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Mineral nutrient content and turnover rate of Mesembryanthemum crystallinum in the north-western desert of Egypt
Journal of Arid Environments (1992) 22: 219-230
Mineral nutrient content and turnover rate of Mesembryanthemum crystallinum in the north-western desert of Egypt Salama M. EI-Darier Department ofBotany, Faculty of Science, Moharram Bay, Alexandria University, Alexandria, Egypt (Received 4 December 1989, accepted 24 July 1990) A knowledgeof the nutrient content of different plant organs is a necessary basis for the evaluationof the role of different nutrient elements in functioning of the ecosystem. The annual species Mesembryanthemum crystallinum concentrates excess Na, K and Ca in the vegetative organs at the standing dead stage. The total content of the different elements exhibited two distinct peaks. The first was during the late vegetative stage and included the elements Na, K, Ca and Mg while the second, during the standing dead stage, was made up of Ca and Mg. The turnover rate and time calculated in the present study indicate that M. crystallinum is well adapted to the environment of the western Mediterranean desert of Egypt. The accumulation of Ca and Na in the litter alters the habitat, inhibiting the growth of someof the associated annuals and permitting increased invasion by M. crystallinum year after year.
Introduction Annual species are a major component of desert plant communities. Several studies have reported on their production and composition, but very few have dealt with their role in the nutrient budgets of desert ecosystems (e.g. Gaballah, 1986). Information on the seasonal changes in the nutrient content of different plant organs is necessary for the elucidation of the role of different nutrient elements in the functioning of the ecosystem (Tolsma et al., 1987). The present study evaluates this role in the introduced annual species Mesembryanthemum crystallinum L. (iceplant) which is common in the mediterranean coastal lands of Egypt. The site selected for study is a transition area between the Abu-Sir ridge and the saline depression, 45 km west of Alexandria and 4 km north of Burg el-Arab village. The soil is a sandy loam. The pH of the soil solution ranges from 7'2 to 7'8, and the mean loss on ignition is about 2 '06% in samples collected at 30 em depth. The area belongs to the zone of 'arid climate with mild winters and warm summers' (UNESCO, 1977). The average annual rainfall is about ISO mm and falls mainly between November and March. The most common perennials in the study area are Eminium spiculatum (Blume) Ktze., Arisarum vulgare Targ.- Tozz., Launaea nudicaulis (L.) Hook. f., Plantago lanceolata L., Fagonia cretica L., Salvia lanigera Poiret, Plantago albicans L., Thymelaea hirsuta (L.) Endl., Asphodelus microcarpus Salzm. & Viv. ; and Salsola longifolia. Forssk. Mesembryanthemum crystallinum L., Plantago coronopus L., Anacydus alexandrinus Willd., Trigonella stellata Forssk. and Carthamus glaucus Bieb. are the most common annuals. Nomenclature follows Tackholm (1974). 0140-1963/92/030219 + 12 $03-00/0
© 1992 Academic Press
Limited
220
S. M. EL-DARIER
Methods Plant materials were sampled from 15 randomly distributed 30 x 30 em quadrats. The sampling period extended from winter (February) to late summer (August). At each harvest, samples of soil were collected at 30 em depth beneath the plants. Samples of plant material were washed, separated into different organs (roots, stems, leaves and reproductive parts), dried at 65°C, weighed and ground. The ground material was digested with triple acid reagent HN0 3 : H 2S04 : HCI0 4 10: 1: 1. The samples of soil were extracted with neutral molar ammonium acetate solution [NH 4 (COOCH 3 ) ) . The samples of plant material and soil were analysed for their content of K, Na, Ca and Mg using an atomic absorption spectrophotometer. The total ash content was determined by ashing the samples for 3 h at 500°C. All these procedures follow Allen et at. (1974). The turnover rate for each of these elements was calculated as the per cent of the total uptake of that element during the growing season. The turnover time is the reciprocal of turnover rate and is a measure of the time needed to make available the nutrients present in the litter. The turnover rate and time were calculated according to Tyler (1971). The phenological states (early vegetative, late vegetative, reproductive and standing dead) were recorded at each sampling. The percentages estimated for different sample quadrats are then used to calculate the average phenological state of the plant at each sampling. Analysis of variance was used to assess the significance of variation in the concentration of different nutrients with date and organ. The least significant difference (L.S.D.) test was then applied to evaluate the differences between the members of pairs of the mean concentration of different nutrients in different dates. All statistical tests follow Steel & Torrie (1960).
Results
Nutrient concentration The concentration of each of the nutrient elements (mg s': inM. crystallinum varied with organ and time (Table 1). In general, the stems attained the highest concentration of all elements. The reproductive organs attained high concentrations ofNa and Mg, while the leaves attained high concentrations of Ca and K. Stems and roots reached their highest concentrations ofNa, K and Ca in the early vegetative stage, but Ca reached a peak in the leaves and reproductive organs at the standing dead stage, and Mg peaked in the leaves and stems at the late vegetative stage. The variations in nutrient concentration with organ were highly significant (p = < 0'01) as evaluated by the F-test (except for Ca in leaves). The difference in concentration between each date and any other date as evaluated by the L.S.D. test was in most cases significant. The L.S.D. test indicated that the difference in Ca concentration in both leaves and stems and that of Mg in roots were significant in all dates. The differences in Na and Mg concentration in the reproductive organs were also significant for all dates. The trends of temporal variation in the relative concentration of most elements in leaves and roots of M. crystallinum (calculated as a percentage of its maximum during the study period) (Fig. 1) were nearly parallel. The concentration increased from a minimum at the beginning of the growing season to a maximum at the early vegetative stage, except for Na and Ca in leaves which attained their maximum at the standing dead stage. An opposite trend was exhibited in the stems; the concentration attained a maximum at the beginning of the growing season, gradually decreased to a minimum at the end of the early vegetative stage, and then increased to high values at the late vegetative stage; Mg, however, attained its maximum at the late vegetative stage. Fluctuations in the concentrations of both Na and
MINERAL NUTRIENT CYCLING
221
K in the reproductive organs were small, but Ca increased and Mg decreased at the end of the growing season.
