Water use assessment in alley cropping systems within ... - Springer Link

Agroforest Syst (2012) 84:243–259 DOI 10.1007/s10457-011-9458-4

Water use assessment in alley cropping systems within subtropical China Ying Zhao • Bin Zhang • Robert Hill

Received: 21 April 2011 / Accepted: 1 November 2011 / Published online: 11 November 2011 Ó Springer Science+Business Media B.V. 2011

B. Zhang Key Laboratory of Crop Nutrition and Fertilization of Ministry of Agriculture of China, Institute of Agricultural Resources and Regional Planning, Chinese Academy of Agricultural Sciences, Beijing 100081, People’s Republic of China

monoculture peanut cropping (P), monoculture younger trees (T1), monoculture older trees (T2), peanut intercropped with younger trees (T1P), and peanut intercropped with older trees (T2P). A multilayered water balance model, with water movement between soil layers, was implemented by the measurement of soil water potential using sets of tensiometers during the periods from March 1999 to December 2002. The spatial and temporal variations of soil water regime indicated that the trees used soil water below the 60-cm soil depth and alleviated the water stress. The direction of soil water movement indicated that soil water moved to the tree row, which indicated that trees competed with peanuts for water, especially during the seasonal drought period. Water competition was related to the tree spacing and tree age. Compared to the tree monoculture systems, the alley cropping system significantly influenced water budget components and water use patterns, as indicated by the increased evapotranspiration (6–11%), and decreased net drainage (7–45%), water storage (6–29%), and runoff (50–60%). Furthermore, alley cropping systems encouraged the rapid growth of trees, and depressed the biomass and yield of peanuts by 20–50% associated with tree shading effects. The results suggest that competition for water and light must be taken into account when optimizing the alley cropping system.

R. Hill Department of Environmental Science and Technology, University of Maryland, College Park, MD 20742, USA

Keywords Alley cropping systems Water balance Water competition Evapotranspiration

Abstract Alley cropping systems may influence soil water movement and the water budget because of its complex interactions between crop and tree rooting systems. The objective of this paper was to evaluate water balance and water competition in an alley cropping system, consisting of deciduous tree wild jujube (Choerospondias axillaris) and economic crop peanut (Arachis hypogaea) within subtropical China. Five treatments (20- by 6-m plots) with three replications were included in this study. The treatments were

Y. Zhao B. Zhang State Key Laboratory of Soil and Sustainable Agriculture, Institute of Soil Science, Chinese Academy of Sciences, P.O. Box 821, Nanjing 210008, People’s Republic of China Y. Zhao (&) Key Laboratory of Plant Nutrition and Agrienvironment in Northwest China, Ministry of Agriculture, Northwest Agriculture and Forestry University, Yangling 712100, Shaanxi, People’s Republic of China e-mail: [email protected]

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Introduction Under the increasing pressures of growing population and industrial encroachment of cultivated lands, China has initiated programs to convert rolling topography land areas into arable land. However, problems with this approach are that many of these land areas experience uneven rainfall distribution that causes soil erosion in the rainy season and seasonal drought in the dry season within subtropical China (Zhang and Zhang 1995). Alley cropping systems where the agricultural crops are grown between widely spaced rows of trees may be an alternative land use to local monoculture cropping as the trees may prevent soil erosion and use soil water from deep soil layers (Xie 1989; Rao et al. 1998). Alley cropping systems have been intensively studied in Africa and Asia since the mid-1980s with respect to multipurpose tree screening, system management and component interactions (Kang et al. 1990; Puri and Nair 2004). Field research has shown that alley cropping systems might improve the system productivity and soil fertility (Kang and Wilson 1987), prevent runoff and soil erosion (Ghosh et al. 1989; Narain et al. 1998; Rao et al. 1998), and use water stored deep in the soil profile (Smith et al. 1997). The root uptake of water and nutrients from deep soil layers and the transfer and release to superficial soil layers has been shown to be beneficial to alley crops (Carbon et al. 1980). However, the alley cropping systems have also shown negative impacts on crop production. The trees and crops competed for light aboveground and for water and nutrients underground if the intercropped components were not compensatory in time and space (Sanchez 1995). The net benefits of alley cropping systems were dependent on the type of trees and management practices such as increasing tree space and pruning to minimize competition (Ong et al. 2002). The competition for water, nutrients and light between the system components was the main reason for the decreasing crop yields (Sanchez 1995), and water competition might be of great concern when adapting alley cropping systems in subtropical China. Numerous researchers have reported on water use in agroforestry (Kang et al. 1990; Van Noordwijk and Lusiana 1999; Kho 2000). The magnitude of water competition between the components in alley cropping systems is mainly dependent on the architecture of roots (Mclntyre et al. 1997). Root structure

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Agroforest Syst (2012) 84:243–259

observation, field water balance method, water simulation models and recently developed methods of sap flow measurements have been previously applied to identify the processes of water competition between tree and crop and to determine the competitiveness of tree roots (Maraux et al. 1998). Livesley et al. (2004) reported that soil water content beneath an alley cropping system increased with distance from the tree row. The possible reasons were attributable to preferential water uptake beneath the canopy and reduced rainfall input through canopy interception, but these hypotheses were not confirmed either by experiment or by modeling. Simulation models based on soil– plant-environment interactions might be helpful in understanding the process and function of alley cropping system. Special features of intercrop modeling are the resource partitioning between the components (Caldwell and Hansen 1993) and the ability to simulate the responses of plants to intercropping environmental conditions. Water balance models are well known and have been summarized in a number of publications for forestry systems (Landsberg and Waring 1997; Whitehead 1998) and for agroforestry systems (Schlegel et al. 2004). The assessment of resource competitiveness among water, fertilizer and light were previously seen as paradoxical because of differences in species and monitoring times (Liu et al. 1996; Jose et al. 2000; Ye et al. 2001), and required a thorough understanding of resources, particularly water utilization between the plant species. Water competition in earlier dry seasons has been observed, but little information exists on how to arrange alley cropping systems so as to reduce water competition and still fully use the potential natural resources. Soil water content is expected to vary spatially with distance from a tree row and those differences would exist according to tree species or tree age. The competition for water in the alley cropping systems are related to the root systems and their water uptake capabilities from deep soil layers. Partitioning of soil water among components of agroforestry systems may be determined by knowing the root distribution, canopy structure and, therefore, tree management (Carbon et al. 1980). By modifying the spatial arrangement of trees and/or by the temporal separation of maximum water needs activity by tree and crop roots, it is possible to maximize crop water use by exploiting times when the trees have a need for less water.


