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凋落叶分解过程中的元素释放不仅有助于植物的生长发育[1],还是改良森林生态系统结构功能的关键过程[2]。研究氮沉降对凋落物分解过程中元素释放的影响,对于了解森林生态系统养分动态和元素循环对氮沉降的响应具有重要意义。现有的研究更加关注氮沉降对凋落叶分解过程中Ca、N、P元素释放的影响,其影响机制也比较明确。研究表明,氮沉降通过改变凋落叶及环境中的N含量,造成凋落叶分解过程中养分元素的不平衡,影响分解者对养分的需求,以及改变酶活性等方式影响Ca、N、P元素的释放[3-5]。而氮沉降对凋落物分解过程中K、Ca、Mg元素释放影响的研究还相对缺乏,且不同区域间的研究结果还存在较大差异。如,Munasinghe等[6]研究表明,氮沉降抑制了美国弗吉尼亚州阔叶混交林凋落叶中Ca元素的释放;而李仁洪等[7]的研究表明氮沉降促进了华西雨屏区慈竹凋落叶分解过程中K、Ca、Mg元素的释放;张林等[8]研究发现,氮沉降对亚热带常绿阔叶甜槠林中凋落叶K、Ca、Mg元素无显著影响。
华西雨屏区主要森林植被类型为常绿阔叶林,受邛崃山脉和岷江山脉的影响,该区域大气氮沉降以湿沉降为主[9-10],2008—2010年年均氮湿沉降量为9.5 g·m−2[11],远高于同期中国50个森林站点观测的氮沉降年平均值1.66 g·m−2[12],这一特性为研究氮沉降对常绿阔叶林凋落物分解过程中K、Ca、Mg元素释放的影响提供了天然实验室。前期研究表明,氮沉降抑制了华西雨屏区天然常绿阔叶林凋落叶的分解[13],抑制了分解过程中Ca、N元素的释放,促进了P元素的释放[14],但氮沉降对该区域凋落叶分解过程中K、Ca、Mg元素释放的影响还不清楚。基于此,本研究以华西雨屏区天然常绿阔叶林为研究对象,研究了模拟氮沉降对凋落叶分解过程中K、Ca、Mg元素浓度和残留率的影响,旨在了解模拟氮沉降对凋落叶分解过程中K、Ca、Mg元素浓度、释放模式以及释放速率有何影响?为全面揭示该区域常绿阔叶林在氮沉降持续增加背景下养分元素的循环过程提供参考。
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研究区位于四川省雅安市雨城区碧峰峡风景区(102°59ʹ E,30°03ʹ N),气候类型为亚热带季风型气候,最高气温25 ℃,最低气温6 ℃,年均气温16 ℃,年平均降水量1 770 mm,年蒸发量1 010 mm。区域大气氮沉降以湿沉降为主[9],2008—2010年氮湿沉降量约为9.5 g·m−2·a−1[11]。研究区内植被丰富,乔木层植物以海桐(Pittosporum tobira Thunb.)、硬斗石栎(Lithocarpus hancei Benth.)、木荷(Schima superba Gardn.)、润楠(Machilus pingii Cheng ex Yang.)和青榨槭(Acer davidii Frarich.)等为主。试验地位于中坡,坡度较小,土壤类型为黄壤。
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2013年10月在研究区选择具有代表性的天然常绿阔叶林,在该林分中随机设置12个3 m×3 m的小样方,每个样方间设置3 m的缓冲带。在阔叶林中收集主要树种(木荷、硬斗石栎和海桐)的新鲜凋落叶,将其充分均匀混合带回实验室自然风干,然后把风干的凋落叶在烘箱中于65 ℃下烘干至恒质量,计算出水分转化系数,接着测定凋落叶的初始质量及养分元素浓度(表1)。称取20.0 g风干后的凋落叶,装入准备好的尼龙网分解袋中。尼龙网分解袋大小为20 cm×20 cm,贴地面层孔径为0.05 mm,表面层孔径为1.00 mm。2013年11月,去除样方内土壤表面的凋落物,将准备好的凋落物分解袋随机均匀地放置在12个样方土壤表面。在每个样方中放置18个凋落袋(1年×6次×3个重复),12个样方共计放置216个凋落袋。
表 1 华西雨屏区凋落叶初始养分元素浓度
Table 1. Initial nutrient elements concentration of litter in rainy area of western China
g·kg−1 碳C 氮N 磷P 钾K 钙Ca 镁Mg 430.73±8.71 8.32±1.14 0.42±0.04 13.25±0.24 2.45±0.06 1.36±0.07 本试验氮沉降水平参考2008—2010年研究区氮湿沉降量(9.5 g·m−2·a−1)[11],设置对照、增加50%、150%和300%4个水平,即为对照(0 g·m−2·a−1, CK)、低氮(5 g·m−2·a−1, L)、中氮(15 g·m−2·a−1, M)和高氮沉降(30 g·m−2·a−1, H),每个氮沉降水平设置3个重复。大气湿氮沉降中的氮元素主要形式是NH4+和NO3−[12],因此以NH4NO3作为氮源,从2013年11月中旬起,每隔半个月进行人工模拟氮沉降。具体方法是:将每次每个样方所需的NH4NO3溶解在2 L清水中,在样方内均匀喷洒,对照样方中施等量清水。
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从2013年11月中旬起,每2个月收集一次凋落物分解袋,每次随机从每个样方中采集凋落袋3袋。将凋落袋迅速带回室内,先将袋外泥土去除,在烘箱中于65 ℃下烘干至恒质量后,称量凋落叶并计算其质量损失率[13],然后将凋落叶粉碎,再利用原子吸收分光光度计法测定K、Ca、Mg元素浓度及其残留率。
