The Sources of Lead Pollution in a Tropical Soil Chronosequence Based on Lead Isotope

Soil is important contributor to global biogeochemical cycles and often receives anthropogenic Pb contamination. Hainan soil chronosequence developed on basalt had provided a good opportunity to identify and quantify the relative contributions of Pb sources in remote tropical areas. The results revealed that Pb concentrations and isotopic ratios of the soils were clearly affected by anthropogenic source. The Pb concentrations and percentage changes of Pb/Th ratios showed significantly Pb enrichment. The low 206 Pb/ 207 Pb values of upper soils indicated a significant addition of extraneous Pb, whereas deeper soils showed a dominantly basaltic source. The 208 Pb/ 206 Pb vs. 206 Pb/ 207 Pb diagram of soils clearly indicated inputs of parent material and anthropogenic Pb sources. We also calculated the mass fractions of anthropogenic-derived Pb ( P b f n i a n t h opog e ) based on isotope mass balance. The values showed a generally decreasing trend with soil depth, implying a significant addition of anthropogenic Pb in top soils. The contribution of anthropogenic Pb in Hainan soil chronosequence highlighted the significance of anthropogenic contamination to soils globally.


Introduction
Soil is an important contributor to global biogeochemical cycles and has played an important role in the ecosystem (Brantley et al., 2007;Ryu et al., 2020;Vogel et al., 2021). In recent decades, rapid industrialization and urbanization have introduced great amount of contaminants, including heavy metals, from various sources into soils (Oluyemi et al., 2008;Awokunmi et al., 2010;Chai et al., 2015;Adedeji et al., 2020).
Heavy metals are major contaminants in the soil environment (Peng et al., 2019;Li et al., 2020). They are resistant to biodegradation, making their accumulation in the soil environment extremely persistent (Bolan et al., 2014). Lead (Pb) is a natural constituent of the earth's crust and is commonly found in soils, plants and water (Lei et al., 2016). Pb is an especially poisonous element and is one of the most widely studied metals in environmental sciences (Zahran et al., 2009). Consumption of Pb even at very low concentrations can cause diseases of nervous, digestive and reproductive systems, and do great harms to human health (Zhang et al., 2008).
Emissions of Pb have been significantly enhanced by various anthropogenic activities (Mukai et al., 2001). Thus, Pb contamination in soils environment has become an important issue for many countries and causes both human health and environment concerns (Cheng et al., 2010;Stefanowicz et al., 2010;Ajmone-Marsan et al., 2010;Kamenov et al., 2014).
The isotopic signatures of Pb in different environments reflect the mixing of different sources, and source apportionment of Pb can be quantified by Pb isotopic methods (Marcantonio et al., 2000;Bird, 2011;Li et al., 2020). Pb isotopic analysis has been increasingly applied for Pb contamination studies of air (Grousset et al., 2005;Hyeong et al., 2011), soils (Wong et al., 2006), sediments (Hosono et al., 2010), and plants (Marcantonio et al., 2000;Klaminder et al., 2008). The use of lead isotopic ratios was shown useful to identify different Pb origins of urban soils in Hongkong (Ettler et al., 2004). Monna et al. (2000) developed Pb isotope ratio based on models 4 to calculate relative contributions of various Pb sources in sediments. Thus, Pb isotopes have been proven to be valuable tracers for understanding anthropogenic lead pools and earth surface processes related to regolith development (Blichert-Toft et al., 2016;Jeon et al., 2020).
In different environments, soils are often receiving Pb from different sources (Zhang et al., 2007a;Li et al., 2011;Kapusta et al., 2015). In urban and rural areas, the sources of lead include not only anthropogenic sources, but also natural sources, such as rock weathering (Monastra et al., 2004;Li et al., 2020). Although Hainan Island is located in one of the relative clean and remote areas in China, soils may have been contaminated by anthropogenic Pb. We established a soil chronosequence developed on basalt, with a strong contrast of Pb isotopic ratios between bedrock (i.e., basalts) and anthropogenic Pb sources. This may provide an excellent opportunity to study the relative contributions of various Pb sources in rural soils. Therefore, the main objectives of this study were: (1) to investigate Pb contamination of rural soils in Hainan Island, (2) to examine the isotopic ratio of Pb and identify possible sources of the contaminant in soils, and (3) to estimate relative contributions of natural and anthropogenic Pb sources of soil chronosequence in order to reveal the effect of different soil horizons and development stage on Pb contributions.

