the impacts of parent material and landscape position on drought and biomass production of wheat...

13
SOIL TECHNOLOGY vol. 6, p. 337-349 Cremlingen 1993 ] The Impacts of Parent Material and Landscape Position on Drought and Biomass Production of Wheat Under Semi-Arid Conditions C.S. Kosmas, N.G. Danalatos, N. Moustakas, B. Tsatiris Ch. Kallianou & N. Yassoglou Sunnnary The effect of drought on the biomass production of rainfed wheat was inves- tigated along catenas in the semi-arid climatic zone of Greece. These catenas are located in hilly areas with rolling to- pography and soils formed on Tertiary and Quaternary deposits of marl, con- glomerates and shale-sandstones. To- tal above ground biomass production was measured on specific hillslope com- ponents (shoulder, backslope and loots- lope) in three successive growing peri- ods and was related to the soil prop- erties, landscape position and climatic data. Crop-water use, calculated ac- cording to a broadly used simplified methodology, was logarithmically corre- lated with biomass production. Gravel and stones on the soil surface appeared to be extremely important in dry years by conserving appreciable amounts of soil water from evaporation through sur- face mulching and preventing large ar- eas from desertification. Stony soils along slope catenas of conglomerates and shale-sandstones, despite their nor- ISSN 0933-3630 (~)1993 by CATENA VERLAG, 38162 Cremlingen-Destedt, Germany 0933-3630/93/5011851/US$ 2.00 + 0.25 mally low productivity, may supply ap- preciable amounts of previously stored water to the stressed plants and to se- cure a not negligible biomass produc- tion even in extremely dr/years. Soils formed on marl are free of coarse frag- ments and despite their considerable depth and high productivity in normal and wet years, they are very susceptible to desertification, being unable to sup- port any vegetation in particularly dry years due to adverse soil physical proper- ties and the absence of gravel and stone mulching. 1 Introduction There are evidences that the earth is experiencing a long-timescale warming as a result of increasing concentrations of carbon dioxide and other radiatively active gasses in the atmosphere (Parry & Carter 1991). The expected cli- matic changes might affect both the to- tal biomass production and the geo- graphic distribution of natural, semi- natural and man-made agricultural sys- tems (Grainger 1992), especially in the arid and semi-arid areas of the Mediter- ranean Region. Dr/land soils on hilly areas are particularly vulnerable to ero- SOIL TECHNOLOGY--A cooperating Journal of CAT~NA

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Page 1: The impacts of parent material and landscape position on drought and biomass production of wheat under semi-arid conditions

SOIL TECHNOLOGY vol. 6, p. 337-349 Cremlingen 1993 ]

T h e I m p a c t s of Parent Mater ia l and Landscape Pos i t ion

on D r o u g h t and B iomass P r o d u c t i o n of W h e a t U n d e r Semi -Ar id Condi t ions

C.S. Kosmas, N.G. Danalatos, N. Moustakas, B. Tsatiris Ch. Kallianou & N. Yassoglou

S u n n n a r y

The effect of drought on the biomass production of rainfed wheat was inves- tigated along catenas in the semi-arid climatic zone of Greece. These catenas are located in hilly areas with rolling to- pography and soils formed on Tertiary and Quaternary deposits of marl, con- glomerates and shale-sandstones. To- tal above ground biomass production was measured on specific hillslope com- ponents (shoulder, backslope and loots- lope) in three successive growing peri- ods and was related to the soil prop- erties, landscape position and climatic data. Crop-water use, calculated ac- cording to a broadly used simplified methodology, was logarithmically corre- lated with biomass production. Gravel and stones on the soil surface appeared to be extremely important in dry years by conserving appreciable amounts of soil water from evaporation through sur- face mulching and preventing large ar- eas from desertification. Stony soils along slope catenas of conglomerates and shale-sandstones, despite their nor-

ISSN 0933-3630 (~)1993 by CATENA VERLAG, 38162 Cremlingen-Destedt, Germany 0933-3630/93/5011851/US$ 2.00 + 0.25

mally low productivity, may supply ap- preciable amounts of previously stored water to the stressed plants and to se- cure a not negligible biomass produc- tion even in extremely d r / y e a r s . Soils formed on marl are free of coarse frag- ments and despite their considerable depth and high productivity in normal and wet years, they are very susceptible to desertification, being unable to sup- port any vegetation in particularly dry years due to adverse soil physical proper- ties and the absence of gravel and stone mulching.

