- Email: [email protected]
Root and shoot growth ofProsopis chilensisin response to soil impedance and soil matric potential
Journal of Arid Environments (1998) 40: 43]52 Article No. ae980426
Root and shoot growth of Prosopis chilensis in response to soil impedance and soil matric potential
A. A. Salih Land & Water Research Center, Agricultural Research Corporation, P.O. Box 126, Wad Medani, Sudan (Received 21, October 1997, accepted 13 May 1998) The suitability of Prosopis chilensis for reforestation under conditions of high soil bulk density and high moisture stress was examined in a greenhouse study at the University of California, Riverside. Root growth was greatly reduced at high bulk density (2.1 Mg my3 ) and high moisture stress (y1.0 MPa). However, adverse effects of high bulk density were greatly reduced at low moisture stress. Rate of root growth was adequate for good tree establishment in soils with medium bulk density (1.7 Mg my3 ). Prosopis chilensis could be successfully grown on dense soils provided good moisture conditions were available for sufficient time. q 1998 Academic Press Keywords: Prosopis chilensis; bulk density; moisture stress; root growth; shoot growth
Introduction Desertification is a global ecological, social and economic problem typical of the arid territories of the world. Controlling desertification is essential for preservation of biodiversity and land productivity. Afforestation using drought-tolerant tree species is an important method for combating desert encroachment. However, inadequate soil moisture during seedling establishment is a major obstacle facing afforestation, particularly on dense soils. Prosopis species, renowned for drought tolerance and fast growth, are commonly used in afforestation programmes in many arid zones (Fagg & Stewart, 1994). The trees have great diversity and potential. They play an important role in the rural economy of many of the world’s arid and semi-arid areas. In addition to combating desertification, Prosopis species provide fuelwood, charcoal, building material and high quality fodder (Felker et al., 1984). In sub-Saharan Africa, including Sudan and neighbouring countries, Prosopis species, mainly P. chilensis (Molina) emend. Bukart (Mustafa, 1988) and P. juliflora (Fagg & Stewart, 1994), are commonly used in afforestation programmes. In many of these 0140]1963r98r010043 q 10 $30.00r0
q 1998 Academic Press
44
A. A. SALIH
areas the climate is characterized by hot, dry summers and a monsoon type rainy season of 250]500 mm. Soil survey data indicate that many of the drought stricken areas in this region have soils that are typified by subsoils with variable but usually high bulk densities ranging between 1.7 and 2.0 Mg my3 (Agrar UND, 1975). For afforestation plantings to be grown successfully on these soils, the transplants must rapidly produce an extensive root system that can penetrate the subsoil with relatively high densities during the short rainy season and thus increase the seedlings’ chances to survive till the following rainy season. A compacted zone at a shallow depth preventing root penetration is highly detrimental to plant growth and yield especially when plants depend on infrequent precipitation for their water supply (Unger & Kasper, 1994). Soil compaction also influences plant growth and yields by affecting nutrient availability, water infiltration, aeration, plant diseases and yield quality. Compacted zone strength is considerably influenced by soil bulk density and water content (Taylor & Gardner, 1963). However, it may be possible through management to circumvent the adverse effects of compaction by growing crops when the soil is sufficiently moist due to timely precipitation or applied irrigation water. This investigation was designed to study the effects of soil impedance and soil matric potential on Prosopis chilensis seedlings growth, in an endeavor to provide information on the potential use of P. chilensis in reforestation in arid regions with compact soils. Materials and methods Greenhouse experiment The experiment was conducted at the University of California, Riverside. The soil utilized was San Emigdio sandy loam (coarse, loamy, mixed (calcareous) thermic Typic Xerofluvent). Containers 15.5 cm in diameter, 35 cm in height with a total volume of 6600 cm3 were employed. To achieve the bulk densities to be used, a procedure developed by Said (1980) was applied. The curve relating bulk densities to moisture contents is shown in Fig. 1. A prototype container was utilized to study the effect of the compaction process on the soil below the compacted layer. It was found that the compaction process slightly increased bulk density of the underlying layer to 1.54 Mg
Figure 1. Compactability curve for San Emigdio sandy loam obtained from Riverside, California showing the relation between bulk density and moisture content for standard compaction procedure.
ROOT AND SHOOT GROWTH OF PROSOPIS CHILENSIS
45
my3 , compared to 1.5 Mg my3 for the control. Air-dried soil was added to fill the lower 10 cm of the container to give a bulk density of 1.5 mg cmy3 . The compacted layer (10 cm) was formed in three increments of 3.3 cm each to ensure a uniform compaction. The uniformity of compaction was confirmed with penetrometer (Eijkelkamp) measurements. Four bulk densities (1.7, 1.9, 2.1 and 1.5 Mg my3 ) were established. A 10 cm soil layer was placed on top of each compacted layer. Soil moisture treatments were made by use of a desorption curve of the soil from y0.01 to y1.5 MPa matric potential. The curve was obtained for soil samples packed at the same bulk density as that of the control (1.5 Mg my3 ). Four moisture treatments were selected to provide a wide range of soil moisture conditions. Water was applied when soil moisture potential reached y0.05, y0.1, y0.5 or y1.0 MPa as measured with tensiometers and gypsum blocks (both calibrated for the soil) located just above the compact layer. The tensiometers measurements were bound up to 0.1 MPa, therefore gypsum blocks were added to measure all the range of soil moisture treatments. Some additional measuring units were placed in the lower part of the container to monitor soil moisture changes. Moisture treatments were imposed 3 weeks after planting. Six P. chilensis seeds inoculated with the proper rhizobium were directly sown in each container. Two weeks later the seedlings were thinned to four. The surface of each container was covered with polyethylene to minimize evaporation and allow equal distribution of irrigation water. Since P. chilensis is adapted to hot environments, the greenhouse temperature was maintained at 328C during day and 258C during the night. The first sample was taken when the plants were 6-weeks-old and about 25 cm high, with a well developed root system in the upper 10 cm layer (indicated by moisture removal). Subsequent samples were taken at 2-week intervals for a total of five samples. The shoot system was cut above the soil surface, weighed fresh, dried at 658C for 5 days and its dry weight determined. The container was opened and the soil column was divided into three equal parts, each 10 cm long. Each part of the soil column was then soaked in water to free the root system. Root systems were carefully washed and stored separately in plastic vials containing a solution of FAA (formaldehyde acetic acid). Root length was determined by the intersection method developed by Newman (1965). Root samples were then dried at 658C for dry weight measurements. Soil strength was measured using separate containers with soils compacted to 1.7, 1.9 and 2.1 Mg my3 at different moisture levels (ranging from 3]15% by weight). Measurements were made after harvest with a hand operated penetrometer (Eijkelkamp) to a depth of 30 cm. Experimental design and statistical analysis The containers were arranged in a 4 = 4 factorial randomized block design with five replications. All data were subjected to analysis of variance. Regression analysis and means differentiation were performed using Duncan’s Multiple Range Test (Steel & Torrie, 1980).
