Dittman Jason A, Driscoll Charles T, Groffman Peter M, Fahey Timothy J
Department of Civil and Environmental Engineering, 151 Link Hall, Syracuse University, Syracuse, New York 13244, USA.
Ecology. 2007 May;88(5):1153-66. doi: 10.1890/06-0834.
The factors controlling spatial and temporal patterns in soil solution and streamwater chemistry are highly uncertain in northern hardwood forest ecosystems in the northeastern United States, where concentrations of reactive nitrogen (Nr) in streams have surprisingly declined over recent decades in the face of persistent high rates of atmospheric Nr deposition and aging forests. Reactive nitrogen includes inorganic species (e.g., ammonium [NH4+], nitrate [NO3-]) and some organic forms (e.g., amino acids) available to support the growth of plants and microbes. The objective of this study was to examine controls on the spatial and temporal patterns in the concentrations and fluxes of nitrogen (N) species and dissolved organic carbon (DOC) in a 12-year record of soil solutions and streamwater along an elevational gradient (540-800 m) of a forested watershed at the Hubbard Brook Experimental Forest (HBEF) in the White Mountains of New Hampshire, USA. Dissolved organic N and DOC concentrations were elevated in the high-elevation spruce-fir-white birch (SFB) zone of the watershed, while NO3- was the dominant N species in the lower elevation hardwood portion of the watershed. Within the soil profile, N retention was centered in the mineral horizon, and significant amounts of N were retained between the lower mineral soil and the stream, supporting the idea that near- and in-stream processes are significant sinks for N at the HBEF. Temporal analysis suggested that hydrologic flow paths can override both abiotic and biotic retention mechanisms (i.e., during the non-growing season when most hydrologic export occurs, or during years with high rainfall), there appears to be direct flushing of N from the organic horizons into the stream via horizontal flow. Significant correlations between soil NO3- concentrations, nitrification rates and streamwater NO3- exports show the importance of biological production as a regulator of inorganic N export. The lack of internal production response (e.g., mineralization, nitrification) to a severe ice storm in 1998 reinforces the idea that plant uptake is the dominant regulator of export response to disturbance.
在美国东北部的北方硬木森林生态系统中，控制土壤溶液和溪水化学性质时空格局的因素极不确定。在该地区，尽管大气活性氮（Nr）持续高沉降且森林老化，但近几十年来溪流中活性氮的浓度却出人意料地下降了。活性氮包括无机形态（如铵离子[NH4+]、硝酸根离子[NO3-]）以及一些可用于支持植物和微生物生长的有机形态（如氨基酸）。本研究的目的是，在美国新罕布什尔州怀特山脉哈伯德布鲁克实验森林（HBEF）的一个森林流域，沿着海拔梯度（540 - 800米），通过12年的土壤溶液和溪水记录，研究氮（N）形态和溶解有机碳（DOC）浓度及通量的时空格局控制因素。流域高海拔云杉 - 冷杉 - 白桦（SFB）区域的溶解有机氮和DOC浓度升高，而NO3-是流域低海拔硬木区域的主要氮形态。在土壤剖面内，氮的保留集中在矿质层，大量的氮保留在下部矿质土壤和溪流之间，这支持了近溪流和溪流内过程是HBEF中氮的重要汇的观点。时间分析表明，水文流动路径可以超越非生物和生物保留机制（即，在大多数水文输出发生的非生长季节，或在降雨量大的年份），似乎存在通过水平流动将氮从有机层直接冲入溪流的情况。土壤NO3-浓度、硝化速率与溪水NO3-输出之间的显著相关性表明，生物生产作为无机氮输出调节剂的重要性。1998年一场严重冰暴后缺乏内部生产响应（如矿化、硝化），强化了植物吸收是干扰后输出响应的主要调节因素这一观点。
