Effects Of Simulated Nitrogen Deposition On Soil Microbes During Litter Decomposition

Global change caused by the atmosphere nitrogen deposition has become a popular issue concerned by the society. Compared with the aboveground, the influence of nitrogen deposition on the belowground ecological process is more complex. As the main decomposers of the belowground ecological process, soil microorganisms play an important role in the material cycle and the energy flow in the litter decomposition process. In this research, soil and litter of Pinus massoniana, Cunninghamia lanceolata and Schima superba, three typical tree species in the low-subtropical were chosen, and the samples were treated with different levels of nitrogen (30 kg N ha-1 a-1,60 kg N ha-1 a-1,120 kg N ha-1 a-1) under the condition of constant temperature and humidity. Phospholipid fatty acids (PLFAs) method was used to measure the soil microbes, and the impacts of nitrogen addition impact on litter decomposition, soil respiration, soil microbial biomass and soil microbial community structure were conducted. The results was summarized as follows:(1) Nitrogen input can change soil environment. Soil ammonium nitrogen, nitrate nitrogen and soil C/N ratio increased with the elevated nitrogen deposition. Soil pH, available nitrogen and total nitrogen were gradually decreased, and soil organic matter and total phosphorus changed little.(2) Litter decomposition was influenced by nitrogen input. Pinus massoniana litter decomposition rate was large at the beginning, then became slow, with the opposite for Cunninghamia lanceolata and Schima superba litter. Furthermore, the Cunninghamia lanceolata litter decomposition rate was the highest and Pinus massoniana litter showed the lowest among all the three litter, with the shortest turnover time for Cunninghamia lanceolata litter and the longest for Pinus massoniana litter. Nitrogen content in the litter increased with litter decomposition, however the carbon content and C/N ratio decreased. Sampling time, litter and the interaction between sampling time and litter had significant effects on the litter residual ratio and the nutrient content. Moreover, significantly negative correlation was found between decomposition rate and nutrient content (nitrogen and C/N) of litter.(3) Soil respiration was affected by nitrogen input. Soil respiration rate was high and accumulated quickly at the beginning of litter decomposition, and then slowed down and became stable, causing the accumulated carbon from soil respiration be slow. Litter could enhance soil respiration, with soil respiration rate and the accumulated carbon from soil respiration for broad-leaf litter treatment being higher than the coniferous litter treatment. Sampling time, litter, sampling time and litter, and the interaction among the three factors had significant effects on the soil respiration. Soil respiration showed significantly positive correlation with soil pH and available nitrogen, while negative correlation with ammonium nitrogen and nitrate nitrogen.(4) Nitrogen input can influence the soil microbe biomass. After 232 days incubation, soil microbial nitrogen (MBN) increased, but soil microbial carbon (MBC) and the ratio of MBC/MBN decreased. Sampling time, litter, sampling time and litter, sampling time and nitrogen levels, and the interaction among three factors had significant effects on the soil microbial biomass. Besides, soil respiration and soil chemical and physical properties had significant correlation with soil microbial biomass.(5) Soil microbial community structure was influenced by nitrogen input. After 232 days of litter decomposition, soil microbial PLFAs (except for Actinomycete PLFAs), soil Fungal/Bacterial (F/B) ratios and soil Gram-positive/Gram-negative (G+/G-) ratios decreased. At the beginning of litter decomposition, the soil microbial PLFAs was dominated by 11 Me18:1 w7c,18:1 w9c,16:1 w5c,18:12OH, a15:0. However, after 232 days of litter decomposition, 10Me 17:0, i17:0, i16:0,16:12OH, 15:0, i 15:0, cyl7:0 became the major soil microbial PLFAs. Sampling time, sampling time and litter and the interaction between sampling time and nitrogen levels (except for Actinomycete PLFAs, Arbuscular Mycorrhizal Fungi(AMF) PLFAs and F/B) had significant effects on the soil microbial biomass. Soil microbial PLFAs, the ratio of F/B and G+/G-showed significantly positive correlation with soil pH, available nitrogen and total nitrogen, while negative correlation with ammonium nitrogen, nitrate nitrogen and organic matter. In addition, Soil microbial PLFAs (except for Actinomycete PLFAs) showed significantly negative correlation with the nitrogen and carbon content of litter and MBN, while showed positive correlation with litter decomposition ratio, soil respiration, C/N of litter, MBC and MBC/MBN.