Glassbox experiment To observe changes in distribution and growth of P. chilesis root systems in response to soil compaction, double plated boxes measuring 35 = 30 cm and 1 cm wide were constructed. Two soil layers, compacted at bulk densities of 1.45 and 1.71 Mg my3 and of 2 cm thickness each, were formed 12 cm from the surface in separate glassboxes. Glass beads 0.2 mm in diameter were used to fill the box below and above the compacted layer. Six P. chilensis seeds were sown 4 cm apart just below the surface of
46
A. A. SALIH
Table 1. Regression equations relating root length density (Y, cm cm y 3 ) of Prosopis chilensis to bulk density (BD, Mg m y 3 ) and matric potential (MP, MPa) of San Emigdio sandy loam obtained from Riverside, California, for five sampling dates
Sample number
Regression equations
R2
1 2 3 4 5
Y s 0.515 y 0.156BD y 3.63 = 10y2 (BD) 2 (MP) Y s 0.507 y 0.130BD y 3.52 = 10y2 (BD) 2 (MP) Y s 0.890 y 0.316BD y 0.13(MP) Y s 1.179 y 0.395BD y 0.57(BD)(MP) q 2.14(BD) 2 (MP) Y s 0.687 y 0.060(BD) 2 y 5.58 = 10 2 (BD)(MP)
0.79*** 0.68*** 0.83*** 0.89*** 0.80***
*** Significant at the 0.001 level.
each box. Hogland solution No. 1 was used to irrigate the plants. A daily record of root length was obtained by marking the glass surface. Results and discussion Root length density The regression equations relating the variables bulk density (BD) and matric potential (MP) to root length density in the compacted layer are shown in Table 1. Increasing bulk density or decreasing matric potential had a negative effect on root length density. On four of the sampling dates (equations (1), (2), (4) and (5)), root length density showed a quadratic response to bulk density and matric potential. These four equations show that the interaction of bulk density and matric potential had a significant effect on LSD. For equation (3), the variable matric potential has an F-value slightly higher than that of the interaction of bulk density and matric potential (22.54 compared to 22.05). The interaction effect was excluded because it does not add to the overall significance of the equation. The correlation coefficient for the five sampling dates was significant at the 0.001 level. The high bulk density soil layer (2.1 Mg my3 ) totally restricted root penetration at the lowest matric potential (y1.0 Mpa) even though the plants were 42-days-old. At a matric potential of y0.05 MPa and a bulk density of 1.5 Mg my3 , root length density was 0.31 cm cmy3 . Root length density decreased to 0.05 cm cmy3 as bulk density increased to 2.1 Mg my3 and matric potential decreased to y1.0 MPa (Fig. 2). Root length density for the compacted soil layers increased with time. The effect of matric potential on modifying the adverse affects of high bulk density can be readily seen from Fig. 2. At a bulk density of 2.1 Mg my3 , root length density was 0.05 cm cmy3 at a matric potential of y1.0 MPa and 0.18 cm cmy3 at a matric potential of y0.05 MPa on the first sampling date. The corresponding values for the fifth sampling date were 0.22 and 0.45 cm cmy3 . Root length density on the second, third and fourth sampling dates displayed similar trends (data not shown). The results consistently show that the adverse effects of high bulk density on root length densities can be ameliorated by increasing matric potential. Dry weight accumulation The regression equations relating shoot dry weight (DW) of P. chilensis for the five sampling dates are presented in Table 2. On three out of the five sampling dates
Figure 2. Three-dimensional representation of root length density of Prosopis chilensis as affected by bulk density and soil matric potential for (a) the first and (b) the last sampling dates (soil obtained from Riverside, California).
