模拟N/S沉降对米槠天然林土壤N净转化速率和N_2O排放的影响

Soil Net N Transformation Rates and N_2O Emission Affected by Simulated N /S Deposition for a Naural Forest of Castanopsis carlesii of Northern Midsubtropics of China

[目的]研究N/S沉降对中亚热带米槠天然林土壤硝化作用和N2O排放的影响。[方法]室内用NH4NO3/K2SO4模拟N/S沉降,在30℃和60%WHC水分条件下,恒温培养15 d。在培养的第1、5、10和15天测定土壤NH+4-N、NO-3-N和pH,在培养的第3、4、5和10天采集气体样品测定N2O浓度。[结果]N沉降和NS复合沉降在0.05水平显著促进土壤的矿化和硝化,且促进效果随沉降强度或NS复合沉降中N的比例增大而增大,但S沉降的促进作用不显著;土壤净矿化速率和净硝化速率与土壤pH具有极显著的负相关性。土壤在自然状态和N/S沉降的影响下,有机氮净矿化速率与净硝化速率之间均存在显著相关性,且矿化速率大于硝化速率。N沉降和NS复合沉降显著提高土壤的硝化率在N/S沉降下,该区土壤中的无机氮仍以NH+4-N为主,硝化作用弱,铵态氮的供给不成为土壤硝化作用的限制因素。NS复合沉降促进土壤N2O的排放,具体机理有待进一步揭示。[结论]N沉降和NS复合沉降明显促进土壤的矿化和硝化作用,S沉降对土壤的矿化和硝化作用的影响不大;NS复合沉降影响土壤的N2O排放,且其影响效应与沉降中N、S的比例相关,在有关酸沉降对土壤N2O排放的影响方面,S沉降应受到关注。

[ Objective ] The impacts of nitrogen and/or sulfur deposition on the soil nitrification and nitrous oxide emissions were investigated for Castanopsis carlesii community which is a typical vegetation form of evergreen broadleaved forests in the northern Midsubtropics of China. [ Method] Ammonium nitrate and potassium sulfate were used in lab incubation experiments to simulate nitrogen and/or sulfur deposition under the conditions of 30 ~C of room temperature and 60% of soil water holding capacity（WHC）. Soil pH values and the contents of soil ammonium nitrogen, nitrate nitrogen were determined at the 1st, 5th, 10th and 15th day after incubation, and nitrous oxide concentrations were deter- mined at 3rd, 4th, 5th and lOth day. [ Result] Soil mineralization and nitrification were promoted for nitrogen deposition, or N and S mixed deposition while no significant difference showed for sulfur deposition. There was a very pronounced negative relationship between soil net min- eralization rate and soil pH value as well as between soil net nitrification rate and pH value. There was a significant positive correlation rela- tionship between soil net mineralization rate and soil net nitrification rate either for controlled soil or soil amended with N and/or S. While the ratio of NO3^- -N to （ NH4^ ＋ NO3^- ） -N was increased with nitrogen deposition, or N and S mixed deposition, NH4 -N was still the dominant forms in the soil inorganic nitrogen species under the influences of acid deposition in this area. The nitrification is weak in this area and the supply of ammonium would not become the limiting factor of soil nitrification. Mixed nitrogen and sulfur deposition promoted the emissions of nitrous oxide, which mechanism need further elucidation in the future. [ Conclusion] The results suggested that nitrogen deposition or N and S mixed deposition would stimulate soil mineralization and nitrification, while sulphur depositon had no effect on the above processes. Mixed ni- trogen and sulfur input promoted the emissions of nitrous oxide, which might be related to the nitrogen proportion in the mixed deposition, and so the role of sulphur can not be ignored when we investigate acid deposition effects on nitrous oxide emission.