为理解氮沉降持续增加对次生林生态系统碳循环相关过程的影响,2013年5月开始对瓦屋山常绿阔叶次生林进行模拟氮沉降试验,氮沉降水平分别为对照(CK,0 kg hm-2 a-1)、低氮(LN,50 kg hm-2 a-1)和高氮(HN,150 kg hm-2a-1).于2013年9月至2014年8月,每月下旬采用LI-8100开路式土壤碳通量测量系统测定土壤呼吸速率.结果显示:瓦屋山常绿阔叶次生林土壤呼吸速率季节变化明显,7月最高(2.77μmol m-2 s-1),1月最低(0.46μmol m-2 s-1).该次生林累积土壤碳排放量为591 g m-2 a-1.土壤呼吸与土壤温度呈极显著(P<0.01)正指数相关,与土壤湿度无显著关系(P>0.05).模拟氮沉降显著(P=0.027)抑制了土壤呼吸,高氮处理年均土壤呼吸速率较对照极显著(P<0.01)下降31.4%.基于土壤温度的土壤呼吸Q10值为2.63,低氮和高氮处理Q10值分别较对照增加16%和33%.综上所述,在该生态系统土壤氮含量较高的背景下,模拟氮沉降可能加剧了土壤碳限制,通过减缓植物根系生长和降低土壤微生物生物量抑制了土壤CO2排放.
Bamboos are one of the fast-growing and multiple use species in the world,and thus bamboo forests/plantations play an important role in C sequestration at regional and global levels.We studied aboveground litterfall,litter decomposition and nutrient dynamics for two years in two subtropical bamboo ecosystems in Southwest China so as to test the hypothesis that litter quality determine the rate and nutrient dynamics during decomposition of different litter fractions.Mean annual total aboveground litter production ranged from 494 to 434 g m^(-2)in two bamboo stands(P stand,dominated by Pleioblastus amarus and H stand,hybrid bamboo dominated by Bambusa pervariabilis x Dendrocalamopsis daii).Bulk(~80%)of litter production was contributed by leaf litter in two stands followed by twigs and sheathes.Different litter fractions represented considerable variations in the rates of mass loss and nutrient release.Variation of the mass remaining after 2 years of decomposition was significantly explained by initial C/N ratio and initial P concentration.Initial concentrations of N,P,Ca,and Mg explained 57.9%,95.0%,99.8%and 98.1%,respectively,of the variations of these elements mass remaining after 2 years of decomposition.The patterns of nutrient dynamics and the final amount remaining were mainly determined by their initial litter substrate quality in these two subtropical bamboo plantations.
TU Li-HuaHU Hong-LingHU Ting-XingZHANG JianLI Xian-WeiLIU LiXIAO Yin-LongCHEN GangLI Ren-Hong
森林细根分解是陆地生态系统碳循环的重要过程之一,其分解速率受到大气氮沉降增加的潜在影响.2007年11月至2013年1月,对华西雨屏区苦竹人工林进行每月1次的模拟氮沉降试验,设对照(CK,0 g N·m-2·a-1)、低氮(5 g N·m-2·a-1)、中氮(15 g N·m-2·a-1)和高氮(30 g N·m-2·a-1)4个处理.2011年1月起,采用分解袋法研究苦竹细根分解.结果表明:苦竹细根分解呈现出先快后慢的趋势,在分解第1年质量损失达60%,分解第2年质量残留率变化较为平缓.对照处理细根质量损失50%和95%分别需要1.20和5.17 a.总体上,负指数模型低估了各处理细根分解速率.模拟氮沉降显著抑制了苦竹细根分解,相对于对照,高氮处理细根在分解2 a后残留量增加51.0%.模拟氮沉降显著增加了凋落物碳、氮和磷元素的残留率.与对照相比,模拟氮沉降处理4.5 a后,中氮和高氮处理土壤pH值显著降低,高氮处理土壤有机碳、总氮、铵态氮和硝态氮含量以及苦竹细根生物量显著增加.
In order to understand the effects of increasing atmospheric nitrogen(N) deposition on the subtropical bamboo ecosystem, a nearly six-year field experiment was conducted in a Pleioblastus amarus plantation in the rainy region of SW China, near the western edge of Sichuan Basin. Four N treatment levels—control(no N added), lowN(50 kg N ha-1a-1), medium-N(150 kg N ha-1a-1),and high-N(300 kg N ha-1a-1)—were applied monthly in the P. amarus plantation starting in November 2007. In June 2012, we collected intact soil cores in the bamboo plantation and conducted a 30-day laboratory incubation experiment. The results showed that the soil N net mineralization rate was 0.96 ± 0.10 mg N kg-1day-1, under control treatment. N additions stimulated the soil N net mineralization, and the high-N treatment significantly increased the soil N net mineralization rate compared with the control. Moreover, the soil N net mineralization rate was significantly and positively correlated with the fine root biomass, the soil microbial biomass nitrogen content and the soil initial inorganic N content, respectively,whereas it was negatively correlated with the soil p H value.There were no significant relationships between the soil N net mineralization rate and the soil total nitrogen(TN)content and the soil total organic carbon content and the soil C/N ratio and the soil microbial biomass carbon content, respectively. These results suggest that N additions would improve the mineral N availability in the topsoil of the P. amarus plantation through the effects of N additions on soil chemical and physical characteristics and fine-root biomass.
Yin-long XiaoLi-hua TuGang ChenYong PengHong-ling HuTing-xing HuLi Liu
凋落物基质质量是影响凋落物分解速率的决定性因子之一,研究旨在探究模拟氮沉降对苦竹林凋落物基质质量的影响。2007年11月至2010年12月每月1次连续对华西雨屏区苦竹人工林进行了模拟氮沉降试验,施氮水平分别为:对照(0 g N·m-2·a-1)低氮(5 g N·m-2·a-1),中氮(15 g N·m-2·a-1)和高氮(30 g N·m-2·a-1)。在施氮2 a后,于2010年1月开始收集各样方的凋落物样品,连续收集12个月,分析测定凋落物基质质量。结果表明:施氮显著增加了凋落叶中N、P元素含量,中氮处理显著增加了凋落枝中N元素含量,中氮和高氮处理均显著增加了凋落枝中P元素含量;施氮对凋落物中C元素含量影响很微弱,显著降低了凋落叶中的C/N,中氮处理显著降低了凋落枝中的C/N,对木质素和纤维素含量均未造成显著影响。由于模拟氮沉降增加了苦竹凋落物的N、P含量,降低了其C/N,因此氮沉降可能会促进苦竹凋落物的初期分解速率。
