长白山森林生态系统大气氮素湿沉降通量和组成的季节变化特征

2009—2010年期间,利用雨量计收集法在长白山森林生态系统定位站开展定位观测,分析降水中氮素浓度,研究了该区域大气氮素湿沉降通量和组成的季节变化特征。结果表明,各形态氮素月均浓度之间差别较大,具有明显的季节性;其降水中浓度主要受降水量和降水频次的影响。全年氮素湿沉降中TN、TIN和TON的沉降量分别为27.64 kg N hm-2a-1、11.05 kg N hm-2a-1和16.59 kg N hm-2a-1,TON为沉降主体,占60.02%;其大气氮沉降量主要由降水量和降水中氮素浓度共同决定。该地区氮湿沉降量已处于我国中等水平,考虑到氮素的干湿沉降比例,本区域的年氮沉降量已接近或超过本区域的营养氮沉降临界负荷,存在一定的环境风险。该地区生长季(5—10月)的氮沉降量(16.59 kg N hm-2a-1)占全年氮沉降量的比例达到73.20%。生长季的氮沉降对于促进植物生长直接生态意义重大,而非生长季的氮沉降对于大量补充次年植物生长初期所需养分的间接生态意义明显。

Seasonal variations of nitrogen flux and composition in a wet deposition forest ecosystem on Changbai Mountain

In the last 150 years, human activities such as fossil fuel usage and centralized agricultural schemes have resulted in the emission of large amounts of reactive nitrogen. Most of this reactive nitrogen falls to the ground via wet deposition as precipitation or dust fall. Nitrogen deposition plays a key role in the nitrogen biogeochemical cycles. Hence, quantification of nitrogen deposition in different areas is important for eco-security assessment. The Changbai Mountain region is a typical temperature forest in China; however, data regarding nitrogen deposition in this region are scarce. Investigation of nitrogen deposition in the Changbai Mountain region can provide a scientific basis for understanding nitrogen cycling in similar natural ecosystems. We conducted our present study at the Changbai Mountain Research Station of Forest Ecosystems. During 2009-2010, we collected rain and snow samples by using a rain gauge. We analyzed the concentrations of total nitrogen (TN), inorganic nitrogen (TIN), and total organic nitrogen (TON). We evaluated seasonal variations in nitrogen deposition fluxes and composition. We showed that the average nitrogen concentrations varied from month to month and were higher in winter than in summer. The amount and frequency of precipitation were the main impact factors. We observed monthly variations in TN and TIN. TON accounted for the highest proportion of TN (30.03-92.88%), whereas NO^-3 was the dominant form of TIN. The values of TN, TIN, and TON were 27.64 kg N hm^-2 a^-1, 11.05 kg N hm^-2 a^-1, and 16.59 kg N hm^-2 a^-1, respectively. The amount of nitrogen deposition varied according to the amount of precipitation and the nitrogen concentration of this precipitation. Our results indicate that nitrogen deposition in the Changbai Mountain region has reached a moderate level compared with other parts of China. However, according to the proportions of wet and dry nitrogen deposition, total nitrogen deposition has approached or even exceeded the critical load of nutrient nitrogen deposition, and this poses an environmental risk. The nitrogen deposition during the growing season (May to October) accounted for 73.20% of the annual nitrogen deposition, namely, 16.59 kg N hm^-2 a^-1, and was important for stimulating plant growth. Nitrogen deposition during the non-growing season can replenish the nutrition required by plants at the start of the next growing season and therefore should also be considered.