1981-2019年华北平原农田土壤有机碳储量的时空变化及影响机制

农业土壤具有可观的固碳及减碳潜力，有助于减缓人类温室气体排放导致的气候变化。为了更好地了解华北平原土壤有机碳储量动态及其驱动因子，结合荟萃分析、随机森林机器学习模型和卫星遥感数据，研究了1981-2019年间中国华北平原农田土壤有机碳储量的时空变化及其驱动因子。结果表明，1981-2019年间华北平原0-20 cm农田土壤有机碳储量约为（523.10±79.36） Tg C （（14.56±1.66） Mg C/hm²），并以5.94 Tg C/a （0.12 Mg C hm^-2 a^-1）的年固持速率稳步增长，占比约为中国农田每年新增土壤有机碳的23.3%。其中，常规农田管理措施，包括无机肥施用、有机肥施用和秸秆还田，对土壤有机碳增长的贡献平均为25.1%，即1.49 Tg C/a （0.03 Mg C hm^-2 a^-1）。相比对照组，氮磷钾无机肥施用可提高22.7%-26.0%的土壤有机碳固定速率，有机肥可提高48.3%，秸秆还田可提高23.4%。同时，上述常规农田管理措施对土壤有机碳的积累作用受到土壤本身理化性质的调控，在温度和降水较高的气候条件下更显著。值得注意的是，无论是无机肥施用、有机肥施用还是秸秆还田，当投入量超过农作物和土壤微生物对碳和养分的需求时，土壤有机碳累积速率会显著下降。这也导致2000年后土壤有机碳固持速率明显减缓，由9.4 Tg C/a下降为3.5 Tg C/a。总的来说，过去几十年农田管理措施的改进显著提高了华北平原农田土壤有机碳的增加速率，而未来华北平原农田系统固碳潜力仍然可观，但亟待明确在保证粮食产量的同时不同气候和土壤环境条件下最佳固碳所需的化肥、有机肥和秸秆投入量。

Spatial-temporal changes in and influencing mechanisms for cropland soil organic carbon storage in the North China Plain from 1981 to 2019

Agricultural soils have significant carbon sequestration and reduction potentials to help mitigate climate change caused by human greenhouse gas emissions. To better understand the dynamics of soil organic carbon (SOC) storage and its driving factors in the North China Plain (NCP), this study combined meta-analysis, random forest machine learning, and satellite remote sensing data to study the spatio-temporal changes of cropland soil organic carbon storage and the contributions of its driving factors from 1981 to 2019. The results show that the cropland soil organic carbon storage of the top 0-20 cm in the NCP was (523.10±79.36) Tg C ((14.56±1.66) Mg C/hm²) from 1981 to 2019, with a growing rate of 5.94 Tg C/a (0.12 Mg C hm^-2 a^-1), which accounted for about 23.3% of the total annual increase of SOC in China''s cropland. The total contribution of conventional cropland management options, including inorganic fertilizer use, organic fertilizer application and straw returning to SOC accumulation was 25.1%, 1.49 Tg C/a (0.03 Mg C hm^-2 a^-1). Compared with control experiments, inorganic fertilizer use (including nitrogen, phosphorus and potassium) increased the SOC sequestration rate by 22.7%-26.0%, organic fertilizer application increasing by 48.3%, and straw returning increasing by 23.4%. Meanwhile, the effects of such conventional management on SOC accumulation were modulated by soil physical and chemical properties. This phenomenon was more obvious under high temperature and precipitation conditions. It is worth noting that when the use of inorganic/organic fertilizer or straw returning exceeded the carbon and nutrient demands of crops and soil microbes, the SOC accumulation decreased significantly. Consequently, the SOC accumulation slowed down from 9.4 Tg C/a to 3.5 Tg C/a during the 2000s. In summary, the increase of cropland SOC in the NCP could be largely attributed to the improvement of cropland management in the past few decades, and the carbon sequestration potential in the NCP cropland is considerable. Nevertheless, it is essential to clarify the input amount of organic fertilizer, inorganic fertilizer, and straw for optimal carbon sequestration under varied climate and soil conditions, while ensuring crop yield.