Distribution Pattern of Soil Organic Carbon and Its Regional Humication Constant in the Coastal Monsoon Region of Eastern China

. Soils are an important pool for storing organic carbon. Soil organic carbon (SOC) content is 16 generally considered as an important indicator to evaluate farmland soil quality. The loss of SOC causes 17 soil degradation and reduces the sustainability of farmland. In order to reveal the distribution pattern of 18 SOC in the coastal monsoon area of eastern China, and to clarify the macro dominant factors of SOC 19 accumulation caused by temperature and precipitation, this paper analyzed the distribution pattern of SOC 20 in the coastal monsoon area of eastern China by using the SOC data collected from the national soil testing 21 and formula fertilization data set, and discussed the effects of temperature and precipitation on SOC content. 22 According to the provincial administrative divisions, the distribution of SOC in the coastal monsoon areas 23 of eastern China from Heilongjiang Province to Hainan Province was calculated. According to the annual 24 average temperature <10.18 ℃ , 10.18 ℃ ~ 20.95 ℃ , >20.95 ℃ , annual average with the impact.

Distribution pattern of soil organic carbon and its regional As one of the main terrestrial carbon pools, global climate change has an important impact on soil 53 carbon pool. SOC has been widely studied as a key determinant of regional food security and global food 54 crisis warming (Diel and Franko, 2020;Nave et al., 2018). Due to the long-term cultivation of human beings, 55 the physical and chemical properties of soil change, which accelerates the decomposition of mineral and 56 soil organic matter, and ultimately leads to the loss of soil productivity (Padilha et al., 2020). Especially in 57 the global warming environment, some scholars expect to accelerate the decomposition and transformation 58 of SOC, but the decomposition rate of SOC is not clear (Yan et al., 2017). Some studies have shown that 59 the impact of precipitation on s SOC is mainly produced by changing the function and structure of terrestrial 60 ecosystem, because water is the basic driving force of almost all chemical and biological processes, 61 including plant growth and survival (Gong et al., 2020), photosynthesis (Hui et al., 2018), and microbial 62 activity (Ma et al., 2020) and soil respiration (Wu et al., 2020). Within tolerable limits, the biological 63 processes that promote the decomposition of organic carbon will be faster at higher temperatures, with an 64 average rainfall of 1750 mm to 2500 mm, and the SOC storage is relatively small (Vitharana et al., 2019). 65 At the regional scale, the effect of precipitation on microbial metabolic limitation strongly restricts the 66 stability of SOC. With the increase of precipitation, the increase of SOC decomposition rate may be the 67 result of the increase of microbial nutrition limitation (Cui et al., 2019). However, some scholars believe 68 that at the regional scale, soil physical and chemical properties and terrain factors may be greater than the 69 dynamic impact of climatic factors on SOC. Soil pH value is the key control factor of SOC dynamic change, 70 temperature and humidity are the secondary driving forces of SOC dynamics at regional scale (Xu et al.,

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The meteorological data used in this study mainly included annual average temperature and annual 159 average precipitation, which were from "China Meteorological science data sharing service network" 160 (http://cdc.nmic.cn). The annual average temperature and annual average precipitation data in the climate 161 data mainly used the average value from 2000 to 2016, and the meteorological data of each county (city, 162 district) used the meteorological data of corresponding meteorological observation station. Accumulated 163 temperature≥10℃ refers to the total temperature when the daily average temperature exceeds or equals to 164 10℃. Water temperature ratio (P/T) refered to the ratio of annual average precipitation to annual average 165 temperature.

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Data analysis 168 In this study, ArcGIS software was used for semi variance analysis to determine the optimal theoretical 169 semi variance model, and the optimal parameters of Kriging interpolation were obtained. Then spatial 174 Results 175

Distribution pattern of SOC in East China
It could be seen from Table 2

Effects of hydrothermal conditions on SOC distribution pattern 267
Effect of temperature on SOC distribution pattern 268 Generally speaking, at higher temperature, the physical and chemical reaction process in the soil was 269 faster, the microbial activity was strong and the metabolism was vigorous, the soil animals were active, and 270 the crop growth rate was also fast. Therefore, the decomposition rate of SOC and nutrient were also fast, 271 and the content of SOC and nutrient was relatively low. On the contrary, SOC was easy to accumulate (Gao  Annual average temperature/(℃) temperature and SOC content was the best, while the correlation between P/T and SOC content was poor.

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The correlation between annual mean temperature and SOC content was slightly lower than that between 303 annual precipitation and SOC content, and the contribution of precipitation index to SOC accumulation was 304 higher.

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The region of above 20.75 ℃ mainly included Fujian Province, Guangdong Province and Hainan 306 Province, the SOC content was negatively correlated with the annual mean temperature and the annual 307 accumulated temperature of ≥10℃, r = -0.5690 ** , r = -0.6701 ** , respectively, while the SOC content was 308 significantly correlated with the annual average precipitation (r = 0.4517 * ) and the water temperature ratio 309 (r = 0.4471 * ); The results showed that: in the region with annual average temperature > 20.75 ℃, 310 temperature index was the main negative correlation contribution factor of SOC content accumulation, 311 while precipitation index had no strong effect on SOC content accumulation. Among them, the negative 312 correlation effect of annual accumulated temperature ≥ 10 ℃ on the accumulation of organic carbon content 313 was slightly higher than the annual average temperature, which played a major negative correlation role.  semi-arid 0 ~ 400 mm, semi-humid area 400~800 mm and humid area > 800 mm), this paper continued to 328 discuss the main influencing factors of SOC under different regional precipitation, as shown in Table 4.