Nutrient content The nutrient content (mg m- z) was calculated by multiplying the nutrient concentration (mg g-l) by its dry weight (g m- z) at different phenological stages where each metre contains an average of 20 individuals (Fig. 2). The major contributions to the nutrient content in leaves of M. crystallinurn were those of Ca and Na. The maximum content of both elements was achieved at the standing dead stage. As may be expected, the two elements exhibited their minimum content at the beginning of the growing season when the biomass accumulation was low. Table 1. Variations in the mean concentration of different nutrients (mg e:' dry weight) in different organs of Mesembryanthemum crystallinum. Meanswithcommon letters arenotsignificantly different at the0·05 probability level Ca
Mg
Total ash
14'40 a 33'20 14·20 AB
26·20 67'40 26·00 AB
0'86 abc 3'34 0·38
388'7 399'4 158·7
37·40 ac 49'40 a" 39'66
17·40 be 29'40 25'78 C
29·80 47'00 48'16
1·08 ad 0'88 2'86
350·4 355'7 163'0
Lv St Rt Rd
50'00 d 28·90 17·00 A
26'10 18·50 a* 11'60 D
51'20 25·00 17'40 C
2·50 e 0·52 a* 1'82
350·7 328·9 169·0
17 April
Lv St Rt Rd
35·20 be 50'40 a' 16'40 A 50·20
16·20 bd 30'90 13·90 AE 21·90
43-00 55'80 18·00 C 34·50 A'
2'90 5'22 1·00 2·02
401'0 315'4 145·0 433·6
20 May
Lv St Rt Rd
40·00 44·40 26·80 54·50
14'60 ad 25·30 13·30 BDE 22·70 A'B'
25'20 40'60 24'00 D 37·80
0·80 bdf 2'31 0'72 2'88
499'8 360'1 149·9 466·0
1 July
Lv St Rt Rd
43-80 35·15 34'20 51'70
19·00 c 18·10 a' 21'00 22·50 A'C'
78'60 31'10 24'20 AD 34·00 A'
2·56 e 1·09 1·79 2'96
390'5 301·9 160·9 462·6
3 August
Lv St Rt Rd
51·50 d 39'70 38·00 53·20
22'40 26·50 24·70C 22'70 B*C*
81'80 39'20 26'10 B 59'00
0·66 cf 0·54 a' 2·00 1·00
361-3 308'1 160·0 465·2
Organ
Na
15 February
Lv St Rt Rd
35·00 ab 60'90 24·60
11 March
Lv St Rt Rd
28 March
Date
K
Lv, leaves; St, stems; Rt, roots; Rd, reproductive.
S. M. EL-DARIER
222
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Figure 1. Variations in the relative concentration (mg g-l) of nutrient elements in different organs of Mesembryanthernurn crystal/inurn. - - - . Leaves, _......... stems, • - - - roots, - - - - - • reproductive.
The stems attained their maximum contents of Na and Ca at the late vegetative stage, and that of K at the standing dead stage. In the roots, the maximum content of the three elements was attained at the standing dead stage. On the other hand, in the reproductive organs as well as in the leaves, Ca exhibited the highest content followed by Na and K at the end of the growing season. The Mg content exhibited a characteristic change with organ and time. The leaves attained the highest Mg content compared to all other organs, which also coincided with the reproductive organs in the time of their maximum content. The maximum content was attained at the beginning of the standing dead stage in July. The maximum content in stems was at the peak of vegetative activity in April, while in roots it was attained at the standing dead stage. The trend of temporal variations of total nutrients in the organ biomass of M. crystal/inurn may be indicated by their ash content (g m -2) (Fig. 3). The ash content (g m -2) of the roots was generally lower than that of all the other organs. The content in roots
MINERAL NUTRIENT CYCLING
223
exhibited a progressive increase from the minimum during the leafing-out stage to the maximum at the standing dead stage. The leaves attained much higher ash content compared to all other organs, followed by stems and reproductive organs. The leaves and stems attained their maximum values during the peak of vegetative activity, while the maximum in the reproductive organs and roots was attained at the standing dead stage. The contributions to the total ash content (g m - 2) differed with plant organ and Leaves
K
Na
+12·0
+24·0
+8·0
+8·0
Ca
+160
Mg
+08
+80
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Figure 2. Variations in the nutrient content of different organs of M. crystal/inurn in relation to the
annual mean content.
224
S. M. EL-DARIER
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Figure 3. Variations in the mean ash content (g m -2) and the contribution of the different organs to its total.
phenological stage. Leaves provided the largest contribution to the total ash content throughout the growing season (about 53% of the total ash content). During the early vegetative stage in March, the contribution by stems and roots was about 32% and 15% respectively. During the maximum vegetative activity and until the end of the growing season in August, the largest contribution was by leaves, followed by stems and reproductive organs (about 53%, 25% and 14%respectively), while the roots had a much lower contribution (about 6'6%). The mineral nutrient content of the litter is indicative of the rate with which nutrients are made available once more for plant uptake. The measure of such nutrient turnover rate is a good indicator of the mobility of different nutrients (Table 2). It is obvious that relatively large amounts of nutrients are present in the litter, especially ofCa, Na and K. The relative mobility of elements may be arranged as Ca > Na > K > Mg.
225
MINERAL NUTRIENT CYCLING
Table 2. Mean turnover rate, turnover time (year) and amount returned to the soil (g m- z year-I) for nutrients in theecosystem of Mesembryanthemum crystallinum
Turnover rate Turnover time Amount returned
to
the soil
Na
K
Ca
Mg
0'74 1'35 2'78
0·72 1'38
0·89 1'12 3·82
0'19 5'26 0'38
1'36
Nutrient uptakeand allocation The uptake of different nutrient elements by plant species during a certain time interval was calculated as the difference between the nutrient contents in the plant at the beginning and at the end of the interval. The nutrient uptake expressed as a ratio of the maximum attained in any month for Na, K, Ca and Mg exhibited two peaks (Fig. 4). The first peak was during April (maximum vegetative activity) and comprised the four elements which attained their maximum uptake during this month. The second peak was exhibited in July (the standing dead stage) and comprised mainly Ca and Mg. Characteristic trends of variation may be noted. The uptake of all elements increased sharply from early to late vegetative stage. Subsequently, Ca and Mg exhibited a sharp decline during the reproductive stage and then increased again to the second peak during the standing dead stage. On the other hand, Na and K decreased gradually to a low value during the standing dead stage and then increased again at the end of the growing season. The amount of annual uptake of each nutrient by M. crystallinurn expressed as a percentage of its total annual uptake of all nutrients, and the percentages of its allocation to
100
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Figure 4. Temporal variations in the nutrient uptake (mg m- 2 month - - - - Na, - - - - K, - ......... - Ca, - - . - - Mg.
t
')
of M. crystallinum.
