Abstract:

An in-depth understanding of the spatial distribution characteristics of soil total phosphorus (TP) and its main controlling factors is of great significance for the development of scientific fertilization strategies, improvement of phosphorus fertilizer utilization, prevention and control of surface source pollution, and promotion of sustainable agricultural development. Based on data collected from 4409 top soil (0‒20 cm) sample points in the Sichuan Basin from 2017 to 2019, this study systematically analyzed the spatial distribution pattern of soil total phosphorus content and its key influencing factors in the region using a combination of geostatistical methods, random forest model, analysis of variance (ANOVA), regression analysis, and other means. The results of the study showed that the total phosphorus content in the surface layer of cropland in the Sichuan Basin ranged from 0.30 to 3.92 g·kg⁻¹, with an average value as high as 1.59 g·kg⁻¹, which was a first-class (very high) level according to the relevant standards and significantly exceeded the national average level. This was mainly related to the widespread distribution of phosphorus-rich purple parent rock in the Sichuan Basin, long-term high-intensity agricultural fertilization, and the high level of the phosphorus-rich purple parent rock. This is mainly related to the widespread distribution of phosphorus-rich purple parent rocks in the Sichuan Basin, long-term high-intensity agricultural fertilization, atmospheric phosphorus deposition, and soil acidification. In terms of the coefficient of variation, the coefficient of variation of total phosphorus in the surface layer of cultivated land in the study area was 32.27%, indicates a medium level of variation. The spatial distribution showed significant regional differences, with an overall trend of higher content in the northwest and lower content in the southeast. The high-value areas were mainly concentrated in the Chengdu Plain, extending to the central part of the Sichuan Central Hilly Area and the mountainous areas in the southern part of the Basin. The spatial differentiation pattern of total phosphorus content in soils in the Sichuan Basin is results from the combined effects of natural factors and anthropogenic activities. For cultivated soils, anthropogenic factors such as fertilizer application and straw return to the field significantly increase soil phosphorus levels. Although phosphorus fertilizer application, type of cropland use, phosphorus returned by straw to the field, and roots also had a significant effect on soil total phosphorus content, these anthropogenic factors were not the dominant factors in the formation of the current spatial distribution pattern of soil total phosphorus content in the study area, as shown in the analytical results of this study. At the regional scale, the spatial distribution of soil total phosphorus content was mainly regulated by climatic factors (especially rainfall and temperature) and intrinsic soil properties (mainly pH, sand content, and clay content). Among them, rainfall was identified as the most critical dominant factor, with its influence far exceeding that of other factors, because precipitation increases soil acidity by increasing the amount of acid entering the soil and leaching non-acidic cations such as Ca²⁺, Mg²⁺, and K⁺ from the soil. Rainfall dominates the spatial distribution of soil total phosphorus content by regulating soil pH: strong rainfall reduces soil pH (loss of salinity ions), prompting the desorption and loss of phosphorus in the iron-aluminum-bound state, whereas weak rainfall maintains an alkaline environment in which phosphorus is preserved in the form of stabilized calcium phosphate, leading to a gradient decrease in total phosphorus content with increasing rainfall. Soil sand content is the second most important influencing factor after rainfall, and soils with high sand content usually have a weak ability to retain fertilizers and water. In areas with high rainfall (annual rainfall>1040 mm), the soil sand content becomes the primary factor influencing the spatial distribution of total phosphorus, which significantly exacerbates the risk of water loss of phosphorus by decreasing the adsorption capacity of the soil and increasing the permeability of the pore space, resulting in a significant negative correlation between total phosphorus content and sand. The main factor in the spatial distribution of phosphorus was an increase in temperature. The increase in temperature, on the other hand, enhanced phosphorus desorption, accelerated microbial mineralization, and promoted the conversion of insoluble phosphorus to fast-acting forms, which together led to a significant decrease in total phosphorus content with the increase in mean annual temperature. Based on the above findings, this study proposes key management insights and recommendations: future farmland management activities, such as fertilizer application and straw return, must consider regional climatic conditions (especially rainfall) and soil background properties (e.g., texture and pH), implement precise nutrient management strategies by zoning and classification, and scientifically and reasonably control the total amount of phosphorus fertilizer input. In the low-rainfall, fine-textured area (e.g., northwestern), which has relatively low rainfall and low soil sand content, which usually means high clay content and strong fertilizer retention, the core of management is to improve the utilization rate of fertilizers and avoid excessive accumulation of phosphorus in the soil due to over-fertilization. In the high-rainfall, coarse-texture area (southeast), which has abundant rainfall, high soil sand content, and weak fertilizer retention, the management of the primary task is to strengthen soil and water conservation. It is necessary to vigorously promote measures such as contour planting, furrow cover, and construction of vegetation buffer zones to minimize the scouring of surface soil by rainfall runoff, thus effectively blocking and controlling the migration of phosphorus to water bodies with surface runoff and leaching, and significantly reducing the risk of agricultural surface source pollution. In this study, it was clear that rainfall is the primary natural driver shaping the spatial distribution pattern of total phosphorus in surface soils of arable land in the Sichuan Basin, and its role is mainly realized by regulating soil pH and driving phosphorus leaching, with soil sand content being the second most important factor. This understanding highlights the fact that, at the regional scale, the spatial variation of soil phosphorus is mainly controlled by natural factors, and that to improve the utilization of phosphorus fertilizers and reduce the risk of environmental pollution, it is necessary to develop a system that is suitable for the specific climatic conditions of the local area.

Key words: soil total phosphorus, Sichuan Basin, spatial distribution, controlling factors, rainfall

摘要：

深入了解土壤全磷的空间分布特征及其主控因素，对于制定科学的施肥策略、提高磷肥利用率以及防控面源污染和促进农业可持续发展具有重要意义。基于2017-2019年在四川盆地内实地采集获得的4409个表层土壤（0-20 cm）样点数据，应用地统计学方法、随机森林模型、方差分析、回归分析等方法分析该区域内土壤全磷含量空间分布特征及不同影响因素对耕层土壤全磷含量的影响程度。 结果表明，四川盆地土壤全磷含量变幅在0.30-3.92 g·kg⁻¹之间，平均值为1.59 g·kg⁻¹，属于一级（极高）水平。盆地内耕地表层土壤全磷含量整体呈现西北高、东南低的空间分布趋势。降雨是控制四川盆地耕地表层土壤全磷含量空间分布的主控因素，其主要通过调控土壤pH、促进土壤磷素淋失等影响土壤全磷含量的空间分布格局；其次为土壤砂粒含量。在未来施肥和秸秆还田等农业活动中，应充分考虑气候因素及土壤质地、pH等土壤性质，合理控制施肥用量；在降雨量小、土壤砂粒含量低的地区应提高肥料利用率，避免磷素过量积累；在降雨量大、土壤砂粒含量高的地区采取水土保持措施，以减少磷素流失至水体中，降低环境污染风险，推动农业可持续发展。

关键词: 土壤全磷, 四川盆地, 空间分布, 主控因素, 降雨