Our previous research has shown that the alley cropping system consists of Choerospondias axillaris and peanut in subtropical China was beneficial in decreasing soil nitrogen loss, however, negative in maintaining peanut crop yield owing to the shading effect (Zhao et al. 2006). The objectives of this study were to assess water balance and water competition in this alley cropping system by an integrated method including: (1) measurement of sap flow of trees, (2) comparison the temporal and spatial distributions of soil water with distance from a tree row, (3) visualization of the direction of soil water movement, and (4) calculation of water balance in the alley cropping system. This study also determined the erosion control effect by monitoring runoff and soil loss, and evaluated water uptake of tree’s root systems by investigating root distribution and dynamics.

Materials and methods Experimental site and design The experimental site is located at the Red Soil Ecological Experimental Research Station, Chinese

245

Academy of Sciences (28°150 N, 116°550 E, 55 m asl) (Fig. 1). The climate at the research area is representative of subtropical China with a minimum and maximum monthly average temperature of 5.9°C in January and 30.0°C in July, respectively. The average annual potential evaporation is about 880 mm, which is much less than the annual rainfall 1795 mm. Rainfall is mostly concentrated from April to early July and accounts for about 50% of the average annual precipitation. Seasonal drought occurs frequently from late July to September during periods of less than 20% of the average annual precipitation, but the potential evaporation accounts for 45% of the annual total. The experimental site was on a gentle slope of 5° and the land use was grassland with sparse Mason pine (Pinus massoniana L.) before March 1999 when the experiment started. The soil is an Alumi-Orthic Acrisol (FAO/UNESCO 1988). The soil is 450 cm deep, low in pH and nutrients and water retention capacity (Table 1) and is well structured as indicated by the sub-angular aggregates in the soil profile. Soil saturated hydraulic conductivity ranges between 7.0 and 26.3 cm d-1. Available soil water between matic potential -100 (pF = 1.0) to -15,000 cm (pF = 4.2) ranged from 18% in the surface soil (0–30 cm) to 10% in the deeper soil profile.

Fig. 1 An illustration of China indicating the location of the experimental site at the Red Soil Station

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Table 1 Soil physical and chemical properties in the 0–200 cm soil profile of the research area at the initiation of the experiment CEC (cmol kg-1)

Ks (cm day-1)

h (cm3 cm-3) pF = 1.0

h (cm3 cm-3) pF = 4.2

26.3

–

–

Depth (cm)

Sand (%)

Silt (%)

Clay (%)

BD (g cm-3)

pH

SOC (%)

0–7

19

43

38

–

4.3

0.42

8–22

17

43

40

1.35

4.4

0.24

6.2

22.0

41

23

23–45

14

40

46

–

4.5

0.15

7.6

12.3

–

–

46–95

12

36

52

1.2

4.5

0.12

9.2

9.3

44

26

96–150

11

36

53

1.27

4.5

0.11

9.9

7.0

44

29

151–200

10

36

54

1.46

4.5

0.25

15.8

12.3

44

34

6.2

BD bulk density, SOC soil organic carbon, CEC cation exchange capacity, Ks saturated hydraulic conductivity, pF log matric potential in hPa, – data not available Table 2 Descriptions of the treatments utilized in the experimental study Code

Treatment

Descriptions

P

Peanut

40 rows; 30 holes row-1; 2 peanuts in a hole, peanut spacing 0.5 m 9 0.2 m

T1

4 year C. axillaris

T2

9 year C. axillaris

3 tree rows; 3 trees in a row; tree spacing 6 m 9 2 m 3 tree rows; 3 trees in a row; tree spacing 6 m 9 2 m

T1P

4 year C. axillaris ? peanut

3 tree rows; 3 trees in a row; tree spacing 6 m 9 2 m; first peanut row 0.75 m from tree row; peanut spacing 0.5 m 9 0.2 m

T2P

9 year C. axillaris ? peanut

3 tree rows; 3 trees in a row; tree spacing 6 m 9 2 m; first peanut row 0.75 m from tree row; peanut spacing 0.5 m 9 0.2 m

The experiment involved five treatments, including two alley cropping systems and three controls (Table 2). The alley cropping systems were composed of peanut (Arachis hypogaea) intercropped with younger trees (T1P) and older trees (T2P); the controls were monoculture peanut cropping (P), monoculture younger trees (T1) and monoculture older trees (T2). The trees are deciduous wild jujube (C. axillaris) which grow rapidly, produce edible fruits and regenerate readily after being pruned for forage and fuel wood, and therefore are recommended for use in the local alley cropping systems (Liu et al. 1996). Peanut is a cash crop, being widely adopted by local farmers in the low hills. The young and old trees were 4 and 9 years old, respectively, at the time of transplanting from the nursery in 1999. Trees with different ages

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were chosen as they might cause large variations in soil water regime due to different water uptake potentials (Wei et al. 2007). The trial plots were established in a randomized complete block design with three replicates (Fig. 2). The plots, 6 m wide on the contour and 20 m long along the slope, were enclosed with cement plates, extending 50 cm into the soil and projecting 30 cm above the ground to reduce interferences between plots. The distances between the plots were 6 m, with 2 m from each adjacent area acting as guard areas. The trees were transplanted at an 6-m row spacing along the contour and 2 m within the row. The trees were separated with a piece of plastic sheathing along the tree row vertically buried to a depth of 70 cm to minimize the interference of the tree roots in the tree row. Peanuts were planted at a distance of 75 cm apart from the adjacent tree row in the alley cropping systems at the same density as adopted by local farmers in the peanut alone plot (spacing 50 cm between crop rows and 20 cm between crop hills). Peanuts were sowed in early April and harvested in early August and weed control was done by hand. Fertilizers were applied only to peanut rows at rates of 173 kg ha-1 N, 43 kg ha-1 P and 112 kg ha-1 K. Measurement of runoff and soil loss A tipping bucket system was equipped at the lower outlet of each plot to measure runoff and soil loss. The tipping bucket system was constructed following the design of Khan and Ong (1997). The tipping numbers were recorded using magnetic counter, read after rainfall events and converted to the runoff volume in mm. Two mesh bags with apertures smaller than 106 lm were placed on one side of the buckets to collect sediments. Water from the buckets was


