Litter decomposition and nutrient distribution in humus profiles in some mediterranean forests in southern Tuscany

Forest Ecology and Management, 57 ( 1993 ) 99-114

99

Elsevier Science Publishers B.V., Amsterdam

Litter decomposition and nutrient distribution in humus profiles in some mediterranean forests in southern Tuscany Bas van Wesemael Laboratoryfor Physical Geographyand Soil Science, Universityof Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam, Netherlands (Accepted 31 July 1992)

ABSTRACT Van Wesemael, B., 1993. Litter decomposition and nutrient distribution in humus profiles in some mediterranean forests in southern Tuscany. For. Ecol. Manage., 57:99-114. Decomposition of leaf litter and the distribution of elements in the humus layer were studied in mediterranean deciduous, sclerophyllous and coniferous forests on acid rocks. The results indicate a clear difference in relative decomposition rate between pine needles (Pinus pinaster: O.12 year -I ) and leaves of deciduous and sclerophyllous species (Quercus cerris, Quercussuber and Arbutus unedo: 0.30 year-~ ). The concentrations of N, P, S and Ca increase upon decomposition, whereas that of K decreases by initial leaching, and those of Mg, Mn (Fe, A1 ) remain unchanged except for an increase resulting from mineral contamination. In deciduous and sclerophyllous litter, absolute amounts of N, P, S and Ca increase until a critical concentration level is reached, after which net mineralization occurs. For pine needles net mineralization was not observed within 915 days. In analogy with the situation during the litter bag experiments, elemental concentrations are highest in the lower more decomposed pan of the humus profiles. In deciduous and sclerophyllous forests net mineralization of N, P, S and Ca starts in the lower part of the fermentation layer. In the coniferous forest elemental concentrations are much lower and no indications of N, P, S and Ca mineralization were found in the ectorganic horizons.

INTRODUCTION

In mediterranean-type ecosystems growth of natural vegetation is often limited by availability of water and nutrients (Specht and Moll, 1983). The sclerophyllous leaves with thick cuticula are considered to be a physiological adaptation against drought and low nutrient status (Rundell, 1988). The gradual litter fall and slow decomposition of the litter promote evenly distributed nutrient fluxes back into the soil (Schlesinger and Hasey, 1981; Mitchell Correspondence to: B. van Wesemael, Laboratory for Physical Geography and Soil Science, University of Amsterdam, Nieuwe Prinsengracht 130, 1018 VZ Amsterdam, Netherlands.

© 1993 Elsevier Science Publishers B.V. All rights reserved 0378-1127/93/$06.00

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et al., 1986; Rundell, 1988). Although it seems possible that conditions are adequate for the development of mor- or moder-type humus forms in the mediterranean zone, this aspect has received little attention. This can partly be explained by the widespread degradation of both soils and vegetation in the mediterranean basin as well as by the frequent occurrence of forest fires (Read and Mitchell, 1983; Specht and Moll, 1983 ), as a result of which welldeveloped humus forms are scarce. In recent studies it was demonstrated that in mature mediterranean forests on acid soils the accumulation of organic matter is considerable and that the dominant humus form is a moder, although mor-type humus also occurs (Pedroli et al., 1988; Sevink et al., 1989; Van Wesemael and Veer, 1992 ). In temperate forests on acid soils retarded decomposition of organic matter is an important factor in both soil formation and nutrient cycling (Duchaufour, 1976). The increase of decomposition with depth and the reduced bioturbation result in the development of humus profiles with distinct layers of partly decomposed organic matter. The resulting vertical spatial separation of mineralization, causing proton consumption in L and F horizons, and root uptake, causing proton production in H and Ah horizons, may lead to acidification of the underlying mineral soil (Ulrich et al., 1979; Verstraten et al., 1990). The role of retarded litter decomposition in nutrient cycling is known from numerous studies. Since most of these studies were carried out under field conditions using litter bags, they represent the net effect of release, uptake and external input and loss of nutrients. Nevertheless, these studies demonstrated that some nutrients are first immobilized in microbial tissue and are released only when a critical concentration is reached (Gosz et al., 1973, Staaf and Berg, 1982). The aim of this study was to investigate decomposition and mineralization of litter and their implications for the development of humus profiles in some mediterranean forests. Furthermore, the consequences of differences in litter quality for nutrient cycling and soil formation are discussed. RESEARCH AREA

