FUNDAMENTOS DE ESTA SUPREMA SALA: Primero:

Soil is one of the most complex mixtures on this planet, not only from a biological, but also from a physical and chemical viewpoint. Consequently, the questions regarding the removal and replenishment of S in soils cannot be totally answered because it is impossible to develop an analytical technique to study the nature of soil S that leaves the soil fabric undisturbed. Without the identification of specific S compounds in soils, the common approach is to classify groups of S compounds into general forms allowed for by current extraction and analytical techniques.

2.2 . 1 The Forms of S i n Soil

Most agricultural soils or mineral soils have total S contents ranging from 50 to 1 000mglkg in the surface 1 5 cm (Freney and Williams, 1 983; Syers and Curtin, 1 987). The New Zealand Soil Bureau reference topsoils have total S values of 1 1 20-

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1 630mg/kg for Yellow-brown loams, and 1 3 0 to 63 0mgS/kg for other soils (New Zealand Soil Bureau, 1 968). Ghani et a/. , ( 1 99 1 ) reported a total S value of 930mg/kg for the topsoil of the Horotiu Yellow-brown loam under pasture, a value below the New Zealand Soil Bureau reference soils for Yellow-brown loams, and values ranging from 1 85 to 6 1 5mg/kg for other soil types, within the New Zealand Soil Bureau reference soils. In contrast, Perrott and Sarathchandra ( 1 987) in their work with soils under established pasture, reported total S values that were generally 20% higher for the same soil groups with mean values of 1 570mg/kg for the Yellow-brown loams and 550mg/kg for the other soils excluding recent and organic soils. These workers presumed that the higher soil S contents reflected histories of greater superphosphate

application to soils in their survey.

The proportions of S in organic or inorganic form vary according to soil type (Metson, 1 979a; 1 979b), depth in profile (Williams, 1 974; Haynes and Williams, 1 992), climate and cultural conditions (Bettany et ai., 1979, 1 980). In most soils, more than 90% of the S is organic and is unavailable for plant uptake until it has been mineralised (decomposed by soil organisms) to inorganic S. In situations where inputs of S from other sources are very low, organic S is the main source of plant available S (sulphate­ S) in soil (Bettany et al. , 1 980; Freney 1 986).

2.2. 1 . 1 Plant Available S (Sulphate-S)

Sulphur is absorbed by plant roots almost exclusively as sulphate-S (S042"), thus the influence of other components in the S cycle (Fig. 2. 1 ) on this pool are of great importance. This S fraction however, represents only a relatively small proportion of total S,. generally less than 5% in New Zealand pastoral soils (Perrott and Sarathchanda, 1 987; Ghani et a/. , 1 988; Sakadevan, 1 99 1 ).

Sulphate-S is derived from wet and dry deposition of mainly sulphate-S and sulphur dioxide, weathering of soil parent rocks (oxidation of reduced inorganic forms of S, ego sulphide) and mineralisation of organic S (Roy and Trudinger, 1 970). Weathering reactions are thought to be a minor input of S in current topsoils (Met son, 1 979a), mainly because mineral sulphides are quickly weathered in aerobic, topsoil environments.

The soil sulphate-S fraction includes the readily soluble inorganic sulphate-S as well sulphate-S adsorbed to soil colloids. In most soils, adsorbed sulphate-S is readily available to plants through desorption and is the main source of plant-available S (Barrow, 1 967; Barrow, 1969; Hasan et aI. , 1 970; Westerman, 1 974). Soils high in

amorphous iron and aluminium oxides or allophane have considerable capacity to sorb sulphate-S through anion exchange sites (Barrow, 1 969), which becomes less available for plant uptake. In soils with low phosphate retention (less than 70%), adsorbed sulphate-S may accumulate during periods of slow plant growth and/or minimal leaching, and decrease during periods of vigorous plant growth and/or leaching (Nguyen and Gob, 1990).

