Background

Across the tropics, the share of secondary versus primary forests is strongly increasing. The high rate of biomass accumulation during this secondary succession relies on the availability of essential nutrients, such as nitrogen (N). Nitrogen primarily limits many young secondary forests in the tropics. However, recent studies have shown that forests of the Congo basin are subject to high inputs of atmospheric N deposition, potentially alleviating this N limitation in early succession.

Methods

To address this hypothesis, we assessed the N status along a successional gradient of secondary forests in the Congo basin. In a set-up of 18 plots implemented along six successional stages, we quantified year-round N deposition, N leaching, N₂O emission and the N flux of litterfall and fine root assimilation. Additionally, we determined the N content and C:N stoichiometry for canopy leaves, fine roots, and litter, as well as δ¹⁵N of canopy leaves.

Results

We confirmed that these forests receive high amounts of atmospheric N deposition, with an increasing deposition as forest succession proceeds. Additionally, we noted lower C:N ratios, and higher N leaching losses, N₂O emission, and foliar δ¹⁵N in older secondary forest (60 years). In contrast, higher foliar, litter and root C:N ratios, and lower foliar δ¹⁵N, N leaching, and N₂O emission in young (< 20 years) secondary forest were observed.

Conclusions

Altogether, we show that despite high N deposition, this early forest succession still shows conservative N cycling characteristics, which are likely indicating N limitation early on in secondary forest succession. As secondary succession advances, the N cycle gradually becomes more open.

中文翻译：

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尽管早期连续非洲热带低地森林的大气沉降量很高，但仍存在保守的 N 循环

背景

在整个热带地区，次生林与原始林的份额正在急剧增加。在这种次生演替过程中，生物量的高速率积累依赖于必需营养素的可用性，例如氮 (N)。氮主要限制了热带地区许多年轻的次生林。然而，最近的研究表明，刚果盆地的森林受到大气氮沉降的大量输入，可能会在早期连续缓解这种氮限制。

方法

为了解决这个假设，我们沿着刚果盆地次生林的连续梯度评估了 N 状态。在六个连续阶段实施的 18 个样地的设置中，我们量化了全年 N 沉降、N 浸出、N ₂ O 排放以及凋落物和细根同化的 N 通量。此外，我们确定了冠叶、细根和枯枝落叶的 N 含量和 C:N 化学计量，以及冠叶的 δ ¹⁵ N。

结果

我们证实这些森林接受了大量的大气氮沉降，随着森林演替的进行，沉降增加。此外，我们注意到较老的次生林（60 年）较低的 C:N 比和较高的 N 浸出损失、N ₂ O 排放和叶面 δ ^{15 N。}相比之下，在年轻（<20 年）次生林中观察到较高的叶面、凋落物和根系 C:N 比，以及较低的叶面 δ ¹⁵ N、N 淋失和 N _{2 O 排放。}

结论

总而言之，我们表明，尽管氮沉积量很高，但这种早期森林演替仍显示出保守的氮循环特征，这可能表明次生林演替早期的氮限制。随着二次演替的推进，N循环逐渐变得更加开放。