## 1 Introduction  
Arsenic (As), a potentially toxic element to humans, is bioaccumulative and non-biodegradable in the natural environment [1; 2; 3]. Karst terrains occupy about 7-10% of the planet [4]. China has approximately 2 million square kilometers of karst landforms, distributed in various provinces and regions, with the majority located in the southern part of the country, particularly widespread in Guangxi, Guizhou, Yunnan, and Chongqing. Because of the high average background values of karstic landscape-formed soils, they are frequently enriched in As [5; 6]. Since As can enter the food chain and endanger human health [7; 8], its excess in karst soils has drawn considerable attention from the public in an effort to protect the environment, the ecology, and public health in karst regions [10; 11; 12]. The natural presence of As in soils is primarily influenced by the composition of geological parent materials [13; 14]. It has been shown that there is a strong correlation between As in soils in northwest Greece and geological parent materials and anthropogenic sources of pollution [4]. In karst areas, the heredity of a parent rock with a high heavy metal content (e.g., black shale, sulfide deposits, and coal measure strata) and secondary enrichment caused by the leaching and weathering of a parent rock with a low heavy metal content (e.g., carbonate rocks) during soil development may lead to an As content that is higher than the normal background [15]. This natural process may result in wide areas of soil exceeding the environmental quality standards for As [16]. Consequently, a certain proportion of cultivated fields in karst regions may fall under strict supervision and be unsuitable for agricultural production, leading to significant public concern and constraining the local agricultural economy [5; 17]. The physico-chemical characteristics and chemical forms of heavy metals largely determine the potential mobility and bioavailability of As [18; 19]. In comparison with the total content of As, the bioavailability of As is more closely related to plant absorption [20; 21]. The primary reasons for soil heavy metal elements in karst regions are pedogenic processes, which cause the high geological background values in the area [22; 23]. In their natural state, the reactivity of most heavy metals is relatively low. Nonetheless, there have been notable alterations in the physico-chemical characteristics of soils as a result of frequent human production activities, for example, in northwestern Greece [4], southern China [22], and southwestern China [23]. As a result, the amount of certain heavy metals that are readily available has increased, which poses a major risk to the safety of agricultural products [24]. To date, the majority of relevant studies concerning As bioavailability and transfer in the soil-plant system have primarily focused on localized areas or limited regions with anthropogenic contamination [25; 26]. However, the bioavailability of soil As and the potential high transfer risk of As from the soil to rice remain unclear, particularly in regions with a naturally high As background. Guangxi, a region of China with one of the most extensive distributions of karst regions and with a significant carbonate rock development [27], faces significant environmental concerns related to the heavy metal contamination of soil. Recent studies have verified that the combination of secondary enrichment and parental rock inheritance is responsible for the high accumulation of As in Guangxi's karstic soil [6]. Fe-Mn nodules are present in many agricultural soils in the karst areas of Guangxi, China [28; 29]. Fe-Mn nodules share a common origin with the surrounding soils and are formed in situ by the cementation of Fe-(oxyhydr)oxides to the soil matrix components.