In a small area along the upper course of the Farma river in southern Tuscany (Italy) well-developed forests occur on homogeneous parent material. Sites were selected in undisturbed deciduous, sclerophyllous and coniferous forests. The vegetation and soils of this area have been described by Pedroli et al. ( 1988 ), while Van Wesemael and Veer ( 1992 ) give a detailed description of the forest stands considered. The deciduous oak forest has a dense tree layer dominated by Quercus cerris ( Rubia peregrina-Quercus cerris association). The sclerophyllous oak forest consists of an open tree layer with Quercus suber and some Q. cerris, and a dense shrublayer with Arbutus unedo ( Vi-

DECOMPOSITION AND MINERALIZATION OF LITTER

101

burno-Quercetum ilices). The coniferous forest is a former heathland of the Tuberario lignosae-Callunetum type, nowadays dominated by pine trees (Pinus pinaster) and with a shrublayer of Arbutus unedo and Erica arborea. The humus profiles are of the moder type with a well-developed ectorganic layer (litter, fermentation and humus horizons). In this layer three different horizons could be distinguished, being composed of fresh and slightly discoloured leaves or needles (L&F 1 ), strongly fragmented litter with both droppings and mycelia (F2), and humified amorphous material with many droppings (H). In deciduous and sclerophyllous forests the humus profile is usually thinner than in coniferous forests (Van Wesemael and Veer, 1992 ). However, in the latter soil horizon differentiation is less prominent - - the F2 horizon being relatively thick, while the H is thin or even lacking. The mineral soils are derived from low-grade metamorphic sandstones and phyllites of the triassic verrucano formation. They are non-calcareous, medium textured and have a thin greyish AEh horizon. The boundary between the ectorganic layer and the AEh horizon is abrupt and wavy. Fine roots are concentrated in the H horizon and in the upper 5 cm of the mineral soil. The soils are classified as dystric cambisols (Food and Agriculture Organization (FAO), 1988 ) or as dystric xerochrepts (Soil Survey Staff, 1990 ). The climate is attenuated mesomediterranean (UNESCO-FAO, 1963). Mean annual precipitation is 988 m m with a maximum in October/November and in May. The average maximum temperature is 5.6 °C in January and 27.0°C in July (Pedroli et al., 1988 ). METHODS

Litter bags Leaves and needles were collected from several trees in the forest stand in which they were subsequently incubated. Deciduous leaves and pine needles were collected in autumn by shaking the trees. Sclerophyllous leaves could not be collected in sufficient quantities by shaking the trees. Therefore brown leaves, about to fall, were picked from the trees. A known air-dry mass of leaves (10 g) was confined in a 20 c m × 2 0 cm polyethylene netting. Mesh size was 1.0 mm for the bottom and 2.0 mm for the top. Two mesh sizes were used in order to prevent excessive loss of litter without excluding mesofauna (Swift et al., 1979 ). Each litter bag was filled with leaves of a single species. The 15 bags per species (three replicates) were installed at random in small plots in the forests from which the leaves were sampled. This was done according to the following scheme: Q. cerris leaves in the deciduous forest; Q. suber and Arbutus unedo leaves in the sclerophyllous forest and P. pinaster needles in the coniferous forest. The litter bags were anchored on the forest floor in December 1987 after

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the peak in litter fall of deciduous and pine trees. After 4, 8, 12, 17 and 30 months litter bags of each species (three replicates ) were removed at random. They were dried for 48 h at 70 °C and the weight of the leaves was determined. For elemental analysis the contents of the three bags per species were combined and ground (0.5 m m mesh).

Litter fall Litter was collected from August 1987 to August 1988 using two 0.86 m 2 and one 0.18 m 2 rectangular litter traps in each stand. For chemical analyses litter of 2 monthly periods was bulked in each stand and finely ground (0.5 m m mesh). The weighted mean elemental concentrations in total litter fall (leaves, twigs < 2 cm diameter, and reproductive parts ) were calculated from the amount and elemental concentrations.

Organic horizons An iron frame (25 cm × 25 cm ) was driven through the ectorganic horizons into the mineral soil. Each ectorganic layer (L&F 1, F2 and H) was sampled. For each stand a composite sample was taken from four randomly chosen points. After removal of gravel and roots, samples were air dried and finely ground (0.5 m m ).