s. M. EL-DARIER
226
K
No
Ca
St~'~'"' '
Roots~
Reproductive organs
Figure 5. Allocation of nutrient uptake (mg m -2 year-I) to the different organs of M. crystal/inurn.
different organs are presented graphically in Fig. 5. Allocation of the four elements to different organs varied markedly. In general, the allocation of these elements (Na, Ca, K, Mg) to leaves (about 46'91, 57'77, 40'51, 46'33% respectively) was higher than their allocation to other organs. The allocation to stems came second (about 27'08, 21'59, 31'31, 32'76%, respectively). The reproductive organs came third for Na, Ca and Mg (13'22, 12·33, 10'59%, respectively) and fourth for K. In general, Ca and Na contributed the major part of the amount taken up by this species (42'56 and 37·44 gm- 2 year-I) followed by K (about 18'86gm- 2 year-I), while the contribution by Mgwas much less (about 1'98 g m- 2 year'. Discussion Therophytes are characteristic of desert and agroecosystems. The main adaptational trait of these plants is their short life cycle. An adequate amount of rainfall (about 15-25 ern) is necessary for their germination; further rain is usually necessary for completion of their life cycle (Kershaw, 1973).
MINERAL NUTRIENT CYCLING
227
Knowledge of the nutrient dynamics of desert annuals is meagre, although there have been a considerable number of studies on perennial species. In the present study, the nutrient concentration in different organs of M. crystal/inum differed with phenological stage. The maximum concentration of most elements in the different organs was attained during the early or late vegetative stages, and declined to the minimum in the standing dead stage, with the exception of the less mobile elements Na and Ca in leaves, and K, Ca and Mg in reproductive organs, where their maxima were attained at the standing dead stage. Gaballah (1986) reported that the maximum concentration of most nutrients in the vegetative organs of four common annual species (Rumex pictus, Cutandia dichotoma, Ifioga spicata and Adonis dentatus) at El-Omayed, Egypt was achieved during the vegetative stage, while the minimum was just before the end of the growing season, except for Na and Ca in lfioga spicata which attained their maxima at the end of the growing season. Fakhry (1989) reported that N, P, K, Na, Ca and Mg exhibited their maximum concentration in annual species (pooled samples) in a desert ecosystem at the vegetative stage. These findings confirm the ability of the annual species to take advantage of favourable conditions of soil moisture. Such phenomena were interpreted by Millar et al. (1965). The higher concentration of Na and Ca in the vegetative organs at the standing dead stage indicates the ability of M. crystallinum to concentrate excess nutrients not in demand by plant organs in standing dead parts. Ca is the most immobile of the essential elements (Bollard, 1960) and is not readily redistributed in plants (Biddulph, 1959); older leaves may have large Ca reserves while younger leaves on the same plant exhibit a deficiency (Nason & McElroy, 1963 as quoted by William, 1969). Tolsma et al. (1987) also reported that in savanna species the macronutrient elements N, P and K are translocated out of the leaf long before leaf abscission, while Na, Ca and Fe are accumulated in older leaves. This trend was also demonstrated by other authors (e.g. Small, 1972; Abdel-Razik, 1980; Nilsen, 1981; El-Darier, 1984, 1988; Fakhry, 1989). Data on ash content can give some information about nutrient supply where the plant grows (Larcher, 1975). In the present study, the leaves produce most of the ash where the elements stored preferentially are K and Mg during the vegetative stage and Na and Ca in the standing dead stage. Stems and roots store mainly Na, K, Ca and Mg during the early vegetative stages, and reproductive organs contain relatively large amounts of Ca, K and Mg. Mesembryanthemum crystallinum does not exhibit major changes in nutrient allocation during development. For instance, the percentage of contribution by different organs to the total ash content did not alter substantially during the growing season, except for a gradual shift of biomass and nutrients to the reproductive structures. This may constitute an adaptation to the uncertain environment (Mulroy & Rundel, 1977). The total uptake of the different elements exhibited two distinct peaks, during the late vegetative and standing dead stages. The first was contributed by the four elements (K, Na, Ca, Mg), while the second was contributed by Ca and Mg. Gaballah (1986) presented similar results on annual species grown in desert ecosystems. Abdel-Razik (1980), in a study on the shrub Thymelaea hirsuta and the perennial herb Asphodelus microcarpus in the western desert of Egypt, reported that the total amount of nutrient taken up by the two species attained different peaks which comprised mainly Ca and K. Similar results were reported by Sharaf (1983) on the perennial grass Ammophila arenaria and the perennial subshrub Ononis vaginalis on the coastal dunes of the western desert of Egypt. These results as well as those of the present study may indicate that Ca is the only element with high uptake value in different life-forms, a phenomenon which may be considered a characteristic feature of desert plants growing in ecosystems with calcareous soils (Wallace & Romney, 1972; El-Ghonemy et al., 1977; Whittaker et ai., 1979). The fact that the nutrient elements are used in different proportions by the species, and that the ecosystem stability is related to nutrient cycling characteristics is not well recognised. Thus the depiction by models of nutrient cycling would help in understanding better this process and the rates by which elements are allocated in different plant parts or
228
S. M. EL-DARIER
returned to the environment. A nutrient cycling model was constructed as a set of in -and outflows (Fig. 6). The compartments represent the different components of the ecosystem under study. Each component includes a value for the mean biomass of the component (g m- 2) , and a set of values of the concentration ofNa, K, Ca and Mg (mg g-l) at maximum vegetative growth in April. The arrows represent the annual rates of flow between the components by uptake, translocation and restitution taking place in the ecosystem. The total annual uptake for Na, K, Ca and Mg was about 37'44,18'88,42'58 and 2'0 g m- 2 respectively. A percentage of the resulting uptake of these elements was retained by roots (about 13%, 17%,8% and 10% respectively), and the remainder was translocated and allocated to different organs of the shoot system. Most (about 54%, 48%, 63% and 52% respectively for Na, K, Ca, Mg) was allocated to the leaves. The stems also received quite
r----------------------------l Retoined
I I
I I II I I I
No = 4·95 K =2'13 Co =5·25 Mg=0·21 (g m-2 yeor-I)
I I
Retained No =17·54 K = 7·55 Co =24·60 Mg =0·92 (g m- z year-I)
Retained
I r
IL
Reproductive orgons (36gm-z) No = 50·2 Co = 34.5 K=21.9Mg=2.02 ( _I) mg g
No = 10·14 Co = 9·19 K = 5·90 Mg = 0·65 (g m- z yeor- I)
_
Tronslocotion rote(g m- z year-I) No = 32·65 K = 15·60 Co = 39·04 Mg = \·78
Restitution (g m- 2 yeor" )
Retoined Roots (51.4 g m- 2 ) No = 16·4 Co = 18·0 K=13·9Mg=I·0 (mg g-I)
(g m- 2 yecr'")
No K Co Mg
Returned =Content No = 27·95 K = \3·69 Co = 38·23 Mg = 0·39
Standing dead ports (581.2 q m- 2 ) No = 48·10 Co =65·78 K = 23·56 Mg = 00·67 (mil II-I)
= 4·78 = 3·27
=3·53
=0·20
Uptake (g m- 2 yecr'")
-. No K Co Mg
= 37·44 = 18·88 = 42·58 = 2·0
Soil minerolomoss (30
em
depth)
(39 x 104 ) g m- z No =0·892 Co = 5·21 K=4·52 Mg =0·21 (mg g-I)
Figure 6. Box-and-arrow diagram ofthe nutrient flux of M. crystallinum, (Figuresbetweenbrackets represent the biomass in g m- 2 of all flows; uptake, translocation and restitution are in g m- 2 year-I.)