1 I n t r o d u c t i o n

There are evidences that the earth is experiencing a long-timescale warming as a result of increasing concentrations of carbon dioxide and other radiatively active gasses in the atmosphere (Parry & Carter 1991). The expected cli- matic changes might affect both the to- tal biomass production and the geo- graphic distribution of natural, semi- natural and man-made agricultural sys- tems (Grainger 1992), especially in the arid and semi-arid areas of the Mediter- ranean Region. Dr/ land soils on hilly areas are particularly vulnerable to ero-

SOIL TECHNOLOGY--A cooperating Journal of CAT~NA

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338 Kosmas, Danalatos, Moustakas, Tsatiris, Kallianou & Yassoglou

sion, especially when their vegetation cover has been degraded.

Rainfed wheat production on hilly lands in the semi-arid climatic zone of Greece is largely dependent on the amount and distribution of precipita- tion. Hilly soils on Tertiary and Qua- ternary formations of marl, conglomer- ates and shale-sandstones usually have a restricted effective soil depth due to erosion and limiting subsurface layers such as petrocalcic horizon, gravelly and stony layer and/or shallow bedrock. Therefore, the tolerance of these soils to erosion is low and, under hot and dry cli- matic conditions and severe soil erosion, rainfed vegetation can no longer be sup- ported, leading to desertification (Yas- soglou 1989). The vulnerability of hilly lands to desertification also depends on their landscape position and aspect.

Landscape position and degree of ero- sion are closely connected (Daniels et al. 1985). Douglas et al. (1984) demonstrated that landscape position and slope aspect greatly affected win- ter wheat yields in eastern Oregon and Washington. Stone et al. (1985) found that significant differences in corn yield among landscape positions were much more consistent among landscape posi- tions than erosion classes. Because of lateral surface and subsurface flow wa- ter is not evenly distributed in the land- scape positions during rainfall. Gary et al. (1991) demonstrated that introduc- tion of a topographic factor in the wheat yield-total water use relation increased the correlation coefficient.

Sloping soils on conglomerate and shale-sandstone hills usually contain high amounts of stones and gravel. Pre- vious studies have shown that rock frag- ments can hold substantial amounts of water, which are available to plants

{Hanson & Blevins 1979). Ravina & Magier (1984) concluded that coarse fragments contribute to improved soil physical conditions by acting as "skele- ton" which resists to soil compaction. In a study by Magier & Ravina (1984), or- chard trees planted on soils with high amounts of rock fragments were larger, had a better developed root system and produced higher yields than on soils without rock fragments.

The purpose of the present work is to investigate the relationship among biomass production of rainfed wheat and soil properties associated with different parent material and landscape position such as soil depth, texture, gravel and stone content, available soil water and degree of erosion; and under semi-arid conditions such as those prevailing in southeastern Greece.

2 M a t e r i a l s a n d m e t h o d s

The study was conducted in three lo- cations along catenas, about 100 km north of Athens. All sites are situ- ated in hilly areas with rolling topog- raphy and soils formed on Tertiary and Quaternary deposits of marl, conglomer- ates, and shale-sandstones. Thirty eight soil sites were selected along the study catenas on distinct landscape positions which were identified as shoulder, back- slope and footslope, classified according to Ruhe's scheme (1960). At each sam- pling site, the landscape position, slope grade, rooting depth, percent of gravel and stones, presence of limiting soil hori- zons and the depth to the bedrock were measured. Soil samples were addition- ally collected from each soil horizon for laboratory determinations.

The soil samples were analyzed for organic C using the modified Walkey-

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Biomass production wheat, semi-arid conditions 339

Black wet oxidation procedure (Nelson & Sommers 1982). The Bouyoucos hy- drometer method was used for the tex- tural analysis. Water retention from 0 to -33kPa (the last value commonly taken as the field capacity) was measured in undisturbed soil samples and from -300 to -1500 kPa (the latter taken as the per- manent wilting point) in disturbed sam- ples. The amount of available water to the plants was then determined as the difference between water retained at -33 and -1500 kPa (Klute 1986). The effec- tive rooting depth was calculated from the observed rooting depth, corrected for the volumetric content of the coarse fragments.

