Soil Phosphorus

Rattan Lal, B.A. Stewart, Rattan Lal, B.A. Stewart

• Publisher: CRC Press
• Year: 2016
• eBook ISBN: 9781315355085
• Edition: 1
• Topic: Biological Sciences
• Subtopic: Biology
• Index: Biological Sciences

Phosphorus is an essential plant nutrient, but global population growth has dramatically reduced the availability of phosphorus fertilizer resources. Despite this scarcity, there remain numerous problems associated with the excessive and inappropriate use of phosphorus leading to non-point source pollution and eutrophication of natural waters. Identifying appropriate systems for managing soil phosphorus and reducing the risks of eutrophication are needed to minimize the environmental risks. This book focuses on the availability and recycling of phosphorus; regulatory and policy issues of sustainable phosphorus use; and water quality management in agroecosystems pertaining to phosphorus. Sections are dedicated to global phosphorus reserves; cycling and pathways of phosphorus; phosphorus in agriculture; human dimensions and policy intervention; and research and development priorities.

Phosphorus is a finite but crucial resource and is an essential element to all life. Sub-optimal availability and nutrient imbalance in the root zone can adversely impact plant growth, and the quality of food and feed grown on these soils. However, the proven reserves of phosphorus can hardly be adequate for a few centuries only. Yet, its misuse and mismanagement has caused severe problems of eutrophication of water and pollution of the environment. Thus, judicious management of soil phosphorus is essential. This volume is specifically devoted to availability and recycling of phosphorus, regulatory/policy issues of sustainable use of phosphorus, and management in agroecosystems in the context of maximizing the use efficiency and minimizing the environmental risks of water quality.

1	The Global Phosphorus Cycle
	Gabriel M. Filippelli

CONTENTS

1.1Introduction

1.2Global Phosphorus Cycling

1.2.1Natural (Prehuman) Phosphorus Cycle

1.2.1.1Apatite Dissolution: Phosphate Rock and Synthetic Hydroxylapatite

1.2.1.2Apatite Dissolution: Marine Sediments and CFAP

1.2.1.3Phosphorus Cycling in Soils

1.2.1.4Riverine Transport of Particulate and Dissolved Phosphorus

1.2.1.5Marine Sedimentation

1.2.2Modern Phosphorus Cycle

1.3Ecosystem Dynamics and Soil Development

1.3.1Examples of the Lake History Approach to Terrestrial P Cycling

1.4Modern Phosphorus Deposit Resources and the Impending P Crisis

1.5Conclusions and Remaining Questions

References

1.1INTRODUCTION

Phosphorus (P) is a limiting nutrient for terrestrial productivity, and thus, it commonly plays a key role in the net carbon uptake in terrestrial ecosystems (Tiessen et al. 1984; Roberts et al. 1985; Lajtha and Schlesinger 1988). Unlike nitrogen (another limiting nutrient but one with an abundant atmospheric pool), the availability of new P in ecosystems is restricted by the rate of the release of this element during soil weathering. Because of the limitations of P availability, P is generally recycled to various extents in ecosystems depending on climate, soil type, and ecosystem level (e.g., Filippelli et al. 2006; Porder et al. 2007; Filippelli 2008). The release of P from apatite dissolution is a key control on ecosystem productivity (Cole et al. 1977; Tiessen et al. 1984; Roberts et al. 1985; Crews et al. 1995; Vitousek et al. 1997; Schlesinger et al. 1998), which in turn is critical to terrestrial carbon balances (e.g., Kump and Alley 1994; Adams 1995). Furthermore, the weathering of P from the terrestrial system and transport by rivers is the only appreciable source of P to the oceans. On longer timescales, this supply of P also limits the total amount of primary production in the ocean (Holland 1978; Broecker 1982; Smith 1984; Filippelli and Delaney 1994). Thus, understanding the controls on P weathering from land and the transport to the ocean is important for models of global change. In this chapter, I will present the current state of knowledge of the natural (prehuman) and modern (synhuman) global P mass balances, including an in-depth examination of climatic and geologic controls on ecosystem dynamics and soil development.

