Atmospheric nitrogen (N) deposition in Mediterranean sclerophyllous forests of Holm oak (, ) in Spain often exceeds empirical critical loads established for ecosystem conservation. There are still uncertainties on the capacity of canopy retention and uptake of the atmospheric N deposited of these forests. Studying and analysing all the forest nitrogen-cycle processes is essential to understand the potential effect of N deposition in these ecosystems. This study conducted a year-long short-term fertilisation experiment with labelled ammonium (N-NH) and nitrate (N-NO) to estimate foliar N absorption rates and assess the influence of leaf phenology and meteorological seasonal variations. Fertilising solutions were prepared to simulate low and high wet N deposition concentration, based on data reported from previous studies. Additionally, ecophysiological and meteorological measurements were collected to explore potential relationships between absorption rates, plant activity, and weather conditions. The results showed that Holm oak leaves were able to absorb both oxidised and reduced N compounds, with higher rates of NH absorption. N recovery of both NH and NO was higher in the low concentration treatments, suggesting reduced effectiveness of absorption as concentration increases. Foliar absorption rates were leaf-age dependent, with the highest values observed in young developing leaves. Foliar uptake showed seasonal changes with a clear reduction during the summer, linked to drought and dry weather conditions, and showing also smaller leaf net assimilation and stomatal conductance. During the rest of the year, foliar N absorption was not clearly associated to plant physiological activity but with environmental conditions. Our findings suggest that Holm oak canopies could absorb an important part of the incoming N deposition, but this process is compound, season and leaf phenology dependent. Further research is therefore needed to better understand and model this part of the N cycle.

中文翻译：

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地中海阔叶常绿森林叶面氮吸收：标记氮施肥实验

西班牙地中海硬叶林圣栎 (, ) 的大气氮 (N) 沉降经常超过为生态系统保护而确定的经验临界负荷。这些森林的冠层保留能力和吸收大气中氮沉积的能力仍然存在不确定性。研究和分析所有森林氮循环过程对于了解氮沉降对这些生态系统的潜在影响至关重要。本研究进行了为期一年的标记铵（N-NH）和硝酸盐（N-NO）短期施肥实验，以估计叶面氮吸收率并评估叶物候和气象季节变化的影响。根据先前研究报告的数据，制备施肥溶液来模拟低和高湿氮沉降浓度。此外，还收集了生态生理学和气象学测量结果，以探索吸收率、植物活动和天气条件之间的潜在关系。结果表明，圣栎树叶能够吸收氧化态和还原态氮化合物，且氨氮吸收率更高。在低浓度处理中，NH 和 NO 的 N 回收率较高，表明随着浓度的增加，吸收的有效性降低。叶吸收率取决于叶龄，在幼嫩发育的叶子中观察到最高值。叶面吸收呈现季节性变化，夏季明显减少，这与干旱和干燥的天气条件有关，并且还显示叶网同化作用和气孔导度较小。在这一年的其余时间里，叶面氮吸收与植物生理活动没有明显相关，但与环境条件相关。我们的研究结果表明，圣栎树冠可以吸收进入的氮沉积的重要部分，但这个过程是复合的、季节和叶子物候相关的。因此，需要进一步的研究来更好地理解和模拟氮循环的这一部分。