ROOT AND SHOOT GROWTH OF PROSOPIS CHILENSIS 47
48
A. A. SALIH
Table 2. Regression equations relating the dry weight (Y, g) of Prosopis chilensis to bulk density (BD, Mg m y 3 ) and matric potential (MP, Mpa) of San Emigdio sandy loam obtained from Riverside, California, for five sampling dates
Sample number
Regression equation
R2
1 2 3 4 5
Y s 0.213 = 10 2 y 6.61BD y 1.14(BD) 2 (MP) Y s 0.249 = 10 2 y 5.46BD y 4.37(BD)(MP) Y s 0.172 = 10 2 y 5.95(BD)(MP) Y s 0.198 = 10 2 y 2.60(BD)(MP) Y s 0.317 = 10 2 y 24.1(BD)(MP)
0.78*** 0.60*** 0.55** 0.46** 0.77***
**, *** Significant at the 0.01 and 0.001 levels, respectively.
(equations (3), (4) and (5)) shoot dry weight showed a quadratic response to bulk density and matric potential. The interaction of bulk density and matric potential was significant in all cases. Increasing bulk density or decreasing matric potential reduced dry weight. Shoot dry weight at the highest bulk density (2.1 Mg my3 ) and lowest matric potential (y1.0 MPa) were 3.14, 6.8 and 7.6 g per seedling at 42, 56 and 63 days after planting, respectively. The effect of bulk density and matric potential were invariably significant at the 0.001 probability level. With the exception of the first sampling date, the interaction effect of bulk density and matric potential was significant. At the first sampling date the plants were small and could apparently obtain most of their requirements from the uncompacted top 10 cm soil layer. However, as the plants grow their requirements for water and nutrients increase and the root system has to explore more soil volume. This may explain the increase in importance of the interaction effect of bulk density and matric potential with time. These results are consistent with those reported by Trouse (1971). Moderate soil compaction, which did not inhibit penetration, resulted in significant reduction in plant growth. The reduction was attributed to reduced root elongation and downwards penetration resulting in inadequate water supply and nutrients. Extension of the root system in a downward direction is possible only if the roots are able to expand the pores or displace the soil. Root dry weight, irrespective of treatments, displayed a consistent increase with time. However, differences between treatments were less apparent as the matric potential decreased to y0.5 and y1.0 MPa (Table 3). This may due to an increase in soil strength at low matric potential to values that are limiting to root growth even at the medium bulk density of 1.7 Mg my3 . Prosopis chilensis roots growing at a relatively low matric potential would probably be restricted by soil layers at strengths considerably lower than their critical level. This is consistent with the observation that resistance to penetration increased with decreased moisture content and increased bulk density (Fig. 3). The same penetration resistance could be achieved by different combinations of moisture content and bulk density. These findings are in conformity with the the concept of excessive soil strength introduced by Taylor et al. (1963).
Rate of shoot and root growth Prosopis chilensis root and shoot growth as a function of time for two soil layers compacted at bulk densities of 1.45 and 1.71 Mg my3 are presented in Fig. 4 and Table 4. Twenty-eight days after seeding the plant heights averaged 9.2 and 12.0 cm for the 1.71 and 1.45 Mg my3 soil layers, respectively. The tap roots reached the compacted soil layer in about 8 days. For the low bulk density the tap root continues to grow through the soil layer at essentially the same rate as for the glass beads. However, the
ROOT AND SHOOT GROWTH OF PROSOPIS CHILENSIS
49
Table 3. F-values and their significance levels for Prosopis chilensis shoot and root dry weight (g) for five sampling dates as affected by bulk density (BD), matric potential (MP) and their interaction (BD = MP). The soil used was San Emgdio sandy loam from Riverside, California
Sample number
Source of variation
1
BD MP BD = MP
2
BD MP BD = MP
3
BD MP BD = MP
4
BD MP BD = MP
5
BD MP BD = MP
F-values Shoot dry weight Root dry weight 44.31*** 41.18*** 2.00 34.02*** 182.66*** 11.18*** 26.10*** 194.05*** 11.40*** 38.23*** 244.75*** 5.19*** 12.41*** 313.54*** 4.39***
38.54*** 59.37*** 3.11* 23.50*** 102.13*** 5.88** 12.03*** 176.19*** 7.03*** 35.05*** 66.92*** 10.62*** 32.14*** 201.85*** 9.95***
*, **, *** Significant at the 0.05, 0.01 and 0.001 probability levels, respectively.
Figure 3. Effect of bulk density and water content on soil strength of San Emigdio sandy loam obtained from Riverside, California as measured by the penetrometer. Bulk density: (I) s 2.1 Mg my3 ; (e) s 1.7 Mg my3 ; (`) s 1.5 Mg my3.
50
A. A. SALIH
Figure 4. Prosopis chilensis root tip location and plant height during the first 30 days of root elongation through mediums with soil layers that were compacted at different bulk densities. Each data point represent the arithmetic mean of six plants (soil obtained from Riverside, California).
root growing in the 1.71 Mg my3 soil layer was excluded from passing through for 2 weeks. Its rate of growth decreased to less than 0.14 cm dayy1 compared to 1.5 cm dayy1 for growth in the glass beads above the soil layer. The shape of the root system was also affected. Several secondary roots were initiated above the compacted soil layer and extended along its surface. Roots which managed to penetrate the compacted layer resumed growth at more or less the same rate as noted prior to meeting the high density soil layer. A similar finding was reported for cotton (Taylor, 1969). A high strength soil pan excluded cotton root growth through the pan for 20 days. The results presented in these studies, relative to climatic and soil conditions prevailing in arid and semi-arid regions of sub-Saharan Africa, suggest that P. chilensis species with a high rate of root development could be successfully grown in afforestation and desert encroachment programmes. Although the amount of rainfall in these regions is variable, the practice of planting crops at the first significant rains could be adapted. As the rainy season usually begins in June, reaching its peak in August, by early July enough moisture is available for transplanting, and within a period of 30 days depth of rooting would be 51 cm (using 1.7 cm dayy1 as a guide). During this period total root length density for the y0.05 MPa soil matric potential increased from 0.19 to 0.49 cm cmy3 (Table 3), or 150% in the upper portion of the root system. In these regions, soils often have compacted layers 35 to 100 cm thick located at 35 to more than 50 cm below the surface. Bulk densities of 1.7 and 1.9 Mg my3 can be used as guides since roots were able to penetrate these soil layers provided reasonable moisture was available in the compacted layer for a sufficient amount of time.
