High negative surface charge increases the acidification risk of purple soil in China

Highlights

• Richer mineral composition induces high content of surface charges on purple soil.

• Purple soil contains large amount of Al³⁺ after electrodialysis treatment.

• Purple soil has a deeper degree of acidification than variable charge soils.

Abstract

Purple soils, developing from rapid physical weathering of nutrient rich sedimentary rock and classified as Inceptisols or Entisols in United States Department of Agriculture (USDA) Taxonomy, are the most important agricultural soils in the Sichuan basin of southwestern China. A portion of purple soils has been acidified and the acidification risk of these soils need further understanding. Here, electrodialysis (ED) was used to simulate the process of natural acidification of soils. We investigated the acidification risk of purple soils by ED treatments and compared its acidification characteristics with that of variable charge soils (Oxisol and Alfisol). Our results showed that the degree of acidification of purple soils after ED treatments was much higher than that of variable charge soils, although the purple soils contained more exchangeable base cations. There was no significant correlation between the purple soil acidity before and after ED treatments, however, the pH of purple soils after the ED treatments had a significant negative correlation (r = −0.680, P < 0.01) with the soil cation exchange capacity (CEC). We concluded that CEC (i.e., effective negative surface charges) played a decisive role in determining the acidification potential of purple soils. High content of clay minerals and low content of Fe/Al oxides in the purple soils lead to the higher amount of effective negative surface charge than that of variable charge soils, which induced the rise in the amount of H⁺ and Al³⁺ absorbed on the surface of purple soils and increased the acidification degree of purple soils remarkably. Therefore, compared to the variable charge soils, the purple soils had a higher acidification risk. These findings will provide a useful reference for the understanding of the acidification characteristics of purple soils.

Introduction

Purple soils, a soil group in Chinese Soil Genetic Classification (CSGC), are formed from the fast-physical weathering of sedimentary reddish or purple mudstone or sandstone of the Triassic to Cretaceous system (Zhong et al., 2019). In CSGC, Purple soils are classified into different soil genus based on their color difference. The main genus of purple soils consist of red purple soil (Fig. 1A) developed from the purplish red sandstones of the Cretaceous Jiaguan Formation (K₂j) (Fig. 1a), the brown purple soil (Fig. 1B) developed from the purple rocks of the Jurassic Penglaizhen Formation (J₃p) (Fig. 1b), the reddish brown purple soil (Fig. 1C) developed from the purple rocks of the Jurassic Suining Formation (J₃s) (Fig. 1c), the grayish brown purple soil (Fig. 1D) developed from the purple rocks of the Jurassic Shaximiao Formation (J₂s) (Fig. 1d), and the dark purple soil (Fig. 1E) developed from the purple rocks of the Truassic Feixianguan Formation (T₁f) (Fig. 1e). Purple soils, characterized by the absence of distinct pedogenic horizons, have a considerably higher formation rate than that of other soils (Li et al., 2009a, Zhou et al., 2014). They are unique in China and mainly distributed in the hilly area in the Sichuan basin of southwestern China with an area of 160,000 km² (Fan et al., 2015, Zhu et al., 2009) (Fig. 2). Depending on horizons and characteristics diagnostic (e.g., cambic horizon), purple soils are classified as Cambosols or Primosols in Chinese Soil Taxonomy (CST) (Ci et al., 2018), and Inceptisols (Shi et al., 2004) or Entisols (Wei et al., 2006) in USDA Taxonomy. Generally, most of the purple soils in the lower part of hill have a cambic horizon since the purple soil in the lower part of hill have higher degree of soil development than that in the upper area. Due to rich mineral nutrients, the areas with purple soil are widely cultivated and regarded as the most important agricultural soil in Sichuan basin of southwestern China (Zhu and Zhu, 2015). However, these regions are mostly slope land and rain intensively, which influence soil properties such as soil nutrition via runoff, drainage, soil erosion (Khan et al., 2016, Wang et al., 2009). Therefore, most of the studies on purple soil focused on the soil conservation (Chen et al., 2017, Li et al., 2009b, Shi et al., 2004) and nutrient management (Jiang et al., 2010, Wang et al., 2009, Zhang et al., 2014).

Based on soil pH, purple soils are classified into three subgroups in CSGC Taxonomy: acidic (pH < 6.5), neutral (pH 6.5–7.5) and calcareous purple soil (pH > 7.5) (Zhao et al., 2014). Among them, the neutral purple soil account for almost 80% (Zhou et al., 2014). Recently, Zhang et al. (2017) studied the impacts of long-term nitrogen fertilization on the acidification of a slightly calcareous purple soil and found that soil pH and exchangeable base cations declined in the past 25 years. Li et al. (2019a) reported that the soil in the purple hilly area of southwest China was dramatically acidified from 1981 to 2012, with a decline of 0.3 pH units. Our previous study also reported that a portion of neutral purple soil has been acidified and the proportion of acidic purple soil increased (Li et al., 2012b). These studies suggested that purple soil is facing the problem of acidification, which could result in the Al³⁺ toxicity and deficiencies of base cations (e.g., K⁺, Ca²⁺ and Mg²⁺), thus, limiting crop growth and decreasing crop yield (Shi et al., 2017). Therefore, more attention should be paid to the ongoing acidification of purple soil. More importantly, the acidification process of purple soil likely differs from other highly weathered acidic soils (e.g., Alfisol and Oxisol) developing from tropical and subtropical regions (Cheng et al., 2018). In the work of Cheng et al. (2018), they reported that the acidification potential of purple soil was higher than that of Alfisol and Oxisol even though the purple soil was rich in base cations. It is important to uncover the acidification mechanism in purple soil which is rarely known until now. By doing so, appropriate management strategies can then be put in place.

Electrodialysis (ED) has been used to extract exchangeable cations from soil and purify soil or mineral samples for decades (Wilson, 1929). Based on the theory of ED, it can be used to simulate soil acidification (Li and Xu, 2013). Additionally, compared to leaching-induced acidification, the ED technique is capable of inducing acidification process much faster and at lower pH value (Mattson, 1933). This means that ED is a reliable and efficient approach to study the acidification characteristics of soil under rapid and strong acidification process (Li et al., 2012a).

To our knowledge, no study has explored the acidification characteristics of purple soil, and the impacts that acidification will have on purple soil are far from clear. In this study, we aim to assess the acidification risk of purple soil by comparing the acidification characteristics of purple soil before and after ED treatment.