Julian Campo (2023) [1] presented an interpretive viewpoint in the "News & Views" section regarding the paper by Wang et al. (2023) [2], concluding that the total cropland carbon sink would increase under climate warming conditions. Their basis is derived from the findings of Wang et al. (2023), which suggest that the replenishment of soil organic carbon lost through erosion would increase. We don’t deny this conclusion and acknowledge its innovativeness. However, the authors propose that the global carbon sink in croplands, resulting from water erosion, shows a net increase of 7% under warming conditions after accounting for the decrease in preserved deposited carbon. Furthermore, this finding can be incorporated into global models. Evidently, the authors of these two articles misunderstand the relationship between organic matter production and mineralization. As temperature rises, mineralization increases exponentially, whereas productivity exhibits a linear increase as temperature rises. Statistics indicate that global soil organic carbon (SOC) is highest in the cold temperate zone (CTZ) and gradually decreases towards the equator, suggesting that soil carbon sink will decrease as temperatures rise. The publication of these two articles will greatly mislead environmental science.
After comprehensively analyzing the latitudinal distribution characteristics of soil and modern sediments, as well as the characteristics of organic matter accumulation in various geological historical periods, this comment concludes that only the CTZ exhibits high carbon sink characteristics. Specifically, there are several aspects to this.
1. Wang et al. (2023) attempted to prove that the intensity of carbon sinks is positively correlated with the annual average temperature, which is a monotonically increasing relationship. However, in the fitting of the relationship between carbon sink intensity and annual mean temperature presented in their Figure 2d , the data exhibits significant dispersion and an extremely low goodness of fit (R2=0.016). The conclusion drawn is that "the total C sink by water erosion on croplands was predicted to increase by 7.4 ± 6.2%." However, with an uncertainty or error of 6.2%, which is nearly equal to the mean value of 7.4%, using such a level of fit to represent the global situation is clearly erroneous.
2. The conclusion of Wang et al. (2023) contradicts the characteristics of modern sedimentation and soil carbon sequestration.
The temperature range mainly discussed in the paper is between 0℃ and 25℃, which is equivalent to the present climate range from CTZ to the equator. According to their logic, the closer to the equatorial tropical region, the higher the SOC should be. However, this is clearly contrary to the statistical trend of the relationship between modern SOC content and temperature, except within the higher latitude cold zones (>70°) and the internal regions of the CTZ (segments CD & EF in Figure 1b). As shown in Figure 1b, the SOC in the CTZ is 2-4 times higher than that in the middle and low latitudes with high temperatures. The SOC is highest in the CTZ, and low in the mid-latitudes. Among them, the SOC in the plains of Brazil is only 3-5 kg/m2, which is 1/4 of the 12-20kg/m2 in the CTZ. Meanwhile, the coverage area of wetlands and peatlands in CTZ is the highest in Figures 1a and 1c. Therefore, the overall trend is that the higher the temperature, the lower the carbon sink intensity.
Although there is a small anomalous high value of SOC and TOC in wetlands near the equator (segments AB in Figure 1b), this does not contradict the characteristic of high carbon sink associated with cold and wet climates. This is because Africa and western South America are highlands near the equator, where their high elevation breaks the zonal pattern and the larger elevation differences can also produce rapid carbon sinks. Similar to the eastern edge of the Qinghai-Tibet Plateau, it is not high in latitude and is located in the subtropical zone. However, considering its high altitude, it can actually be converted to the middle or CTZ. The richest plateau peat in China is distributed in this area. The total amount of peat between 3400-4800 m above sea level reaches 48%. These data primarily originate from the publicly available global soil organic carbon database GSOCmap Web Service, which is highly credible. At the same latitude, water convergence factors also have a significant impact on carbon sink. For example, the SOC content in the Sanjiang Plain of Heilongjiang Province, China, is the lowest (1.2-2.0%) in areas with flat terrain within the plain, while the high-value areas (4.6-7.1%) are all located in mountainous regions such as Qingheishan in the north of Hegang, the mountainous areas from Qitaihe to Shuangyashan, and Bukai Mountain in the north of Hulin[3](Yang et al.,2022). This comment has measured the SOC content across different latitude zones in China (Table 1). The results show that SOC decreases from the CTZ to the tropical zone, with high SOC values found in mountainous areas within the CTZ. Since Wang's paper (2023) is a study from the perspective of global soil carbon sinks, its conclusions should not contradict the statistical results obtained in this comment using the global soil database GSOCmap.