ROOT AND SHOOT GROWTH OF PROSOPIS CHILENSIS
51
Table 4. Prosopis chilensis dry weight of total roots (g) for five sampling dates as affected by bulk density and soil matric potential of San Emigdio sandy loam from Riverside, California
Sample number: Treatment w
1
2
3
4
5
A1 B1 A2 B1 A3 B1 A4 B1
4.49 a 3.37 b 2.73 c 2.87 c 2.93 ab 3.26 a 2.58 b 1.87 c 2.72 a 2.48 a 1.80 b 1.79 b 2.40 a 1.93 b 1.61 c 1.28 d
5.75 a 4.48 b 3.84 b 2.81 c 3.41 a 2.68 b 2.47 b 2.49 b 2.61 a 2.60 a 2.37 a 2.12 b 2.02 a 1.64 ab 1.48 ab 1.63 b
8.52 a 7.59 ab 7.28 b 4.94 c 5.12 b 4.83 c 4.70 c 4.49 d 3.63 b 3.97 b 3.22 b 3.69 b 3.14 b 2.93 b 2.61 b 2.86 b
10.35 a 9.61 b 8.31 c 7.79 d 4.95 b 5.94 c 5.58 c 4.87 d 3.82 a 5.68 b 4.81 ba 3.69 a 2.74 a 4.63 b 3.97 b 3.26 a
11.03 a 9.14 b 7.17 c 6.25 d 5.49 a 4.88 ab 4.79 ab 4.60 b 4.70 a 4.63 a 4.12 a 3.96 a 4.23 a 4.25 a 4.08 a 3.18 a
A1 B2 A2 B2 A3 B2 A4 B2 A1 B3 A2 B3 A3 B3 A4 B3 A1 B4 A2 B4 A3 B4 A4 B4
w Bulk density (Mg my3 ): A1, 1.5; A2, 1.7; A3, 1.9; A4, 2.1. Soil matric potential (MPa): B1, y0.05; B2, y0.10; B3, y0.50; B4, y1.0. Means followed by the same letter are not significantly different at the 0.05 level according to Duncan’s Multiple Range Test.
Conclusions Increasing bulk density or decreasing soil matric potential had a negative effect on root length density and shoot dry weight of Prosopis chilensis. The effect of bulk density was more pronounced at high soil matric potential. Differences in total dry weight of roots growing in soil layers with different bulk densities became insignificant as soil matric potential decreased to y0.5 and y1.0 MPa. A soil layer of bulk density 1.7 Mg my3 prevented root growth from passing through the layer for a period of 2 weeks. Several secondary roots, which were initiated from a mat of roots growing on the surface of the compacted layer, were able to pass through at several points. Prosopis chilensis could be successfully used for afforestation on soils with high subsoil bulk density provided that reasonable soil moisture is available in the compacted soil layer at transplanting and for a sufficient time. The author wishes to thank Prof. A. G. T. Babiker, Dr I.A. Ali of the Arid Land Research Center of Tottori University and Mr A. ArWahab for useful comments and critical reading of the manuscript.
References Agrar UND (1975). Feasibility study for mechanized farming in Southern Darfur, Southern Kordofan, and Bahr El Ghazal. Hydrotechnik GMBH.
52
A. A. SALIH
Fagg, C.W. & Stewart, J.L. (1994). The value of Acacia and Prosopis in arid and semi-arid environments. Journal of Arid Environments, 27: 3]25. Felker, P., Clark, P.R., Osborn, J.F. & Cannell, G.H. (1984 ). Prosopis pod production}comparison of North American, South American, Hawaiian, and African germplasm in young plantations. Economic Botany, 38: 36]51. Mustafa, A.R.F.M. (1988). Prosopis chilensis, A tree for Sudan’s dry zone. In: Habitat, M.A. & Saavedra, J.C. (Eds), The Current State of Knowledge on Prosopis juliflora, pp. 163]167. International Conference on Prosopis, Recife, Brazil, 25]29 August 1986. 554 pp. Newman, E.I. (1966). A method of estimating the total length of roots in a sample. Journal Applied Ecology, 3: 139]145. Said, D.L. (1980). A root chamber study of cowpeas grown in soils with layers compacted at different bulk densities. Ph.D. thesis, University of California, Riverside. Steel, R.G.D. & Torrie, J.H. (1980). Principles and Procedures of Statistics. A biometrical approach (2nd Edn). New York: McGraw-Hill Book Co. 598 pp. Taylor, H.M. (1969). The rhizotron at Auburn, Alabama. A plant root observation laboratory. Auburn University Agriculture Experimental Station Circular, 71. Taylor, H.M. & Gardner, H.R. (1963). Penetration of cotton seedlings tap root as influenced by bulk density, moisture content and strength of soil. Soil Science, 96(3): 153]156. Taylor, H.M., Mathers, A.C. & Lotspiech, F.B. (1963). Pans in the Southern Great Plains soils. 1. Why root-restricting pans occur. Agronomy Journal, 56: 328]332. Trouse, Jr., A.C. (1971). Soil conditions as they affect plant establishment, root development and yield. In: Barnes, K.K. et al. (Eds), Compaction of Agricultural Soils, pp. 241]252. An ASAE monograph. Joseph, MI: American Society of Agricultural Engineers. 407 pp. Unger, P.W. & Kasper, T.C. (1994). Soil compaction and root growth. Agronomy Journal, 86: 759]766.
