Late-rotation fertilization of Norway spruce stands is a frequently used management tool in Fennoscandia to increase timber yields. Meanwhile, the growing demand for renewable resources has sparked great interest in earlier and repeated fertilizer application but it remains unclear how this affects carbon dioxide (CO₂) fluxes in the understory, especially forest floor respiration (R_ff). This study investigated the effects of forest fertilization on R_ff and net forest floor exchange (NFFE) in young, nitrogen (N) limited Norway spruce stands in southern Sweden. In a short-term dose experiment, R_ff and NFFE were recorded during 2016 after varying doses of N (0, 150, 300, or 450 kg ha^-1 of N, hereafter N0, N150, N300, N450) were added to circular, 3-m-diameter plots in April. In a second, long-term experiment, two stand-level fertilizer applications with 150 kg ha^-1 of N on each occasion were performed in 2014 and 2016 and R_ff was measured at semi-regular intervals from mid-2013 to the end of 2017. In the dose experiment, fertilization increased R_ff by 23 %, 81 % and 55 % in the N150, N300 and N450 treatments, respectively. Under well-lit conditions, the N300 and N450 treatments significantly enhanced photosynthetic CO₂ uptake of the forest floor vegetation by 97 % and 66 %, respectively, while the N150 treatment had no significant effect. The results of the long-term experiment indicate an initial stimulation of R_ff, but this effect was transient. Our findings imply that fertilization in young Norway spruce stands, using the N150 dose (the typical dose used in Swedish forestry), may cause a transient burst in R_ff that is far outweighed by nutrient-driven increases in forest floor photosynthesis under favourable light conditions prior to canopy closure.

中文翻译：

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森林施肥暂时增加了瑞典挪威云杉幼苗林分的土壤 CO2 外排


挪威云杉林分的晚轮作施肥是芬诺斯坎迪亚常用的管理工具，可以提高木材产量。与此同时，对可再生资源不断增长的需求引发了人们对早期和重复施肥的极大兴趣，但目前尚不清楚这如何影响林下二氧化碳 （CO₂） 通量，尤其是森林地面呼吸 （R_ff）。本研究调查了森林施肥对瑞典南部年轻、氮 （N） 限制的挪威云杉林分的 R_ff 和净森林地面交换 （NFFE） 的影响。在短期剂量实验中，在 2016 年 4 月将不同剂量的 N（0、150、300 或 450 kg ^ha-1 的 N，以下简称 N0、N150、N300、N450）添加到直径为 3 m 的圆形地块后，记录了 R_ff 和 NFFE。在第二个长期试验中，在 2014 年和 2016 年进行了两次林分肥料施用，每次施用 150 kg ^ha-1 的氮，并从 2013 年年中到 2017 年底以半规则的间隔测量 R_ff。在剂量实验中，施肥使 N150、N300 和 N450 处理的 R_ff 分别增加了 23 %、81 % 和 55 %。在光照充足的条件下，N300 和 N450 处理显著提高了森林地面植被对 CO₂ 的光合吸收，分别提高了 97 % 和 66 %，而 N150 处理没有显著影响。长期实验的结果表明 R_ff 的初始刺激，但这种效果是短暂的。 我们的研究结果表明，使用 N150 剂量（瑞典林业使用的典型剂量）对挪威云杉幼苗进行施肥可能会导致 R_ff 的短暂爆发，这远远超过在树冠关闭前有利的光照条件下营养驱动的森林地面光合作用的增加。