247

Fig. 2 A schematic of the experimental layout (left) and the location of tensiometers (right) in the field

directed into the sediment tank constructed under the tipping bucket system. The sediment tank was 1.5 m long, 1 m wide and 50 cm deep and water flowed over the edges. The sediments were weighted after being oven-dried at 105°C. Measurement of tree sap flow Tree sap flow was measured using a thermal dissipation technique (Granier 1987; Lu 1997) to determine transpiration. Heat-dissipation probes were used and inserted into the stem and vertically placed 100 mm apart between two probes. Each probe contained a heater and a copper–constantan thermocouple. The upper probe is heated at constant power while the lower one is not powered and measures the ambient temperature of the wood. The sap flux density in the vicinity of the heated probe is related to the temperature difference between the two probes. Three trees in the block at the lowest slope position were selected according to trunk diameter. The probes were connected to a datalogger (Delt T2e, Delt T Inc. UK) and data were recorded every 30 min. Measurement of plant growth performances Tree stem height and diameter at breast height (dbh), and canopy width as well as peanut growth performance and yield were measured each year using core methods (Liu et al. 1996). In 2003, a soil core method was used to

measure root distribution of crop and trees (Jose et al. 2000). Root distribution was described using an average of 3–4 replicates in terms of total length, density and root weight measured with time within different depths of the soil profile. The biomass of leaves, branches, trunks and roots of C. axillaris were sampled and calculated according to the relationship between tree performance and its biomass (Feng et al. 1993). Measurement of soil water regime Soil water content and soil water potential were measured using a neutron moisture gauge and septum stopper tensiometers, respectively. Sets of stopper tensiometers and neutron access tubes were installed on either side of the central tree row in each plot, situated 50 cm away from the tree, at distances of 0, 1, 2 and 4 m from a tree row in the T2 and T2P plots and at distances of 0, 1 and 2 m in the P, T1 and T1P plots. In the peanut alone treatment (P), the sets of access tubes and tensiometers were installed at equivalent locations similar to those in the younger trees treatment (T1 and T1P). Soil water content was measured using the neutron moisture gauge after field calibration in the specified soil. The neutron access tubes were 200 cm deep. Soil water content was recorded every 5 days at an interval of 10 cm within the upper 100 cm soil depth, and at 20 cm intervals from 100 to 200 cm soil depths. Additional measurements of soil water content in the

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top 20 cm was determined gravimetrically. Soil water potential was measured using septum stopper tensiometers manufactured by the Institute of Soil Science, CAS (Nanjing, China). The tensiometer was designed to use a hypodermic needle to measure the suction in the tube through a special septum type stopper (Wei et al. 2007). A special silicon stopper was used to close the tube and the stopper remained airtight. The porous ceramic cup enabled measurements of less than -80 kPa to be made. An electronic hand held meter (SOILSPEC101, Australia) was equipped with a pressure transducer and was connected to the tensiometer by inserting the fine needle into the stopper to measure negative soil water potential. The soil depths of the tensiometer sets were 10, 30, 100, 150 and 180 cm. The tensiometers were installed close to the neutron access tubes and spaced 10 cm in between. Soil water potential was manually recorded at 9:00 a.m. every 2 days.

slope, qv is the vertical water flow, and c is direction angle of qr (0–360°) with 90° being the vertical direction downward and c value increased with the counter clockwise direction (Fig. 3). Soil water flow direction was visualized using isolines of the c values interpolated by ordinary kriging. Water flux was calculated using Darcy’s law (Eq. 3) (Landsberg 1986):

Determination of water fluxes and directions along the slope

k ð hÞ ¼ a

A resultant total water flux (qr) was calculated using the water flux in a vertical direction to water level and along the slope direction according to the geometrical relationship (Fig. 3; Harr 1977; Sun 1994). The resultant water flux and direction angle were calculated by the following equations (Eqs. 1, 2): qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qr ¼ ðqd þ qv sin aÞ2 þðqv cos aÞ2 ð1Þ c ¼ cos 1 ðqv cos a=qr Þ a

ð2Þ

where qr is the resultant total water flux (cm h-1), a is the slope angle in degree, qd is the water flow along the

q ¼ kðhÞ

ow oz

ð3Þ

where q is the water flux (cm h-1), k(h) is the unsaturated hydraulic conductivity, W is water potential and z is the distance between soil depths in a tensiometer set or the distance between the tensiometer sets. k(h) was determined in situ by the instantaneous profile method and expressed as an exponential function (Lei et al. 1988; Soares and Almeida 2001): b h hs

ð4Þ

where h is the measured soil water content, hs is the saturated water content, and a, b are empirical coefficients related to soil texture. Calculation of system evapotranspiration Evapotranspiration (ET) was estimated using a soil water balance model which is expressed as follows (Eq. 5) (Palomo et al. 2002): ET ¼ P R CI Rs D DS

ð5Þ

where P (mm) is precipitation measured by the weather station located 500 m away from the 270o vertical, upward

1m

0.5 m

2m

Ground surface

225o

315o

tree 0m

upslope 180o

0o(360o) downslope

4m

qr

qv 900

qd

qr

45o

135o

90o vertical, downward

Fig. 3 Graphical representation of vertical (qd), downslope (qv), and resultant (q r) water fluxes (left) and the direction of the resultant water flux (right)

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249

experimental plot; R (mm) is runoff measured using the tipping bucket system; DS (mm) is change in soil water storage in the depth of 0–200 cm calculated by the neutron moisture gauge measurements; CI (mm) is tree rainfall interception estimated by analytical model and statistical data in this area (Yi et al. 1996); and D (mm) is net deep drainage (the difference of drainage component and capillary rise at the 200 cm soil depth) (Soares and Almeida 2001) was estimated using Eq. 6: ow oz

