Effects Of Simulated Nitrogen Deposition On Soil Carbon Cycling Processes Of Pinus Fabulaeformis Forests In Warm Temperate Of China

China has become one of the three biggest area of nitrogen deposition in the world. There has been a lasting controversy over the point that if nitrogen deposition can enhance forest soil carbon sequestration and how is the potential of forest soil carbon sequestration under the drive of nitrogen deposition. Chinese pine(Pinus tabulaefortnis) is endemic to China, researches on the effects of simulated nitrogen deposition on soil carbon cycling processes in Chinese pine forests can not only fill the gap of nitrogen deposition studies in warm temperate area, but also lay a solid foundation for researches on the nitrogen deposition and global change in this region.Through in situ simulated nitrogen deposition, nitrogen levels were implemented as:NO (0kg N·hm-2·a-1), N1(50kg N·hm·a-1), N2(100kg N·hm·a-1), and N3(150kg N·hm·a-1). We compared and investigated the responses and underlying mechanisms of soil carbon cycling key processes to nitrogen deposition in both plantation (PF) and natural forests (NF) of Chinese pine. The main results were as follows:(1) Nitrogen deposition had no significant effect on soil microbial biomass in PF, but significantly decreased soil microbial biomass at0-10cm depth in NF. Litter treatment significantly affected hydrogen peroxidase, cellulase, urease and invertase activities. It showed that soil enzyme activities were higher in control (CK.) subplot than those in both litter and root exclusion (LRE), and only litter exclusion (LE) subplots. Hydrogen peroxidase activity was increased at N1level, but decreased at N2and N3levels. Nitrogen deposition inhibited polyphenol oxidase, peroxidase and invertase activities, but had no significant effects on cellulase, urease and phosphatase activities. (2) Nitrogen deposition significantly promoted P. tabulaeformis decomposition in PF. The decomposition of Quercus liaotungensis was suppressed by nitrogen deposition in NF. Nitrogen deposition promoted P. tabulaeformis decomposition at the beginning of decomposition and suppressed its decomposition at the end of decomposition in NF. Nitrogen deposition facilitated P. tabulaeformis-Q. liaotungensis mixed litter decomposition before12months and then had no significant effect on mixed litter decomposition. K was almost realeased while other nutrients were showed enrichment during decomposition. At the beginning of early nutrition control stage, the litter nutrient release was very fast. The accumulation of lignin at the end of decomposition led to slow release of litter nutrients.(3) Litter treatment significantly affected soil respiration. Soil respiraiton in LRE and LE subplots was significantly lower than that in CK subplot. Nitrogen deposition reduced soil respiration in LRE subplot in both PF and NF, however, soil respiration was not significantly different between different nitrogen levels. Nitrogen deposition increased soil respiration in LE subplot in both PF and NF, and soil respiration increased with nitrogen levels in PF while soil respiration was the highest in the N2level in NF. Nitrogen deposition also increased soil respiration in CK subplot in both PF and NF. Temperature sensitivity of soil respiration Q10was significantly higher in CK subplot than that in both LRE and LE subplots. Nitrogen deposition decreased Q10in both PF and NF.(4) Nitrogen deposition decreased soil organic carbon at different depths. The decrease of soil organic carbon at0-20cm depth was larger than that at20-40cm and40-60cm depth and soil organic carbon decreased with nitrogen levels. Nitrogen deposition significantly increased soil total nitrogen at0-20cm depth in PF, but had no significant effect on the soil total nitrogen at0-20cm depth in NF.Nitrogen deposition reduced soil C storage by promoting soil respiration in Chinese pine forests. How soil carbon pool respond to nitrogen deposition under the background of future high nitrogen deposition is worth further study.