华西雨屏区苦竹林土壤呼吸对模拟氮沉降的响应

RESPONSE OF SOIL RESPIRATION TO SIMULATED NITROGEN DEPOSITION IN PLEIOBLASTUS AMARUS FOREST, RAINY AREA OF WEST CHINA

2007年11月至2008年11月,对华西雨屏区苦竹(Pleioblastus amarus)人工林进行了模拟氮沉降试验,氮沉降水平分别为对照(CK,0gN·m–2·a–1)、低氮(5gN·m–2·a–1)、中氮(15gN·m–2·a–1)和高氮(30gN·m–2·a–1)。每月下旬,采用红外CO2分析法测定土壤呼吸速率,并定量地对各处理施氮(NH4NO3)。结果表明:2008年试验地氮沉降量为8.241g·m–2,超出该地区氮沉降临界负荷。在生长季节,苦竹林根呼吸占总土壤呼吸的60%左右。模拟氮沉降促进了苦竹林土壤呼吸速率,使苦竹林土壤每年向大气释放的CO2增加了9.4%～28.6%。在大时间尺度上(如1a),土壤呼吸主要受温度的影响。2008年6～10月,土壤呼吸速率24h平均值均表现为:对照<低氮<中氮<高氮。氮沉降处理1a后,土壤微生物呼吸速率和土壤微生物生物量碳、氮增加,并且均与氮沉降量具有相同趋势。各处理土壤呼吸速率与10cm土壤温度、月平均气温呈极显著指数正相关关系,利用温度单因素模型可以解释土壤呼吸速率的大部分。模拟氮沉降使得土壤呼吸Q10值增大,表明氮沉降可能增强了土壤呼吸的温度敏感性。在氮沉降持续增加和全球气候变暖的背景下,氮沉降和温度的共同作用可能使得苦竹林向大气中排放的CO2增加。

Aims Our objectives were to determine the effect of increased nitrogen deposition on total soil respiration and microbial respiration ofPleioblastus amarus forest, and whether increased nitrogen deposition alters the temperature sensitivity of soil respiration. Methods For one year starting November 2007, we conducted a simulated nitrogen deposition field experiment in P. amarus forest, Rainy Area of West China. The levels of nitrogen deposition were control （CK, 0gN·m^-2·a^-1）, low nitrogen （5 gN·m^-2·a^-1）, medium nitrogen （15 gN·m^-2·a^-1） and high nitrogen （30 gN·m^-2·a^-1）. At the end of each month, soil respiration was measured by infrared gas analyzer, and NH4NO3 was added to N-treated plots. Natural wet nitrogen deposition was measured in 2008, and 0--20 cm horizon soil was collected for soil respiration sampling in November 2008. These soil samples were maintained in the laboratory at 20 ℃, and we measured soil microbial respiration （by infrared gas analyzer） and microbial biomass carbon and nitrogen. Important findings The wet nitrogen deposition of Liujiang, Hongya County is 8.241 g.m^-2; it exceeds the local nitrogen deposition critical loads. Root respiration accounts for about 60% of the total soil respiration of P. amarus forest from April to October. Nitrogen deposition promotes soil respiration ofP amarus forest, and CO2 release from forest soil is increased by 9.4%-28.6%. Microbial respiration, microbial biomass carbon and nitrogen were stimulated by nitrogen deposition, and respiration rate rises with increased nitrogen deposition. The soil respiration rate exhibited positive exponential relation- ships with air temperature and soil temperature at 10 cm depth. Exponential relationships between temperature and soil respiration were highly significant in all plots. Nitrogen deposition may increase temperature sensitivity of soil respiration. With increasing rates of anthropogenic N deposition and global warming, nitrogen combined with temperature may increase the release of CO2 from P. amarus forest soil.