D ¼ qDt ¼ kðhÞ Dt

resource A to the monoculture crop (Kho 2000). In this paper, we evaluated the water resources. Analysis of variance (GLM Univariate, SPSS) was performed for the effect of alley cropping treatment and space differences on water use. Results Runoff and soil erosion

ð6Þ Runoff and soil loss varied with year and land use (Table 3). In general, the largest annual runoff and soil loss occurred in 2000, followed by decreasing amounts in 2002 and 2001. The time period when soil erosion exhibits the greatest potential for loss occurs during the first flowering period of peanut in early June and after its harvest in late summer. We noticed that although the annual runoff coefficient (ratio between runoff and precipitation) was low (Table 3), the runoff coefficient was much higher during the April to May when runoff often occurred. The forestry systems (T1 and T2) produced 40–50% greater runoff than the alley cropping systems (T1P and T2P), while the latter caused 250–350% greater soil loss than the former. Mean runoff in three years (2000–2002) decreased with 62.6% in T1P than T1 and with 56.1% in T2P than T2. Compared with the peanut monoculture system (P), the peanut alley cropping systems (T1P and T2P) decreased soil loss by 11–23% and increased runoff by 6–21%. Compared with P, the mean soil loss decreased with 25.3% in T1P and with 32.0% in T2P. Those results were consistent with a previous report by Zhang and Zhang (1995). The greater runoff in the forestry systems might be attributed to lower coverage

where Dt is the interval of time, others variables are the same as in Eq. 3; Rs (mm) is the inner horizontal flow and was calculated according to Eq. 6, where z is the horizontal distance and k(h) in the horizontal direction is assumed to the same as the vertical direction. Utilizing Rs, lateral water flow with different distances from the tree row is particularly taken into account in Eq. 5. Finally, the ET was calculated with Eq. 5 after estimating the values of the other components of the soil water balance equation. Resource competitiveness The resource interaction (TA) is a measured of the net effect of trees on the availability of a resource A. The resources interaction to the crop was analyzed to determine the competitiveness between tree and crop in the agroforestry system. TA is defined as TA ¼

AAF AS AS

TA [ 1

ð7Þ

where, AAF is availability of resource A to the crop in the agroforestry system, and AS is availability of

Table 3 Runoff and soil sediment losses for the peanut monoculture crop (P), the young tree monoculture (T1), the mature tree monoculture (T2), and the two alley cropping systems (T1P and T2P) during the 2000, 2001, and 2002 experimental periods Treatment

2000 (rainfall 1928.4 mm)



Runoff (mm)

Runoff (mm)

Runoff (mm)

RC (%)

Soil loss (t km-2 a-1)

RC (%)

Soil loss (t km-2 a-1)

RC (%)

Soil loss (t km-2 a-1) 212.3 ± 34.5

P

146.9 ± 5.3

7.6

677.7 ± 22.1

19.9 ± 4.9

1.2

58.2 ± 14.8

66.6 ± 9.6

3.9

T1

284.6 ± 23.0

14.8

148.7 ± 17.6

67.5 ± 22.2

4.2

29.7 ± 10.1

150.8 ± 54.8

8.9

66.8 ± 25.0

T2

303.6 ± 9.7

15.8

150.8 ± 14.4

83.3 ± 28.4

5.2

35.9 ± 12.8

145.1 ± 47.6

8.6

62.7 ± 21.8

T1P

116.7 ± 8.7

6.1

605.7 ± 70.6

11.1 ± 4.4

0.7

29.8 ± 12.3

25.9 ± 2.8

1.5

73.0 ± 8.1

T2P

137.4 ± 3.4

7.1

520.3 ± 14.7

21.8 ± 5.6

1.4

55.1 ± 15.1

28.6 ± 3.9

1.7

69.7 ± 9.9

Mean ± standard deviation of three replicates of the field treatment RC runoff coefficient

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250


and the formation of surface seals which could decrease infiltration. Tillage for peanut cropping may increase infiltration, but may increase soil loss. The low soil loss in the alley cropping system, compared with P treatment, might be the result of the residual coverage provided by the tree rows that would hinder soil loss. Sap flow The sap flow of C. axillaris varies diurnally with rainfall (Fig. 4). The average sap flow during the observation period was 10.5, 26.9, 29.0 and 39.8 l day-1 in T1, T1P, T2 and T2P, respectively. The sap flow was greater in the alley cropping systems than in the forestry monoculture systems. The differences between tree alley and monoculture systems were greater in the young forestry systems with the flow rate twice greater in T1P than T1. Sap flow has been shown to be closely related to soil water potential at 100 cm soil depths (r = 0.737 and P \ 0.01, Zhao et al. 2005). The results indicated that the alley cropping systems mainly used deeper soil water by a more extensive and deeper root system.

60

T1

T1P

Temporal variation of soil water regime The seasonal variation of soil water content exhibited three phases corresponding to precipitation and temperature. Figure 5 shows the soil water content for the 1-m distance from the tree row and represents the most intensive interaction zone between peanuts and trees. The first phase from March to mid-June was characterized by surplus rainfall and moderate temperatures in the rainy season. The profiled soil water content was found to be stable during this period, e.g., in the topsoil (0–30 cm) around 0.30 cm3 cm-3 with little variation (CV = 5.2%). The second phase from mid-June to late September was characterized by the highest temperatures, but the lowest rainfall in the dry season when soil water content began to dramatically decrease. The average profiled soil water content, especially in the upper soil horizon (0–60 cm) was lower than previously in the first phase and had larger variation (CV = 8.6%). For instance, the 0–30 cm soil water content was as low as 0.21 cm3 cm-3 in early July when the crop began to grow and might suffer water stress. The third phase from late

T2

T2P

Rainfall

Sap flow (l day -1)

40 20 0 60 40 20

Rainfall (mm)

0 60

40

20

0 2001-6-15

2001-6-29

2001-7-13

2001-7-27

2001-8-10

2001-8-24

2001-9-7

Time (days) Fig. 4 Sap flow of trees (C. axillaris) in the monoculture tree systems (T1, T2) compared with the alley cropping systems (T1P, T2P) and corresponding rainfall

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251

water from the deeper soil depths. Changes in soil water storage varied with distance from the tree row (Fig. 6). The general pattern was of increasing storage in spring season, alternate periods of accretion and depletion from March to June, followed by depletion of stored water from June to September (i.e., seasonal drought period), and then accretion in winter season. The average soil water storage (0–200 cm) of T2P in 0-, 1-, 2-, and 4-m distance from the tree row was 642.0 (CV = 7.7%), 670.2 (CV = 6.4%), 669.0 (CV = 5.7%) and 678.4 mm (CV = 3.9%), respectively, indicating that there was an increased trend with the increased distance from the tree row. The average soil water storage for P, T1, T2, T1P and T2P in 1-m distances from tree row was 668.0 (CV = 3.4%), 680.9 (CV = 3.8%), 656.6 (CV = 5.1%), 662.5 (CV = 3.7%) and 670.2 mm (CV = 6.4%), respectively, indicating that the wettest soil profiles was in the young tree monoculture system and driest soil profile was in the more mature tree monoculture system.

