Abstract Nitrogen (N) availability relative to plant demand has been declining in recent years in terrestrial ecosystems throughout the world, a phenomenon known as N oligotrophication. The temperate forests of the northeastern U.S. have experienced a particularly steep decline in bioavailable N, which is expected to be exacerbated by climate change. This region has also experienced rapid urban expansion in recent decades that leads to forest fragmentation, and it is unknown whether and how these changes affect N availability and uptake by forest trees. Many studies have examined the impact of either urbanization or forest fragmentation on nitrogen (N) cycling, but none to our knowledge have focused on the combined effects of these co‐occurring environmental changes. We examined the effects of urbanization and fragmentation on oak‐dominated (Quercus spp.) forests along an urban to rural gradient from Boston to central Massachusetts (MA). At eight study sites along the urbanization gradient, plant and soil measurements were made along a 90 m transect from a developed edge to an intact forest interior. Rates of net ammonification, net mineralization, and foliar N concentrations were significantly higher in urban than rural sites, while net nitrification and foliar C:N were not different between urban and rural forests. At urban sites, foliar N and net ammonification and mineralization were higher at forest interiors compared to edges, while net nitrification and foliar C:N were higher at rural forest edges than interiors. These results indicate that urban forests in the northeastern U.S. have greater soil N availability and N uptake by trees compared to rural forests, counteracting the trend for widespread N oligotrophication in temperate forests around the globe. Such increases in available N are diminished at forest edges, however, demonstrating that forest fragmentation has the opposite effect of urbanization on coupled N availability and demand by trees.
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机译：摘要 近年来，全球陆地生态系统中氮素相对于植物需求的可用性一直在下降，这种现象被称为氮贫营养化。美国东北部的温带森林经历了生物可利用氮的急剧下降，预计气候变化将加剧这种情况。近几十年来，该地区也经历了快速的城市扩张，导致森林破碎化，目前尚不清楚这些变化是否以及如何影响林木对氮的可用性和吸收。许多研究已经研究了城市化或森林破碎化对氮（N）循环的影响，但据我们所知，没有一项研究关注这些同时发生的环境变化的综合影响。我们研究了城市化和破碎化对从波士顿到马萨诸塞州中部（MA）的城市到农村梯度的橡树（栎属）森林的影响。在城市化梯度的八个研究地点，沿着从发达边缘到完整森林内部的 90 m 样带进行了植物和土壤测量。城市林的净氨化率、净矿化率和叶面氮含量显著高于农村，而城乡林净硝化率和叶面碳氮含量无差异。在城市地点，森林内部的叶面氮和净氨化和矿化高于边缘，而农村森林边缘的净硝化和叶面碳氮高于内部。这些结果表明，与农村森林相比，美国东北部的城市森林具有更大的土壤氮有效性和树木对氮的吸收，抵消了全球温带森林中广泛的氮肥营养化趋势。然而，有效氮的增加在森林边缘减少，这表明森林破碎化对树木耦合氮的可用性和需求具有相反的影响。
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