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In the region of 0 mm ~ 400 mm precipitation, SOC content had a very relationship correlation with 330 annual average precipitation, annual average temperature, P/T and annual accumulated temperature of 331 ≥10℃, r = 0.6922 ** , r =-0.8908 ** , r =0.9031 ** , r =0.8808 ** , respectively. The correlation between SOC 332 content and P/T was the best, which indicated that the coupling effect of annual average precipitation and 333 annual average temperature played an important role in SOC accumulation.

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In the region of 400 mm ~ 800 mm precipitation, SOC content had a very significantly relationship with 335 annual mean temperature, P/T and annual accumulated temperature ≥10℃, r = 0.8576 ** , r = -0.8233 ** , r = 336 0.8731 ** , respectively. The correlation between SOC content and annual average precipitation was r = 337 0.1000, which indicated that SOC content was not correlated with annual average precipitation, and SOC 338 content was negatively correlated with P/T. Therefore, in the area of 400 ~ 800 mm precipitation, 339 temperature index played a major contribution factor to SOC accumulation. Donggang City, Fengcheng City, Zhenxing District and Zhen'an District, was low, the precipitation could 347 still reach more than 800 mm. In order to eliminate the influence of the above 12 counties (cities, districts) 348 on the overall statistical results, we removed the data of the above 12 counties (cities, districts) in the area 349 with precipitation > 800 mm, we found that the correlation between SOC content and annual average 350 precipitation, annual average temperature, P/T and annual accumulated temperature ≥10℃ was better than 351 that before removing the data, at this time, the SOC content had a very significant relationship with the 352 annual average temperature and annual accumulated temperature ≥10℃, r = -0.6933 ** r = -0.7058 ** , 353 respectively, and also had a significant relationship with the annual average precipitation and P/T, r = -354 0.5782 ** , r =-0.5172 ** , respectively. This showed that the temperature factor was still the main factor of 355 SOC accumulation in the area with precipitation > 800 mm, followed by precipitation factor. Among them, 356 the accumulation effect of annual accumulated temperature ≥10℃ on organic carbon content was slightly 357 higher than the annual average temperature.

366 Effects of hydrothermal conditions on SOC distribution pattern 367
Based on the principal component analysis results of SPSS 15.0, the author selected annual accumulated 368 temperature ≥10℃ and annual average precipitation as two independent variables of organic carbon model.

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After multiple combinations of variables, the correlation of different combinations was repeatedly screened, 370 and the following models were obtained.

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Then: log log log log log log log log Among them, C was the organic carbon content (g·kg -1 ),α, β, γ and ε were constants, T was the annual 381 accumulated temperature of ≥10℃ (℃), P was the annual average precipitation (mm).

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"Humification coefficient" refers to the ratio of added organic carbon to a more stable soil carbon pool 383 (Poeplau et al., 2015). That was to say, the ratio of the amount of plant residues (measured by carbon) added to the soil one year after decomposition to the original amount was generally used to measure the degree of 385 humus formation by biochemical and chemical action of animal, plant and microbial residues under the 386 action of microorganisms. In this paper, we redefined a new constant K, the humification constant of 387 regional organic carbon for many years, which was a constant under the comprehensive influence of 388 hydrothermal conditions. 389 SPSS 15.0 multiple regression was used to obtain the following analysis results, as shown in Table 5. α 390 = -6.4467, β = 1.8001, γ = 1.3525, ε = 0.6550 were calculated. The p-values of α, β, γ and ε were all 0 or 391 Closed to 0, with extremely significant correlation. According to the regression statistics in Table 6 and Table 7, R 2 = 0.9998 (r = 0.9999), and the standard 401 error was 0.0672, P < 0.01, with extremely significant correlation.

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According to

Effect of temperature on SOC accumulation 408
In this paper, from the farthest Daxinganling area of Heilongjiang Province to Sanya City of Hainan 409 Province, the straight-line distance is more than 5000 kilometers，the annual average temperature varies   Schematic diagram of the study area and distribution of sample points Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors.

Figure 2
Distribution of SOC Northeast China Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Distribution of SOC in North China Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Distribution of SOC in East China Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Distribution of SOC in Guangdong Province Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Distribution of SOC in Guangdong Province Note: The designations employed and the presentation of the material on this map do not imply the expression of any opinion whatsoever on the part of Research Square concerning the legal status of any country, territory, city or area or of its authorities, or concerning the delimitation of its frontiers or boundaries. This map has been provided by the authors. Relationship between SOC content and annual average precipitation (n=780) Figure 10 Relationship between SOC content and annual average temperature in the region with annual precipitation > 800mm (n=311) Figure 11 Relationship between SOC content and p/t (n=780) Figure 12