Root and shoot growth of Prosopis chilensis in response to soil impedance and soil matric potential
A. A. Salih Land & Water Research Center, Agricultural Research Corporation, P.O. Box 126, Wad Medani, Sudan (Received 21, October 1997, accepted 13 May 1998) The suitability of Prosopis chilensis for reforestation under conditions of high soil bulk density and high moisture stress was examined in a greenhouse study at the University of California, Riverside. Root growth was greatly reduced at high bulk density (2.1 Mg my3 ) and high moisture stress (y1.0 MPa). However, adverse effects of high bulk density were greatly reduced at low moisture stress. Rate of root growth was adequate for good tree establishment in soils with medium bulk density (1.7 Mg my3 ). Prosopis chilensis could be successfully grown on dense soils provided good moisture conditions were available for sufficient time. q 1998 Academic Press Keywords: Prosopis chilensis; bulk density; moisture stress; root growth; shoot growth
Introduction Desertification is a global ecological, social and economic problem typical of the arid territories of the world. Controlling desertification is essential for preservation of biodiversity and land productivity. Afforestation using drought-tolerant tree species is an important method for combating desert encroachment. However, inadequate soil moisture during seedling establishment is a major obstacle facing afforestation, particularly on dense soils. Prosopis species, renowned for drought tolerance and fast growth, are commonly used in afforestation programmes in many arid zones (Fagg & Stewart, 1994). The trees have great diversity and potential. They play an important role in the rural economy of many of the world’s arid and semi-arid areas. In addition to combating desertification, Prosopis species provide fuelwood, charcoal, building material and high quality fodder (Felker et al., 1984). In sub-Saharan Africa, including Sudan and neighbouring countries, Prosopis species, mainly P. chilensis (Molina) emend. Bukart (Mustafa, 1988) and P. juliflora (Fagg & Stewart, 1994), are commonly used in afforestation programmes. In many of these 0140]1963r98r010043 q 10 $30.00r0
q 1998 Academic Press
44
A. A. SALIH
areas the climate is characterized by hot, dry summers and a monsoon type rainy season of 250]500 mm. Soil survey data indicate that many of the drought stricken areas in this region have soils that are typified by subsoils with variable but usually high bulk densities ranging between 1.7 and 2.0 Mg my3 (Agrar UND, 1975). For afforestation plantings to be grown successfully on these soils, the transplants must rapidly produce an extensive root system that can penetrate the subsoil with relatively high densities during the short rainy season and thus increase the seedlings’ chances to survive till the following rainy season. A compacted zone at a shallow depth preventing root penetration is highly detrimental to plant growth and yield especially when plants depend on infrequent precipitation for their water supply (Unger & Kasper, 1994). Soil compaction also influences plant growth and yields by affecting nutrient availability, water infiltration, aeration, plant diseases and yield quality. Compacted zone strength is considerably influenced by soil bulk density and water content (Taylor & Gardner, 1963). However, it may be possible through management to circumvent the adverse effects of compaction by growing crops when the soil is sufficiently moist due to timely precipitation or applied irrigation water. This investigation was designed to study the effects of soil impedance and soil matric potential on Prosopis chilensis seedlings growth, in an endeavor to provide information on the potential use of P. chilensis in reforestation in arid regions with compact soils. Materials and methods Greenhouse experiment The experiment was conducted at the University of California, Riverside. The soil utilized was San Emigdio sandy loam (coarse, loamy, mixed (calcareous) thermic Typic Xerofluvent). Containers 15.5 cm in diameter, 35 cm in height with a total volume of 6600 cm3 were employed. To achieve the bulk densities to be used, a procedure developed by Said (1980) was applied. The curve relating bulk densities to moisture contents is shown in Fig. 1. A prototype container was utilized to study the effect of the compaction process on the soil below the compacted layer. It was found that the compaction process slightly increased bulk density of the underlying layer to 1.54 Mg
Figure 1. Compactability curve for San Emigdio sandy loam obtained from Riverside, California showing the relation between bulk density and moisture content for standard compaction procedure.
ROOT AND SHOOT GROWTH OF PROSOPIS CHILENSIS
45
my3 , compared to 1.5 Mg my3 for the control. Air-dried soil was added to fill the lower 10 cm of the container to give a bulk density of 1.5 mg cmy3 . The compacted layer (10 cm) was formed in three increments of 3.3 cm each to ensure a uniform compaction. The uniformity of compaction was confirmed with penetrometer (Eijkelkamp) measurements. Four bulk densities (1.7, 1.9, 2.1 and 1.5 Mg my3 ) were established. A 10 cm soil layer was placed on top of each compacted layer. Soil moisture treatments were made by use of a desorption curve of the soil from y0.01 to y1.5 MPa matric potential. The curve was obtained for soil samples packed at the same bulk density as that of the control (1.5 Mg my3 ). Four moisture treatments were selected to provide a wide range of soil moisture conditions. Water was applied when soil moisture potential reached y0.05, y0.1, y0.5 or y1.0 MPa as measured with tensiometers and gypsum blocks (both calibrated for the soil) located just above the compact layer. The tensiometers measurements were bound up to 0.1 MPa, therefore gypsum blocks were added to measure all the range of soil moisture treatments. Some additional measuring units were placed in the lower part of the container to monitor soil moisture changes. Moisture treatments were imposed 3 weeks after planting. Six P. chilensis seeds inoculated with the proper rhizobium were directly sown in each container. Two weeks later the seedlings were thinned to four. The surface of each container was covered with polyethylene to minimize evaporation and allow equal distribution of irrigation water. Since P. chilensis is adapted to hot environments, the greenhouse temperature was maintained at 328C during day and 258C during the night. The first sample was taken when the plants were 6-weeks-old and about 25 cm high, with a well developed root system in the upper 10 cm layer (indicated by moisture removal). Subsequent samples were taken at 2-week intervals for a total of five samples. The shoot system was cut above the soil surface, weighed fresh, dried at 658C for 5 days and its dry weight determined. The container was opened and the soil column was divided into three equal parts, each 10 cm long. Each part of the soil column was then soaked in water to free the root system. Root systems were carefully washed and stored separately in plastic vials containing a solution of FAA (formaldehyde acetic acid). Root length was determined by the intersection method developed by Newman (1965). Root samples were then dried at 658C for dry weight measurements. Soil strength was measured using separate containers with soils compacted to 1.7, 1.9 and 2.1 Mg my3 at different moisture levels (ranging from 3]15% by weight). Measurements were made after harvest with a hand operated penetrometer (Eijkelkamp) to a depth of 30 cm. Experimental design and statistical analysis The containers were arranged in a 4 = 4 factorial randomized block design with five replications. All data were subjected to analysis of variance. Regression analysis and means differentiation were performed using Duncan’s Multiple Range Test (Steel & Torrie, 1980).
