased significantly under each treatment,with decreases of 57.32%,47.88%and 52.82.In 18 a stand,compared with the growing season,the biomass of all groups of soil microorganisms under CK treatment was not significantly increased in non-growing season,but significantly increased under LN and HN treatment.Under each treatment,the F/B ratio increased by 65.86%,73.55%and 7.21%,while the G+/G-ratio significantly decreased by 36.69%,32.97%and56.00%,respectively.The above results indicated that the seasonal increases of microbial biomass were inhibited by nitrogen treatment in 11 a stand,but were promoted in 18 a stand,and the HN treatment could affect the seasonal variation of microbial community structure.Nitrogen addition had a significant effect on the AMF vesicle,mycelia and total infection rate of Chinese fir roots as well as the content of easily extracted glomalin in soil,and the infection rates were highest under LN treatment in growing season.However,in the non-growing season,the response of different age stands to nitrogen deposition was different.In 11 a stand,the total AMF infection rate was the highest under LN treatment;in 18 a stand,the total AMF infection rate gradually decreased with the increase of nitrogen addition.Compared with the growth season,the total AMF infection rate in non-growth season significantly increased under CK treatment and significantly decreased under LN treatment,indicating that nitrogen application inhibited the seasonal increase of AMF.Nitrogen addition had a significant effect on the manganese content in order 4 root of Chinese fir,and the manganese content of order 4 root in 18 a stand decreased under the LN treatment,and significantly increased under the HN treatment.In non-growing season,nitrogen application significantly reduced soil manganese content.Compared with growing season,the manganese content of order 4 root under CK,LN and HN treatment increased significantly（66.16%,33.18%and 31.58%,respectively）in non-growth season.In 18 a stand,the manganese content of 1-2 oeder root increased significantly under CK and HN treatment,with increases of 33.76%and 32.64%,respectively;the manganese content of 3 order root only increased significantly under CK treatment;the manganese content of 4 order root increased significantly under each treatment,with increases of 24.28%,29.42%and 18.53%,respectively.These results indicated that nitrogen application could inhibit the seasonal increase of root manganese content to a certain extent.In summary,Nitrogen application can promote soil phosphorus nutrient depletion and lead to soil acidification.Short-term nitrogen application had little effect on soil microbial community structure,but significant effect on AMF,indicating that functional groups directly related to roots in microbial community were more susceptible to nitrogen addition,indicating that the initial nitrogen-restricted AMF in the forest was transformed into carbon-restricted by nitrogen application.Nitrogen application can promote the consumption and loss of soil manganese and manganese uptake by roots.In addition,nitrogen application could affect the seasonal variation of soil nutrient content,soil microbial community characteristics,AMF infection rate and root manganese content. |
