| In the late 18th century to the late 20th century, researchers discovered that the global nitrogen fixation rate can be controlled. At present, the amount of active nitrogen produced by human activities exceeds the amount of active nitrogen in natural land processes. Following Europe and the United States, China is considered as the world’s third-largest region providing nitrogen deposition. However, nitrogen deposition is a serious problem implicated in global climate change. Excess nitrogen deposition has caused a series of ecological problems. The characteristics and processes of various ecological systems are greatly affected by an increase in nitrogen deposition. Global climate change is an undisputed phenomenon that affects precipitation patterns at the global and regional scales. This phenomenon causes unbalanced regional rainfall, which results in relatively large seasonal variation; this imbalance constitutes increased rainfall in the northern hemisphere and decreased precipitation in subtropical regions. Rainfall is the most important source of soil moisture and can alter soil aeration conditions. Rainfall affects the decomposition of surface litter and soil organic matter, soil enzyme activity, plant root growth, as well as the structure and function of microorganism and vegetation communities; rainfall also plays an important role in controlling biological and chemical processes underground. Nitrogen deposition and changes in rainfall exert major effects on forest ecosystem processes. In this study, the evergreen broad-leaved forest in Rainy Area of Western China is the research object. Through simulating nitrogen deposition and rainfall, we studied the effects of simulated nitrogen deposition and rainfall on the key processes and characteristics of carbon and nitrogen cycling in the ecosystem of evergreen broad leaved forest in Rainy Area of Western China. Two factors, nitrogen deposition and precipitation, were set up in six treatments. Eighteen plots were established and divided into six treatments with three replicates:comparison(CK), nitrogen deposition 15 g N m-2·a-1(N), water reduction 10% (R), water addition 10%(A), nitrogen deposition 15 g N m-·2-a-1 and water reduction 10% (NR), nitrogen deposition 15 g N m-2·a-1 and water addition 10%(NA).The main results are as follows:1. The soil organic carbon content in different soil layers showed the higher in summer and autumn, the lower content in spring and winter in evergreen broad-leaved forest in Rainy Area of West China. The soil organic carbon content was higher in 0-1 Ocm soil layer than that of in 10-20cm soil layer. The average soil organic carbon content in order was as follows:R<NR<CK<A<N<NA in 0-10cm soil layer and R<NR<A<CK<NA<N in 10-20cm soil layer. Simulated nitrogen deposition and water addition promoted the accumulation of the soil organic carbon. Water reduction inhibited the accumulation of the soil organic carbon. The soil C/N value of 0-10cm soil layer was significantly higher than that of 10-20cm, and the C/N value of the soil showed a trend of increasing with the soil depth. The water reduction reduced the soil C/N, and the water addition increased the soil C/N. Water addition increased the content of soil TOC and water reduction reduced the content of soil TOC in the same simulated nitrogen deposition conditions; simulated nitrogen deposition increased soil TOC in the same rainfall condition. Nitrogen deposition and precipitation changes had significant influence on the content of DOC and MBC (P＜0.05), of LC was not significant (P>0.05); and the interaction between nitrogen deposition and precipitation changes did not have a significant effect on the soil TOC, DOC, MBC and LC content (P>0.05).2. The change of soil inorganic nitrogen was 14.66-18.37 mg·kg-1 in evergreen broad-leaved forest in Rainy Area of West China. The soil NH4+-N and NO3--N content is higher in 0-10cm soil layer than that of in 10-20cm soil layer. Simulated nitrogen deposition promoted the accumulation of the soil inorganic nitrogen, and decreased the soil pH value; while water addition inhibited the accumulation of the soil inorganic nitrogen. The soil NH4+-N, MO3-N with total nitrogen, organic matter, and water content had a significant relation in different treatments. Water addition decreased the content of soil NH4+-N and NO3--N content in different soil layers in the same simulated nitrogen deposition condition; and nitrogen deposition increased the content of NH4+-N and NO3+-N in different soil layers in the same rainfall condition. Nitrogen deposition and precipitation changes had significant influence on the content of NH4+-N and NO3+-N (P＜0.05), and the interaction between nitrogen deposition and precipitation changes did not have a significant effect on the content of NH4+-N and NO3--N (P>0.05).3. Simulated nitrogen deposition and water addition significantly inhibited soil respiration, while water reduction increased the soil respiration rate significantly. Water reduction increased soil CO2 flux by 576 g·m-2·a-1, simulated nitrogen deposition and water addition treatments reduced soil CO2 flux by 726 and 803 g·m-2·a-1, respectively. The CK, N, R, A, NR andNA treatments of soil respiration rate of the Q10 values were 2.77.2.44,3.18,2.39.2.58 and 2.22 respectively, it showed that the water reduction increased the temperature sensitivity of soil respiration, while nitrogen deposition and water addition reduced the temperature sensitivity of soil respiration. Soil temperature has a greater influence on soil respiration rate than soil moisture. Simulated nitrogen deposition and water addition reduced the carbon and nitrogen content of the soil microbial biomass. while water reduction increased the carbon and nitrogen content of the microbial biomass. And the interaction between nitrogen deposition and precipitation changes did not have a significant