MINERAL NUTRIENT CYCLING
229
large amounts (about 31%, 38%, 23% and 36% respectively), while the reproductive organs received the least (about 15%, 14%, 13% and 12% respectively). The turnover rate of different nutrients may be defined as the amount of nutrients returned from the plant to the soil pool by leaf fall, expressed as a percentage of total nutrient uptake. This measure of nutrient turnover shows that non-foliar tissues (roots and stems) transported to leaves the equivalent of 47%,40%, 58% and 46% of the total uptake ofNa, K, Ca and Mg at the maximum vegetative activity. These amounts increased to about 89%, 74% and 72% for Ca, Na and K and decreased to about 19% for Mg at the standing dead stage, and were subsequently released in litter. Generally, most trees transfer large quantities of Ca, returning about 55 to 95% of their annual Ca uptake to the forest floor in leaf fall (Hartmann, 1963); conifers usually return a smaller percentage of their annual uptake than do broad-leaf species. The turnover rates estimated in oak forest were about 34%, 19'7%,23'8% and 18·8% respectively for K, Ca, Mg and N (Rochow, 1975). Some species of sand dunes (Ammophila arenaria and Ononis vaginalis) return about 8% and 13% ofCa and about 4% and 37% ofK (Sharaf, 1983). Fakhry (1989) estimated the amount ofN returned to the soil pool as about 23% of the total N taken up for different life forms in the western desert of Egypt. The turnover time calculated in the present study was mostly lower than that calculated in several other ecosystems. Holmgren & Brewster (1972) calculated an average turnover time of 14 years for above-ground plant detritus in a salt desert shrub ecosystem in western Utah. About 4·1 years was calculated for litter in a taU-grass prairie (West, 1981). In a deciduous forest, Cromack (1973) calculated a 2·5 year turnover time for everything in the litter compartment except the phenols and stable carbon products. The present study indicates that M. crystallinum in the western Mediterranean desert of Egypt exhibits a degree of resilience (rapid turnover and recycling rates) in relation to its environment. The same conclusion was obtained by Fakhry (1989) for herbaceous species grown in the same region. The accumulation ofCa, Na and K is a prominent feature in the standing dead leaves of M. crystallinum. William (1969) reported that Ca content of foliage is an inherent species characteristics which is relatively independent of site. Beneficial effects of Ca addition to litter include neutralisation of decomposition products, improvement of soil physical conditions, and encouragement of active bacterial populations (Lutz & Chandler as quoted by William, 1969). Such modifications in the habitat characteristics inhibit the growth ofsome of the associated annuals, especially grasses, and stimulate greater invasion by M. crystallinum year after year. Therefore, one may suggest thatM. crystallinum has an important role in the Ca and Na economy of desert ecosystems. Such features of high nutrient uptake, high and rapid turnover rate, and modification of the site for its own benefit, make it possible for this species to survive the limited nutrient resources in desert ecosystems. I am very grateful to Professor M. Ayyad for revising the manuscript and making valuable comments.
References Abdel-Razik, M. S. (1980). A study on productivity and nutrient turn-over of somecommon plant species in the western Mediterranean desert of northern Egypt. Ph.D. Thesis, University of Alexandria. 89 pp. Allen, S., Grimsbay,H. M., Parkinson, J. A. & Quarmby, C. (1974). Chemical Analysis ofEcological Materials. Oxford and London: Blackwell. 565 pp. Biddulph, O. (1959). Translocationof inorganic solutes. In Plant Physiology, Vol. 2, pp. 553-603. New York: Academic Press. Bollard, E. G. (1960). Transport in the xylem. Annual Review of Plant Physiology. 11: 141-166.