Soil sulphate-S levels are often subject to seasonal fluctuation depending upon the net balance between addition from rainfall, irrigation water, mineralisation of organic matter, applied fertilisers, and losses from leaching, plant and micro-organism uptake (Williams, 1 968). The importance of these processes are reflected in the data of many workers such as Nguyen ( 1 982), Goh and Gregg, ( 1 982a, 1 982b) Cornforth et aI.,

( 1 983), Nguyen et ai., ( 1 989a, 1989b), and Ghani et aI. ( 1 990). In the North Island, the amounts of extractable soil sulphate-S present in spring are generally lower than in autumn, possibly due to the increase in leaching loss of sulphate-S and the slow rate of mineralisation during the winter time (Nguyen, 1 982; Nguyen et aI. , 1 989a, 1 989b; Cornforth et aI., 1983). Ghani et ai., ( 1 990) found that amounts of soil sulphate-S can decrease in short spaces of time, particularly, after rainfall events causing drainage.

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Short-term Variability vs Long-term Steady-State of the sulphate-S pool.

Soil sulphate-S pool size varies significantly over short periods of time (Blair 1 979). However, in the absence of external inputs of sulphate-S, and with steady removal by leaching and transfers by plants and animals, Watkinson and Perrott ( 1 990) postulated that a quasi steady-state exists in soil producing relatively constant concentrations of sulphate-So The results of Ghani et ai. , ( 1 990) showed variability with season in the amount of soil sulphate-S, but also illustrated that in constant soil conditions (eg. absence of recent fertiliser application and heavy rain), sulphate-S was steady within I mg Slkg for several months due to the significant amounts of sulphate-S released by mineralisation. Such results indicate the magnitude and speed of mineralisation processes operating under field conditions.

In some grazed pasture systems, a total of 1 5 to 30kg S/ha may be lost from the system, via the sulphate-S pool each year (Saggar et ai. , 1 990b; Sakadevan, 1 99 1 ). For steady-state conditions to apply, mineralisation must equal the sum of immobilisation plus leaching and plant uptake. For the system to reach and sustain steady-state, regular S fertiliser applications are required.

2.2. 1.2 Organic S

From the preceding discussion, it is obvious that the nature of organic S and the dynamics of the mineralisationlimmobilisation processes, is central to predicting plant uptake and leaching losses. However, little is known of the macro-molecular nature of organic S in soils (Freney and Stevenson, 1 966; Freney, 1 967; Freney and Williams, 1 983). Organic S mainly originates as plant and animal residues which are subsequently decomposed and re metabolised by soil microorganisms. Freney ( 1 967) and Lowe ( 1 969a, 1 969b) reported a wide variety of S compounds that were produced by organisms either in or on soils. Most of these were susceptible to decomposition,

did not accumulate in their mono-molecular form, and were not readily identifiable in the soils.

Although more than 90% of total S may be present in the organic form, on an annual basis, only a small percentage of this fraction, as little as 2%, may enter the active S cycling pool which supplies S, available for plant uptake (Till and May, 1 97 1 ; Goh and Gregg, 1 982a, 1 982b; Chapman, 1 987a, 1 987b).

Under long-term grassland, there can be a build up in organic matter content whilst, in contrast, under arable cultivation there is appreciable breakdown of soil organic matter (Jackman, 1 964; McLaren and Swift, 1 977; Keer et ai. , 1 990; Haynes and Williams, 1 992; Hedley et ai. , 1 995).

Sulphur in soil organic matter or extracts of soil can be separated into two major groups, (i) Ill-reducible S, and (ii) C-bonded S, based on susceptibility to reduction by reducing agents. Such fractionation indicate the chemical form of S, but produce little if any information about the size or nature of the 'labile pool' (rapidly cycled pool) of mineralisable organic S present in the soils (Haynes and Williams, 1 992) and therefore have not been very successful for predicting S mineralisation rates (Freney, 1 986).