Chemical analyses Organic matter content was determined by loss on ignition after heating at 500°C for 16 h. Total nitrogen was determined by the Kjeldahl method including addition of salicylic acid (Bremner and Mulvaney, 1982). Samples were digested with concentrated HNO3 and HC1 in a microwave oven (Bettinelli et al., 1989 ) and subsequently total K, Na, Ca, Mg, Fe, Mn and A1 were determined by atomic absorption/emission spectrophotometry and P was determined colorimetrically with a continuous flow autoanalyzer. Total sulphur was determined by inductively coupled plasma atomic emission spectrometry after digestion of the organic material in concentrated HNO3 (Novozamsky et al., 1986).

Calculations Remaining mass in the litter bags (%RM) was calculated from the weight of litter (Xt) at each sampling period (t) and initial mass (Xo) % R M = X' × 100

Xo

( 1)

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DECOMPOSITION AND MINERALIZATION OF LITTER

Similarly the percentage of the initial element amount remaining (%RA) was calculated from the initial mass (Xo), the initial concentration (Co) and the mass and concentration remaining at time t (X, and 6",): %RA

C,×Xt =Co~ X 100

(2)

The relative decomposition constant (k) of leaf litter confined in litter bags can be estimated using an exponential model (Olson, 1963; Wieder and Lang, 1982)

x,

In (~oo)= - k t

(3)

RESULTS AND DISCUSSION

Dry mass loss

Absolute decomposition rates of confined liter decreased and variation in losses from individual bags increased with time (Fig. 1 ). After 1 year the weight loss rate of all litter types decreased. Field observations indicate that in this period litter bags became covered with fresh litter, and may be contaminated by decomposition products of fresh litter (Gosz et al., 1973 ). Relative decomposition rates were estimated by fitting an exponential model (eqn. (3)) to the mean values of the masses at each sampling (X,). The rel100

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B. VAN WESEMAEL

ative decomposition constant (k) was calculated from the relationship between t and In (XffXo) by means of linear regression (Wieder and Lang, 1982 ). In Table 1 it can be seen that these relationships are highly significant ( P < 0.001 ) for all litter types. Decomposition of the dominant litter type is most rapid in the deciduous oak forest (Q. cerris), although the difference with the sclerophyllous oak forest (Q. suber and Arbutus unedo leaves) is small (Table 1 ). The decomposition rate of the pine needles (P. pinaster) in the coniferous forest is much lower. Since the decomposition in the first year is much faster than over the whole period, relative decomposition constants from other studies have to be compared over the same incubation period. The k values of deciduous and sclerophyllous leaves in Table 1 are larger than those reported by Schlesinger and Hasey (1981) for a mediterranean shrubland (Saliva mellifera and CeanoTABLE 1

Litter decomposition constants from litter bag experiments compared with those from a mass balance approach of the ectorganic layer Time (years)

ka

1 2.5

r 2

n

0.50 0.32

1.00 0.75

4 6

1 2.5 1 2.5

0.47 0.32 0.38 0.30

0.99 0.80 0.91 0.93

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1 2.5

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0.90 0.65

4 6

(year -I )

LiHer bags Deciduous forest Q. cerris Q. cerris Sclerophyllous forest Q. suber Q. suber Arbutus unedo Arbutus unedo

Coniferous forest Pinus pinaster Pin us pinaster

Mass balance of ectorganic layer

Deciduous Sclerophyllous Coniferous

kb

0.26 0.19 0.17

aCalculated from the remaining mass fraction (Xt/Xo): In (Xt/Xo ) = - kt. bCalculated from the ratio of the yearly input of litter (L) and the amount of organic matter on the forest floor (X): k = L / X (Van Wesemael and Veer, 1992 ).