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Cromack, K. (1973). Litter production and decomposition in a mixed hardwood watershed and a white pine watershed at Coweeta Hydrologic Station, N.C. Ph.D. Dissertation, University of Georgia, Athens. EI-Darier, S. M. O. (1984). Nutrient cycling in olive trees. M.Sc. Thesis, University of Alexandria. 93 pp. El-Darier, S. M. O. (1988). Nutrient cycling, energy flow and water relations ofthe ecosystem offig orchards in the western Mediterranean desert of Egypt. Ph.D. Thesis, University of Alexandria. 147 pp. El-Ghonemy, A. A., El-Gazzar, A. M. Wallace, A. & Romney, E. M. (1977). Mineral element composition of the natural vegetation along a transect at Mareotis, Egypt. Soil Science, 124: 16-26. Fakhry, A. M. (1989). Seasonal variations in nutrient contents in a plant community of the western Mediterranean region of Egypt at El-Ornayed. M.Sc. Thesis, University of Alexandria. 84 pp. Gaballah, M. S. (1986). A study on primary productivity and nutrient content of some therophytes in the non-saline depressions of the north-western coastal desert of Egypt. M.Sc. Thesis, University of Alexandria. 135 pp. Hartmann, F. (1963). Zur Frange der Nahrstoffbilanz im Waldboden. Allgemeine Forstzeitung, 74: 71-75. Holmgren, R. C. & Brewster, S. F. [r (1972). Distribution of organic matter reserve in a desert shrub community. USD A Forest Service Research Paper INT-30. Kershaw, K. A. (1973). Quantitative and Dynamic Plant Ecology, (2nd Edn) New York: Elsevier. 308 pp. Larcher, W. (1975). Physiological Plant Ecology. Berlin, Heidelberg and New York: SpringerVerlag. 252 pp. Millar,C. E., Turk, L. M. & Foth, H. O. (1965). Fundamentals ofSoil Science. New York, London: John Wiley. 491 pp. Mulroy, T. W. & Rundel, P. W. (1977). Annual plants; adaptations to desert environments. BioScience, 27: 109-114. Nilsen, E. T. (1981). Productivity and nutrient cycling in the early postburn chaparral species Lotus
scoparius. Proceedings of the Symposium on Dynamics and Management of Mediterranean-type Ecosystems. United States Department of Agriculture. Rochow, J. J. (1975). Mineral nutrient pool and cycling in a missouri forest. JournalofEcology, 63:
985-994. Sharaf, A. (1983). A study on nutrient cycling of some common species of the calcareous sand dune habitat in the Mediterranean coastal desert of Egypt. Ph.D. Thesis. Tanta University. 111 pp. Small, E. (1972). Photosynthetic rates in relation to nitrogen recycling as an adaptation to nutrient deficiency. Canadian Journal of Botany, 50: 2227-2233. Steel, G. D. & Torrie, J. H. (1960). Principles and Procedures ofStatistics. New York, Toronto and London: McGraw-Hill. 481 pp. Tackholm, V. (1974). Students'Flora of Eg)'pt. Cairo: Cairo University Press. 888 pp. Tolsma, D. J., Ernst, W. H. 0., Verweij, R. A. & Vooijs, R. (1987). Seasonal variation of nutrient concentrations in a semi-arid savanna ecosystem in Botswana. Journal of Ecology, 75: 755-770. Tyler, G., (1971). Distribution and turnover of organic matter and minerals in a shore meadow ecosystem. Gikos, 22: 265-291. UNESCO (MAB) (1977). Development of arid and semi-arid land. Obstacles and prospects. Technical Note 6: 42 pp. Wallace, A. & Romney, E. M. (1972). Radioecology and ecophysiology of desert plants at Nevada test site. U.S. AtomicEnergy Commission, Office of Information Services, TID. 25954. 439 pp. West, N. E. (1981). Nutrient cycling in desert ecosystems In Arid-Land Ecosystem: Structure, functioning and management, Vol 2: Goodal & Perry (Eds). Cambridge: Cambridge University Press. pp. 301-324. Whittaker, R. H., Likens, G. E., Bormann, F. H., Eaton, J. S. & Siccama, T. G. (1979). The Hubbard Brook ecosystem study: forest nutrient cycling. Ecology, 60: 203-220. William, A. T. (1969). Accumulation and cycling of calcium by dogwood trees. Ecological Monographs, 39: 101-118.
Mineral nutrient content and turnover rate of Mesembryanthemum crystallinum in the north-western desert of Egypt Salama M. EI-Darier Department ofBotany, Faculty of Science, Moharram Bay, Alexandria University, Alexandria, Egypt (Received 4 December 1989, accepted 24 July 1990) A knowledgeof the nutrient content of different plant organs is a necessary basis for the evaluationof the role of different nutrient elements in functioning of the ecosystem. The annual species Mesembryanthemum crystallinum concentrates excess Na, K and Ca in the vegetative organs at the standing dead stage. The total content of the different elements exhibited two distinct peaks. The first was during the late vegetative stage and included the elements Na, K, Ca and Mg while the second, during the standing dead stage, was made up of Ca and Mg. The turnover rate and time calculated in the present study indicate that M. crystallinum is well adapted to the environment of the western Mediterranean desert of Egypt. The accumulation of Ca and Na in the litter alters the habitat, inhibiting the growth of someof the associated annuals and permitting increased invasion by M. crystallinum year after year.
Introduction Annual species are a major component of desert plant communities. Several studies have reported on their production and composition, but very few have dealt with their role in the nutrient budgets of desert ecosystems (e.g. Gaballah, 1986). Information on the seasonal changes in the nutrient content of different plant organs is necessary for the elucidation of the role of different nutrient elements in the functioning of the ecosystem (Tolsma et al., 1987). The present study evaluates this role in the introduced annual species Mesembryanthemum crystallinum L. (iceplant) which is common in the mediterranean coastal lands of Egypt. The site selected for study is a transition area between the Abu-Sir ridge and the saline depression, 45 km west of Alexandria and 4 km north of Burg el-Arab village. The soil is a sandy loam. The pH of the soil solution ranges from 7'2 to 7'8, and the mean loss on ignition is about 2 '06% in samples collected at 30 em depth. The area belongs to the zone of 'arid climate with mild winters and warm summers' (UNESCO, 1977). The average annual rainfall is about ISO mm and falls mainly between November and March. The most common perennials in the study area are Eminium spiculatum (Blume) Ktze., Arisarum vulgare Targ.- Tozz., Launaea nudicaulis (L.) Hook. f., Plantago lanceolata L., Fagonia cretica L., Salvia lanigera Poiret, Plantago albicans L., Thymelaea hirsuta (L.) Endl., Asphodelus microcarpus Salzm. & Viv. ; and Salsola longifolia. Forssk. Mesembryanthemum crystallinum L., Plantago coronopus L., Anacydus alexandrinus Willd., Trigonella stellata Forssk. and Carthamus glaucus Bieb. are the most common annuals. Nomenclature follows Tackholm (1974). 0140-1963/92/030219 + 12 $03-00/0
© 1992 Academic Press
Limited
220
S. M. EL-DARIER
Methods Plant materials were sampled from 15 randomly distributed 30 x 30 em quadrats. The sampling period extended from winter (February) to late summer (August). At each harvest, samples of soil were collected at 30 em depth beneath the plants. Samples of plant material were washed, separated into different organs (roots, stems, leaves and reproductive parts), dried at 65°C, weighed and ground. The ground material was digested with triple acid reagent HN0 3 : H 2S04 : HCI0 4 10: 1: 1. The samples of soil were extracted with neutral molar ammonium acetate solution [NH 4 (COOCH 3 ) ) . The samples of plant material and soil were analysed for their content of K, Na, Ca and Mg using an atomic absorption spectrophotometer. The total ash content was determined by ashing the samples for 3 h at 500°C. All these procedures follow Allen et at. (1974). The turnover rate for each of these elements was calculated as the per cent of the total uptake of that element during the growing season. The turnover time is the reciprocal of turnover rate and is a measure of the time needed to make available the nutrients present in the litter. The turnover rate and time were calculated according to Tyler (1971). The phenological states (early vegetative, late vegetative, reproductive and standing dead) were recorded at each sampling. The percentages estimated for different sample quadrats are then used to calculate the average phenological state of the plant at each sampling. Analysis of variance was used to assess the significance of variation in the concentration of different nutrients with date and organ. The least significant difference (L.S.D.) test was then applied to evaluate the differences between the members of pairs of the mean concentration of different nutrients in different dates. All statistical tests follow Steel & Torrie (1960).