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质量损失率=(M0-Mt)/M0×100%
式中:Mt为凋落叶在t时刻的质量,M0为凋落叶初始干质量。
养分残留率=(Ct×Mt)/(C0×M0)×100%
养分释放率=100%−(Ct×Mt)/(C0×M0)×100%
式中:Mt为该阶段凋落叶干质量,M0为凋落叶初始干质量;Ct为t时刻凋落叶养分浓度,C0为初始养分浓度。
利用Microsoft Excel 2013进行分类汇总,采用SPSS 22.0统计软件,对每次取样的凋落叶质量损失率、K、Ca、Mg浓度以及残留率进行单因素方差分析,利用重复测量方差分析检验各处理间凋落叶质量损失率、K、Ca、Mg浓度以及残留率的差异性,通过Pearson相关性分析检验各处理下凋落叶质量残留率与K、Ca、Mg元素残留率的相关关系。利用Excel 2013和SigmaPlot12.5制作相关图表。本研究所使用的凋落叶质量残留率数据为本课题组前期研究结果[13]。
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由图1a可以看出,各处理的凋落叶K元素浓度动态变化均一致,表现为在整个分解过程中一直呈下降趋势。分解1年后,L、M和H处理的K元素浓度分别比CK高17.76%、52.52%和68.66%。重复测量方差分析表明(表2),氮沉降各处理的K元素浓度均显著高于CK(P<0.05)。这表明,氮沉降显著抑制了凋落叶分解过程中K元素浓度的降低。
图 1 各处理凋落叶分解过程中K、Ca、Mg元素浓度的变化
Figure 1. Dynamic of K,Ca, Mg concentration in all treatments during decomposition
表 2 各处理凋落叶质量损失率、K、Ca、Mg元素浓度及其残留率的显著性
Table 2. The significant of concentration and remaining of K, Ca, Mg elements and mass loss rate in various treatments
处理
Treatments质量损失率
Mass loss rate/%浓度Concentration/(g·kg−1) 残留率Remaining/% K Ca Mg K Ca Mg CK 22.71±1.52a 7.04±0.27a 3.74±0.13a 1.48±0.12a 44.65±1.87a 116.81±5.36a 85.56±4.91a L 19.98±0.66b 7.84±0.18b 3.24±0.10b 1.39±0.08a 50.46±1.02b 104.55±3.51b 82.04±4.72a M 18.75±0.75c 8.36±0.28c 2.92±0.09c 1.50±0.12a 53.71±1.77c 95.99±3.00c 88.22±6.72a H 17.59±0.67d 8.77±0.23d 2.66±0.09d 1.37±0.15a 56.84±1.54d 89.33±3.11d 84.29±7.44a 注:不同小写字母表示差异达到P=0.05显著水平。
Note:Different lowercase letters indicate significant difference at P=0.05 level.各处理的凋落叶Ca元素浓度动态变化均表现为相同趋势,在凋落叶分解的前4个月,各处理的Ca元素浓度整体呈上升趋势,4~10个月表现为下降趋势,10~12个月呈上升趋势(图1b)。分解1年后,L、M和H处理的Ca元素浓度分别比CK低12.98%、16.37%和27.43%。重复测量方差分析表明(表2),氮沉降各处理的Ca元素浓度显著低于CK(P<0.05)。这表明,模拟氮沉降显著促进了凋落叶分解过程中Ca元素浓度的降低。
CK、L和H处理的凋落叶Mg元素浓度动态变化均一致,表现为在分解前2个月呈增加趋势,2~10个月呈下降趋势,10~12个月呈增加趋势;M处理的凋落叶Mg元素浓度表现为分解前2个月呈增加趋势,2~6个月呈下降趋势,6~12个月呈增加趋势(图1c)。分解1年后,L和M处理的Mg元素浓度分别比CK高15.86%和18.08%,H处理的Mg元素浓度比CK低15.08%。重复测量方差分析表明(表2),氮沉降各处理的Mg元素浓度与CK之间差异均不显著(P>0.05)。这表明,模拟氮沉降对凋落叶分解过程中Mg元素浓度无显著影响。
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由图2a可以看出,各处理凋落叶K元素释放率均随着分解时间的延长而增加,表明凋落叶分解过程中K元素的释放表现为直接释放模式。分解1年后,L、M和H处理的K元素残留率分别比CK高3.91%、10.27%和13.91%。重复测量方差分析表明(表2),氮沉降各处理的K元素残留率显著高于CK(P<0.05)。这说明,氮沉降显著抑制了凋落叶分解过程中K元素的释放。
图 2 各处理凋落叶分解过程中K、Ca、Mg元素释放率的变化
Figure 2. Dynamic of K,Ca, Mg release rate in all treatments during decomposition
