| The increasing atmospheric nitrogen(N) deposition is rapidly and profoundly altering the terrestrial ecosystem. However, it is not clear how the increasing soil available N caused by N deposition influence soil enzyme activity and microbial characteristics. Based on pot experiments and N gradient addition experiment(0, 2.8, 5.6, 11.2, 22.4 and 44.8g N m-2.yr-1), the effect of N addition on soil enzymes(invertase, urease and alkaline phosphatase) and microbial characteristics(microbial biomass carbon, microbial biomass nitrogen and microbial community) in Pinus tabuliformis seedling soil was studied. The main conclusions were drawn as follows:(1) Different N addition levels had different influence on the biomass of Pinus tabuliformis seedlings. In general, with the increase of N addition, the individual biomass, the aboveground biomass and aboveground biomass had a trend of increasing at first and then decreasing with a peak at 5.6 g N m-2.yr-1 treatment. The aboveground biomass decreased but belowground biomass had no significant changes after reached the peak, and thus the total biomass increased at 2.8-11.2 g N m-2.yr-1 levels, and decreased at 22.4- 44.8 g N m-2.yr-1 levels.(2) Different N addition levels had different influence on enzyme activity and microbial characteristics in rhizosphere soil of Pinus tabuliformis seedlings. Invertase, urease and alkaline phosphatase activities were promoted by low N addition and suppressed by high N addition. For example, urease activity significantly increased at all N treatments except at 2.8 g N m-2.yr-1. Invertase and alkaline phosphatase activity has the unimodal trend, and reached the peak at 11.2 g N m-2.yr-1and 5.6 g N m-2.yr-1,respectively. There was no significant change in microbial biomass carbon and microbial biomass nitrogen at low N addition(2.8 to 5.6 g N m-2.yr-1), but significantly increased at 11.2-22.4 g N m-2.yr-1, and reduced at the high N addition(44.8 g N m-2.yr-1); N addition levels had no significant influence on bacteria and G-PLFAs, but increased the fungi, actinomycetes and G + PLFAs at low N addition and then decreased them at higher N addition.(3) N addition significant influenced the rhizosphere soil enzyme activity and microbial characteristics of Pinus tabuliformis seedlings. N addition(< 11.2 g N m-2.yr-1) improved the invertase and urease activity of rhizosphere soil, but the three kinds of enzyme activities no longer increased, even decreased caused by excessive N addition(≥22.4 g N m-2.yr-1). Microbial biomass carbon increased significantly at 5.6 g N m-2.yr-1. However. The microbial biomass nitrogen increased significantly at or more than 11.2 g N m-2.yr-1. Correlation analysis showed that the invertase and urease activities, microbial biomass carbon and microbial biomass nitrogen had significant correlation relationship; Organic carbon, total nitrogen contents were the main factors influencing the soil enzyme activity and microbial biomass.(4) The effect of N addition on enzyme activity and microbial characteristics in rhizosphere soil was significantly different from in non-rhizosphere soil. Soil organic carbon, total nitrogen accumulation rate were 97.73% and 41.47%, respectively with the highest at5.6 g N m-2.yr-1. The p H in rhizosphere soil was significantly lower than that in non-rhizosphere soil. The three kinds of enzyme activity in rhizosphere was higher than those in non-rhizosphere soil in all N treatments, except that urease activity in rhizosphere soil was lower than that in non-rhizosphere at low N addition. The enrichment rate of invertase, urease and alkaline phosphatase reached maximum with 71.1%, 50% and 97.6% at 5.6-11.2 g N m-2.yr-1 treatment, respectively. Microbial biomass carbon had no obvious increase or decreasing trend, while microbial biomass nitrogen in rhizosphere soil was higher than that in non-rhizosphere. Microbial biomass carbon and microbial biomass nitrogen were highest with 26.9% and 75.7% at 22.4 g N m-2.yr-1 and 11.2 g N m-2.yr-1 level, respectively.The total biomass, soil enzyme activities, microbial biomass C, microbial biomass N and PLFAs reached the higest at 5.6-11.2 g N m-2yr-1 treatment, indicting that the N deposition level(2.06 g N m-2.yr-1) in the Loss Plateau is favorable to the growth of Pinus tabuliformis seedling and to the improving of soil enzyme activities and microbial biomass, and that N addition can be used for seedling and soil management in this region. |
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