温带两种林型对氮沉降的再分配及其生长季动态与影响因子

氮(N)沉降对森林生态系统的结构与功能具有重要的影响,而森林对到达林地的N沉降量及其分配格局的影响尚不清楚。量化了2012—2013年5—10月两个生长季蒙古栎林和杂木林的林内树干径流和穿透雨及其林外大气降雨总氮(TN)、可溶性氮(DN)和颗粒态氮(PN)沉降通量的季节动态,旨在比较两种林型对N沉降的再分配格局及其季节变化,分析影响其变异的主要因子。结果表明:林外大气降雨、蒙古栎林林内、杂木林林内(树干径流+穿透雨)TN沉降平均值分别为:8.49、15.97、13.39 kg hm~(-2)a~(-1),其中DN分别占其TN的76.35%、82.79%和75.02%,PN分别占其TN的26.35%、17.21%和24.98%,蒙古栎林和杂木林林内穿透雨TN沉降量分别占其TN的95.5%和94.5%。蒙古栎林和杂木林冠层淋溶TN沉降量分别为7.48kg hm~(-2)a~(-1)和4.90 kg hm~(2)a~(-1);其中,前者的DN高于后者,但PN呈相反趋势。两种林型的N沉降组分具有明显的季节动态:沉降量均集中在生长季中期(6—8月),生长季前期和末期较低。林外降雨量分别与林外大气降雨、蒙古栎林和杂木林林内的树干径流和穿透雨中的TN、DN浓度呈显著负指数函数关系(P0.001)。连续降雨天数对蒙古栎林、杂木林林内TN、DN浓度的影响表现为连续降雨2 d以内为富集作用,之后为稀释作用。研究表明林冠对大气氮沉降有显著富集作用,其富集强度及时间动态与森林类型和降雨特征有关,建议氮沉降试验应考虑林冠的富集效应。

Redistribution of nitrogen deposition and its influencing factors during the growing season in two temperate forests Northeast China

Nitrogen (N) deposition plays an important role in forest structure and function, but the redistribution pattern of N deposition and its components through the forest canopy is not well understood. In the study, we measured the concentrations of total N (TN), dissolved N (DN), and particulate N (PN) in rainfall outside of the stands, and from stemflow and throughfall during the growing seasons (May to October) of two consecutive years (2012-2013) in two temperate stands (i.e., Mongolia oak (Quercus mongolica) stand and mixed stand) in the Maroershan region, Northeast China. The objectives were to compare the redistribution patterns of the TN, DN, and PN depositions and their seasonal dynamics in the two stands, and explore potential driving factors of the variability in N deposition. The results showed that the mean TN depositions from outside of the stands, and inside of the oak stand and mixed stand were 8.49 kg hm^-2 a^-1, 15.97 kg hm^-2 a^-1, and 13.39 kg hm^-2 a^-1, respectively; among which DN and PN accounted for 76.35%, 82.79%, and 75.02%, and 26.35%, 17.21%, and 24.98% of the TN deposition, respectively. The TN deposition of throughfall accounted of 95.5% and 94.5% of TN inside the oak stand and mixed stand, respectively. The oak and mixed stand canopies leached TN deposition of 7.48 kg N·hm^-2 a^-1 and 4.90 kg N hm^-2 a^-1, respectively; and the former accumulated more DN but less PN than the latter. There was a significant seasonal pattern of N deposition for the two stands, in which the deposition fluxes were concentrated in the mid-growing seasons (June-August), and declined in the early- and late-growing seasons. The concentrations of TN and DN outside the stands, and in the throughfall and stemflow of the oak and mixed stands were all significantly related to the rainfall in a negative exponential function (P < 0.001). The days of continuous rain events influence the concentrations of TN and DN inside the two stands as enrichment within two days of continuous rainfall and dilution in the latter stages. This study indicated that the forest canopy significantly enriched the atmospheric N deposition, and the enrichment strength and its temporal dynamics varied with the characteristics of stands and rainfall. It was suggested that the enrichment effect of forest canopy should be taken into account in the simulation experiments of N deposition in the future.