Glassbox experiment To observe changes in distribution and growth of P. chilesis root systems in response to soil compaction, double plated boxes measuring 35 = 30 cm and 1 cm wide were constructed. Two soil layers, compacted at bulk densities of 1.45 and 1.71 Mg my3 and of 2 cm thickness each, were formed 12 cm from the surface in separate glassboxes. Glass beads 0.2 mm in diameter were used to fill the box below and above the compacted layer. Six P. chilensis seeds were sown 4 cm apart just below the surface of
46
A. A. SALIH
Table 1. Regression equations relating root length density (Y, cm cm y 3 ) of Prosopis chilensis to bulk density (BD, Mg m y 3 ) and matric potential (MP, MPa) of San Emigdio sandy loam obtained from Riverside, California, for five sampling dates
Sample number
Regression equations
R2
1 2 3 4 5
Y s 0.515 y 0.156BD y 3.63 = 10y2 (BD) 2 (MP) Y s 0.507 y 0.130BD y 3.52 = 10y2 (BD) 2 (MP) Y s 0.890 y 0.316BD y 0.13(MP) Y s 1.179 y 0.395BD y 0.57(BD)(MP) q 2.14(BD) 2 (MP) Y s 0.687 y 0.060(BD) 2 y 5.58 = 10 2 (BD)(MP)
0.79*** 0.68*** 0.83*** 0.89*** 0.80***
*** Significant at the 0.001 level.
each box. Hogland solution No. 1 was used to irrigate the plants. A daily record of root length was obtained by marking the glass surface. Results and discussion Root length density The regression equations relating the variables bulk density (BD) and matric potential (MP) to root length density in the compacted layer are shown in Table 1. Increasing bulk density or decreasing matric potential had a negative effect on root length density. On four of the sampling dates (equations (1), (2), (4) and (5)), root length density showed a quadratic response to bulk density and matric potential. These four equations show that the interaction of bulk density and matric potential had a significant effect on LSD. For equation (3), the variable matric potential has an F-value slightly higher than that of the interaction of bulk density and matric potential (22.54 compared to 22.05). The interaction effect was excluded because it does not add to the overall significance of the equation. The correlation coefficient for the five sampling dates was significant at the 0.001 level. The high bulk density soil layer (2.1 Mg my3 ) totally restricted root penetration at the lowest matric potential (y1.0 Mpa) even though the plants were 42-days-old. At a matric potential of y0.05 MPa and a bulk density of 1.5 Mg my3 , root length density was 0.31 cm cmy3 . Root length density decreased to 0.05 cm cmy3 as bulk density increased to 2.1 Mg my3 and matric potential decreased to y1.0 MPa (Fig. 2). Root length density for the compacted soil layers increased with time. The effect of matric potential on modifying the adverse affects of high bulk density can be readily seen from Fig. 2. At a bulk density of 2.1 Mg my3 , root length density was 0.05 cm cmy3 at a matric potential of y1.0 MPa and 0.18 cm cmy3 at a matric potential of y0.05 MPa on the first sampling date. The corresponding values for the fifth sampling date were 0.22 and 0.45 cm cmy3 . Root length density on the second, third and fourth sampling dates displayed similar trends (data not shown). The results consistently show that the adverse effects of high bulk density on root length densities can be ameliorated by increasing matric potential. Dry weight accumulation The regression equations relating shoot dry weight (DW) of P. chilensis for the five sampling dates are presented in Table 2. On three out of the five sampling dates
Figure 2. Three-dimensional representation of root length density of Prosopis chilensis as affected by bulk density and soil matric potential for (a) the first and (b) the last sampling dates (soil obtained from Riverside, California).