Results
Nutrient concentration The concentration of each of the nutrient elements (mg s': inM. crystallinum varied with organ and time (Table 1). In general, the stems attained the highest concentration of all elements. The reproductive organs attained high concentrations ofNa and Mg, while the leaves attained high concentrations of Ca and K. Stems and roots reached their highest concentrations ofNa, K and Ca in the early vegetative stage, but Ca reached a peak in the leaves and reproductive organs at the standing dead stage, and Mg peaked in the leaves and stems at the late vegetative stage. The variations in nutrient concentration with organ were highly significant (p = < 0'01) as evaluated by the F-test (except for Ca in leaves). The difference in concentration between each date and any other date as evaluated by the L.S.D. test was in most cases significant. The L.S.D. test indicated that the difference in Ca concentration in both leaves and stems and that of Mg in roots were significant in all dates. The differences in Na and Mg concentration in the reproductive organs were also significant for all dates. The trends of temporal variation in the relative concentration of most elements in leaves and roots of M. crystallinum (calculated as a percentage of its maximum during the study period) (Fig. 1) were nearly parallel. The concentration increased from a minimum at the beginning of the growing season to a maximum at the early vegetative stage, except for Na and Ca in leaves which attained their maximum at the standing dead stage. An opposite trend was exhibited in the stems; the concentration attained a maximum at the beginning of the growing season, gradually decreased to a minimum at the end of the early vegetative stage, and then increased to high values at the late vegetative stage; Mg, however, attained its maximum at the late vegetative stage. Fluctuations in the concentrations of both Na and
MINERAL NUTRIENT CYCLING
221
K in the reproductive organs were small, but Ca increased and Mg decreased at the end of the growing season.
Nutrient content The nutrient content (mg m- z) was calculated by multiplying the nutrient concentration (mg g-l) by its dry weight (g m- z) at different phenological stages where each metre contains an average of 20 individuals (Fig. 2). The major contributions to the nutrient content in leaves of M. crystallinurn were those of Ca and Na. The maximum content of both elements was achieved at the standing dead stage. As may be expected, the two elements exhibited their minimum content at the beginning of the growing season when the biomass accumulation was low. Table 1. Variations in the mean concentration of different nutrients (mg e:' dry weight) in different organs of Mesembryanthemum crystallinum. Meanswithcommon letters arenotsignificantly different at the0·05 probability level Ca
Mg
Total ash
14'40 a 33'20 14·20 AB
26·20 67'40 26·00 AB
0'86 abc 3'34 0·38
388'7 399'4 158·7
37·40 ac 49'40 a" 39'66
17·40 be 29'40 25'78 C
29·80 47'00 48'16
1·08 ad 0'88 2'86
350·4 355'7 163'0
Lv St Rt Rd
50'00 d 28·90 17·00 A
26'10 18·50 a* 11'60 D
51'20 25·00 17'40 C
2·50 e 0·52 a* 1'82
350·7 328·9 169·0
17 April
Lv St Rt Rd
35·20 be 50'40 a' 16'40 A 50·20
16·20 bd 30'90 13·90 AE 21·90
43-00 55'80 18·00 C 34·50 A'
2'90 5'22 1·00 2·02
401'0 315'4 145·0 433·6
20 May
Lv St Rt Rd
40·00 44·40 26·80 54·50
14'60 ad 25·30 13·30 BDE 22·70 A'B'
25'20 40'60 24'00 D 37·80
0·80 bdf 2'31 0'72 2'88
499'8 360'1 149·9 466·0
1 July
Lv St Rt Rd
43-80 35·15 34'20 51'70
19·00 c 18·10 a' 21'00 22·50 A'C'
78'60 31'10 24'20 AD 34·00 A'
2·56 e 1·09 1·79 2'96
390'5 301·9 160·9 462·6
3 August
Lv St Rt Rd
51·50 d 39'70 38·00 53·20
22'40 26·50 24·70C 22'70 B*C*
81'80 39'20 26'10 B 59'00
0·66 cf 0·54 a' 2·00 1·00
361-3 308'1 160·0 465·2
Organ
Na
15 February
Lv St Rt Rd
35·00 ab 60'90 24·60
11 March
Lv St Rt Rd
28 March
Date
K
Lv, leaves; St, stems; Rt, roots; Rd, reproductive.
S. M. EL-DARIER
222
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Figure 1. Variations in the relative concentration (mg g-l) of nutrient elements in different organs of Mesembryanthernurn crystal/inurn. - - - . Leaves, _......... stems, • - - - roots, - - - - - • reproductive.
The stems attained their maximum contents of Na and Ca at the late vegetative stage, and that of K at the standing dead stage. In the roots, the maximum content of the three elements was attained at the standing dead stage. On the other hand, in the reproductive organs as well as in the leaves, Ca exhibited the highest content followed by Na and K at the end of the growing season. The Mg content exhibited a characteristic change with organ and time. The leaves attained the highest Mg content compared to all other organs, which also coincided with the reproductive organs in the time of their maximum content. The maximum content was attained at the beginning of the standing dead stage in July. The maximum content in stems was at the peak of vegetative activity in April, while in roots it was attained at the standing dead stage. The trend of temporal variations of total nutrients in the organ biomass of M. crystal/inurn may be indicated by their ash content (g m -2) (Fig. 3). The ash content (g m -2) of the roots was generally lower than that of all the other organs. The content in roots
MINERAL NUTRIENT CYCLING
223
exhibited a progressive increase from the minimum during the leafing-out stage to the maximum at the standing dead stage. The leaves attained much higher ash content compared to all other organs, followed by stems and reproductive organs. The leaves and stems attained their maximum values during the peak of vegetative activity, while the maximum in the reproductive organs and roots was attained at the standing dead stage. The contributions to the total ash content (g m - 2) differed with plant organ and Leaves
K
Na
+12·0
+24·0
+8·0
+8·0
Ca
+160
Mg
+08
+80
+4·0 692
9·48
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-80
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4·0
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F M AM.\.A
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Mg
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Month
Figure 2. Variations in the nutrient content of different organs of M. crystal/inurn in relation to the
annual mean content.
