Distribution of nitrogen-15 tracers applied to the canopy of a mature spruce-hemlock stand, Howland, Maine, USA
Distribution of nitrogen-15 tracers applied to the canopy of a mature spruce-hemlock stand, Howland, Maine, USA
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DOI：
10.1007/s00442-009-1325-x
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发表时间：
2009-06-01
期刊：
影响因子：
2.7
通讯作者：
Gaige, Elizabeth
中科院分区：
文献类型：
--
作者：
Dail, David Bryan;Hollinger, David Y.;Gaige, Elizabeth
在氮素有限的生态系统中，通过氮沉降施肥可以促进植物生长，从而影响碳固存。在许多N沉积-C螯合实验中，N直接添加到土壤中，绕过冠层过程，并可能有利于土壤对N的固定。为了了解增加的氮沉降对低肥力未管理森林的影响，并更好地模拟自然氮沉降过程，我们在21公顷云杉-铁杉林的树冠层中直接添加了18公斤氮公顷（-1）年（-1）溶解的NH 4 NO3。在两个0.3公顷的子地块中，添加的N被同位素标记为（NH 4）-N-15（+）或（NO3）-N-15（-）（1%最终富集）。在生态系统库中，我们分别回收了38%和67%的（NH 4）-N-15（+）和（NO3）-N-15（-）。在植物生物量中可回收的N-15中，只有3-6%在活叶和树干木材中回收。树枝、树枝和树皮是NO3（-）和NH 4（+）最重要的植物汇，它们分别占N-15回收量的25-50%。森林地面和土壤N-15的保留是小的相比，以前的研究，凋落物层和腐殖化O层的重要汇NH 4（+）（9%）和NO3（-）（7%）。保留冠层元素（表面的分支和树干）提供了一个很大的汇N可能已经通过物理化学过程，而不是由N同化表明在木材组织中的回收率差。因此，以这种特殊方式添加的降水携带的氮的冠层保留可能不会成为植物有效氮数年。尽管在这片森林中有很大的冠层氮保持潜力，但由于氮的增加，新木材生长中的C螯合量仅与16 g C m（-2）年（-1）相似，或比该地点目前的年净C螯合量高出约10%。
In N-limited ecosystems, fertilization by N deposition may enhance plant growth and thus impact C sequestration. In many N deposition-C sequestration experiments, N is added directly to the soil, bypassing canopy processes and potentially favoring N immobilization by the soil. To understand the impact of enhanced N deposition on a low fertility unmanaged forest and better emulate natural N deposition processes, we added 18 kg N ha(-1) year(-1) as dissolved NH4NO3 directly to the canopy of 21 ha of spruce-hemlock forest. In two 0.3-ha subplots, the added N was isotopically labeled as (NH4)-N-15 (+) or (NO3)-N-15 (-) (1% final enrichment). Among ecosystem pools, we recovered 38 and 67% of the N-15 added as (NH4)-N-15 (+) and (NO3)-N-15 (-), respectively. Of N-15 recoverable in plant biomass, only 3-6% was recovered in live foliage and bole wood. Tree twigs, branches, and bark constituted the most important plant sinks for both NO3 (-) and NH4 (+), together accounting for 25-50% of N-15 recovery for these ions, respectively. Forest floor and soil N-15 retention was small compared to previous studies; the litter layer and well-humified O horizon were important sinks for NH4 (+) (9%) and NO3 (-) (7%). Retention by canopy elements (surfaces of branches and boles) provided a substantial sink for N that may have been through physico-chemical processes rather than by N assimilation as indicated by poor recoveries in wood tissues. Canopy retention of precipitation-borne N added in this particular manner may thus not become plant-available N for several years. Despite a large canopy N retention potential in this forest, C sequestration into new wood growth as a result of the N addition was only similar to 16 g C m(-2) year(-1) or about 10% above the current net annual C sequestration for this site.