effect on the soil CO2 efflux in this forest ecosystem.4. The seasonal variation of soil microbial biomass was obvious in the different soil layers, which was higher in autumn and winter; and lower in spring and summer. The number of soil bacteria, fungi and actinomycetes in 0-10cm soil layer was higher than that of 10-20cm. In the 0-20cm soil layer, the average soil microbial biomass in CK, R, A, N, NR and NA treatments were 6.44,7.27,4.98,5.42,5.43 and 4.81 cfu·g-1·106 respectively, and performance:R>CK>NR>N>A>NA. Total microbial biomass increased by 12.89% compared with CK treatment in R treatment, while A, N, NR and NA treatments were decreased by 22.67%,15.84%,15.68% and 25.31% respectively compared with CK treatment. Water reduction increased the total microbial biomass in the soil; while simulated nitrogen deposition and water addition reduced the total amount of microbes in the soil. Simulated nitrogen deposition treatment reduced the total amount of microorganism in soil in the same rainfall condition. Water reduction increased the total amount of microbes in the soil, and water addition reduced the total amount of microorganism in soil in the same simulated nitrogen deposition conditions. Nitrogen deposition and precipitation changes had a significant impact on the total amount of microorganisms and bacteria in the surface soil layer (P＜0.05). and their interaction effect on the microbial flora is not significant (P>0.05).5. The 6 kinds of soil enzyme activity were higher in 0-10cm soil layer than that in 10-20cm soil layer, and it had obvious seasonal variation in evergreen broad-leaved forest in Rainy Area of West China. The cellulase and acidic phosphatase activity was higher in the summer, the invertase and urease activity was higher in autumn, the peroxidase and polyphenol oxidase activity was higher in winter. Nitrogen deposition and precipitation changes did not change the vertical distribution pattern of soil enzyme activity in the original. On the whole, simulated nitrogen deposition inhibited the activities of invertase, cellulase, peroxidase and polyphenol oxidase, and promoted the activities of urease, acid phosphatase. Water reduction make activities of the urease, acid phosphatase, soil invertase and cellulase increased, and water addition make the activities of urease, acid phosphatase, soil invertase and cellulase decreased. Simulated nitrogen deposition had a significant effect on the activities of soil enzyme (P<0.05). And the interaction between nitrogen deposition and precipitation changes did not have a significant effect on the activities of soil enzyme in evergreen broad-leaved forest in Rainy Area of West China.6. The leaf decomposition rate was higher than the twig, and the litter decomposition was faster in summer than other seasons in an evergreen broad-leaved forest in the Rainy Area of western China.. After the decomposition of 24 months, the qualities of leaf and twig residual rates were 38.03% and 80.42%, respectively. It took 1.48years to decompose 50% of leaf litter mass, and 5.58 years shorter than twig. The decomposition coefficient of each treatment was as follows:k(A)>k(CK)>k(NA)>k(N)>k(R)>k(NR), and the residual rate of twig was N>NR>R>NA>CK>A. The simulated nitrogen deposition, water reduction and water addition treatments to decompose 50% of leaf litter mass took 1.86,2.35 and 1.36a, respectively; and while to decompose 50% of twig litter mass took 8.97,7.93 and 6.38a, respectively. Each treatment of the leaf litter decomposition of 95% required 6.01-11.55a, while twig litter decomposition of 95% required 26.93-38.04a. Under the same nitrogen deposition condition, water addition treatment promoted the decomposition of leaf litter, while water reduction treatment reduced the decomposition of leaf litter; under the same precipitation condition, nitrogen deposition reduced the decomposition of leaf litter. Thus it can be seen, nitrogen deposition and precipitation changes had a significant effect on litter decomposition, and their interaction effect was not significant (P>0.05). Considering that nitrogen deposition increases continuously and global climate change occurs, water addition treatment promoted the decomposition of litter, nitrogen deposition and water reduction treatment reduced the decomposition of litter in an evergreen broad-leaved forest in the Rainy Area of western China, the interaction between nitrogen deposition and precipitation changes did not have a significant effect on litter decomposition in this forest ecosystem.7. The C, N and P concentrations of leaf litter fluctuated 0.42-1.15 g·kg-1,5.86-9.56 g·kg-1 345.68-478.53 g·kg-1 respectively. The C, N and P concentrations of leaf litter were increased in the simulated nitrogen deposition and water reduction treatments in an evergreen broad-leaved forest in the Rainy Area of western China, and the concentration of C, N and P decreased in water addition treatments. The litter nutrient element of evergreen broad-leaved forest release rate was as follows:N<C<P. The C and N are both direct release mode, and P showed enrichment - release mode in different treatments. Simulated nitrogen deposition and water reduction inhibited the release of P, N and C in the leaves of evergreen broad leaved forest, and water addition promoted the release of P, N and C. The correlation between C, N and P elements residual rate and dry matter residue rate was significant (P<0.5). After the decomposition of 24 months, the C/N value shows: NA<N<CK<R<A<NR; Simulated nitrogen deposition treatment reduced the C/N value by 4.25%, and water reduction and water addition treatments increased C/N value 1.13% and 4.02% respectively. The interaction between nitrogen deposition and precipitation effect on the release of N and P element was significant (P＜0.05), and not significant on the release of C element (P>0.05). |
PDF Full Text Request