Nair Richard K F, Perks Michael P, Mencuccini Maurizio
School of Geosciences, University of Edinburgh, Crew Building, Edinburgh, EH9 3FF, Midlothian, UK.
Forest Research, Northern Research Station, Roslin, EH25 9SY, Midlothian, UK.
Glob Chang Biol. 2017 Apr;23(4):1711-1724. doi: 10.1111/gcb.13450. Epub 2016 Aug 23.
Nitrogen (N) deposition (N ) drives forest carbon (C) sequestration but the size of this effect is still uncertain. In the field, an estimate of these effects can be obtained by applying mineral N fertilizers over the soil or forest canopy. A N label in the fertilizer can be then used to trace the movement of the added N into ecosystem pools and deduce a C effect. However, N recycling via litter decomposition provides most of the nutrition for trees, even under heavy N inputs. If this recycled litter nitrogen is retained in ecosystem pools differently to added mineral N, then estimates of the effects of N on the relative change in C (∆C/∆N) based on short-term isotope-labelled mineral fertilizer additions should be questioned. We used N labelled litter to track decomposed N in the soil system (litter, soils, microbes, and roots) over 18 months in a Sitka spruce plantation and directly compared the fate of this N to an equivalent amount in simulated N treatments. By the end of the experiment, three times as much N was retained in the O and A soil layers when N was derived from litter decomposition than from mineral N additions (60% and 20%, respectively), primarily because of increased recovery in the O layer. Roots expressed slightly more N tracer from litter decomposition than from simulated mineral N (7.5% and 4.5%) and compared to soil recovery, expressed proportionally more N in the A layer than the O layer, potentially indicating uptake of organic N from decomposition. These results suggest effects of N on forest ∆C/∆N may not be apparent from mineral N tracer experiments alone. Given the importance of N recycling, an important but underestimated effect of N is its influence on the rate of N release from litter.
氮（N）沉降推动森林碳（C）固存，但其效应大小仍不确定。在野外，可以通过在土壤或林冠上施用矿物氮肥来估算这些效应。然后可以使用肥料中的氮同位素标记来追踪添加的氮进入生态系统库的移动情况，并推断出碳效应。然而，即使在大量氮输入的情况下，通过凋落物分解进行的氮循环也为树木提供了大部分养分。如果这种循环的凋落物氮在生态系统库中的保留方式与添加的矿物氮不同，那么基于短期同位素标记矿物肥料添加量对氮对碳相对变化（∆C/∆N）的影响估计就值得怀疑。我们使用氮同位素标记的凋落物在一个 Sitka 云杉人工林中追踪了 18 个月土壤系统（凋落物、土壤、微生物和根系）中分解的氮，并将这种氮的归宿与模拟氮处理中等量的氮直接进行了比较。到实验结束时，当氮来自凋落物分解时，在 O 层和 A 层土壤中保留的氮是来自矿物氮添加量的三倍（分别为 60%和 20%），这主要是因为在 O 层中的回收率增加。与模拟矿物氮相比，根系从凋落物分解中表达出略多的氮示踪剂（分别为 7.5%和 4.5%），并且与土壤回收率相比，在 A 层中表达的氮比例高于 O 层，这可能表明从分解中吸收了有机氮。这些结果表明，仅从矿物氮示踪剂实验中可能无法明显看出氮对森林∆C/∆N 的影响。鉴于氮循环的重要性，氮的一个重要但被低估的影响是其对凋落物氮释放速率的影响。