Plants (Sorghum vulgare) were found to utilise S from both the ill-reducible as well as the C-bonded S fractions (Freney et ai. , 1 975). Incubation experiments using labelled S

C

SS), show that both Ill-reducible and C-bonded S fractions can be transformed to sulphate-S via mineralisation (eg., Bettany et al. , 1 974; Goh and Tsuji, 1 979; Fitzgerald et ai. , 1 984; McLaren et ai. , 1 985; Strickland and Fitzgerald, 1 985; Ghani et al. , 1 988). Similarly 3SS labelled SO/- enters both soil organic S fractions when soils are incubated (McLaren et al. , 1 985).

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HI-reducible S

HI-reducible S compounds include S atoms that are not directly bonded to C but are linked to C via an oxygen atom (C-O-S, ester sulphate-S), or nitrogen atom (C-N-S, sulphamate). This fraction is thought to be generated predominantly by soil micro-flora (Fitzgerald, 1 978; David et aI. , 1 984;) which metabolise organic residues in the presence of adequate S (Saggar et al. , 1 98 1 b). Mechanisms of ester formation, transformation, and its significance in the S cycle were reviewed by Fitzgerald ( 1 976). However, what proportion of organic sulphate-S are metabolic by-products, components of dead celis, or products formed by reaction of inorganic sulphate-S with humic constituents, remains unclear.

Organic sulphate-S are thought to be associated with mainly the higher molecular weight fraction of soil organic matter (Bettany et aI. , 1 973 ; Schoneau and Bettany, 1 987; Swift et al. , 1 988; Keer et al. , 1 990; Haynes and Williams, 1 992), and are considered to be the most labile fraction. Bettany et al. ( 1 973) suggested that higher degrees of humification as a result of increased biological activity, will form higher molecular weight, strongly condensed stable humic acids with considerably active, hydrolysable side chain components. Thus residual organic S in soils reflects a fraction which has been protected from microbial decomposition, either spatially protected within aggregates or associated with complex organics.

Carbon-bonded S

C-bonded S is believed to be more associated with the highly condensed aromatic humus core and a more stable organic S form than the organic sulphate-S fraction (Bettany et al. , 1 973 ; Bettany et al. , 1 979). This S fraction is normally calculated as the difference between total organic S and organic sulphate-So Soil C-bonded S may be

derived directly from both leaf litter and root inputs, as well as microbial protein synthesis.

C-bonded S can be fractionated into two groups, (i) Raney-Nickel reducible organic S, believed to be mainly amino acid S (Lowe and DeLong, 1 963 ; Freney et ai. , 1 975) and, (ii) ' inert' C-bonded S fraction which is highly resistant to chemical degradation, and is probably not a significant source of plant-available S (Biederbeck, 1 978).

2.2. 1 .3 Microbial Biomass-S

Soil microbial biomass is a living part of soil organic matter, and with plant roots, are the driving force behind mineralisationlimmobilisation processes in soils. The microbial biomass-S fraction compromises only c. 0.4 to 4% of total S (Saggar et aI. , 1 98 1 a; Chapman, 1 987a; Gupta and Germida, 1 988a; Ghani et aI. , 1 990).

Soil moisture and temperature are factors that influence microbial activities and consequently, soil microbial biomass-S concentrations (Ghani et aI. , 1 990). Degradation of soil organic compounds where S is a constituent is controlled firstly, by the energy needs of the microbes which they can obtain from energy released when C is respired and secondly, by the S requirements of the microbes for metabolic, structural and/or other physiological needs. The enzymatic activities of microorganisms in the degradation of soil organic S compounds would be difficult if not impossible to relate with S mineralisation rates because of the unknown identity of all the selective and non­ selective enzymes involved in degradation. In addition, substrate accessibility is complex and probably one of the more important factors in the undisturbed soil fabric.

In document LA APLICACIÓN DEL FACTOR MITIGACIÓN DE DAÑO EN LOS DESPIDOS LABORALES EN LA CUANTIFICACIÓN DEL LUCRO CESANTE (página 181-185)