The impact of harvesting intensity, vegetation control and fertilization on soil N availability 5–20 years post-harvest was determined at 18 jack pine and black spruce forests in northern Ontario, Canada. The study sites are affiliated with the North American long-term soil productivity (LTSP) study. Boreal forest growth is commonly N limited and young stands approaching crown closure acquire most of their N from the soil. The study sites represented a range of soil types and were harvested with three levels of organic matter removal (stem-only harvest, full-tree harvest and full-tree harvest with organic horizon removal by blading). Three study sites had multiple applications of glyphosate to control competing vegetation and one site included a trial with two applications of N, P, K fertilizer to the full-tree harvest treatment. We used short-term anaerobic laboratory incubations and in-situ closed top tube growing season incubations to determine mineralizable N and net N mineralization, respectively. Profile (organic plus mineral horizons) mineralizable N pools were higher in operational clearcut (stem-only and full-tree harvesting) treatments compared to the uncut reference forest 10–14 years post harvest. Full-tree harvesting with blading significantly reduced profile mineralizable N pools at this time compared to operational harvesting, while the same treatment at one upland site also reduced net N mineralization pools. Profile mineralizable N pools and microbial biomass C and N were lower in full-tree compared to stem-only harvesting treatments, suggesting that increased slash retention with stem-only harvesting was playing a role in enhanced soil N availability at a time when plantations were approaching a period of greater N demand. Fertilization and vegetation control treatments had few effects on N indices, although net N mineralization pools for the stem-only and full-tree harvest that also had the repeated herbicide treatment were consistently, but not significantly, lower than non-herbicided treatments. Soil N indices were useful predictors of dominant height growth for jack pine to a greater degree than for black spruce, and soil N concentrations were as effective at predicting height growth as more resource intensive measurements. Our results point to the importance of continued monitoring of soil and stand conditions at these LTSP sites as treatment effects on soil N cycling and site productivity will likely continue to evolve. At this stage of stand development, full-tree harvesting plus organic horizon removal, followed by operational harvesting has led to lower soil N availability than operational tree-length harvesting, and stands with understory vegetation control are now showing indications of reduced N availability.

在加拿大安大略省北部的 18 处针叶松和黑云杉林中测定了收获强度、植被控制和施肥对收获后 5-20 年土壤氮可用性的影响。这些研究地点隶属于北美长期土壤生产力 (LTSP) 研究。北方森林的生长通常受氮限制，接近树冠关闭的年轻林分从土壤中获取大部分氮。研究地点代表了一系列土壤类型，收获时采用了三个有机物去除水平（仅茎收获、全树收获和全树收获，通过铲刀去除有机层）。三个研究地点多次施用草甘膦来控制竞争植被，一个地点包括一项试验，在整棵树收获处理中施用两次 N、P、K 肥料。原位封闭式顶管生长季节孵化，分别确定可矿化 N 和净 N 矿化。在采伐后 10-14 年，与未砍伐的参考森林相比，在操作性砍伐（仅茎和全树收获）处理中，剖面（有机加矿物层）可矿化氮库更高。与操作采伐相比，此时采用叶片的全树采伐显着减少了剖面可矿化的 N 库，而在一个高地地点的相同处理也减少了净 N 矿化库。与仅茎收获处理相比，全树中可矿化的氮库和微生物生物量 C 和 N 较低，表明在种植园接近对氮需求更大的时期时，仅茎收获增加的斜线保留在提高土壤氮的可用性方面发挥了作用。施肥和植被控制处理对 N 指数的影响很小，尽管也进行了重复除草剂处理的仅茎和全树收获的净 N 矿化库始终低于非除草剂处理，但不显着。与黑云杉相比，土壤氮指数更能有效地预测长针松的主要高度增长，并且土壤氮浓度在预测高度增长方面与资源密集型测量一样有效。我们的结果表明，在这些 LTSP 地点继续监测土壤和林分条件的重要性，因为处理对土壤氮循环和地点生产力的影响可能会继续发展。在林分开发的这个阶段，全树收获加上有机层去除，然后是操作性收获导致土壤氮的可用性低于林木长度的操作性收获，并且有林下植被控制的林分现在显示出氮可用量减少的迹象。