Effects Of Simulated Nitrogen Deposition On Belowground Carbon Cycling Processes And Soil Biochemical Characteristics In A Secondary Evergreen Broad-leaved Forest On Wawu Mountain

Atmospheric nitrogen ?N? deposition is refers to the process that reactive N produced by anthropogenic activities deposits to terrestrial or marine ecosystems from the atmosphere via a series of pathways, it is an important part of the N cycles. Due to the fast development of agriculture and industry in the world, as well as the global expansion of population, the atmospheric reactive N from anthropogenic activities increased rapidly, resulting in a sharp increase in global atmospheric N deposition, and having signally affected the ecological processes and features of the ecosystems. In recent years, due to the increasing of human needs, the original evergreen broad-leaved forest in most regions of China have been severely damaged and replaced by a large area of secondary forest which has become the main component of Chinese forest resources. In the global scale, the secondary forest is a significant carbon ?C? sink and under the influence of increased N deposition. To investigating the effects of increased atmospheric N deposition on underground carbon cycle processes and soil biochemical characteristics in a secondary evergreen broad-leaved forest, and providing theoretical basis and data support for deeper understanding the potential impact of atmospheric N deposition on the carbon balance and soil nutrient in secondary forest. From April 2013, a field experiment including three N treatments, i.e., control ?CK, no N addition?, low-N (LN,50 kg N·hm^-2·a^-1) and high-N (HN, 150 kg N·hm^-2·a^-1) was conducted in this secondary forest, to research the responses of the underground C cycling processes and soil biochemical characteristics to increased N deposition. The main results were as follows:1 There was an obvious seasonal change in soil respiration rate of this secondary forest, with the minimum (0.48±0.03?mol CO₂·m^-2·s^-1) in January and the maximum (3.32±0.30?mol CO₂·m^-2·s^-1) in July. Its annual soil carbon emissions was 610.0±60.7 g C·m^-2·a^-1. Soil respiration rate showed a highly remarkable positive exponential relationship with soil temperature ?P<0.001, R2=0.783? and a remarkable negative linear correlation with soil moisture ?P=0.016, R2=0.080?. The Q₁₀ value of soil respiration calculated from soil temperature was 3.06±0.26. Simulated N deposition significantly inhibited soil respiration rate of this secondary forest and increased its temperature sensibility coefficient ?Q10?.The soil CO₂ emissions and Q10 value in HN plots were significantly36.6% lower and 13.4% higher than the control, respectively.2 After 2 years of decomposition, foliar litter mass remaining of Lithocarpus hancei ?LH? and Schima sinensis ?SS? in control plots were 41.1% and 38.7%, respectively. The decomposition coefficients k of LH and SS in control plots were 0.43±0.04 and 0.41±0.03, and respectively reduced by 12.8% and 19.8% in HN plots. Simulated N deposition inhibited the degradation of lignin and promoted cellulose decomposition of these two kinds of foliar litter. C, N, P, K, Mg and Mn elements of these two kinds of foliar litter almost released all throughout the processes of decomposition, while Ca accumulated at the early stages and released at later stages. Percent of initial C, N, P, K, Ca, Mg and Mn remaining for LH litter were 34.5%,44.6%,67.1%,61.1%,16.7%,9.9% and 25.0%, respectively; N deposition showed some depressing effect on the release of C, N and K, accelerated the release of Mg and Mn, but had little effect on P and Ca. The residual rate of C, N, P, K, Ca, Mg and Mn in SS litter were 34.5%,37.7%,77.2%,45.9%,18.2%,6.3% and 19.7%, respectively; N deposition showed some suppression effect on the release of C, N and K, accelerated the release of Mn, but had little effect on P, Mg and Ca.3 The root biomass of diameter <2 mm and >2 mm of Castanopsis platyacantha ?CP? in the 0-10 cm soil layer were 1305.9±137.1 g·m^-3 and 2222.0±321.7g·m^-3, respectively, and showed a downtrend with the increase of N deposition. In the HN plots, root biomass of diameter <2 mm and >2 mm respectively decreased 19.7% and 37.4% compared with the control. The root length density ?RLD?, root surface area ?RSA? and root volume ?RV? of CP root declined with the increase of soil depth. In the HN plots, the RLD, RSA and RV in 0-15 cm soil layer severally increased 10.1% ,8.4% and 13.3%, and respectively boosted 11.9%,44.6% and 58.3% for 30-45 cm soil layer, and showed less change for 30-45 cm soil layer.4 The soil total organic carbon ?TOC?, dissolved organic carbon ?DOC?, total nitrogen ?TN?, dissolved organic nitrogen ?DON?, microbial biomass carbon ?MBC? and microbial biomass nitrogen ?MBN? contents in organic horizon in this secondary forest were 107.06±6.63,0.84±0.08,5.93±0.36, 0.10±0.01,5.40±0.97 and 0.23±0.01 g·kg^-1, respectively, and the nitrate nitrogen ?NO₃^--N?, ammonium nitrogen ?NH₄⁺-N?, available phosphorus ?AP? and available potassium ?AK? contents were 31.67±4.29, 20.41±4.00,2.04±0.07 and 142.16±8.70 mg·kg-1, severally. HN treatment generally increased the soil TOC, DOC, TN, DON, NO₃^--N, NH₄⁺-N and AP content, and decrease the MBC and MBN content, but had less influence on AK content. The soil pH and C/N value were 3.91±0.01 and 18.29±0.24, respectively. N deposition significantly reduced the soil pH value, but had no effect on soil C/N.5 The soil urease, nitrate reductase, protease, sucrase and acid phosphatase activities in this secondary forest were 2.97±0.20,0.68±0.04,1.04±0.04,14.24±0.81 and 3.85±0.26?mol·g^-1·h^-1, respectively. Compared with the control plots, protease and acid phosphatase activities in HN plots were severally boosted 20.2% and 17.8%, while urease and sucrase activities changed less ?<7%?. Soil nitrate reductase activity in the LN significantly reduced 13.6%, but there was little change in HN plots.The results showed that N deposition to some extent inhibited the decomposition of leaf litter in this secondary forest by inhibiting the degradation of lignin and reduced the root biomass of CP by affecting the C allocation of plant to root. Additional, due to microbial activity was also suppressed, soil carbon emissions rate reduced, thereby resulting in the soil C storage in this secondary forest increased.