ROOT AND SHOOT GROWTH OF PROSOPIS CHILENSIS 47
48
A. A. SALIH
Table 2. Regression equations relating the dry weight (Y, g) of Prosopis chilensis to bulk density (BD, Mg m y 3 ) and matric potential (MP, Mpa) of San Emigdio sandy loam obtained from Riverside, California, for five sampling dates
Sample number
Regression equation
R2
1 2 3 4 5
Y s 0.213 = 10 2 y 6.61BD y 1.14(BD) 2 (MP) Y s 0.249 = 10 2 y 5.46BD y 4.37(BD)(MP) Y s 0.172 = 10 2 y 5.95(BD)(MP) Y s 0.198 = 10 2 y 2.60(BD)(MP) Y s 0.317 = 10 2 y 24.1(BD)(MP)
0.78*** 0.60*** 0.55** 0.46** 0.77***
**, *** Significant at the 0.01 and 0.001 levels, respectively.
(equations (3), (4) and (5)) shoot dry weight showed a quadratic response to bulk density and matric potential. The interaction of bulk density and matric potential was significant in all cases. Increasing bulk density or decreasing matric potential reduced dry weight. Shoot dry weight at the highest bulk density (2.1 Mg my3 ) and lowest matric potential (y1.0 MPa) were 3.14, 6.8 and 7.6 g per seedling at 42, 56 and 63 days after planting, respectively. The effect of bulk density and matric potential were invariably significant at the 0.001 probability level. With the exception of the first sampling date, the interaction effect of bulk density and matric potential was significant. At the first sampling date the plants were small and could apparently obtain most of their requirements from the uncompacted top 10 cm soil layer. However, as the plants grow their requirements for water and nutrients increase and the root system has to explore more soil volume. This may explain the increase in importance of the interaction effect of bulk density and matric potential with time. These results are consistent with those reported by Trouse (1971). Moderate soil compaction, which did not inhibit penetration, resulted in significant reduction in plant growth. The reduction was attributed to reduced root elongation and downwards penetration resulting in inadequate water supply and nutrients. Extension of the root system in a downward direction is possible only if the roots are able to expand the pores or displace the soil. Root dry weight, irrespective of treatments, displayed a consistent increase with time. However, differences between treatments were less apparent as the matric potential decreased to y0.5 and y1.0 MPa (Table 3). This may due to an increase in soil strength at low matric potential to values that are limiting to root growth even at the medium bulk density of 1.7 Mg my3 . Prosopis chilensis roots growing at a relatively low matric potential would probably be restricted by soil layers at strengths considerably lower than their critical level. This is consistent with the observation that resistance to penetration increased with decreased moisture content and increased bulk density (Fig. 3). The same penetration resistance could be achieved by different combinations of moisture content and bulk density. These findings are in conformity with the the concept of excessive soil strength introduced by Taylor et al. (1963).
Rate of shoot and root growth Prosopis chilensis root and shoot growth as a function of time for two soil layers compacted at bulk densities of 1.45 and 1.71 Mg my3 are presented in Fig. 4 and Table 4. Twenty-eight days after seeding the plant heights averaged 9.2 and 12.0 cm for the 1.71 and 1.45 Mg my3 soil layers, respectively. The tap roots reached the compacted soil layer in about 8 days. For the low bulk density the tap root continues to grow through the soil layer at essentially the same rate as for the glass beads. However, the
ROOT AND SHOOT GROWTH OF PROSOPIS CHILENSIS
49
Table 3. F-values and their significance levels for Prosopis chilensis shoot and root dry weight (g) for five sampling dates as affected by bulk density (BD), matric potential (MP) and their interaction (BD = MP). The soil used was San Emgdio sandy loam from Riverside, California
Sample number
Source of variation
1
BD MP BD = MP
2
BD MP BD = MP
3
BD MP BD = MP
4
BD MP BD = MP
5
BD MP BD = MP
F-values Shoot dry weight Root dry weight 44.31*** 41.18*** 2.00 34.02*** 182.66*** 11.18*** 26.10*** 194.05*** 11.40*** 38.23*** 244.75*** 5.19*** 12.41*** 313.54*** 4.39***
38.54*** 59.37*** 3.11* 23.50*** 102.13*** 5.88** 12.03*** 176.19*** 7.03*** 35.05*** 66.92*** 10.62*** 32.14*** 201.85*** 9.95***
*, **, *** Significant at the 0.05, 0.01 and 0.001 probability levels, respectively.
Figure 3. Effect of bulk density and water content on soil strength of San Emigdio sandy loam obtained from Riverside, California as measured by the penetrometer. Bulk density: (I) s 2.1 Mg my3 ; (e) s 1.7 Mg my3 ; (`) s 1.5 Mg my3.
50
A. A. SALIH
Figure 4. Prosopis chilensis root tip location and plant height during the first 30 days of root elongation through mediums with soil layers that were compacted at different bulk densities. Each data point represent the arithmetic mean of six plants (soil obtained from Riverside, California).
root growing in the 1.71 Mg my3 soil layer was excluded from passing through for 2 weeks. Its rate of growth decreased to less than 0.14 cm dayy1 compared to 1.5 cm dayy1 for growth in the glass beads above the soil layer. The shape of the root system was also affected. Several secondary roots were initiated above the compacted soil layer and extended along its surface. Roots which managed to penetrate the compacted layer resumed growth at more or less the same rate as noted prior to meeting the high density soil layer. A similar finding was reported for cotton (Taylor, 1969). A high strength soil pan excluded cotton root growth through the pan for 20 days. The results presented in these studies, relative to climatic and soil conditions prevailing in arid and semi-arid regions of sub-Saharan Africa, suggest that P. chilensis species with a high rate of root development could be successfully grown in afforestation and desert encroachment programmes. Although the amount of rainfall in these regions is variable, the practice of planting crops at the first significant rains could be adapted. As the rainy season usually begins in June, reaching its peak in August, by early July enough moisture is available for transplanting, and within a period of 30 days depth of rooting would be 51 cm (using 1.7 cm dayy1 as a guide). During this period total root length density for the y0.05 MPa soil matric potential increased from 0.19 to 0.49 cm cmy3 (Table 3), or 150% in the upper portion of the root system. In these regions, soils often have compacted layers 35 to 100 cm thick located at 35 to more than 50 cm below the surface. Bulk densities of 1.7 and 1.9 Mg my3 can be used as guides since roots were able to penetrate these soil layers provided reasonable moisture was available in the compacted layer for a sufficient amount of time.
