It is well established that nutrient addition influences ecosystem features such as productivity, carbon storage, soil acidification and biodiversity. Less studied are long-term effects of sustained fertiliser application on forest soil characteristics and nutrient supplies, and especially direct and indirect mechanisms underlying changes. We investigated effects of 3 decades versus 1 decade of optimised fertiliser application on soil properties and nutrient supplies in a 30-year-old nutrient optimisation experiment in a Norway spruce plantation in northern Sweden. We tested for direct and indirect effects of fertiliser use through structural equation models and correlations among tree and soil variables. Results showed that soil characteristics, especially organic carbon and nutrient concentrations, were significantly affected by 10- and 30-year fertiliser application. Soil C/N was similar for the short-term versus controls, but decreased for the long-term versus short-term treatment. Although not explicitly measured, it was clear from our analyses and earlier studies at the site that litter accumulation played a key role in explaining these changes in soil properties, while foliar stoichiometry data suggest long-term effects of litter quality. Nutrient supply rates increased more after 30 than 10 years of fertiliser application. Summarized, we showed that the interplay of direct and indirect effects can yield nonlinear patterns over time, as exemplified by soil C/N. Furthermore, we conclude that lagged, indirect effects of fertilisation through altered litter quantity and quality dominate changes in soil characteristics in this forest. These soil characteristics have further relevance to nutrient availability, suggesting that nutrient optimisation can influence soil fertility also indirectly.

中文翻译：

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长期优化施肥后树木生长的增加间接改变了北方森林的土壤特性

众所周知，养分添加会影响生态系统特征，例如生产力、碳储存、土壤酸化和生物多样性。研究较少的是持续施肥对森林土壤特征和养分供应的长期影响，特别是潜在变化的直接和间接机制。我们在瑞典北部的挪威云杉种植园进行了 30 年的养分优化实验，研究了 3 年与 1 年优化肥料施用对土壤特性和养分供应的影响。我们通过结构方程模型以及树木和土壤变量之间的相关性测试了肥料使用的直接和间接影响。结果表明，土壤特征，尤其是有机碳和养分浓度，施肥 10 年和 30 年显着影响。短期与对照相比，土壤 C/N 相似，但长期与短期处理相比，土壤 C/N 下降。虽然没有明确测量，但我们的分析和现场早期研究清楚地表明，凋落物积累在解释土壤特性的这些变化方面发挥了关键作用，而叶面化学计量数据表明凋落物质量的长期影响。施肥 30 年多于 10 年，养分供应率增加更多。总而言之，我们表明，直接和间接影响的相互作用会随着时间的推移产生非线性模式，如土壤 C/N 所示。此外，我们得出结论，通过改变凋落物数量和质量，施肥的滞后、间接影响主导了这片森林土壤特性的变化。