各处理凋落叶Ca元素释放动态整体一致,Ca元素在分解前2个月表现为释放状态,之后的2~12个月由富集变为释放状态(图2b),凋落叶分解过程中Ca元素释放表现为释放-富集的交替模式。分解1年后,L、M和H处理的Ca元素残留率分别比CK低6.39%、6.51%和15.93%。重复测量方差分析表明(表2),氮沉降各处理的Ca元素残留率显著低于CK(P<0.05)。这说明,模拟氮沉降显著促进了凋落叶分解过程中Ca元素的释放。
各处理凋落叶Mg元素释放动态均一致,在分解前2个月呈富集状态,随后均表现为释放状态(图2c),凋落叶分解过程中Mg元素的释放表现为富集-释放模式。分解1年后,L、M和H处理的Mg元素残留率分别比CK高16.31%、21.44%和0.58%。重复测量方差分析表明(表2),氮沉降各处理与CK之间无显著差异。这说明,模拟氮沉降对凋落叶分解过程中Mg元素的释放无显著影响。
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将凋落叶质量残留率与K、Ca、Mg元素残留率进行相关性分析。从图3a中可以看出,各处理凋落叶质量残留率与K元素残留率均呈极显著的正相关,表明氮沉降没有改变凋落叶质量残留率与K元素残留率的正相关关系;CK处理的凋落叶质量残留率与Ca元素残留率呈显著的正相关,而L、M和H处理凋落叶质量残留率与Ca元素残留率的相关性不显著(图3b),说明氮沉降降低了凋落叶质量残留率与Ca元素残留率的正相关关系;CK、L和H处理的凋落叶质量残留率与Mg元素残留率呈显著的正相关,M处理凋落叶质量残留率与Mg元素残留率的相关性不显著(图3c),表明L和H处理并未改变凋落叶质量残留率与Mg元素残留率的正相关关系,M处理降低了两者的正相关关系。
模拟氮沉降对华西雨屏区天然常绿阔叶林凋落叶分解过程中K、Ca、Mg元素释放的影响
Effects of Simulated Nitrogen Deposition on the Releases of Potassium, Calcium, and Magnesium During Litter Decomposition in a Natural Evergreen Broadleaved Forest in the Rainy Area of Western China
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摘要:
目的 研究氮沉降背景下凋落叶分解过程中钾(K)、钙(Ca)、镁(Mg)元素的释放动态,揭示森林生态系统在氮沉降持续增加背景下养分元素的循环过程。 方法 在华西雨屏区天然常绿阔叶林中设置对照(CK)、低氮(L)、中氮(M)和高氮沉降(H)4个处理,以NH4NO3为氮源,采用尼龙网袋法对凋落叶进行模拟氮沉降分解试验,研究了凋落叶分解过程中K、Ca、Mg元素浓度及残留率,探讨氮沉降对凋落叶分解过程中养分释放的影响。 结果 经过1年的分解,模拟氮沉降显著抑制了凋落叶分解过程中K元素浓度的下降,显著促进了Ca元素浓度的下降,对Mg元素浓度无显著影响。在各处理中,K元素呈净释放模式,Ca元素表现为释放-富集的交替模式,Mg元素呈富集-释放模式,模拟氮沉降未改变凋落叶分解中K、Ca、Mg元素的释放模式。分解1年后,L、M和H处理的K元素残留率分别比CK高3.91%、10.27%和13.91%,模拟氮沉降显著抑制了凋落叶分解过程中K元素的释放;L、M和H处理的Ca元素残留率分别比CK低6.39%、6.51%和15.93%,模拟氮沉降显著促进了凋落叶分解过程中Ca元素的释放;L、M和H处理的Mg元素残留率与CK差异不显著,模拟氮沉降对凋落叶分解过程中Mg元素的释放无显著影响。 结论 模拟氮沉降未改变凋落叶分解过程中K、Ca、Mg元素的释放模式,但对凋落叶分解过程中K、Ca、Mg元素的释放速率产生了不同的影响。 Abstract:Objective To study the release dynamics of potassium, calcium, and magnesium during litter decomposition under simulated nitrogen deposition, so as to better understand the nutrient cycling process of forest ecosystem under continuously increasing nitrogen depositions. Method NH4NO3 was used to establish the low level (L), medium level (M), high (H) level and the control (CK) treatments of nitrogen deposition in a natural evergreen broadleaved forest in the Rainy Area of Western China. Nylon mesh bag method was employed to simulate litter decomposition under nitrogen deposition. Experiment continued for one year and the concentration, remaining percentages and release patterns of potassium, calcium, and magnesium during litter decomposition were studied. Result After one year of decomposition, the simulated nitrogen