September to following March was characterized by the lowest temperatures and medium amounts of rainfall. The soil water contents were in between the first and the second phases.

Spatial variation of soil water regime Soil water content increased with increased soil depth (Fig. 5). In comparison with the peanut monoculture (P), soil water content in the alley cropping system (e.g., T2P) was higher in the topsoil, but lower at the 100-cm soil depth. These differences indicated that the alley cropping system could retain more water in the soil surface partly owing to lower runoff, but used more water at the greater soil depth. Furthermore, soil water content in the alley cropping systems (e.g., T2P) was higher above the 60-cm soil depth than the forestry monoculture systems (e.g., T2), but lower below the 100-cm soil depth, also indicating that trees in the alley cropping systems might be using more

P

0.40

0.40

30 cm

60 cm

T2 0.36

0.36

0.32

0.32

0.28

0.28

0.24

0.24

3

-3

Soil volumetric water content (cm cm )

T2P

0.20 0

50

100

150

0.40

200

250

300

350

0

50

100

150

0.40

100 cm

200

250

300

350

180 cm

250

0.36

0.36

200

0.32

0.32

150

0.28

0.28

100

0.24

0.24

50

0.20

0.20 0

50

100

150

200

250

300

350

Rainfall (mm)

0.20

0 0

50

100

150

200

250

300

350

Time (days) Fig. 5 Soil water content at 1-m distance from the tree row for the peanut monoculture crop (P), the mature tree monoculture (T2) and alley cropping (T2P) treatments in 2000 for the 30-, 60-, 100-, and 180-cm soil depths and corresponding rainfall

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Agroforest Syst (2012) 84:243–259 P

0m

750

T1P T2P

T1

1m

750

700

650

650

600

600

550

550 400

2m

750

4m

750

350 300

700

700

650

650

250 200 150 100

600

600

Rainfall (mm)

Soil water storage (mm)

T2 700

50 550

550 0

50

100

150

200

250

300

350

0 0

50

100

150

200

250

300

350

Time (days) Fig. 6 Soil water content storage at the 0-, 1-, 2-, and 4-m distance from the tree row for the peanut monoculture crop (P), the young tree monoculture (T1), the mature tree monoculture

(T2), and the two alley cropping systems (T1P and T2P) in 2000 and corresponding rainfall

Direction of soil water movement

90° with the increasing soil profile in the dry season, indicating that the soil water moved towards the tree row at the topsoil and downwards at the deeper soil. In contrast, the water movement in T2 treatment was predominately directed towards the tree row for the whole profile, indicating strong water use from the deeper soil layers by the trees. In the T1P treatment, the water moved downwards (45°–180°) in the topsoil, and was highly variable (45°–315°) in the subsoil, especially during the seasonal drought period. In contrast, the direction of water movement in T2P changed frequently for the whole profile for all periods, indicating that the big trees intensified their water use. Compared with the tree monoculture system, the pattern of water movement in the peanut monoculture system was very similar to the peanut alley cropping systems since the investigated soil profile was located in the area of peanut growth (i.e., 1-m distance from the tree low; Fig. 2). For instance, the angles of water flow direction in P, T1P and T2P

The plant species competing for soil water was further inferred from examining the directions of soil water movement. This is demonstrated by the soil pedon at the 1-m distance from the tree row which represents the central area influenced by both tree and crop. Figure 7 shows the water movement direction in the 2-m soil profile during the 1-year time period by the angles of water flow at the 1-m distance from the tree row where trees were located in the upslope, and 0° indicated water moves towards the downslope direction (Fig. 3). In the P treatment, the water moved downwards due to water recharge in the rainy season, especially in the deeper soil (50–200 cm), the lateral flow along slope dominated with direction angle of 45°–90°. In the dry season, however, the water moved upwards to the tree row from deep soil to topsoil with angle from 270° to 135°. In the T1 treatment, the direction of water movement changed from 180° to

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253

P

0 -50

270 225 180

-100

135 90

-150

Direction angle of water flow

Depth (cm)

315

45 0

-200 50

100

150

200

250

300

350

System evapotranspiration

Time (days) T1

0

270 225 180

-100

135 90

-150


Depth (cm)

315

-50

45 0

-200

50

100

150

200

250

300

350

Time (days) T2

0

270 225

-100

180 135 90

-150

45


Depth (cm)

315

-50

0

-200

50

100

150

200

250

300

350

Time (days) T1P

0

270 225 180

-100

135 90

-150

45


Depth (cm)

315

-50

-200

0

50

100

150

200

250

300

350

Time (days) T2P

0 -50

270 225 180

-100

135 90

-150

45


315

Depth (cm)

treatments decreased radially from the deep soil to the topsoil. It was obvious that the alley cropping systems had higher angles ([270°) of upwards flow than the peanut monoculture system, indicating strong tree ‘‘hydraulic lift’’ effects in the alley crop systems which may beneficial to the growth of the shallow-rooting peanut.

