Role of clay minerals in controlling phosphorus availability in a subtropical Alfisol

Highlights

• The fate of P fertilizers in Alfisol is controlled by chemical adsorption reactions.

• P is primarily immobilized by goethite, kaolinite, and illite in Alfisol.

• P adsorbed by illite may serve as an indicator of P availability in Alfisol.

Abstract

Inorganic P is adsorbed by a variety of soil minerals. For acidic subtropical Alfisols with high contents of both (hydr)oxides and clay minerals, there is an underappreciated understanding of the importance of clay minerals to P species and availability varying with soil pH and P concentration. In this study, the rate and equilibrium of P adsorption in a subtropical Alfisol were investigated by combining soil incubation with three types of P fertilizer with multi-surface modelling. Phosphorus species analysis revealed that most of the P was present in the forms of Al-P and Fe-P in the subtropical Alfisol. The multi-surface model calculation showed that goethite, kaolinite, and illite were the most important soil constituents responsible for the immobilization of P. Clay minerals were as important as Al/Fe (hydr)oxides for P adsorption in the subtropical Alfisol. In addition, there was a highly positive linear correlation between P adsorbed by illite and Olsen P, with a slope of approximately to 1. The model analysis also showed that >99% of the P adsorbed by illite was in the form of ${\left(-SO\right)}_{2}P{O}_2^{2-}$. These results suggest the important role of illite in controlling P availability and that P adsorption by illite may serve as an indicator of P availability in the subtropical Alfisol.

Introduction

Phosphorus is an essential macronutrient for plant growth in both terrestrial and aquatic ecosystems and is often a primary limiting factor in agricultural production (Hinsinger, 2001, Raghothama and Karthikeyan, 2005). However, long-term nutrient fertilization in intensive farming systems has led to high accumulation of P in soils, which can lead to serious environmental problems such as eutrophication of surface waters (Jiao et al., 2012, Kier and Kirkland, 2013). In soils, P is directly taken up by crops in the form of orthophosphate anions ($P{O}_4^{3-}$). Phosphorus availability is strongly affected by the adsorption of P onto the positively charged sites of Fe and Al (hydr)oxides and clay minerals and the formation of P-containing minerals (Barrow, 2017, Hiemstra et al., 2013, Holford, 1997, Meyer et al., 2021, Weng et al., 2011). Fe/Al (hydr)oxides have been generally recognized as important agents in controlling P availability in the subtropical Alfisol because of their high positive charge and P adsorption capacity (Antelo et al., 2005; Xu et al., 2019), whereas the role of clay mineral has been largely ignored (Cui and Weng, 2013, Weng et al., 2011, Weng et al., 2012). In actual agricultural production, crops can only utilize a very small fraction (10–20%) of the fertilized P, which is largely determined by the soil properties and fertiliztion strategies (Liu et al., 2017, Meyer et al., 2021). The relationship between individual soil constituents and P availability is still poorly understood and difficult to predict in soils.

It is generally accepted that P availability is highly correlated with P species. The role of P adsorbed by clay minerals in controlling P availability is generally neglected compared with that of Fe/Al oxides (Gérard, 2016, Hinsinger, 2001). A review of experimental studies over a 70 years periods revealed that in most soils, clay minerals should be considered as important P-binding constituents that can possibly outcompete Fe/Al (hydr)oxides P adsorption (Gérard, 2016). Our recent study also demonstrated that myo-inositol hexakisphosphate a common type of organic P in soils, can form inner-sphere complexes and surface precipitates on the surface of kaolinite (Hu et al., 2020). These findings indicate that although the P adsorption capacity of clay minerals is much lower than that of Fe/Al (hydr)oxides, they may also provide a potentially important interface for P adsorption owing to their higher contents in soils, especially in the subtropical Alfisol, the clay minerals content of which is over six times that of Fe/Al (hydr)oxides. The P adsorbed by clay minerals is more easily desorbed than that adsorbed by Fe/Al (hydr)oxides, thereby indicating that the stability of P adsorption to clay minerals is weaker than that of Fe/Al (hydr)oxides; thus P is more readily available to plants (Al-Kanani and MacKenzie, 1991). It has also been demonstrated that there is a high linear correlation between available P and the clay mineral content in soils (Herlihy and McGrath, 2007). These facts indicate that in soil such as the subtropical Alfisol the potential importance of clay minerals to P availability may be greatly underestimated compared with that of Fe/Al (hydr)oxides.

To evaluate the contribution of clay minerals to P availability in the subtropical Alfisol, it is critical to use a reasonable method to identify the P species regarding the soil constituents. In practice, Olsen P is a convenient measure of available P in soil; P species can be classified into Al-P, Fe-P, Ca-P, and occluded P (O-P) by Jackson-Chang’s method (Chang and Jackson, 1957). Such P species can be easily measured by different chemical extractions using acids and bases. A ‘family’ of P is adsorbed by active sites with similar affinities on different soil constituents. It is impossible to accurately reflect the contribution of clay minerals to P availability (Olsen P). However, the shortcomings of chemical methods for the determination of P species can be complemented by the multi-surface model. The model considers that the soil is composed of a series of independent soil constituents, and the amount of P adsorbed by soil is a linear sum of the P adsorbed by each individual soil constituent. Phosphorus species based on the types of soil constituents and dissolved inorganic P can be accurately predicted by the multi-surface model under certain pH levels and P concentrations (Devau et al., 2011, Devau et al., 2010, Weng et al., 2001). Therefore, it is an effective and convenient approach to determine the P species, which can be used to link P adsorption to clay minerals with the available P in the subtropical Alfisol.

In this study, to examine the role of clay minerals in controlling P availability in the subtropical Alfisol, the kinetics and isotherms of P adsorption were investigated by combining soil incubation with three types of P fertilizer with multi-surface modelling. The pH, dissolved organic carbon (DOC) concentration and Olsen P concentration were monitored during incubation. The P species in the subtropical Alfisol depending on the incubation time and P concentration were analyzed first by Jackson-Chang’s method, and then calculated using the multi-surface model. The contributions of different soil constituents to P availability were also compared. The hypotheses of this study are as follows: (1) clay minerals are important for P adsorption and P species in the subtropical Alfisol; (2) the multi-surface model is an effective and convenient method to determine P adsorption by clay minerals, and (3) clay minerals play a key role in controlling the P availability in the subtropical Alfisol.