DECOMPOSITION AND MINERALIZATION OF LITTER

105

thus megacarpus: 0.40 and 0.26 year -1, respectively), and comparable with those from the Hubbard Brook forest of Fagus grandifolia and Acer saccharum (k= 0.37 -0.51 year- l; Gosz et al., 1973 ). The decomposition rate ofP. pinaster needles (0.12 year-1 ) in the mediterranean coniferous forest is much slower than that ofPinus sylvestris needles in temperate coniferous forests (0.26 year-l in 5 years; Staaf and Berg, 1982). The decomposition rates of the ectorganic layer estimated by a mass balance study in the same forests are somewhat smaller (Van Wesemael and Veer, 1992 ). They range from 0.26 year- i for the deciduous oak forest floor to 0.19 year- ~for sclerophyllous oak forest and 0.17 year- l for coniferous forest. The more rapid decomposition from litter bags in the broad leaved forests may be explained by the mass loss by strong fragmentation which was observed in F2 horizons. The exclusion of soil fauna by the mesh size of the litter bags, which is often reported (Swift et al., 1979), seems to be of lesser importance in comparison to the loss of litter considering the relatively high decomposition rates. The high decomposition rates in litter bags could also be attributed to the soil moisture conditions, which are more favourable in the litter bags as a result of artificial compaction of the litter. Element concentrations in confined litter The increase in ash content of confined leaf litter upon decomposition is ascribed to contamination with mineral material by soil fauna, and enrichment of salts by respiration of carbohydrates (Fig. 2 ). A distinctly lower ash content is found in coniferous forests, where decomposition is slower and thick ectorganic horizons prevent contamination with mineral material. In order to facilitate the comparison of gradients in elemental concentrations, all elements are expressed on an ash-free basis. Ca, Mg, K, Mn, Fe and AI are abundant in the mineral soil, and therefore the increase in concentrations of these elements after 500 days is mainly because of contamination by soil minerals (Fig. 2). The concentrations of N, P, S and Ca increase upon decomposition for all litter types (Fig. 2). Initial concentrations of the pine needles are distinctly lower than in the other litter types. The concentrations in decomposing pine needles show an almost linear increase with time. The N, P, S and Ca concentrations in the sclerophyllous and deciduous litter increased linearly during the first year, after which the rates of increase dropped (Fig. 2). Such gradients of N, P and S concentrations are usually attributed to microbial immobilization ( Swift et al., 1979; Waring and Schlesinger, 1985 ). Since these elements are either directly linked to carbon or are part of the structure of organic matter, they are released only when the carbon is consumed by heterotrophic microorganisms. These organisms immobilize limiting elements until a critical concentration is reached. The clear increase of Ca concentra-

106

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tions in all litter types has not been reported for litter bag experiments in temperate forests (Gosz et al., 1973; Staaf and Berg, 1982), and suggests a similar release by microbial activity followed by strong adsorption. The major part of K, Na, Mg and Mn is not structurally bound in organic matter. The retention of these cations is governed by the adsorption complex (Gosz et al., 1973; Staaf and Berg, 1982 ). The decrease in K concentrations is restricted to the initial stage of decomposition, and can be attributed to leaching (Waring and Schlesinger, 1985 ). K, as a monovalent cation, is only weakly bound to the adsorption complex. After this stage, K also seems to reach a critical concentration (Fig. 2 ). The Na concentrations are very low, and the trends seem to be subject to seasonal variation. The bulk of the Na is probably derived from sea spray in atmospheric precipitation, which is highest in autumn and winter (Waring and Schlesinger, 1985). The concentrations of Mg and Mn remain rather constant.

Nutrient release from confined litter In the litter bags filled with Arbutus unedo and Q. cerris, the absolute amounts of N, P and S first increased before they were gradually released. The absolute increase of N, P and S is attributed to atmospheric deposition (Gosz et al., 1973 ). The degree of accumulation of these elements differs and is dependent on the initial concentration in the litter bags (Table 2). The leaves of Q. suber showed net mineralization from the start of the experiment (Fig. 3). For both Arbutus unedo and Q. cerris leaves net mineralization started after 356 days (Fig. 3). In the P. pinaster needles accumulation continued until the end of the experiment. The critical concentration at which net mineralization begins is often expressed as the carbon: element ratio: C: N = 2030 (Campbell, 1978; Waring and Schlesinger, 1985; Stevenson, 1986), C: P = 200, and C: S = 200 (Stevenson, 1986 ). In this study these values are C : N = 3 0 , C : P = 5 5 0 , C : S = 2 5 0 for Q. cerris, Q. suber and Arbutus unedo leaves (Table 3; Fig. 3 ). For the pine needles net mineralization was not observed within the 915 days. The lowest carbon: element ratios in the pine forest are well above these critical ratios (C: N = 48, C: P = 1855, C: S = 377 ). In contrast to the critical C: N and C: S ratios, the critical C: P is distinctly higher than those reported elsewhere. This can be attributed to both the low nutrient status of mediterranean ecosystems (Specht and Moll, 1983) and the strong variation in C: P ratios caused by analytical error at very low P concentrations. In the forests of this study the behaviour of N, P and S is characterized by the same immobilization and release phase as reported in most other forest ecosystems (Gosz et al., 1973; Staaf and Berg, 1982; Mitchell et al., 1986). The length of the immobilization phase is longer for P. pinaster needles (more than 915 days) than reported for P. sylvestris needles in temperate forests (525 days, Staaf and Berg, 1982). The behaviour of Ca is comparable with

