重庆缙云山两种林分土壤呼吸对模拟氮沉降的季节响应差异性

氮沉降对土壤呼吸的影响仍然存在着争论,需要进一步研究。选择重庆缙云山的马尾松林和柑橘林开展了氮添加实验,分别设置3个氮添加水平(低氮T_5:20 g N m~(-2)a~(-1),中氮T_(10):40 g N m~(-2)a~(-1)和高氮T_(15):60 g N m~(-2)a~(-1))和对照(T_0:0 g N m~(-2)a~(-1))共4个水平的处理,各林分每个处理各9次重复,每个处理量分4次,在每个季度开始各施1次。采用ACE(Automated Soil CO_2 Exchange Station,UK)自动土壤呼吸监测系统测定两林分土壤表层(0—10 cm)的呼吸、温度和湿度,分别在当年的7月、9月、11月、第2年的1月、2月、3月、5月、6月各连续测定4d,每天(8:00—18:00)4次,以揭示两种林分土壤呼吸对模拟氮沉降的季节动态响应及其差异性。结果表明:(1)柑橘林与马尾松林林下土壤表层呼吸表现出一致的季节变化动态趋势:夏季春季秋季冬季,但柑橘林土壤呼吸显著高于马尾松林(P0.05)。(2)总体上氮沉降抑制了2种林分土壤表层呼吸,而N沉降量大抑制程度越高。只在冬季土壤湿度低的马尾松林下氮沉降促进了土壤呼吸。(3)土壤温度与土壤呼吸有极显著的正相关指数关系(P0.01),而土壤水分与土壤呼吸有显著的二次模型拟合关系,但均受到氮沉降量处理的影响。综合分析表明,在亚热带山区2类森林下的典型案例结果支持氮沉降抑制土壤呼吸的认识。

Seasonal responses of soil respiration to simulated nitrogen deposition in a citrus plantation and masson pine forest in Mt. Jinyun, Chongqing, China

Disagreements over the effects of nitrogen deposition on soil respiration still exist, thereby necessitating further studies. In the present study, we designed an experiment to explore the seasonal responses of soil respiration to various levels of nitrogen deposition (T₀: 0 g N m^-2 a^-1as the control; T₅: 5 g N m^-2 a^-1 as the low N treatment; T₁₀: 10 g N m^-2 a^-1 as the moderate nitrogen treatment; and T₁₅: 15 g N m^-2 a^-1 as the high nitrogen treatment. All treatments were performed with nine replicates both in the Masson pine forest and in the Citrus plantation at Mt. Jinyun, Chongqing, China. The different levels of N addition were applied four times at the beginning of each season from May 2014. We measured soil respiration, soil temperature, and soil moisture simultaneously by using the Automated Soil CO₂ Exchange Station (ACE, UK) for four consecutive days in July, September, November, January, February, March, May, and June, respectively, at four times each day between 8:00 and 6:00. The results showed that:(1) the two forests underwent similar seasonal changes in soil respiration, with the most change being observed during the summer, followed by spring, autumn, and winter, which yielded the least change. However, the citrus plantation presented significantly higher levels of soil respiration than the Masson pine forest across all seasons (P < 0.05). (2) Generally, nitrogen deposition suppressed soil respiration in both forests and these inhibitory effects were strengthened with increasing levels of nitrogen deposition. The only exception in which nitrogen deposition enhanced soil respiration was in the Masson pine forest during the winter, while soil moisture was very low. (3) Soil respiration showed significantly positive exponential relationships with soil temperature (P < 0.01) and significant quadratic relationship with soil moisture (P < 0.05). Both factors (soil temperature and moisture) are influenced by the levels of nitrogen deposition in subtropical forests. Thus, the results of our study on subtropical forests confirm the theory that nitrogen deposition inhibits soil respiration.