南亚热带森林土壤有机碳组分对模拟酸雨的早期响应

Effects of simulated acid rain on soil organic carbon fractions in southern subtropical forests

关键词:  南亚热带森林  模拟酸雨  有机碳组分  土壤酸化

Key words:  southern subtropical forests  simulated acid rain(SAR)  soil organic carbon fractions  soil acidification

摘要:

应用人工模拟酸雨控制实验,探讨鼎湖山国家级自然保护区三种南亚热带主要植被类型(季风常绿阔叶林、针阔叶混交林和马尾松林)的土壤有机碳组分,包括土壤总有机碳(TOC)、土壤易氧化有机碳(ROC)、土壤不易氧化有机碳(NROC),在不同模拟酸雨处理梯度:对照CK(pH4.5的天然湖水)、pH4.0、pH3.5、pH3.0处理下的响应特征。结果表明:上层土壤(0～20 cm)易氧化有机碳、不易氧化有机碳和总有机碳含量与森林类型密切相关,大小顺序均表现为混交林>阔叶林>马尾松林。经25个月模拟酸雨处理,鼎湖山森林土壤酸化有加剧的趋势; CK、pH4.0、pH3.5、pH3.0四个处理下土壤上层剖面易氧化有机碳含量分别为阔叶林(7.14、8.29、8.74、9.84 g·kg^-1)、混交林(8.58、8.53、10.28、10.36 g·kg^-1)和马尾松林(3.90、4.49、4.74、5.48 g·kg^-1),三个林型土壤易氧化有机碳含量呈现随模拟酸雨强度增加而升高的趋势; 森林土壤总有机碳和不易氧化有机碳含量变化缓慢,在各酸梯度处理下差异不显著(P>0.05)。研究结果显示,长期的酸雨作用使土壤酸化不断加剧,易氧化有机碳对酸雨的响应更敏感,但其在酸雨下积累的趋势不利于土壤总有机碳的存埋,但关于酸雨对土壤总有机碳的影响仍然需要长期的实验监测。

Abstract:

Soil carbon cycling is one of the important links in soil ecosystem,while the soil ecosystem is the final acceptor of the acid rain. Thus,the acid rain will have dramatic impacts on the stability of soil organic carbon and even the forest soil carbon cycling. Soil is also the largest carbon pool in terrestrial ecosystem,and stores approximately 1 500 Pg of C in the upper meter of soil. Even a small change of the contents can result in a significant consequence on carbon cycling in forest. The readily oxidation organic carbon(ROC),as one of the liable soil organic carbon fractions,is more sensitive to environmental changes and its turnover rate is faster than other stable soil organic carbon. Consequently,ROC is considered as an early indicator of changes in total soil organic carbon(TOC). However,there have been very few studies on the fractions of soil organic carbon(liable and stable organic carbon)in response to the acid rain at home and abroad,despite the fact that acid rain has recently become a serious environmental threat in this region. In this paper,four gradients of simulated acid rain(SAR)treatments were designed[control(natural lake water,pH4.5±0.3),pH4.0,pH3.5,pH3.0] to investigate the effects of simulated acid rain on soil organic carbon fractions,including total soil organic carbon,readily oxidation organic carbon and non-readily oxidation organic carbon(NROC),in three forests[i.e. a pine forest(PF),a mixed conifer and broadleaf forest(MF)and broadleaved forest(BF)] of Dinghushan Biosphere Reserve in southern China. The acidic solutions were used a mixture of H₂SO₄ and HNO₃ in a 1:1 mole ratio to the natural lake water. Results showed that ROC,NROC and TOC in the topsoil(0-20 cm)varied with types of forest: MF >BF>PF. After 25 months' SAR treatments,the pH values in the forest topsoil reduced gradually with the increase of acidity,and presented a trend toward increasing soil acidification with SAR. This phenomenon was in consistent with results from other researchers at Dinghushan Nature Reserve. Meanwhile,the concentrations of ROC in BF(7.14 g·kg^-1 for CK,8.29 g·kg^-1 for pH4.0,8.74 g·kg^-1 for pH 3.5,9.84 g·kg^-1 for pH3.0),MF(8.58 g·kg^-1 for CK,8.53 g·kg^-1 for pH4.0,10.28 g·kg^-1 for 3.5,10.36 g·kg^-1 for pH3.0)and PF(3.90 g·kg^-1 for CK,4.49 g·kg^-1 for pH4.0,4.74 g·kg^-1 for pH3.5,5.48 g·kg^-1 for pH3.0),which indicated ROC increased gradually with the decrease of treatment pH value in all forests. Meanwhile,the percentage of ROC to total soil organic carbon was also increased. However,due to the relative stability of soil total organic carbon,the concentrations of TOC and NROC were not significantly different among the four treatments in all forests(P>0.05). The results indicated that the ROC was more sensitive to long-term acid rain treatment,and would have negative effects on the accumulation of soil organic carbon. However,more long-term investigations are needed in order to validate the role of acid rain on soil organic carbon storage.