The total above ground biomass pro- duction (TAGBP) of wheat was mea- sured in all 38 soil sites at the end of three successive growing periods of 1989-90, 1990-91 and 1991-92. From each site, three plots of 1 m 2 each were harvested, and the total biomass was weighed in the spot. Then subsamples were taken to the laboratory and dried at 65°C for moisture content correction.

Daily meteorological data were sup- plied by the nearby Meteorological Sta- tion of Tanagra (Greek National Mete- orological Service). The studied sites, situated at a distance of 2-18 km, be- long to the Thiessen polygon of this sta- tion. The potential evapotranspiration rate (ETo) was calculated from daily val- ues of maximum and minimum air tem- perature, sunshine duration, air humid- ity and wind speed, according to Pen- man (1948; modified by Fr~re 1979). The maximum evapotranspiration rate (ETm) was determined by the poten- tial evapotranspiration rate and the crop (leaf area) coefficient of wheat according to F.A.O. (1977).

In an a t tempt to estimate the actual

evapotranspiration rate (ETa) and to re- late it with the measured biomass pro- duction, a simple water balance model was used to calculate ETa from the mo- mentary soil moisture content in the root zone. The employed algorithm fol- lows the F.A.O. approach (Doorenbos et al. 1977, 1979). The model uses daily values of rainfall and potential evapo- transpiration rates, measured soil pa- rameters such as the field capacity and the permanent wilting point; and crop parameters (coefficient of crop cover, ef- fective rooting depth, depletion fraction of the total soil water storage, the actual crop calendar), and calculates the value of actual evapotranspiration (ETa).

3 R e s u l t s a n d d i s c u s s i o n

3.1 Soi l p r o p e r t i e s

Parent material and landscape position and topography are the main varying soil formation factors, since the study soils were formed under the same cli- matic conditions. The main physical and chemical properties of the plow layer of all study soils are summarized in tab. 1.

In the shale-sandstone catenas, the low carbonate content of the parent ma- terial and the prolonged stability ad- vanced the formation of soils with an argillic horizon. However, the intensive cultivation in the last century resulted in accelerated erosion and finally in the removal of the greater part of the soil and the formation of the present shal- low, very stony and gravelly soils on the shoulders and backslopes to moderately deep to deep, slightly gravelly soils on the footslopes (tab. 1).

Erosion on the lands with conglomer- ates and marl is less pronounced, despite

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340 Kosmas, Da.alatos, Moustakas, Tsatiris, Kallianou & Yassoglou

Soil land- slope clay O.M. CaCOz coarse CEC P Bulk FC** W P scape frag- den- posi- men t si ty t ion* % % % % v / v c m o l / k g m g / k g g / c m a v / v v / v

SHALE-SANDSTONES

B S S l l S 19.7 28.8 2 8 0 26.0 27.5 20.6 1 19 31.5 19 6 BSS13 S 22.1 29 8 2 6 0 26.9 23 6 23.2 1 22 32.1 20 4 BSS4 S 26 7 33.8 2 1 0 26 6 30.0 26.3 1.28 38.9 24.9 BSS7 S 10.2 26 6 2.4 0 29.9 22.5 25 6 1.33 36 1 19.3 BSS14 S 29.5 28.4 2.6 0 24.4 25 2 38 6 1.31 30 0 18.7 BSSI2 S 23.7 27.2 2 9 0 27 5 22 7 27 2 1 32 31.3 19 7 BSB5 B 28 0 31.2 2.6 0 27 4 27.2 15 1 1.27 33.6 21.4 BSB22 B 24 5 29.2 1 9 0 25.6 24.3 20.6 1 37 32.1 22.6 B S B I 5 B 27.5 40.4 2.3 0 19.4 32 1 37.5 1 22 40.9 24.3 BSB14 B 27.5 23 8 2.6 0 26.8 19.5 19.0 1.28 33.5 21.1 BSF31 F 4 7 38.9 1 7 0 10.8 31 2 11.0 1 18 34.9 22.4 BSF6 F 5 5 32.6 1 9 0 18.1 29.5 12 0 1 19 33.0 21.9 BSF16 F 9.2 44.3 1.7 0 11 1 30.8 34 1 1.27 39 3 23 2 BSF17 F 4.5 42.9 1 7 0 7 6 31 5 30 8 1 31 39.4 22 1