224
S. M. EL-DARIER
240
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160
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May
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Figure 3. Variations in the mean ash content (g m -2) and the contribution of the different organs to its total.
phenological stage. Leaves provided the largest contribution to the total ash content throughout the growing season (about 53% of the total ash content). During the early vegetative stage in March, the contribution by stems and roots was about 32% and 15% respectively. During the maximum vegetative activity and until the end of the growing season in August, the largest contribution was by leaves, followed by stems and reproductive organs (about 53%, 25% and 14%respectively), while the roots had a much lower contribution (about 6'6%). The mineral nutrient content of the litter is indicative of the rate with which nutrients are made available once more for plant uptake. The measure of such nutrient turnover rate is a good indicator of the mobility of different nutrients (Table 2). It is obvious that relatively large amounts of nutrients are present in the litter, especially ofCa, Na and K. The relative mobility of elements may be arranged as Ca > Na > K > Mg.
225
MINERAL NUTRIENT CYCLING
Table 2. Mean turnover rate, turnover time (year) and amount returned to the soil (g m- z year-I) for nutrients in theecosystem of Mesembryanthemum crystallinum
Turnover rate Turnover time Amount returned
to
the soil
Na
K
Ca
Mg
0'74 1'35 2'78
0·72 1'38
0·89 1'12 3·82
0'19 5'26 0'38
1'36
Nutrient uptakeand allocation The uptake of different nutrient elements by plant species during a certain time interval was calculated as the difference between the nutrient contents in the plant at the beginning and at the end of the interval. The nutrient uptake expressed as a ratio of the maximum attained in any month for Na, K, Ca and Mg exhibited two peaks (Fig. 4). The first peak was during April (maximum vegetative activity) and comprised the four elements which attained their maximum uptake during this month. The second peak was exhibited in July (the standing dead stage) and comprised mainly Ca and Mg. Characteristic trends of variation may be noted. The uptake of all elements increased sharply from early to late vegetative stage. Subsequently, Ca and Mg exhibited a sharp decline during the reproductive stage and then increased again to the second peak during the standing dead stage. On the other hand, Na and K decreased gradually to a low value during the standing dead stage and then increased again at the end of the growing season. The amount of annual uptake of each nutrient by M. crystallinurn expressed as a percentage of its total annual uptake of all nutrients, and the percentages of its allocation to
100
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80
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Figure 4. Temporal variations in the nutrient uptake (mg m- 2 month - - - - Na, - - - - K, - ......... - Ca, - - . - - Mg.
t
')
of M. crystallinum.
s. M. EL-DARIER
226
K
No
Ca
St~'~'"' '
Roots~
Reproductive organs
Figure 5. Allocation of nutrient uptake (mg m -2 year-I) to the different organs of M. crystal/inurn.
different organs are presented graphically in Fig. 5. Allocation of the four elements to different organs varied markedly. In general, the allocation of these elements (Na, Ca, K, Mg) to leaves (about 46'91, 57'77, 40'51, 46'33% respectively) was higher than their allocation to other organs. The allocation to stems came second (about 27'08, 21'59, 31'31, 32'76%, respectively). The reproductive organs came third for Na, Ca and Mg (13'22, 12·33, 10'59%, respectively) and fourth for K. In general, Ca and Na contributed the major part of the amount taken up by this species (42'56 and 37·44 gm- 2 year-I) followed by K (about 18'86gm- 2 year-I), while the contribution by Mgwas much less (about 1'98 g m- 2 year'. Discussion Therophytes are characteristic of desert and agroecosystems. The main adaptational trait of these plants is their short life cycle. An adequate amount of rainfall (about 15-25 ern) is necessary for their germination; further rain is usually necessary for completion of their life cycle (Kershaw, 1973).
MINERAL NUTRIENT CYCLING
227
Knowledge of the nutrient dynamics of desert annuals is meagre, although there have been a considerable number of studies on perennial species. In the present study, the nutrient concentration in different organs of M. crystal/inum differed with phenological stage. The maximum concentration of most elements in the different organs was attained during the early or late vegetative stages, and declined to the minimum in the standing dead stage, with the exception of the less mobile elements Na and Ca in leaves, and K, Ca and Mg in reproductive organs, where their maxima were attained at the standing dead stage. Gaballah (1986) reported that the maximum concentration of most nutrients in the vegetative organs of four common annual species (Rumex pictus, Cutandia dichotoma, Ifioga spicata and Adonis dentatus) at El-Omayed, Egypt was achieved during the vegetative stage, while the minimum was just before the end of the growing season, except for Na and Ca in lfioga spicata which attained their maxima at the end of the growing season. Fakhry (1989) reported that N, P, K, Na, Ca and Mg exhibited their maximum concentration in annual species (pooled samples) in a desert ecosystem at the vegetative stage. These findings confirm the ability of the annual species to take advantage of favourable conditions of soil moisture. Such phenomena were interpreted by Millar et al. (1965). The higher concentration of Na and Ca in the vegetative organs at the standing dead stage indicates the ability of M. crystallinum to concentrate excess nutrients not in demand by plant organs in standing dead parts. Ca is the most immobile of the essential elements (Bollard, 1960) and is not readily redistributed in plants (Biddulph, 1959); older leaves may have large Ca reserves while younger leaves on the same plant exhibit a deficiency (Nason & McElroy, 1963 as quoted by William, 1969). Tolsma et al. (1987) also reported that in savanna species the macronutrient elements N, P and K are translocated out of the leaf long before leaf abscission, while Na, Ca and Fe are accumulated in older leaves. This trend was also demonstrated by other authors (e.g. Small, 1972; Abdel-Razik, 1980; Nilsen, 1981; El-Darier, 1984, 1988; Fakhry, 1989). Data on ash content can give some information about nutrient supply where the plant grows (Larcher, 1975). In the present study, the leaves produce most of the ash where the elements stored preferentially are K and Mg during the vegetative stage and Na and Ca in the standing dead stage. Stems and roots store mainly Na, K, Ca and Mg during the early vegetative stages, and reproductive organs contain relatively large amounts of Ca, K and Mg. Mesembryanthemum crystallinum does not exhibit major changes in nutrient allocation during development. For instance, the percentage of contribution by different organs to the total ash content