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that of N, P and S, although the loss of Ca in the mineralization phase is smaller. This is probably caused by strong adsorption of divalent Ca ions. Net mineralization occurs directly for Arbutus unedo with the highest initial Ca concentration (Table 2). For Q. suber net mineralization was observed after 237 days and for Q. cerris after 356 days. Pinus pinaster needles do not show net Ca mineralization within 915 days (Fig. 3). The critical C:Ca ratio is approximately 35 for deciduous and sclerophyllous species (Table 3 ). From the start of the experiment the amounts of K and Mg decreased upon decomposition (Fig. 3 ). Loss of K was rapid in the first 4 months after which it was released very slowly from the oak (Q. suber and Q. cerris) leaves with the highest initial concentrations, and was retained by the Pinus and Arbutus

110

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TABLE 3

Carbon: element ratios in decomposing leaves during a litter bag experiment Species

Time (days)

Carbon:elementratio ~ C:N

C:P

C:S

C:Ca

Q. cerris

0 127 237 356 513 910

62 50 39 32 34 27

1060 813 643 531 599 586

n.d. n.d. n.d. n.d. n.d. n.d.

48 35 30 25 26 22

Q. suber

0 127 237 356 513 910

31 28 22 22 21 23

775 635 565 565 544 629

350 318 273 244 231 217

58 44 36 33 31 28

Arbutus unedo

0 127 237 356 513 910

40 32 27 25 25 22

808 666 574 506 532 477

397 333 269 248 240 191

35 30 26 27 27 21

P. pinaster

0 132 242 361 518 915

87 72 65 64 61 48

3267 2874 1956 2318 2418 1855

628 643 575 529 509 377

137 114 106 108 99 91

~See Table 2.

n.d., not determined.

leaves. The amount of Mg was lost at approximately the same rate as the dry weight. Immobilization of Ca and the relatively small losses of K and Mg from all investigated leaf litters are different from the situation in temperate forests (Gosz et al., 1973; Staaf and Berg, 1982). However, the behaviour of these elements is comparable with that in mediterranean shrublands (Schlesinger and Hasey, 1981; Mitchell et al., 1986 ).

Distribution of the major elements in the humus profile The ectorganic horizons are well stratified, and therefore the individual horizons contain organic matter in specific stages of decomposition. The total elemental concentrations in the various horizons of the humus profiles are

DECOMPOSITION AND MINERALIZATION OF LITTER

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given in Table 4. These concentrations were calculated on an ash-free basis in order to correct for the increase of mineral contamination from L to H horizons. The concentration of all elements increases with increasing decomposition stage (from L&F1 to H horizon). This was also observed during the litter bag experiments. In the uppermost ectorganic horizons (L&F1), concentrations of most elements are virtually equal to those in litter fall (cf. Tables 2 and 4). The low K concentrations in the L&F I horizon are a result of leaching, which also occurred during the initial phase of the litter bag experiment (Fig. 3 ). The high concentrations of A1 and Fe are probably a result of contamination by the mineral soil and subsequent adsorption on the organic material, comparable with that at the end of the litter bag experiment. In the ectorganic horizons of the Hubbard Brook forest (Gosz et al., 1976 ) element retention is not as strong as in these mediterranean forests: concentrations are generally lower and Ca, Mg, K and Mn concentrations decrease with depth. The gradients in N, P, S and Ca concentrations in the ectorganic horizons deserve special attention. It was shown in the litter bag experiments that these concentrations, expressed as carbon:element ratios, indicate net mineralization. In all forest types the carbon:element ratios decrease with depth in the ectorganic layer (Table 4). The differences in concentrations of N, P and S between the sites correspond to those in the litter fall (Table 2 ). Concentrations diminish from deciduous through sclerophyllous to coniferous forests. Under deciduous and sclerophyllous forests the critical values of the C: N, C: S and C: Ca ratios, upon which net mineralization starts, are reached in the lower part of the fermentation layer (F2), and the critical C:P ratio is attained deeper in the profile (H horizon). Under pine forest none of the critical ratios is reached in the ectorganic layer, and mineralization of N, P, S and Ca is probably taking place in the organic rich part of the mineral profile (AEh horizon). Such a specific allocation of mineralization processes to different ectorganic and endorganic horizons is typical for slow decomposition of organic matter in mor- and moder-type humus profiles and is a major factor in the acidification of the mineral soil (Ulrich et al., 1979; Verstraten et al., 1990). In the mediterranean coniferous forest decomposition is slow and net mineralization is restricted to the upper part of the mineral profile. Such an extreme situation is not attained in most temperate coniferous forests and is probably a consequence of the limited availability of water during the dry mediterranean summer (Specht and Moll, 1983 ). CONCLUSIONS