M A R L

BMS14 S 11.2 51.5 2.8 32 4 0 37 3 5.9 1 56 35 1 18 7 BMS18 S 9 5 49.4 2 0 ~25 4 0 27 1 3.2 1 45 33.4 18.6 BMS22 S 8.8 26.5 2 1 21 5 0 16 9 3.7 1 61 21.4 10.3 BMS24 S 10.1 39.4 2 1 30 6 0 34 6 4.2 1 55 31 4 14 3 BMB19 B 11 5 43 9 2.1 29.5 0 23 9 5 3 1.42 30 4 15 3 BMB15 B 13 6 61.5 2.7 50.0 0 26.5 9 0 1 53 33.1 19 5 BMBI6 B 9.7 37.4 2.1 19.7 0 23.0 8 7 1.53 30 1 14 9 BMB23 B 9.1 41.2 2.2 31.3 0 27.6 3 9 1 52 31 9 13 9 B M F 1 7 F 4 5 39.7 2.9 11 5 0 25.0 19 9 1 58 31 5 16 5 BMF20 F 3.5 44.5 2 5 25 6 0 27 5 7 6 1.49 32 3 16 3 BMF21 F 5 5 42 8 2.6 32 8 0 29 3 2.2 1.47 28 8 17.6 BMF25 F 4 5 43.5 2.9 27.8 0 30 2 5 7 1.57 29.7 13.8

C O N G L O M E R A T E S

BCS22 S 7.8 38.7 1.5 45 9 22.7 20.6 11 8 1.27 29 6 9.7 BCS25 S 7.2 41.9 1 8 45.5 9 4 30.8 4.2 1 22 29.9 12 0 BCS28 S 12 4 37.2 1.4 37 2 23 5 24.2 5 6 1 28 27 6 11 2 BCF30 S 12.5 39.3 1.6 39 7 21 3 20.5 4 3 1.25 28.5 14 2 BCB23 B 16 7 39.4 1 2 42 6 17.7 20 2 5.6 1 31 28.5 12 7 BCB24 B 11.5 37.5 1 6 29 3 11.9 26 5 2 9 1 26 28.6 13.6 BCB29 B 13.2 44 2 1.5 34.3 18.7 27.3 7 3 1 29 31.5 17.9 BCS31 B 9.8 35.1 1 7 35.2 19.6 22 3 7 8 1 24 26.5 12.5 BCB26 B 14.6 40.8 1.4 45 1 15.9 23.9 10.2 1 25 29.4 12.7 B C F 2 7 F 3.5 48.4 1 7 28.5 10.2 33 2 10 6 1 31 33.7 18.0 BCF32 F 4.2 44.2 1.8 23 5 12.3 25 6 11 1 1.27 32 2 17 7 BCF33 F 5 5 48.3 1.5 26 2 10.5 27.3 4.5 1 28 33.6 18.7

* S -- shoulder , B = b~-,.kslope, F ---- footslope. ** FC = field capaci ty , W P = wi t l ing po in t

T a b . 1: Chemical and physical properties of the plow layer of the studied 8o,ls.

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Biomass production wheat, semi-arid conditions 341

the intensive cultivation. The soils on conglomerates are stony, more perme- able and resistant to erosion than soils on marl. They are moderately deep to deep in the shoulders and backslopes, to deep or very deep in the footslopes.

The soils on marl are fine-textured, calcareous, more compacted, and less permeable than the soils on the shale- sandstone and conglomerate catenas. They are free of gravel and stones and axe normally more susceptible to ero- sion. The observed relatively low ero- sion rates are therefore attributed to their less steep slopes (tab. 1). The soils are moderately deep to deep with a topsoil relatively rich in organic mat- ter and generally more fertile than the gravelly and stony counterparts on the shale-sandstone and conglomerate hills.