did not alter substantially during the growing season, except for a gradual shift of biomass and nutrients to the reproductive structures. This may constitute an adaptation to the uncertain environment (Mulroy & Rundel, 1977). The total uptake of the different elements exhibited two distinct peaks, during the late vegetative and standing dead stages. The first was contributed by the four elements (K, Na, Ca, Mg), while the second was contributed by Ca and Mg. Gaballah (1986) presented similar results on annual species grown in desert ecosystems. Abdel-Razik (1980), in a study on the shrub Thymelaea hirsuta and the perennial herb Asphodelus microcarpus in the western desert of Egypt, reported that the total amount of nutrient taken up by the two species attained different peaks which comprised mainly Ca and K. Similar results were reported by Sharaf (1983) on the perennial grass Ammophila arenaria and the perennial subshrub Ononis vaginalis on the coastal dunes of the western desert of Egypt. These results as well as those of the present study may indicate that Ca is the only element with high uptake value in different life-forms, a phenomenon which may be considered a characteristic feature of desert plants growing in ecosystems with calcareous soils (Wallace & Romney, 1972; El-Ghonemy et al., 1977; Whittaker et ai., 1979). The fact that the nutrient elements are used in different proportions by the species, and that the ecosystem stability is related to nutrient cycling characteristics is not well recognised. Thus the depiction by models of nutrient cycling would help in understanding better this process and the rates by which elements are allocated in different plant parts or
228
S. M. EL-DARIER
returned to the environment. A nutrient cycling model was constructed as a set of in -and outflows (Fig. 6). The compartments represent the different components of the ecosystem under study. Each component includes a value for the mean biomass of the component (g m- 2) , and a set of values of the concentration ofNa, K, Ca and Mg (mg g-l) at maximum vegetative growth in April. The arrows represent the annual rates of flow between the components by uptake, translocation and restitution taking place in the ecosystem. The total annual uptake for Na, K, Ca and Mg was about 37'44,18'88,42'58 and 2'0 g m- 2 respectively. A percentage of the resulting uptake of these elements was retained by roots (about 13%, 17%,8% and 10% respectively), and the remainder was translocated and allocated to different organs of the shoot system. Most (about 54%, 48%, 63% and 52% respectively for Na, K, Ca, Mg) was allocated to the leaves. The stems also received quite
r----------------------------l Retoined
I I
I I II I I I
No = 4·95 K =2'13 Co =5·25 Mg=0·21 (g m-2 yeor-I)
I I
Retained No =17·54 K = 7·55 Co =24·60 Mg =0·92 (g m- z year-I)
Retained
I r
IL
Reproductive orgons (36gm-z) No = 50·2 Co = 34.5 K=21.9Mg=2.02 ( _I) mg g
No = 10·14 Co = 9·19 K = 5·90 Mg = 0·65 (g m- z yeor- I)
_
Tronslocotion rote(g m- z year-I) No = 32·65 K = 15·60 Co = 39·04 Mg = \·78
Restitution (g m- 2 yeor" )
Retoined Roots (51.4 g m- 2 ) No = 16·4 Co = 18·0 K=13·9Mg=I·0 (mg g-I)
(g m- 2 yecr'")
No K Co Mg
Returned =Content No = 27·95 K = \3·69 Co = 38·23 Mg = 0·39
Standing dead ports (581.2 q m- 2 ) No = 48·10 Co =65·78 K = 23·56 Mg = 00·67 (mil II-I)
= 4·78 = 3·27
=3·53
=0·20
Uptake (g m- 2 yecr'")
-. No K Co Mg
= 37·44 = 18·88 = 42·58 = 2·0
Soil minerolomoss (30
em
depth)
(39 x 104 ) g m- z No =0·892 Co = 5·21 K=4·52 Mg =0·21 (mg g-I)
Figure 6. Box-and-arrow diagram ofthe nutrient flux of M. crystallinum, (Figuresbetweenbrackets represent the biomass in g m- 2 of all flows; uptake, translocation and restitution are in g m- 2 year-I.)
MINERAL NUTRIENT CYCLING
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large amounts (about 31%, 38%, 23% and 36% respectively), while the reproductive organs received the least (about 15%, 14%, 13% and 12% respectively). The turnover rate of different nutrients may be defined as the amount of nutrients returned from the plant to the soil pool by leaf fall, expressed as a percentage of total nutrient uptake. This measure of nutrient turnover shows that non-foliar tissues (roots and stems) transported to leaves the equivalent of 47%,40%, 58% and 46% of the total uptake ofNa, K, Ca and Mg at the maximum vegetative activity. These amounts increased to about 89%, 74% and 72% for Ca, Na and K and decreased to about 19% for Mg at the standing dead stage, and were subsequently released in litter. Generally, most trees transfer large quantities of Ca, returning about 55 to 95% of their annual Ca uptake to the forest floor in leaf fall (Hartmann, 1963); conifers usually return a smaller percentage of their annual uptake than do broad-leaf species. The turnover rates estimated in oak forest were about 34%, 19'7%,23'8% and 18·8% respectively for K, Ca, Mg and N (Rochow, 1975). Some species of sand dunes (Ammophila arenaria and Ononis vaginalis) return about 8% and 13% ofCa and about 4% and 37% ofK (Sharaf, 1983). Fakhry (1989) estimated the amount ofN returned to the soil pool as about 23% of the total N taken up for different life forms in the western desert of Egypt. The turnover time calculated in the present study was mostly lower than that calculated in several other ecosystems. Holmgren & Brewster (1972) calculated an average turnover time of 14 years for above-ground plant detritus in a salt desert shrub ecosystem in western Utah. About 4·1 years was calculated for litter in a taU-grass prairie (West, 1981). In a deciduous forest, Cromack (1973) calculated a 2·5 year turnover time for everything in the litter compartment except the phenols and stable carbon products. The present study indicates that M. crystallinum in the western Mediterranean desert of Egypt exhibits a degree of resilience (rapid turnover and recycling rates) in relation to its environment. The same conclusion was obtained by Fakhry (1989) for herbaceous species grown in the same region. The accumulation ofCa, Na and K is a prominent feature in the standing dead leaves of M. crystallinum. William (1969) reported that Ca content of foliage is an inherent species characteristics which is relatively independent of site. Beneficial effects of Ca addition to litter include neutralisation of decomposition products, improvement of soil physical conditions, and encouragement of active bacterial populations (Lutz & Chandler as quoted by William, 1969). Such modifications in the habitat characteristics inhibit the growth ofsome of the associated annuals, especially grasses, and stimulate greater invasion by M. crystallinum year after year. Therefore, one may suggest thatM. crystallinum has an important role in the Ca and Na economy of desert ecosystems. Such features of high nutrient uptake, high and rapid turnover rate, and modification of the site for its own benefit, make it possible for this species to survive the limited nutrient resources in desert ecosystems. I am very grateful to Professor M. Ayyad for revising the manuscript and making valuable comments.
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