Study area and pedon description
Soil chronosequence is often used to illustrate soil genesis, which minimizes variation in many variables that can influence soil development and then allows a stronger focus on specific soil forming factors (Vidic and Lobnic, 1997;Ryu et al., 2020).
Hainan Island is the second largest island in China, located in the north of South China Sea. It is one of the largest areas of Quaternary basalts in China (Fig.1). Hainan Island is located in the tropical area with superior hydrothermal conditions, which has unique chemical weathering and soil forming conditions that obviously different from the temperate areas (Zhang et al., 2007b). It has a mean temperature of 23-24ºC and a mean annual precipitation of 1400 to 1800 mm. Meanwhile, the types of parent rocks (basalt) were very homogeneous and the differences of climate, topography and vegetation were small in Hainan Island. Thus, these homogeneous soil forming factors in Hainan Island provided favorable conditions for us to study the geochemistry of soil chronosequence in tropics.
Three Soil profiles were developed on the basalts that range in age: 180 ka (HB12), 420 ka (HB10) and 2300 ka (HB06), which was determined using K-Ar chronology. Thus, we selected them to establish a soil chronosequence. The sites were minimally eroded gently sloping shield volcano surfaces. The soils support forests that were dominated by Eucalyptus and Casuarinas. Soil pits were hand-dug, described by genetic horizon to fresh rock, and channel sampled to form an integrated homogenous unit for each horizon. The soils were classified as Primosols to Ferralsols (Chinese Soil Taxonomy (CST), 2001), which were equivalent to Entisols to Oxisols (Table 1)

Laboratory analyses and calculations
Soil samples were air-dried, crushed using a wooden pestle and mortar, and then passed through a 2 mm nylon sieve. According to the different requirements of chemical analysis items, soil samples were further milled to pass the different sieves.
The soil pH, soil bulk density, soil organic matter (SOM) and cation exchange capacity (CEC) were measured by methods of Lu (1999). The prepared samples (0.149 mm, 500 mg) were weighed into a disposable graphite crucible with 700 mg of flux (1:1 mix of Li-metaborate and Li-tetraborate) and fused for 30 min in a 1000°C muffle furnace (Li et al., 2014). Then fused samples were dissolved in a solution (10% HNO3 + 1% HF) and determined the Pb La and Th concentrations of the soils at Nanjing University using a JY 38S (JION YVON, France) single channel scanning plasma spectrometer. The standard reference materials were GSR-3, GXR-5 and GXR-6. Analytical uncertainties were less than ±2% for the trace elements. One approach to determine chemical losses during weathering of primary minerals was to determine the ratio of chemical concentration in the weathered mineral to the corresponding concentration in the parent mineral. We use Th as the immobile index element for mass balance calculations in this study because Li et al. (2014) showed that Th is the least mobile elements in soils developed on basalt. The percent change in the ratio relative to the ratio in the parent sample is then defined according to Nesbitt and Markovics (1997) as: where Ri and Rp represent the ratio of element to a conservative element in weathered samples and fresh basalt, respectively.
We calculated the percentage contribution of anthropogenic and natural Pb sources to total Pb in soils based on isotope mass balance (Li et al., 2011

Physicochemical data in soils of Hainan Island
Descriptions and selected physicochemical characteristics of soils were shown in The bulk density was clearly lower in A horizon than in C horizon for each soil profile. The soil organic matter (SOM) showed a decreasing trend with depth, and the cation exchange capacity (CEC) varied from 3.6 cmol + kg -1 to 23.6 cmol + kg -1 in Hainan soils. It is worth noting that the pH, SOM and CEC values all showed a significantly decreasing trend with soil ages for the HB12, HB10 and HB06 profiles.

Pb elemental and isotopic characteristics of soils
The concentrations of Th, La and Pb for the fresh basalt were 1.76 mg kg -1 , 11.7 mg kg -1 and 2.31 mg kg -1 in this study, respectively. The Pb concentrations ranged from 9.1 to 70.5 mg kg -1 ( there was a significant negative correlation between the isotopic ratios and concentration of Pb in soil profiles (Fig. 2). The average contributions of anthropogenic Pb were 22%, 66% and 32% in HB06, HB10 and HB12 soil profiles, respectively. For the old soils (2300 ka), there were much more leaching losses of soluble elements, leading to relative enrichment of lava-derived Pb. So the relative contributions of anthropogenic Pb were lower than those of younger soils (Fig. 4). The significantly lower values of pH, SOM and CEC in the HB06 profile than those of HB12 and HB10 profiles (Table 1), may give rise to strong depletion in the old tropical soils (Li et al., 2014). In addition, the highest change of La/Th ratios indicated significant strong leaching of major element in HB06 ( Furthermore, because the location of HB12 was closest to the sea (Qiongzhou Strait), the annual rainfall was higher than other sites (Fig. 1). This may cause much stronger leaching loss and lead to a relative low contributions of anthropogenic Pb than those of HB10. Thus, the contributions of anthropogenic Pb sources were related to the development stage of soil and the leaching intensity of local area.       Pb shows a trend of increasing first and then decreasing with the increase of soil age.