1.2GLOBAL PHOSPHORUS CYCLING

1.2.1NATURAL (PREHUMAN) PHOSPHORUS CYCLE

The human impact on the global P cycle has been substantial over the last 150 years and will continue to dominate the natural cycle of P on the globe for the foreseeable future. Because this anthropogenic modification began well before the scientific efforts to quantify the cycle of P, we can only guess at the preanthropogenic mass balance of P. Several aspects of the preanthropogenic sources of sinks of P are relatively well constrained (Figure 1.1). The initial source of P to the global system is via the weathering of P during soil development, whereby P is released mainly from apatite minerals and is made soluble and bioavailable (this process will be discussed at considerable length later). In contrast to this process of chemical weathering, the physical weathering and the erosion of material from the continents results in P that is typically unavailable to biota. An exception to this, however, is the role that physical weathering plays in producing fine materials with extremely high surface area/ mass ratios. Phosphorus and other components may be rapidly weathered if this fine material is deposited in continental environments (i.e., floodplains and delta systems) where it undergoes subsequent chemical weathering and/or soil development. Thus, the total amount of P weathered from continents may be very different from the amount of potentially bioavailable P.

Apatite minerals, the dominant source-weathering source of P, widely vary in chemistry and structure and can form in igneous, metamorphic, sedimentary, and biogenic conditions. All apatite minerals contain phosphate oxyanions linked by Ca²⁺ cations to form a hexagonal framework, but they differ in elemental composition at the corners of the hexagonal cell (Table 1.1; McClellan and Lehr 1969). Fluorapatite (FAP) is by far the most abundant of the apatite minerals on land and is an early formed accessory mineral in igneous rocks, appearing as tiny euhedral crystals associated with ferromagnesian minerals (McConnell 1973).

FIGURE 1.1The natural (prehuman) phosphorus cycle, showing reservoirs (in teragram P) and fluxes (denoted by arrows, in teragram P per year) in the P mass balance.

TABLE 1.1 Terminology and Crystal Chemistry of Apatites
Mineral Name	Chemical Composition
FAP	Ca5(PO4)3F
HAP	Ca5(PO4)3OH
Chlorapatite	Ca5(PO4)3Cl
Dahllite	Ca5(PO4,CO3OH)3(OH)
CFAP	Ca5(PO4,CO3OH)3(F)
Source:Phillips, W. R., and D. T. Griffen, Optical Mineralogy: The Nonopaque Minerals. W. H. Freeman and Co., San Francisco, 1981. With permission.

1.2.1.1Apatite Dissolution: Phosphate Rock and Synthetic Hydroxylapatite

The dissolution rate of apatite minerals has been the focus of a variety of relatively disparate studies. For example, the importance of P as a fertilizer and as an integral component of teeth and bone material has led to a wide range of P dissolution and retention studies. Most of the agricultural studies have focused on the dissolution of fertilizers, in which P has already been leached from a phosphate rock ore (usually a mixed lithology of marine sediments containing high percentages of francolite [CFAP], Ca₅(PO₄,CO₃OH)₃(F); Filippelli 2011) and reprecipitated as highly soluble phosphoric salts. Some studies, however, have investigated the dissolution of phosphate rock in laboratory experiments (Smith et al. 1974, 1977; Olsen 1975; Chien et al. 1980; Onken and Matheson 1982). The medical studies have investigated the dissolution behavior of synthetic HAP (similar in composition to dahllite) in acidic solutions, especially to understand the processes of bone resorption and formation, as well as dental caries formation (Christoffersen et al. 1978; Fox et al. 1978; Nancollas et al. 1987; Constantz et al. 1995).

Several researchers have attempted to develop dissolution rate equations to model the apatite dissolution (Olsen 1975; Smith et al. 1977; Christoffersen et al. 1978; Fox et al. 1978; Lerman 1979; Chien 1980; Onken and Matheson 1982; Hull and Lerman 1985; Hull and Hull 1987; Chin and Nancollas 1991). The rate equations from these models are of forms that include zero order, first order, parabolic diffusion, mixed order, and others. One model (Hull and Hull 1987) focuses on surface dissolution geometry, which, the authors argue, fits the experimental results better than previous dissolution models. Clearly missing in these experiments and the dissolution rate equations derived from them are the experimental conditions that replicate the natural dissolution processes and agents in soils, as well as a reasonable range of apatite mineralogy likely to be exposed to natural weathering processes in soils.

1.2.1.2Apatite Dissolution: Marine Sediments and CFAP

Perhaps the most comprehensive and geologically appropriate laboratory examination of apatite dissolution has been performed on CFAP. Researchers focused on the marine P cycle have performed several studies on CFAP dissolution (Lane and Mackenzie 1990, 1991; Tribble et al. 1995), as well as CFAP precipitation in laboratory settings (Jahnke 1984; Van Cappellen and Berner 1988; Van Cappellen 1991; Filippelli and Delaney 1993). These studies have focused on CFAP because of the importance of this authigenic marine mineral phase in terms of the oceanic P cycle (Ruttenberg 1993; Filippelli and Delaney 1996). Several general observations about apatite dissolution/precipitation have been made. For example, the presence of Mg has been determined to retard CFAP formation (Atlas and Pytkowicz 1977), but the presence of other trace elements (e.g., Fe and Mn) appears to have little effect (Filippelli and Delaney 1992, 1993).