8 . 2 Climat ic condi t ions

As mentioned above, the study areas be- long to the semi-arid climatic zone of Greece. However, the large variation in the weather conditions prevailed dur- ing the 3 study periods 1989-90 through 1991-92 made easier the assessment of the main factors affecting the biomass production of rainfed wheat along the various hillslope components, and of the desertification risk in dry years.

The principle climatic parameter af- fecting crop growth and biomass produc- tion can be expressed by simple indices such as the difference between poten- tial evapotranspiration rate (ETo) and rainfall (R). The 10-days average R and ETo values for the three growing peri- ods of wheat are schematically presented in fig. 1. It is apparent that evapo- transpiration largely exceeded rainfall throughout the growing period in 1989- 90 (viz. ETo = 370 vs. R = 95 ram),

whereas in the following growing period the opposite occurred. Actually, based on the long term average climatic data of the area studied, the first year was ex- tremely dry (R = 95 mm vs 370 mm in an average year). The following growing period 1990-91 was exceptionally wet; the rainfall (R = 663 ram) exceeded the potential evapotranspiration and only at the end of this growing period a slight water deficit occurred (fig. 1). Only the third growing period 1991-92 was a typical one for the study area with a total precipitation of 389 ram. As fig. 1 shows, water surplus occurred in the winter months, whereas later in the spring, rainfall lagged considerably be- hind potential evapotranspiration.

8 .8 B i o m a s s produc t ion

The total average above ground biomass production (TAGBP) of wheat is schematically presented in fig. 2 for the various landscape positions / parent ma- terials and for the three growing peri- ods. The observed large year-to-year fluctuation in TAGBP which holds for all parent materials and landscape posi- tions (sign. at a = 0.001) points to the climate and particularly to the crop wa- ter use as the factor most responsible for the biomass production of wheat in the studied areas.

The differences in biomass production observed among soils with the same par- ent material but on different landscape positions were highly significant during the normal and the wet years. The same holds for the gravelly and stony soils (shale-sandstones and conglomer- ates) during the dry year. In that year the biomass production on the marly catenas was almost hill and indepen- dent on the landscape position. Appar-

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342 Kosmas, Danalatos, Moustakas, Tsatiris, Kallianou & Yassoglou

rainfall ~ evapotr/Uon

o I i - -

9 -

a-: 7-" 6-" s-" 4-" 3-" 2-" 1" 0

1 9 8 9 - 9 0

,0,0,0 00, UH 111 H t 1H N D J F M A M

1 9 9 0 - 9 1

4

3

0 I I

N D d F M A M

11 1 9 9 1 - 9 2

41 3~ 2" 1-

o I I

N D J F M A M Month

Fig. 1: Rainfall and potential euapotranspiration in the study area d~ring the three growing seasons o f wheat.

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BJomass productio, wheat, semi-arid conditio.s 343

/ Shoulder ~ Backslope [-----7 Footslope

2 0 0 0 0

1 6 0 0 0

1 2 0 0 0

8000

4000

0

20000

, ~ 1 6 0 0 0 m J=

a¢ 1 2 0 0 0

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20000

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Shale-sandstones

• ;,¢:.:.:::"

n

i j

W Conglomerates

d 1 9 8 9 - 9 0 1 9 9 0 - 9 1

Growing period 1 9 9 1 - 9 2

Fig. 2: The effect of parent material and landscape position on the total above ground biomass production o[ wheat for three growing seasons.

