Morse Jennifer L, Durán Jorge, Beall Fred, Enanga Eric M, Creed Irena F, Fernandez Ivan, Groffman Peter M
Cary Institute of Ecosystem Studies, Millbrook, NY, USA,
Oecologia. 2015 Jan;177(1):17-27. doi: 10.1007/s00442-014-3117-1. Epub 2014 Nov 19.
In northern forests, large amounts of missing N that dominate N balances at scales ranging from small watersheds to large regional drainage basins may be related to N-gas production by soil microbes. We measured denitrification rates in forest soils in northeastern North America along a N deposition gradient to determine whether N-gas fluxes were a significant fate for atmospheric N inputs and whether denitrification rates were correlated with N availability, soil O2 status, or forest type. We quantified N2 and N2O fluxes in the laboratory with an intact-core method and monitored soil O2, temperature and moisture in three forests differing in natural and anthropogenic N enrichment: Turkey Lakes Watershed, Ontario; Hubbard Brook Experimental Forest, New Hampshire; and Bear Brook Watershed, Maine (fertilized and reference plots in hardwood and softwood stands). Total N-gas flux estimates ranged from <1 in fertilized hardwood uplands at Bear Brook to >100 kg N ha(-1) year(-1) in hardwood wetlands at Turkey Lakes. N-gas flux increased systematically with natural N enrichment from soils with high nitrification rates (Bear Brook < Hubbard Brook < Turkey Lakes) but did not increase in the site where N fertilizer has been added since 1989 (Bear Brook). Our results show that denitrification is an important and underestimated term (1-24% of atmospheric N inputs) in N budgets of upland forests in northeastern North America, but it does not appear to be an important sink for elevated anthropogenic atmospheric N deposition in this region.
在北方森林中，从小流域到大型区域流域尺度上主导氮平衡的大量缺失氮，可能与土壤微生物产生的氮气有关。我们沿着氮沉降梯度测量了北美东北部森林土壤中的反硝化速率，以确定氮气通量是否是大气氮输入的一个重要归宿，以及反硝化速率是否与氮有效性、土壤氧气状况或森林类型相关。我们在实验室中采用原状土芯法对N₂和N₂O通量进行了量化，并监测了三种在自然和人为氮富集程度上存在差异的森林中的土壤氧气、温度和湿度：安大略省的土耳其湖流域；新罕布什尔州的哈伯德布鲁克实验林；缅因州的熊溪流域（硬木和软木林分中的施肥和对照样地）。总氮气通量估计值范围从熊溪施肥硬木高地的<1 kg N ha⁻¹ 年⁻¹到土耳其湖硬木湿地的>100 kg N ha⁻¹ 年⁻¹。氮气通量随着硝化速率高的土壤中自然氮富集程度的增加而系统性增加（熊溪<哈伯德布鲁克<土耳其湖），但在自1989年以来添加了氮肥的地点（熊溪）并未增加。我们的结果表明，反硝化作用是北美东北部高地森林氮收支中一个重要但被低估的部分（占大气氮输入的1 - 24%），但它似乎并不是该地区人为大气氮沉降增加的一个重要汇。