0

-200

50

100

150

200

250

300

350

Time (days)

Fig. 7 Contour plots of the resultant water flow directions at 1-m distance from tree row for the peanut monoculture crop (P), the young tree monoculture (T1), the mature tree monoculture (T2), and the two alley cropping systems (T1P and T2P) in 2000 within the 0–200 cm soil depths

Table 4 illustrated the water budget components for different treatments at the different distances from the tree row. In comparison to the tree monoculture system, the alley cropping systems increased the evapotranspiration (ET) by 6–11%, and decreased the net drainage by 7–45% and decreased the water storage by 6–29%. The change in soil water storage for the 0–200 cm soil depth was attributed to annual differences in rainfall. The amount of rainfall was 1928.4 mm in 2000, the change of soil water storage was positive, indicating water was being stored. In contrast, the amount of rainfall was 1614.5 mm in 2001, the negative value for change of soil water storage indicated more water being consumed than stored. The annual water budgets from 2000 to 2002 indicated that 10–15% of rainfall was participated into surface water runoff, 5–20% into canopy interception and 65–85% into infiltration. In the infiltration component, 15–20% of water was drained off the profile, 5–10% was stored within the soil profile, with a residual part of the stored water being used in ET (data not shown). In addition, the water movement from the soil profile by capillary rise accounted for 5–10% of the rainfall quantity indicating that deceases in water evaporation were partly compensated by capillary rise water. The evapotranspiration was at a maximum at the 1-m distance from the tree row indicating that the root density was at a maximum in this zone leading to intensified water use. The introduction of trees in the alley cropping system increased water infiltration in the zone far away from the tree row, which might be beneficial to the peanuts as more water should be available for absorption. The soil within the tree row zone had decreased water infiltration, which further decreased the net drainage because of increasing water by capillary rise over decreasing deep drainage. The analysis of impacts of either alley cropping treatments or space tree row on water use indicated that there

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Table 4 Water balance at the 0, 1, 2, and 4 m distance from tree row along the slope for the peanut monoculture (P), the young tree monoculture (T1), the mature tree monoculture Treatment

Distance (m)

2000 (rainfall 1928.4 mm) ET (mm)

P

T1

T2

T1P

T2P

D (mm)

(T2), and the two alley cropping systems (T1P and T2P) during the 2000, 2001, and 2002 experimental periods 2001 (rainfall 1614.5 mm)

DW (mm)

ET (mm)

D (mm)

DW (mm)

2002 (rainfall 1690.7 mm) ET (mm)

D (mm)

DW (mm)

0

1477.6

199.3

73.1

1351.3

188.5

-40.9

1283.0

184.2

41.7

1

1489.3

155.5

66.5

1349.1

184.6

-35.6

1265.0

178.0

56.2

2

1498.3

105.5

62.2

1371.5

159.4

-37.7

1339.3

128.4

39.8

0

1478.9

210.1

104

1259.1

223.8

-62.3

1216.4

187.6

77.0 61.4

1

1384.1

186.6

96.9

1231.8

225.4

-58.2

1160.0

188.8

2

1332.0

175.4

109.1

1228.1

215.6

-54.6

1133.8

170.4

60.3

0 1

1526.6 1457.4

123.6 139.2

158.8 142.6

1343.3 1292.9

153.8 188.2

-97.3 -89.4

1261.8 1235.1

113.0 120.7

91.0 76.6

2

1270.2

99.7

108.0

1189.3

202.0

-75.1

1156.8

186.7

66.4

4

1229.7

232.6

114.7

1156.7

213.3

-67.9

1112.0

203.3

55.9

0

1526.3

142.7

94.3

1313.5

207.7

-92.5

1269.6

115.5

68.1

1

1594.7

129.7

94.6

1295.9

188.4

-87.7

1267.3

85.2

62.7

2

1515.2

94.1

113.5

1342.8

155.6

-84.9

1265.3

98.8

53.5

0

1582.6

99.3

146.1

1381.2

114.8

-127.3

1293.5

100.3

85.3

1

1627.7

138.0

139.5

1433.8

129.2

-119.1

1332.3

124.0

75.2

2

1456.4

120.5

107.4

1316.9

153.9

-106.4

1271.7

141.0

58.6

4

1420.5

189.7

69.6

1258.4

213.4

-106.8

1283.1

172.7

54.3

Note that the change in soil water storage (DW) were measured; and that evapotranspiration (ET, not including water interception) and net deep drainage (D) were estimated

existed significant (P \ 0.05) treatment differences in ET in 2000, 2001 and 2002, and significant (P \ 0.05) space tree row differences in ET only in 2000, but not in 2001 and 2002. These differences indicated that treatment effects on water use are significant across years, but the effects of space distance from the tree row only have significant effects in the wetter years (e.g., 2000). Crop growth and root distribution The alley cropping systems significantly decreased the yields and biomass of the intercropping peanut (Table 5). The total peanut yield decreased by 8.1–60.0% and the magnitude of the decrease was greater in T2P than in T1P. Peanut yield was the lowest near the tree row and increased with the distance from the tree row. Peanut biomass spatially varied similar to the yields. The alley cropping systems promoted tree growth as indicated by height, diameter, and canopy size (Table 5). The tree biomass increased by 16.8 and 18.4% in T1P and T2P as

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compared with T1 and T2, respectively. The analysis of relative net tree effect on water availability to the crop (Tw) was 0.03 for young trees and 0.10 for mature trees. This difference demonstrated that the alley cropping system had a minor influence on water utilization by peanut. The root distribution excavated in 2003 indicated that the roots of C. axillaris were concentrated in soil above the 40 cm soil depth in the treatments with young trees (T1 and T1P) and above the 60 cm in the treatments with mature trees (T2 and T2P) (Table 6). The root weight for the mature trees was twice that for the young trees. The tree root weight in the 0–30 cm depth was two-fold higher in the alley cropping systems than that in the monoculture forestry systems. The lateral roots of young trees accounted for 50% of root length density above the 30 cm depth, for 15% at 30–40 cm and for 5% below 60 cm in the T1P treatment. The tree taproot reached to 100 cm depth in the T2P treatment and the lateral roots were largely distributed in the upper 60 cm, with 30% of root length density below 60 cm and less than 5% below 80 cm.