ROOT AND SHOOT GROWTH OF PROSOPIS CHILENSIS
51
Table 4. Prosopis chilensis dry weight of total roots (g) for five sampling dates as affected by bulk density and soil matric potential of San Emigdio sandy loam from Riverside, California
Sample number: Treatment w
1
2
3
4
5
A1 B1 A2 B1 A3 B1 A4 B1
4.49 a 3.37 b 2.73 c 2.87 c 2.93 ab 3.26 a 2.58 b 1.87 c 2.72 a 2.48 a 1.80 b 1.79 b 2.40 a 1.93 b 1.61 c 1.28 d
5.75 a 4.48 b 3.84 b 2.81 c 3.41 a 2.68 b 2.47 b 2.49 b 2.61 a 2.60 a 2.37 a 2.12 b 2.02 a 1.64 ab 1.48 ab 1.63 b
8.52 a 7.59 ab 7.28 b 4.94 c 5.12 b 4.83 c 4.70 c 4.49 d 3.63 b 3.97 b 3.22 b 3.69 b 3.14 b 2.93 b 2.61 b 2.86 b
10.35 a 9.61 b 8.31 c 7.79 d 4.95 b 5.94 c 5.58 c 4.87 d 3.82 a 5.68 b 4.81 ba 3.69 a 2.74 a 4.63 b 3.97 b 3.26 a
11.03 a 9.14 b 7.17 c 6.25 d 5.49 a 4.88 ab 4.79 ab 4.60 b 4.70 a 4.63 a 4.12 a 3.96 a 4.23 a 4.25 a 4.08 a 3.18 a
A1 B2 A2 B2 A3 B2 A4 B2 A1 B3 A2 B3 A3 B3 A4 B3 A1 B4 A2 B4 A3 B4 A4 B4
w Bulk density (Mg my3 ): A1, 1.5; A2, 1.7; A3, 1.9; A4, 2.1. Soil matric potential (MPa): B1, y0.05; B2, y0.10; B3, y0.50; B4, y1.0. Means followed by the same letter are not significantly different at the 0.05 level according to Duncan’s Multiple Range Test.
Conclusions Increasing bulk density or decreasing soil matric potential had a negative effect on root length density and shoot dry weight of Prosopis chilensis. The effect of bulk density was more pronounced at high soil matric potential. Differences in total dry weight of roots growing in soil layers with different bulk densities became insignificant as soil matric potential decreased to y0.5 and y1.0 MPa. A soil layer of bulk density 1.7 Mg my3 prevented root growth from passing through the layer for a period of 2 weeks. Several secondary roots, which were initiated from a mat of roots growing on the surface of the compacted layer, were able to pass through at several points. Prosopis chilensis could be successfully used for afforestation on soils with high subsoil bulk density provided that reasonable soil moisture is available in the compacted soil layer at transplanting and for a sufficient time. The author wishes to thank Prof. A. G. T. Babiker, Dr I.A. Ali of the Arid Land Research Center of Tottori University and Mr A. ArWahab for useful comments and critical reading of the manuscript.
References Agrar UND (1975). Feasibility study for mechanized farming in Southern Darfur, Southern Kordofan, and Bahr El Ghazal. Hydrotechnik GMBH.
52
A. A. SALIH
Fagg, C.W. & Stewart, J.L. (1994). The value of Acacia and Prosopis in arid and semi-arid environments. Journal of Arid Environments, 27: 3]25. Felker, P., Clark, P.R., Osborn, J.F. & Cannell, G.H. (1984 ). Prosopis pod production}comparison of North American, South American, Hawaiian, and African germplasm in young plantations. Economic Botany, 38: 36]51. Mustafa, A.R.F.M. (1988). Prosopis chilensis, A tree for Sudan’s dry zone. In: Habitat, M.A. & Saavedra, J.C. (Eds), The Current State of Knowledge on Prosopis juliflora, pp. 163]167. International Conference on Prosopis, Recife, Brazil, 25]29 August 1986. 554 pp. Newman, E.I. (1966). A method of estimating the total length of roots in a sample. Journal Applied Ecology, 3: 139]145. Said, D.L. (1980). A root chamber study of cowpeas grown in soils with layers compacted at different bulk densities. Ph.D. thesis, University of California, Riverside. Steel, R.G.D. & Torrie, J.H. (1980). Principles and Procedures of Statistics. A biometrical approach (2nd Edn). New York: McGraw-Hill Book Co. 598 pp. Taylor, H.M. (1969). The rhizotron at Auburn, Alabama. A plant root observation laboratory. Auburn University Agriculture Experimental Station Circular, 71. Taylor, H.M. & Gardner, H.R. (1963). Penetration of cotton seedlings tap root as influenced by bulk density, moisture content and strength of soil. Soil Science, 96(3): 153]156. Taylor, H.M., Mathers, A.C. & Lotspiech, F.B. (1963). Pans in the Southern Great Plains soils. 1. Why root-restricting pans occur. Agronomy Journal, 56: 328]332. Trouse, Jr., A.C. (1971). Soil conditions as they affect plant establishment, root development and yield. In: Barnes, K.K. et al. (Eds), Compaction of Agricultural Soils, pp. 241]252. An ASAE monograph. Joseph, MI: American Society of Agricultural Engineers. 407 pp. Unger, P.W. & Kasper, T.C. (1994). Soil compaction and root growth. Agronomy Journal, 86: 759]766.