Table 1 Sampling locations and organic carbon content test results of soil and lake surface sediment samples
Many scholars have proved that when the temperature rises, although the primary productivity increases, the mineralization increases exponentially. Finally, the intensity of carbon sink is negatively correlated with temperature, which is consistent with the statistical data in Figure 1. Kai et al (2016)[4] found that soil microorganisms are extremely sensitive to climate warming. A short-term temperature rise of about one and a half years can lead to increased microbial activity and enhanced soil respiration, resulting in a net loss of soil organic carbon. Chen et al. (2024)[5] revealed that soil warming accelerates soil carbon emissions in alpine grasslands. Gudasz et al. (2010)[6] reached a similar conclusion in their study, suggesting that when temperatures rise, primary productivity increases, leading to an increase in the transport of organic carbon in sediments, but the amount of organic carbon buried does not increase accordingly. The mineralization intensity of organic carbon in lake sediments is significantly positively correlated with temperature, indicating that the higher the water temperature, the higher the mineralization rate of organic carbon and the lower the carbon sequestration rate. The mineralization rate increases exponentially with temperature, as shown in the following ormula (Formula 1).
y=100.0362t-1.635 (1)
In the formula (Formula 1), y is the organic carbon mineralization rate (mgC/m2/d). t is the water temperature (℃).
Fig.1 Relationship curve between peatland coverage, SOC, TOC, and latitude[7]-[9]
3. The conclusions reported by Julian Campo (2023) are contradictory to ancient deposits. The carbon sequestration intensity across different geological ages in ancient strata is also highest in CTZ, which is fully consistent with the distribution characteristics of modern wetland sediments and soil TOC.
When discussing climate, it is imperative to study the Earth's past states. The development of peat and coal throughout various geologic time periods offers the best interpretation of soil carbon sinks. As shown in Figure 2, from the Triassic period to the present, coal has mainly developed in CTZ, while evaporite rocks have developed in arid regions of the ±30° mid-latitude subtropical high-pressure belt. Red beds and lateritic bauxite are developed in low-latitude tropical rainforest regions. Coal, evaporative salt rocks, and red beds are three types of climate-sensitive sediments. In tropical regions, red beds are developed, and coal formation is rare. Even if coal beds do develop, it is due to global cooling, which causes the range of coal formation to extend towards lower latitudes.
Fig. 2 Distribution of climate sensitive sediments such as coal in different eras and their relationship with latitude[10]
Finally, taking the Jurassic period as an example, the aforementioned three types of climate-sensitive sediments are used to illustrate that rising temperatures are detrimental to carbon sinks. As shown in Figure 3, the data on coal and red beds mainly come from Zhang et al. (2022)[11]. Due to climate cooling or warming during the Jurassic period, coal development occurred at different latitudes, but coal always remained at high latitudes and red beds at low latitudes. The Jurassic period is divided into five stages: the Badaowan Formation, Sangonghe Formation, Xishanyao Formation, Toutunhe Formation, and Tuchengzi Formation depositional stages, each characterized by distinct climates of cold, warm, icy, hot, and extremely hot, respectively. These varying climatic features are the result of multiple latitudinal shifts in climatic zones. It can be seen that the early Middle Jurassic Xishanyao Formation was colder than the modern climate, and the climate zone shifted southward in the northern hemisphere. The present-day warm-temperate North China was a cool-temperate region during this period, where thick coal seams of the Badaowan Formation were developed. The Late Jurassic Tuchengzi Formation was warmer than modern climate, with the climate zone moving northward, and the cold temperate climate required for coal formation shifting northward. Thick red beds are mainly developed northward to 43°N, and the northernmost reaches 49°N. At this time, red beds were developed throughout the Chinese Mainland, but coal was not developed, and the climate was similar to that of modern Brazil, with a very low SOC. In the blank zone between the red beds and the coal, evaporite rocks are mainly developed.
Fig. 3 Distribution and latitude relationship of coal and red beds in different periods of Jurassic in central and eastern China
In summary, the paper by Wang et al. (2023) and Julian Campo's interpretation are not innovations in environmental science. Rather , they severely violate objective laws and should be corrected.
References
[1] Julian Campo. Warming to increase cropland carbon sink. Nature Climate Change, 2023, 13:121–122.
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[3] Yang Yixun, Jiang Xiaoxu, LI Mingsheng, et al. Spatial Variability of Soil Organic Carbon Content and Density in The Sanjiang Plain[J]. Chinese Journal of Soil Science, 2022, 53(6): 1313 − 1319.
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[11] Zhang, X. Z., Fang, L. H., Wu, T., et al. Palynological assemblages and palaeoclimate across the Triassic-Jurassic boundary in the Haojiagou section, southern Junggar Basin. Chinese Journal of Geology, 57(4),1-16(2022).