255

Table 5 Average yields and biomass of the peanut monoculture (P), the young tree monoculture (T1), the mature tree monoculture (T2), and the two alley cropping systems (T1P and T2P) during the 1999, 2000, 2001, and 2002 experimental periods Treatment

Item

1999

P

Yield of peanut (kg hm-2) -2

Biomass of peanut (kg hm ) T1P T2P T1

Yield of peanut (kg hm-2)

1390.5 ± 24.2

2001

1587 ± 112.9

2002

2360.9 ± 104.7

Average

2289 ± 126.9

1906.85

2473.3 ± 118.1

1101.7 ± 113.78

1870.4 ± 110.5

1779.5 ± 76.8

1806.23

869.1 ± 73.5

1451.3 ± 113.2

1235.1 ± 50.3

876.8 ± 97.3

1108.08

Biomass of peanut (kg hm-2)

2179.5 ± 108.4

1073.3 ± 65.1

906.1 ± 34.4

986.8 ± 45.9

1286.43

Yield of peanut (kg hm-2)

1097.5 ± 109.8

1468.4 ± 52.5

1513.2 ± 50.9

1288.7 ± 42.8

1341.95

Biomass of peanut (kg hm-2)

2342.5 ± 226.5

1119 ± 18.8

1170.7 ± 56.2

1133.4 ± 55.2

1441.40

Height (m)

1.84 ± 0.09

2.17 ± 0.11

3.39 ± 0.36

3.86 ± 0.47

2.82

Diameter (cm)

1.25 ± 0.15

2.74 ± 0.19

3.74 ± 0.41

4.99 ± 0.63

3.18

Canopy width (m) T2

2000

2.53 ± 0.26

3.2 ± 0.31

2.87

Height (m)

2.94 ± 0.12

3.25 ± 0.08

4 ± 0.15

4.56 ± 0.22

3.69

Diameter (cm)

4.79 ± 0.47

6.51 ± 0.23

6.72 ± 0.06

7.56 ± 0.05

6.40

Canopy width (m)

–

–

2.97 ± 0.33

4.19 ± 0.12

3.58

2.1 ± 0.20

2.43 ± 0.12

4.02 ± 0.21

4.57 ± 0.28

3.28

Diameter (cm)

1.43 ± 0.13

3.16 ± 0.26

4.57 ± 0.20

6.11 ± 0.30

3.82

Canopy width (m) Height (m)

– 2.94 ± 0.01

– 3.62 ± 0.16

2.6 ± 0.34 5.35 ± 0.40

3.81 ± 0.37 5.73 ± 0.35

3.21 4.41

5.92 ± 0.67

7.76 ± 0.34

8.81 ± 0.55

10.66 ± 0.76

8.29

3.6 ± 0.22

4.75 ± 0.20

4.18

T1P

Height (m)

T2P

Diameter (cm) Canopy width (m)

–

–

–

–

Mean ± standard deviation of three replicates of the field treatment – data not available

In contrast, the roots of the monoculture peanut were only concentrated in soil above the 40 cm depth with 96% in the 0–20 soil depths and 4% in the 20–40 soil depths with no differences found between different treatments (data not shown, Zhao et al. 2006).

Discussion Compared with the peanut monoculture system, the alley cropping systems had the lower soil losses. This can be partly attributed to the tree canopy intercepting rainfall and, therefore, reducing rain drop impacts (Wei et al. 2007). Moreover, the tree rows could also serve as barriers to overland water flow, resulting in a lower erodibility than in the peanut monoculture system. The greater soil loss and smaller runoff in the alley cropping systems than in the monoculture tree systems may be partially attributed to the large area of tilled soil. Soil tillage breaks down and loosens the surface soil, which normally results in a higher macroporosity, a greater infiltration and, consequently, less runoff. Compared with peanut systems (monoculture

and cropped with trees), the tree monoculture systems produced the greatest runoff but the least soil loss. This difference may be partially attributed to the observed hard seals and crusts on the surface soil between the tree rows. The Ultisol under study is rich in iron and aluminum oxides and the soil aggregates are prone to breaking down by fast wetting through rainfall (Zhang and Horn 2001). Dispersed particles from aggregates may clog soil pores, creating low permeability seals at the soil surface that dry through evaporation to form hard crusts. To reduce the soil loss in the alley cropping system, we suggest reducing the tilled area or improving soil stability by the addition of organic matter. Soil water storage increased with increasing distance from the C. axillaris tree row. Similar observations have been made in sub-humid highlands of western Kenya with grevillea (Livesley et al. 2004). The smaller amount of stored water under the tree canopy was probably a result of the interception loss of rainfall by that canopy and the preferential uptake of water from regions close to the tree stem. Those effects also could be viewed by a decreased coefficient

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256


Table 6 Distribution of root weight of C. axillaris trees in the fine, coarse, skeleton, and total categories for the young tree monoculture (T1), the mature tree monoculture (T2), and the two alley cropping systems (T1P and T2P) Treatments

Soil depth (cm)

Root weight in dry matter (kg) Fine root (\2 mm)

T1

T1P

T2

T2P

Coarse root (2–10 mm)

Skeleton root ([10 mm)

