Soil Biochemistry

Volume 7

Jean-Marc Bollag, G. Stotzky

• Publisher: CRC Press
• Year: 2021
• ISBN:
  9781000447392
• Edition: 1
• Topic: Ciencias biológicas
• Subtopic: Bioquímica

This book describes the interactions between soil minerals and microorganisms to more specialized areas such as the formation of desert varnishes. It is helpful for scientists and students who want to extend their knowledge of and research into soil biochemistry.

1
Biochemistry of Sulfur Cycling in Soil

JAMES J. GERM

IDA University of Saskatchewan, Saskatoon, Saskatchewan, Canada

MILTON WAINWRIGHT

University of Sheffield, Sheffield, England

VANDAKATTU V. S. R. GUPTA*

University of Saskatchewan, Saskatoon, Saskatchewan, Canada

* Present affiliation: Commonwealth Scientific and Industrial Research Organization (CSIRO), Canberra, Australia.

I. INTRODUCTION

Sulfur (S) is an essential element for the growth and activity of organisms. It is abundant throughout the earth’s crust (ca. 0.1%), and in soil it is derived from the atmosphere, weathered rock, fertilizers, pesticides, irrigation water, and such [1]. Since the industrial revolution, the increased burning of fossil fuels has resulted in a greater input of atmospheric S to the soil budget [2–4]. In addition, volatilization of S (as hydrogen sulfide, carbon disulfide, carbonyl sulfide, methyl mercaptan, dimethyl sulfide, dimethyl disulfide, sulfur dioxide) from marine algae, marsh lands, mud flats, plants, and soils contributes to the global circulation of S through the atmosphere.

Sulfur exists in a number of oxidation states (+6 to −2), and for biological systems, it is the most oxidized and most reduced states that are important. The most oxidized form is a component of the nervous system (sulfatides) and connective tissue (sulfated polysaccharides), whereas the most reduced form is required by all microorganisms for storage and transformations of energy, synthesis of amino acids and proteins, enzyme reactions, and as a constituent of coenzymes, ferridoxins, vitamins, and others [5].

Sulfur is considered to be a macronutrient in most ecosystems, but in some ecosystems, the availability of S to the biota is limiting. For example, sulfur-deficient soils (> 100 million ha) are found in many parts of the world [6], and over 8 million ha in western Canada are either deficient or potentially deficient [7]. Plants need substantial amounts of S for growth and grain production, with their requirements varying according to species. As a result, to manage effectively the S needs of crops, it is important to understand the nature and quantities of different S pools in soil and the various transformation processes of the S cycle (Fig. 1). This review discusses the major pools and transformations of the soil S cycle, with particular emphasis given to biological and biochemical processes.

Figure 1 The global sulfur cycle. The fluxes are given in millions of tonnes sulfur per year (tg S yr⁻¹). Numbers in italics indicate the amounts that anthropogenic activities have added; nonitalic numbers denote the transfers estimated to have prevailed before anthropogenic activities had a significant influence on the sulfur cycle. Adapted from Ref. 8.

II. NATURE AND FORMS OF SULFUR IN SOIL

The nature and quantities of various S pools in soil are influenced by pedogenic factors, such as climate, regional vegetation, and local topography. The total S content of soils ranges from 0.002 to 10% [9], with the highest levels being found in tidal flats and in saline, acid sulfate, and organic soils. The total S concentrations in the surface layers of soils from various parts of the world range from 18 to 6400 μg S g⁻¹ for African soils; 42 to 6450 μg S g⁻¹ for Asian soils, 24 to 2000 μg S g⁻¹ for Australian soils; 20 to 4210 μg S g⁻¹ for European soils, 145 to 8000 μg S g⁻¹ for soils in the USSR; 32 to 2300 μg S g⁻¹ for North American soils; and 27 to 1104 μg S g⁻¹ for Central and South American soils [6, 7]. The total S content in western Canadian agricultural soils ranges from 0.01 to 0.1% [6, 7, 10]. Roberts and Bettany [11] reported that the total S in surface horizons increases from semiarid (Brown Chernozems) to more humid soil zones (Black Chernozems), and from the upper to lower slope portions of catenas in Saskatchewan, Canada.

Organic S constitutes more than 90% of the total S present in most surface soils [9, 11, 12]. Thus, a close relation exists between the organic C, total N, and organic S contents of soil. Freney and Williams [9] summarized information on the C:N:S ratios for a wide range of soils from all over the world and reported ratios ranging from 50:4:1 (cultivated brown Chernozems of western Canada) to 271:13:1 (virgin Luvisols of Canada), depending on various pedogenic factors. The mean worldwide C:N:S ratio isapproximately 130:10:1 for agricultural soils and 200:10:1 for native grassland and woodland soils [6, 7, 10]. The differences in the organic C:N:S ratios of various regions and landscapes have only recently been stressed [11, 12]. Organic C:N:S increased from 68:7:1 in Brown Chernozemic soils to 145:11:1 in Gray soils, with a similar trend being noted downslope within a catena [11]. Native grassland soils generally exhibit wider C: N: S ratios than their cultivated counterparts. For example, the C:N:S ratios of the native and cultivated gray wooded soils of western Canada are 271:13:1 and 129: 11:1, respectively [7]. In contrast to C:S ratios, the variation in N:S ratios as the result of cultivation and between soil groups is less, and most agricultural soils have N: S ratios in the range of 6:1 to 10:1. In agricultural soils, the N:S ratio usually tends to decrease with increasing soil depth.

Cultivation of native pasture soils causes a more rapid loss of N than of S, but the loss of carbon-bonded S (C-S) follows more closely that of N [13]. This results in higher HI-S:C-S ratios in cultivated soils than in native pastures. The hydriodic acid-reducible sulfur (HI-S) fraction is organic sulfur that is directly reducible to H₂S by hydriodic acid; it is believed to comprise mainly ester sulfates [14]. Similar observations have been reported by other workers [12, 15–17] and might be explained by the difference between biochemical and biological mineralization of organic compounds in soil.

A. Inorganic Sulfur

Inorganic forms of S account for less than 25% of the total S in most agricultural soils [18]. Sulfide (S²⁻), elemental S (S⁰), sulfite (SO_3²⁻), thiosulfate (SO₂O_3²⁻), tetrathionate (S₄O_6²⁻) and sulfate (SO_4²⁻) are the main forms of inorganic S in agricultural soils. In well-drained soils sulfides account for less than 1% of the total S [19], and measurable quantities of S₂O_3²⁻ and S₄O_6²⁻ are detected only in soils treated with S⁰ fertilizer or exposed to pollutants [20, 21, J. R. Lawrence, Microbial oxidation of elemental sulfur in agricultural soils, PhD dissertation, University of Saskatchewan, Saskatoon, Saskatchewan, Canada, 1987]. There are several forms of SO_4²⁻, including easily soluble SO_4²⁻, adsorbed SO_4²⁻, insoluble SO_4²⁻, and SO_4²⁻ coprecipitated/cocrystallized with CaCO₃. Water-soluble salts of Mg, Ca, and NaSO₄ account for less than 5% of the total S in surface horizons of most well-drained soils, although higher levels may accumulate under arid conditions [10]. Sulfate adsorption is influenced by soil pH, nature of colloidal surfaces, presence...