deposition significantly inhibited the decrease of potassium in the process of litter decomposition, which significantly promoted the decrease of calcium concentration and had no significant effects on magnesium concentration. However, the simulated nitrogen deposition did not change the release pattern of potassium, calcium, and magnesium during litter decomposition. The potassium exhibited a net release pattern in all the four treatments, the calcium exhibited release-enrichment alternate pattern and the magnesium exhibited an enrichment-release pattern. The remaining percentages of potassium in L, M and H treatments increased by 3.91%, 10.27% and 13.91% respectively compared with the CK, showing that the nitrogen deposition significantly inhibited the release of potassium. The remaining percentage of calcium in L, M and H treatments decreased by 6.39%, 6.51% and 15.93% respectively compared with CK, i.e. nitrogen deposition significantly promoted the release of calcium. There was no significant difference in the remaining percentages of magnesium between the control and the nitrogen treatments i.e. simulated nitrogen deposition had no significant effect on the dynamics of magnesium during litter decomposition. Conclusion Simulated nitrogen deposition will not change the release pattern of potassium, calcium, and magnesium, but has different effects on the release rate of potassium, calcium, and magnesium during litter decomposition. -
表 1 华西雨屏区凋落叶初始养分元素浓度
Table 1. Initial nutrient elements concentration of litter in rainy area of western China
g·kg−1 碳C 氮N 磷P 钾K 钙Ca 镁Mg 430.73±8.71 8.32±1.14 0.42±0.04 13.25±0.24 2.45±0.06 1.36±0.07 表 2 各处理凋落叶质量损失率、K、Ca、Mg元素浓度及其残留率的显著性
Table 2. The significant of concentration and remaining of K, Ca, Mg elements and mass loss rate in various treatments
处理
Treatments质量损失率
Mass loss rate/%浓度Concentration/(g·kg−1) 残留率Remaining/% K Ca Mg K Ca Mg CK 22.71±1.52a 7.04±0.27a 3.74±0.13a 1.48±0.12a 44.65±1.87a 116.81±5.36a 85.56±4.91a L 19.98±0.66b 7.84±0.18b 3.24±0.10b 1.39±0.08a 50.46±1.02b 104.55±3.51b 82.04±4.72a M 18.75±0.75c 8.36±0.28c 2.92±0.09c 1.50±0.12a 53.71±1.77c 95.99±3.00c 88.22±6.72a H 17.59±0.67d 8.77±0.23d 2.66±0.09d 1.37±0.15a 56.84±1.54d 89.33±3.11d 84.29±7.44a 注:不同小写字母表示差异达到P=0.05显著水平。
Note:Different lowercase letters indicate significant difference at P=0.05 level. -
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