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344 Kosmas, Danalatos, Moustakas, Tsatiris, KMlianou & Yassoglou

• 1 9 8 9 - 9 0 • 1 9 9 0 - 9 1 0 1 9 9 1 - 9 2

2 0 0 0 0

A

¢8 r,, 1 5 0 0 0

v

10000 ¢n E 0

• - 5 0 0 0 m

• 1 -°- ' ' I .~. .J '5 i t " ° .. ° . ~ ' ° ~

0 . . . . t . . . . a . . . . i . . . . n . . . . i . . . . 1

0 2 0 4 0 6 0 8 0 1 0 0 1 2 0

Effective soil depth (cm)

Fig. 3: P~lation between the total above 9round b:omass production of ra~ttfed wheat and effective soil depth of sods formed on shale-sandstones for three successive years.

ently the soils on the footslopes, i.e. the least eroded areas, are more fertile, deep with a higher water storage capacity. Additionally, they received both surface and subsurface water from the overly- ing landscape components and - - as ex- pected - - they produced the highest biomass. Erosion on the higher positions {shoulders and backslopes), as evidenced by reduced soil thickness and light soil color, resulted in lower TAGBP. The lowest values of TAGBP were recorded on the shoulders (the zone of maximum erosion) in all studied sites, even in the wet period 1990-91, during which prac- tically no water deficit occurred.

The great effect of the effective soil depth on TAGBP can be depicted in fig. 3 for the soils on shale-sandstones, which have a highly variable depth to

bedrock and gravel and stone contents, according to their landscape position. It can be observed that TAGBP increased logarithmically with soil depth point- ing to the impact of the soil depth on both water availability {especially in dry years) and fertility status {especially in the wet years). No similar relations could be established for the soils on marl and conglomerates, in which a deep rooting is not restricted by any limiting subsurface soil layer.

The effect of parent material on biomass production should separately be evaluated for different landscape posi- tions and growing periods:

a) In the wet period 1990-91, a sig- nificant variation in TAGBP oc- curred among the soils on the shoul- ders and backslopes with differ-

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Biomass production wheat, semi-arid conditions 345

ent parent materials. As apparent from fig. 2, the soils on marl were more productive (13,330 kg ha -1) than their stony counterparts, viz. 9,700 kg ha -1 and 10,430 kg ha -1 for shale-sandstones and conglom- erates, respectively (sign. at a - - 0.001). Since water availability was practically optimal in this period, the highest productivity of the soils on marl must be attributed to their higher fertility status than the rest of soils. This is justified by their finer texture, higher organic matter content, and particularly by the ab- sence of coarse fragments (tab. 1). The same holds for the deeper soils on the footslopes, with the soils on marl being the most productive, viz. 19,200 kg ha -1 vs. 18,100 kg ha -1 and 16,270 kg ha -1, for marl, shale-sandstone and conglomerates, respectively, but only the last value is significantly differnt from the first two.

b) Under dry conditions (1989-90) a completely different situation ap- peared. Actually, the catenas on marl were almost bare (viz. 960 kg ha -I biomass on the shoulders and backslopes and 1,355 kg ha -i on the footslopes), whereas an aver- age TAGBP of 4,620 and 11,100 kg ha -i was measured on the shoul-

ders and footslopes of the shale- sandstone sites, respectively (fig. 2).

The above results present an evidence that under conditions of reduced rain- fall, hilly lands on marl formations are more susceptible to desertification than on conglomerate and shale-sandstone formations.

In an attempt to get a better un- derstanding and to quantitatively ex-

plain the fluctuations in biomass pro- duction along the study catenas, the ac- tual crop water use expressed as the "actual evapotranspiration" (ETa) was calculated. In many studies (Rijtema & Aboukhaled 1975, Slabbers 1980), the usefulness of using the ratio of actual over maximum evapotranspira- tion (ETm) in assessing the reduction of biomass production and yield has been stressed. The relation between ETa/ETm and TAGBP based on the data obtained along catenas on marl and shale-sandstones is apparent in fig. 4. The presently available data indicate that in the sampling sites the TAGBP increses logarithmically from zero to its maximum value where ETa -- ETm. Ap- parently, crop transpiration is nil in the zero point, and ETa confines to surface evaporation. Fig. 4 suggests that wa- ter availability, as expressed by the ra- tio ETa/ETm explains in a great degree the variation of TAGBP in the catenas of marls. Moreover, fig. 4 confirms that the plateau TAGBP level on the foot- slopes is higher than on the shoulders, and since ETa = ETm at this level, the difference in TAGBP reflects the differ- ence in the fertility status of the soils on the footslopes.

