Explanations for nitrogen decline—Response
Explanations for nitrogen decline—Response
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氮素下降的解释——响应
DOI：
10.1126/science.abq8690
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发表时间：
2022
期刊：
影响因子：
56.9
通讯作者：
Reich, Peter B.
中科院分区：
文献类型：
--
作者：
Mason, Rachel E.;Craine, Joseph M.;Lany, Nina K.;Jonard, Mathieu;Ollinger, Scott V.;Groffman, Peter M.;Fulweiler, Robinson W.;Angerer, Jay;Read, Quentin D.;Reich, Peter B.
奥尔夫等人。仅选择氮可用性下降的证据子集，并指定不太可能的机制来得出地球大面积氮可用性并未下降的结论。我们不同意将证据分为 Olff 等人的类别。描述;整套观察结果范围更广，无法用作者提出的机制来解释。奥尔夫等人。声称自 1990 年以来氮排放量的下降可以解释氮供应量下降的原因。我们的审查承认排放量减少以及由此导致的生态系统大气中氮沉积量的减少是一个可能的影响因素。然而，我们还提供了氮可用性下降的长期记录，包括自 1930 年左右以来植物叶片中氮浓度的下降 (1, 2) 和自 1900 年代初以来植物花粉中氮浓度的下降 (3)，以及美国新罕布什尔州哈伯德布鲁克的一系列土壤氮可用性指标和溪流中 NO3 的下降，其历史可以追溯到 1960 年代和 1970 年代 (4, 5)。这些观察结果早于氮沉降的减少。此外，正如我们在评论中所解释的那样，在从未经历过氮沉降大幅升高的地方，氮可用性指标出现了下降（1），同时植物中其他元素浓度也下降了（6-8）。奥尔夫等人。然后提出，沉积物和植物中自然丰度氮同位素比（d15N）值的大规模下降可以通过人为氮排放的同位素特征随时间的变化向同位素较轻、还原形式的氮的变化来解释。然而，他们引用的这种转变对植物 d15N 可能产生影响的证据仅涉及非典型环境中的少数案例研究 (9-11)。土壤中歧视 15N 的过程会提高沉积氮的同位素比率；这种过程的影响随着氮供应的增加而增加 (2, 12)。模型显示，沉积氮的同位素特征必须低得令人难以置信，才会导致植物 d15N 以观察到的速率下降 (2)。毫无疑问，大量且管理不善的人为氮输入导致了许多地方的富营养化和生物多样性丧失。然而，大气中二氧化碳含量的上升、变暖和其他一些全球变化同时导致氮供应量（即相对于氮需求的氮供应量）的减少。 Olff 等人指出，人为氮供应量的增加有据可查。并没有统一影响全球生态系统，也不太可能成为所有陆地生态系统氮可用性变化的首要驱动因素。正如我们在评论中指出的那样，对氮可用性下降的根本反应必须是减少二氧化碳排放。我们指出，尽管施肥可能是增加植物、微生物和食草动物氮利用率的一种选择，但在设计干预措施时必须考虑许多因素，以实现明确的目标，而不产生不可接受的负面后果。有必要开展进一步的工作，以更全面地证明氮素可用性下降的程度，阐明根本机制，并确定适当的应对措施。但在此之前，必须承认氮供应量下降的科学证据。
Olff et al. select only a subset of the evidence for declining nitrogen availability and assign unlikely mechanisms to reach the conclusion that nitrogen availability is not declining over large areas of Earth. We disagree that the evidence can be grouped into the categories that Olff et al. describe; the complete set of observations is wider in scope and cannot be explained by the mechanisms that the authors propose. Olff et al. claim that declines in nitrogen emissions since 1990 can explain declining nitrogen availability. Our Review acknowledges reduced emissions, and the resulting reduction in atmospheric deposition of nitrogen onto ecosystems, as a likely contributing factor. However, we also present long-term records of declining nitrogen availability, including declining nitrogen concentrations in plant leaves since around 1930 (1, 2) and in plant pollen since the early 1900s (3), as well as declines in a broad suite of soil nitrogen availability indicators and stream water NO3–at Hubbard Brook in New Hampshire, United States, that date back to the 1960s and 1970s (4, 5). These observations predate reductions in nitrogen deposition. Moreover, as we explain in the Review, declines in nitrogen availability indicators have occurred in places that have never experienced substantially elevated nitrogen deposition (1) and alongside declines in concentrations of other elements in plants (6–8). Olff et al. then propose that large-scale declines in natural abundance nitrogen isotope ratio (d15N) values in sediment and plants can be explained by a change over time in the isotopic signature of anthropogenic nitrogen emissions toward isotopically lighter, reduced forms of nitrogen. However, the evidence they cite of possible effects of this shift on plant d15N refers only to a handful of case studies in atypical environments (9–11). The isotopic ratio of deposited nitrogen is elevated by processes in soil that discriminate against 15N; the effects of such processes increase with increasing nitrogen supply (2, 12). Models show that the isotopic signature of deposited nitrogen would have to be implausibly low to cause plant d15N to decline at the observed rate (2). There is little doubt that massive and poorly managed anthropogenic nitrogen inputs have led to eutrophication and biodiversity loss in many locations. However, rising atmospheric CO2, warming, and several other global changes are concurrently driving a reduction in nitrogen availability (ie, nitrogen supply relative to nitrogen demand). The well-documented increases in anthropogenic nitrogen supply noted by Olff et al. have not affected global ecosystems uniformly and are unlikely to be the overriding driver of changes in nitrogen availability across all terrestrial ecosystems.As we state in our Review, the fundamental response to declining nitrogen availability must be to reduce CO2 emissions. We point out that, although fertilization may be one option for increasing nitrogen availability to plants, microbes, and herbivores, numerous factors must be taken into account when designing interventions that can achieve well-defined goals without unacceptable negative consequences. Further work is necessary to more fully demonstrate the extent of declines in nitrogen availability, to clarify the underlying mechanisms, and to delineate appropriate responses. But before this can happen, the scientific evidence for declining nitrogen availability must be acknowledged.
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