Nitrogen deposition across scales: hotspots and gradients in a California savanna landscape
Nitrogen deposition across scales: hotspots and gradients in a California savanna landscape
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跨尺度的氮沉降：加州稀树草原景观中的热点和梯度
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发表时间：
2015
期刊：
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通讯作者：
M. Cadenasso
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作者：
Elise M. Tulloss;M. Cadenasso
氮(N)沉积是全球变化的关键组成部分。为了预测这些变化的后果，需要量化氮沉积的空间和时间模式。我们研究了2008-2009年雨季在加州中北部的六个橡树稀树草原上的两个空间尺度上的无机氮沉积模式。我们发现了一个区域尺度的沉积梯度，在这种梯度中，离城市和农业源头较近的地点比距离较远的地点获得更高的沉积速率。此外，距离城市污染源较近的站点获得了更大比例的N，作为NO3−-N。在当地的立地尺度上，与邻近的开阔草地相比，林下植被获得了更多的N输入。氮素沉积的时间异质性也被发现。沉积表现出强烈的季节性模式，与这一地中海生态系统独特的干湿循环相对应。虽然这些空间和时间结果是意料之中的，但令人惊讶的是，橡树下热点效应的大小在区域梯度上发生了变化。在区域坡度较高的沉积端，林冠下N的吸收比例高于坡度较低端。地方和区域尺度之间的这种非线性表明了景观背景对于理解站点内动态的重要性，以及多标度研究方法的必要性。将区域尺度的景观背景和当地尺度的植被结构以及时间变异性纳入我们对沉积模式的理解中，对于监测努力以及解决氮沉积增加的潜在生态影响至关重要。
Nitrogen (N) deposition is a key component of global change. In order to predict the consequences of those changes, the spatial and temporal patterns of N deposition need to be quantified. We investigated patterns of inorganic-N deposition across two spatial scales—regional and local—in six oak savanna sites across north-central California during the 2008–2009 wet season. We found a regional-scale deposition gradient in which sites closer to urban and agricultural sources received higher rates of deposition than more remote sites. Additionally, sites closer to urban sources received a greater proportion of N as NO3−-N. At the local, within-site scale, canopy understories received greater N inputs compared to the adjacent open grassland. Temporal heterogeneity in N deposition was also found. Deposition exhibited a strong seasonal pattern, corresponding to the distinct wet-dry cycles characteristic of this Mediterranean ecosystem. While these spatial and temporal results were expected, it was surprising that the magnitude of the hotspot effect under oaks changed across the regional gradient. At the high deposition end of the regional gradient proportionally more N was received beneath canopies than at the low end of the gradient. This nonlinearity between local and regional scales demonstrates the importance of landscape context for understanding within-site dynamics and the need for multi-scalar research approaches. Incorporating regional-scale landscape context and local-scale vegetation structure, as well as temporal variability, into our understanding of deposition patterns is crucial for monitoring efforts as well as addressing potential ecological effects of increased N deposition.