In a study of the pH dependence of CFAP dissolution, Lane and Mackenzie (1990; 1991) used a fluidized bed reactor to determine the dissolution chemistry. As presented in the study by Tribble et al. (1995), a Ca- and F-depleted surface layer appears to form during early dissolution, followed by a later stage when stoichiometric dissolution is achieved. The incongruent initial dissolution is probably due to the removal of most or all of the F, and some Ca, in the depleted surface layer, and the formation of a hydrogen–calcium–phosphate phase, as has also been observed by other researchers (Atlas and Pytkowicz 1977; Smith et al. 1974; Driessens and Verbeeck 1981; Thirioux et al. 1990). This surface-controlled reaction eventually achieves a steady state, whereby the depleted surface layer does not change in depth, and the solid effectively dissolves congruently (Tribble et al. 1995). Ruttenberg (1990; 1992) performed extensive dissolution experiments on FAP, HAP, and CFAP, with a goal of developing an extraction scheme for the characterization of these mineral phases in marine sediments. Although geared toward extraction technique development, these studies showed increased dissolution rate with decreasing grain sizes and higher dissolution rates of HAP and CFAP than that of FAP with a sodium acetate–acetic acid solution at pH 4.0.

1.2.1.3Phosphorus Cycling in Soils

The cycling of P in soils (see Figure 1.1) has received much attention, in terms of both fertilization and natural development of ecosystems. Of the approximately 122,600 Tg P within the soil/biota system on the continents, nearly 98% is held in soils in a variety of forms. The exchange of P between biota and soils is relatively rapid, with an average residence time of 13 years, whereas the average residence time of P in soils is 600 years (Figure 1.1). As noted earlier, the only significant weathering source for phosphorus in soils is apatite minerals. These minerals can be congruently weathered as a result of a reaction with dissolved carbon dioxide:

$${\text{Ca}}_5{\left({\text{PO}}_4\right)}_3\text{OH}+4{\text{H}}_2{\text{CO}}_3\to 5{\text{Ca}}^{\mathrm{2+}}+3{\text{HPO}}_4^{2-}+4{\text{HCO}}_3^{-}+{\text{H}}_2\text{O}$$

In soils, P is released from mineral grains by several processes. First, the reduced pH produced from respiration-related CO₂ in the vicinity of both degrading organic matter and root hairs dissolves P-bearing minerals (mainly apatites) and releases P to the root pore spaces (e.g., Schlesinger 1997). Second, organic acids released by plant roots can also dissolve apatite minerals and release P to the soil pore spaces (Jurinak et al. 1986). Phosphorus is very immobile in most soils, and its slow rate of diffusion from the dissolved form in pore spaces strongly limits its supply to the rootlet surfaces (Robinson 1986). Furthermore, much of the available P in soils is in o...

Table of contents

• Cover
• Half title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Editors
• Contributors
• Chapter 1 The Global Phosphorus Cycle
• Chapter 2 Positive and Negative Effects of Phosphorus Fertilizer on U.S. Agriculture and the Environment
• Chapter 3 Coupled Cycling of Carbon, Nitrogen, and Phosphorus
• Chapter 4 Phosphorus in Soil and Plants in Relation to Human Nutrition and Health
• Chapter 5 Phosphorus Management
• Chapter 6 Phosphorus Effluxes from Lake Sediments
• Chapter 7 Economic and Policy Issues of Phosphorus Management in Agroecosystems
• Chapter 8 Phosphorus Fertilization and Management in Soils of Sub-Saharan Africa
• Chapter 9 Phosphorus and the Environment
• Chapter 10 Enhancing Efficiency of Phosphorus Fertilizers through Formula Modifications
• Chapter 11 Soil Phosphorus Cycling in Tropical Soils: An Ultisol and Oxisol Perspective
• Chapter 12 The Use of Phosphorus Radioisotopes to Investigate Soil Phosphorus Dynamics and Cycling in Soil–Plant Systems
• Chapter 13 Pathways and Fate of Phosphorus in Agroecosystems
• Index