In mediterranean forests on acid soils the decomposition of deciduous (Q.

cerris) and sclerophyllous (Q. suber and Arbutus unedo) leaves is comparable with decomposition in temperate deciduous forests. Pine needles (P. pinaster) showed a distinctly lower decomposition rate, when compared with tem-

DECOMPOSITION AND MINERALIZATION OF LITTER

1 13

perate forests. The increase of N, P, S and Ca concentrations upon decomposition appears to be negatively correlated with the initial concentration. For pine needles net mineralization was not found within 915 days. The concentrations of K, Mg, Na and Mn show less clear trends upon decomposition. Nutrient retention by the vegetation seems to increase from deciduous to sclerophyllous and pine forests as revealed by the slow decomposition and nutrient release rates resulting in low concentrations of major elements in the humus profile. The vertical spatial separation of mineralization in F horizons and root uptake in H and AEh horizons is more prominent in the deciduous and sclerophyllous forests than in the coniferous forest. ACKNOWLEDGEMENTS

The many useful suggestions during the experiments and the correction of the manuscript by Drs. A. Tietema, Prof. Dr. J. Sevink and Prof. Dr. J.M. Verstraten are gratefully acknowledged. Also I want to thank I. van Voorthuysen for the adaptation of the microwave oven digestion method. The many chemical analyses were performed by I. van Voorthuysen and J. Westerveld. The research was funded by a grant from the Dutch Foundation for Scientific Research ( N W O ) .

REFERENCES Bettinelli, M., Baroni, U. and Pastorelli, N., 1989. Microwave oven sample dissolution for the analysis of environmental and biological materials. Anal. Chim. Acta, 225: 159-174. Bremner, J.M. and Mulvaney, C.S., 1982. Nitrogen--total. In: A.L. Page, R.H. Miller and D.R. Keeney (Editors), Methods of Soil Analysis; Part 2 Chemical and Microbiological Properties, 2nd edn. American Society of Agronomy, Madison, Wl, pp. 595-624. Campbell, C.A., 1978. Soil organic carbon, nitrogen and fertility. In: M. Schnitzer and S.U. Khan (Editors), Soil Organic Matter. Elsevier, Amsterdam, pp. 173-271. Duchaufour, P., 1976. Dynamics of organic matter in soils of temperate regions: its action on pedogenesis. Geoderma, 15:31-40. Food and Agriculture Organization, 1988. FAO/UNESCO Soil Map of the World: Revised Legend. FAO, Rome, 138 pp. Gosz, J.R., Likens, G.E. and Bormann, F.H., 1973. Nutrient release from decomposing leaf and branch litter in the Hubbard Brook forest, New Hampshire. Ecol. Monogr., 43:173-19 I. Gosz, J.R., Likens, G.E. and Bormann, F.H., 1976. Organic matter and nutrient dynamics of the forest and forest floor in the Hubbard Brook forest. Oecologia, 22: 305-320. Mitchell, D.T., Coley, P.G.F., Webb, S. and Allsopp, N., 1986. Litter fall and decomposition processes in the coastal fynbos vegetation, south-western Cape, South Africa. J. Ecol., 74: 977-993. Novozamsky, I., van Eck, R., van der Lee, J.J., Houba, V.J.G. and Temminghoff, E., 1986. Determination of total sulphur and extractable sulphate in plant materials by inductivelycoupled plasma atomic emission spectrometry. Commun. Soil Sci. Plant Anal., 17 ( 11 ): 11471157.

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