A similar situation appears for the shale-sandstones. The following points should be stressed. The plateau TAGBP level on the footslopes of shale- sandstones is about the same with that of marls, and much higher than the plateau level on the shoulders of shale- sandstones. An underestimation of the predicted TAGBP, as compared to ex- perimental values, is shown by two clus- ters of points in fig. 4. These points correspond to the biomass production on the shale-sandstone slopes during the dry year (1989-90). Apparently, gravel

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346 Kosmas, Danalatos, Moustakas, Tsatiris, Kallianou & Yassoglou

o f o o t s l o p e z~ shoulder

A a J¢

v

m

m E o

,m

m

20000

1 5 0 0 0

1 0 0 0 0

5000

0

2 0 0 0 0

15000

1 0 0 0 0

5000

0 0.0

Mar l o o

Cb ,,

i

o

S h a l e - s a n d s t o n e s ~

0.2 0 .4 0 .6 0 .8 1.0

E T a / E T m

Fig. 4: Relation between total above ground biomass production along the study catena8 and relative evapotranspiration rate (ETa/ETrn).

and stones, most responsible for the low exploitable soil volume and generally the lower biomass production under normal climatic conditions, play a very impor- tant role against desertification under dry weather conditions. It is believed that surface mulching - - no taken into account in the biomass growth calcula-

tions -- is the main cause for the ob- served underestimations and, according to fig. 4, it may reduce evaporation as much as 20% (for soils on shoulders) to 30% (for soils on the footslopes) of ETm. With a calculated total ETm of 360 mm in the growing period 1989-90, these val- ues correspond to 70 and 105 mm water

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Biomass production wheat, semi-arid conditions 347

D Marl 0 8hale-sandstone

1 . 0 "

¢} 0 8 03

o 0 . 6

0.4"

0.0 0.0

o oo , . s - o.gs.o.s,.l,lx o

R 0.97, n-51 _ . ~ o

O 0 0 0 =

?, ' i • i , i , I

0.2 0.4 0.6 0.8 1.0

ETa/ETm

Fig. 5: Relation between the calculated relative evapotranspiration rate and the relative bioraass production/or different parent materials, landscape positions and three growin9 periods.

used for the above soils, respectively. A noticeable deviation in the points of zero production in the curves of fig. 4 sug- gests a lower overall evaporation com- ponent in the gravelly and stony soils on the shale-sandstones as compared with that in the soils on marl. This confirms a significant surface mulching effect of the gravel and stones.

The higher surface mulching of the stony soils must be attr ibuted to their both high content of surface stones and favorable physical properties. The av- erage bulk density of the Ap horizon of soils on marl was 1.50 g c m -3 ver- sus 1.27 g cm -3 for soils on conglomer- ates and shale-sandstones (tab. 1), mea- sured in the middle of the growing pe- riod. Due to the absence of coarse fragments and the high available stor-

age capacity of the soils on marl (viz. 23% vs. 12.7-14.9% for shale-sandstones and 16.5-18.0% for conglomerates), the percolating water is stored at shallower depths. Furthermore, compaction and lack of stone mulching would not re- strict liquid flow as suction develops dur- ing evaporation and drying off in these soils. These conditions render the marly soils more vulnerable to desertification in drier than normal years as compared to the soils on conglomerate and shale- sandstone formations.

Considering the plateau TAGBP lev- els estimated for each landscape po- sit ion/parent material, the relative TAGBP, i.e. the actual over maximum biomass production, can be correlated with the calculated relative evapotran- spiration (gTa/gTm) (fig. 5). Note,

S O I L T B C H N O L O G Y - - A c o o p e r a t i n g J o u r n & I of C A T B N A

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348 Kosmas, Danala¢os, Moustakas, Tsa¢iris, Kallianou & Yassoglou

however, that the outliers indicated in fig. 4 are excluded from this curve. Un- doubtedly, the impact of gravel and sur- face stones on soil compaction and sur- face mulching on the susceptibility to de- sertification deserves further investiga- tion.

Acknowledgements

This publication was partially financed by the EC Project ~Mediterranean De- sertification and Land Use, MEDA- LUS ~, [Contract no EPOC-CTg0-0014- (SMA)].

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SOIL T~CHNOLOGY--A ¢ o o p e r & t | n g J o u r n a l of CATENA