GCB高被引综述|Carbon for soils, not soils for carbon

题目：

Carbon for soils, not soils for carbon

期刊： 

Global Change Biology

第一作者：

Gabriel Y. K. Moinet

第一发表单位：  

Wageningen University

土壤有机碳（SOC）固存作为应对气候变化和粮食不安全的“双赢”解决方案，得到了越来越多的推广。这一机遇可能不容错过！然而，涉及这两个问题的复杂性要求本研究对任何潜在解决方案进行详细和细致的审查，无论其多么吸引人。在此，本研究对全球SOC固存策略在气候变化缓解和粮食生产方面的好处进行了重新审视。尽管对SOC固存对气候变化的贡献的估计有所不同，但几乎没有考虑到SOC饱和。我们表明，考虑饱和后，SOC固存对气候变化缓解的潜在贡献将在2100年前减少53%–81%。此外，通过审查超过21个meta分析，本研究发现，增加SOC的观察到的产量效应并不一致，从负面到中性到正面不等。本研究发现，只有在特定的土地管理实践在特定条件下应用时，才会证实“双赢”结果的承诺。因此，我们本研究认为，现有的知识基础并不足以证明当前趋势，即全球议程首先关注SOC固存。我们需要从气候智能土壤转向土壤智能农业，适应并适合每个地方的背景，同时同时量化多种土壤功能。只有这样的全面评估才能最大化土地可持续性的协同作用，并满足食品安全的农艺需求。这意味着要远离对农业土壤SOC的全球目标。SOC固存可能会在这一过程中发生，并为气候变化缓解做出贡献，应该被视为一种共同利益。

保持全球变暖低于1.5°C（与《巴黎协议》第2条一致）需要立即采取行动，以在未来几十年内实现净零二氧化碳（CO2）排放（IPCC WG3, 2022）。净零CO2排放的实现意味着人类排放与人类去除CO2之间的平衡。为了将全球变暖限制在1.5°C，几乎所有情景都依赖于从大气中去除CO2。这对于许多将全球变暖限制在2°C以下的情景也是如此（IPCC WG1, 2021），这证实了早期的评估。

在有关二氧化碳去除（CDR）技术的讨论中，土壤有机碳（SOC）固存近年来获得了显著的社会和科学关注。SOC固存的确切气候变化缓解潜力是一个激烈争论的主题。SOC固存常常被宣传为一种“胜利—胜利”的解决方案，有利于气候变化缓解和粮食安全，尽管这些主张值得仔细审查。土壤碳动力学的复杂性，加上土壤、植物和气候之间复杂的相互作用，需要对SOC固存的实际好处和局限性进行详细探讨。

改善土壤功能将导致全球更大且更稳定的产量，从而保障全球食品安全并使全球农业生产适应气候变化。越来越多的热情推动着更加乐观的预测，例如，声称SOC固存将是支持五个（FAO, 2017）、七个（FAO, 2019）甚至多达12个可持续发展目标的关键。因此，最近几篇论文和一些高层国际倡议（2015年在COP21上发起的“4p1000倡议”、2018年COP23期间举行的Koronivia研讨会以及FAO的RECSOIL计划）都呼吁迅速扩大和实施SOC固存实践，以同时解决气候变化和粮食安全的挑战。

然而，早在15年前，Janzen（2006）就指出，存储碳于土壤以应对气候变化缓解和在土壤有机物（SOM）分解过程中释放养分以支持植物生产之间存在权衡。同时，气候变化和粮食安全都被描述为“棘手问题”，强调了其令人生畏的复杂性以及国际社会、政治、经济和科学跨学科努力所需的巨大工作量，以提供即使是部分解决方案。在这样的背景下，声称一种通用解决方案可以同时解决气候变化和粮食不安全的问题应引起一定程度的怀疑。像任何正在进行的科学讨论一样，它需要不懈和严谨的审查。

从这个角度出发，我们重新审视通过SOC固存实现气候缓解的术语和概念，以及支持“双赢”与粮食安全的实证证据。我们批判性地审查了全球SOC固存潜力的估计，并量化SOC固存对在2100年前景下达到净零CO2排放目标的潜在贡献。尽管我们在此关注技术上可实现的SOC固存潜力，但我们也承认实施SOC固存实践面临许多重大的社会、经济和政治障碍，这进一步使SOC固存的全球缓解潜力受到怀疑。然后，我们回顾了增加SOC库存对地上植物生产的影响的现有证据。在这里，我们关注植物生产作为一种关键的土壤功能，因为它与食品安全有明显的联系，尽管我们承认其他功能也同样重要。

（注：以上翻译来着ChatGPT，具体文章内容请以原文内容为准。若解读有误欢迎探讨指正。）

Fig. 1: Visualisation of the impact of three different assumptions regarding C saturation on global soil organic carbon (SOC) sequestration and its potential to mitigate climate change to the horizon 2100. The three assumptions are as follows: An unrealistic constant sequestration rates over time, that is, no saturation limit (green dotted lines and frame); slow exponential decrease in sequestration rates (30 years exponential decay half-life; blue lines and frame); and fast exponential decrease in sequestration rates (10 years half-life; orange lines and frame). For the three assumptions, the temporal dynamics of global sequestration rates (a) and global SOC sequestration potential (b) are depicted for the example of the estimate from Bossio et al. (2020). The contribution of SOC sequestration to a pathway to 1.5°C target is represented for the whole range of literature estimates of sequestration rates in panels (c), (d) and (e), each panel corresponding to one assumption on saturation as indicated by the colours.

Fig. 2: Illustration of the complexity of unravelling causal relations between an increase in soil organic carbon (SOC) stocks and crop yields. Agricultural practices that increase SOC stocks may also enhance crop yields, suggesting that the evolution of SOC and yield in time may correlate with no causal link between them. Moreover, even if a causal link exists, determining which SOC or yield is the cause, and which is the effect, is problematic.

Fig. 3: Conceptual figure illustrating potential conflicts between soil organic carbon (SOC) sequestration and food production. The figure depicts two hypothetical cases in which crop residues are removed from one field after harvest to be applied as OM inputs to another crop field. In panel a, residues are transferred from a sandy soil to a clay soil. After some years, a new equilibrium for SOC stock is reached. The clay soil gains more SOC than the sandy soil loses, due to its higher C stabilisation capacity (Kirschbaum et al., 2020). Therefore the net overall effect is that C is sequestered, to the benefit of climate (provided that no additional N2O or CH4 emissions would arise). The clay soil also sees crop yield increasing, but not as much as the yield in the sandy soil decreases, due to the stronger yield effect of organic amendments in sandy than clay soils (Hijbeek et al., 2017; Zomer et al., 2017). The net effect for yield is that less crops are produced overall. The reciprocal transfer, in panel b, leads to mirrored effects: Small yield loss in the clay soil and high yield gain in the sandy soil, and large CO2 emissions in clay soil and small SOC sequestration in the sandy soil with an overall SOC loss and aggravated climate change, but more food produced overall. Importantly, assuming that each field is owned by a different farmer, someone always loses. This clearly illustrates that local win-win scenarios can occur at the expense of fertility elsewhere.