Fire frequency exerts a fundamental control on productivity and nutrient cycling in savanna ecosystems. Individual fires often increase short-term nitrogen (N) availability to plants, but repeated burning causes ecosystem N losses and can ultimately decrease soil organic matter and N availability. However, these effects remain poorly understood due to limited long-term biogeochemical data. Here, we evaluate how fire frequency and changing vegetation composition influenced wood stable N isotopes (δ¹⁵N) across space and time at one of the longest running prescribed burn experiments in the world (established in 1964). We developed multiple δ¹⁵N records across a burn frequency gradient from precisely dated Quercus macrocarpa tree rings in an oak savanna at Cedar Creek Ecosystem Science Reserve, Minnesota, USA. Sixteen trees were sampled across four treatment stands that varied with respect to the temporal onset of burning and burn frequency but were consistent in overstory species representation, soil characteristics, and topography. Burn frequency ranged from an unburned control stand to a high-fire-frequency stand that had burned in 4 of every 5 years during the past 55 years. Because N stocks and net N mineralization rates are currently lowest in frequently burned stands, we hypothesized that wood δ¹⁵N trajectories would decline through time in all burned stands, but at a rate proportional to the fire frequency. We found that wood δ¹⁵N records within each stand were remarkably coherent in their mean state and trend through time. A gradual decline in wood δ¹⁵N occurred in the mid-20th century in the no-, low-, and medium-fire stands, whereas there was no trend in the high-fire stand. The decline in the three stands did not systematically coincide with the onset of prescribed burning. Thus, we found limited evidence for variation in wood δ¹⁵N that could be attributed directly to long-term fire frequency in this prescribed burn experiment in temperate oak savanna. Our wood δ¹⁵N results may instead reflect decadal-scale changes in vegetation composition and abundance due to early- to mid-20th-century fire suppression.

中文翻译：

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可变火频率下橡树稀树草原的百年尺度木氮同位素轨迹

火灾频率对热带稀树草原生态系统的生产力和养分循环具有根本的控制作用。单独的大火通常会增加植物的短期氮（N）利用率，但是反复燃烧会导致生态系统氮素损失，并最终降低土壤有机质和氮素的利用率。然而，由于有限的长期生物地球化学数据，对这些影响的了解仍然很差。在这里，我们评估火灾发生频率和变化的植被组成的影响木材稳定的N-如何同位素（δ ¹⁵ ñ）跨越时间和空间在世界上（成立于1964年），运行时间最长的规定烧伤实验之一。我们开发了多个δ ^15个ñ纪录对面正是日的烧伤频率梯度大果栎在美国明尼苏达州锡达克里克生态系统科学保护区的橡树大草原上的年轮。在四个处理台上采样了十六棵树木，这些处理台的燃烧时间和燃烧频率随时间而变化，但在树皮上的树种代表，土壤特征和地形都一致。燃烧频率的范围从未燃烧的控制机架到过去55年中每5年燃烧4次的高发射频率机架。由于N贮量和净氮矿化率是目前最低的频繁烧毁看台上，我们推测木δ ^15个ñ轨迹中全部烧掉站通过时间将有所下降，但在速度成正比的火灾发生频率。我们发现，木δ ¹⁵ ñ每个展台内的记录在平均状态和时间趋势上都非常一致。木材逐渐下降δ ¹⁵ ñ发生在本世纪中叶20日在无糖，低，中等火架，而有一个在高火立场没有趋势。三个林分的下降与规定的燃烧开始没有系统地吻合。因此，我们发现在木材变化的证据有限δ ¹⁵ ñ可能在温带橡树稀树草原这个规定的燃烧试验直接归因于长期的火灾频率。我们的木材δ ¹⁵ ñ 取而代之的是，结果可能反映了20世纪初至20世纪中期火灾抑制所造成的植被组成和丰度的年代际变化。