Total

0–20

0.15 ± 0.05

0.16 ± 0.02

3.33 ± 1.35

3.64 ± 1.41

20–40

0.11 ± 0.04

0.27 ± 0.06

2.15 ± 0.72

2.53 ± 0.77

40–60

0.16 ± 0.06

0.17 ± 0.09

0.31 ± 0.09

0.64 ± 0.10

60–80

0.07 ± 0.01

0.09 ± 0.03

0.04 ± 0.04

0.20 ± 0.02

80–100

0

0

0

0

Subtotal

0.49 ± 0.12

0.69 ± 0.13

5.83 ± 1.03

7.01 ± 0.95

0–20

0.16 ± 0.03

0.32 ± 0.06

4.77 ± 0.90

5.25 ± 0.94

20–40

0.16 ± 0.04

0.27 ± 0.12

1.78 ± 0.84

2.20 ± 0.94

40–60

0.14 ± 0.08

0.25 ± 00.08

0.93 ± 0.51

1.32 ± 0.65

60–80

0.18 ± 0.13

0.10 ± 0.05

0.07 ± 0.07

0.36 ± 0.26

80–100 Subtotal

0 0.64 ± 0.28

0 0.94 ± 0.20

0 7.55 ± 0.56

0 9.13 ± 0.66

0–20

0.16 ± 0.04

0.21 ± 0.06

5.63 ± 0.86

6.00 ± 0.87

20–40

0.22 ± 0.05

0.38 ± 0.06

1.64 ± 0.70

2.24 ± 0.79

40–60

0.18 ± 0.07

0.42 ± 0.18

3.41 ± 1.92

4.01 ± 1.73

60–80

0.18 ± 0.03

0.35 ± 0.07

0.25 ± 0.02

0.78 ± 0.10

80–100

0.07 ± 0.04

0.07 ± 0.05

0

0.14 ± 0.07

Subtotal

0.81 ± 0.14

1.43 ± 0.32

10.93 ± 0.61

13.17 ± 0.27

0–20

0.19 ± 0.08

0.22 ± 0.08

9.87 ± 3.32

10.28 ± 3.39

20–40

0.19 ± 0.07

0.44 ± 0.05

5.90 ± 0.89

6.53 ± 0.96

40–60

0.19 ± 0.12

0.54 ± 0.31

3.92 ± 1.94

4.65 ± 2.36

60–80

0.12 ± 0.05

0.24 ± 0.04

0.15 ± 0.04

0.51 ± 0.07

80–100

0.03 ± 0.03

0.21 ± 0.13

1.69 ± 1.66

1.93 ± 1.82

Subtotal

0.72 ± 0.28

1.65 ± 0.31

21.53 ± 5.74

23.90 ± 6.25

Mean ± standard deviation of three replicates of the field treatment

of variation of stored water with the decreased distance from tree row. The alley cropping system exploited more stored water than the monoculture system, especially at the 1-m distance from tree row as indicated by the highest evapotranspiration rates, but the beneficial effects to peanut water use appeared to be compensated by decreased interception and increased capillary rise water. This observation is in agreement with the measurement of sap flow, which demonstrated that the water uptake of trees was stronger in the alley cropping system. Similarly, Howard et al. (1997) has shown that sap flow in roots is capable of extracting 80% of its water from below the crop rooting zone which suggests good potential for below-ground complementarity. Soil water regimes indicated that water content at the soil surface was greater than 0.28 cm3 cm-3 in the

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rainy season and there should be no water limitation for peanut growth. However, the water content decreased to 0.21 cm3 cm-3 in the dry season and may have limited peanut growth thus resulting in the reduced yields observed (Fig. 5). Especially, soil water content rapidly decreased after consecutive dry days in June, suggesting the stress of seasonal drought to peanut growth. The alley crop system retained more soil water in the topsoil that possibly may be attributed to either decreasing evaporation by canopy shading effects, or tree hydraulic lift effects in the alley crop system. The latter point was further validated by soil water dynamics in the soil profile as the soil water content in P was much higher after early July than T2P and T2 at the \60 cm soil depths, especially in the 100 cm soil layers. This observation indicates that tree roots may use the deep soil water,


resulting in higher water contents in the \60 cm soil layer, thereby, helping to alleviate the reduced yields of peanuts due to water stress. These results were consistent with the root measurements presented in Table 6. For instance, the proportion of root weight in certain soil layers to total root weight in the whole soil profile was 45.5, 17.0 and 7.0% in the 0–20, 20–40 and \60 cm layers in the older tree monoculture system, respectively. For the alley cropping systems, the tree root weight proportion was 43.0, 27.3 and 10.2% in the 0–20, 20–40 and \60 cm layers, respectively. This further confirmed that the tree roots in the alley cropping system, compared with the tree monoculture system, can distribute in deeper soil layer. The application of fertilizers to crops in the alley cropping systems improved soil fertility so that in the dry season the tree roots might extend deeper to extract more deep soil water (Zhao et al. 2006). Tree and crop roots have been shown to be the principal users of water and nutrients in the soil (Wu et al. 2000; Jose et al. 2000). The root analysis showed that both peanut and trees had the majority of their root distribution above the 40-cm soil depth and that was the key zone where the most competition for water occurred (Zhao et al. 2006). In contrast, tree roots also had a significant concentration of roots in the 40- to 100-cm depths that would be expected to help buffer the resource competition with the peanuts due to its differences in root distribution. The soil water was lower in T2 than that in T2P above the 60-cm depth, but was higher below the 100-cm depth, indicating the tree roots in the alley cropping system could use more water in the deeper soil. Zhu and Zhu (2003) analyzed the spatial structure of roots between trees and wheat in agroforestry systems and concluded that the niche breadth of trees was more extensive than that of wheat, so trees could use water and nutrients in deep soil layers alleviating competition between trees and wheat. Van Noordwijk and Lusiana (1999) indicated that the ability of a tree’s root system to take up water resources depended on the distribution of the tree and crop roots, soil hydraulic properties, groundwater level and rainfall regime. In the red soil region of subtropical China, the near surface groundwater is accessible to tree roots, which provides a probability for trees using soil water from the deeper layers (Zhang and Zhang 1997). The direction of water flow indicated that the soil water above 30 cm soil depth tended to move

257

downward while water in deeper layers tended to move up towards the tree row. These results demonstrated the positive effects of deep root water uptake and/or its transfer of water in the soil profile through hydraulic lift (Smith et al. 1997). Since a severe drought did not occur during our study, the potential competition for water between trees and crops was not as great as it might have been. However, note that the soil water also moved up towards the tree row during the seasonal drought, suggesting that the competition for water between tree and crop roots had occurred. Similar finding were also reported by others (e.g., Jose et al. 2000). Tree roots had definite advantages in the competition for water resources owing to the size and depth distribution of the tree roots. C. axillaris in the alley cropping system used deeper soil water resources, and was in agreement with previous experiments (Yao 1995). In the dry season, either young or mature C. axillaris in the alley cropping systems was found to use the deep soil water resources that was not available to the peanut crop roots. Previous allied researches have showed that the shading effects in alley cropping systems may have contributed to the physiological variations in soil water uptake that was linked with the depth of fertilization, consequently affected the yields of peanut and its straw, and tree biomass (Wang et al. 2003). There was a significant linear correlation between yield and biomass of the peanut and the photosynthetically active radiation (PAR), indicating that decreased light significantly resulted in the reduced yields and biomass of peanut and must be considered for optimization of the cropping system (P \ 0.01; Zhao et al. 2006). When water, light and fertilizer have been studied as limiting factors to crop growth, whether the light was a main obstacle factor or not has not been clearly determined. Research on the peanut of India and Pennisetum glaucum showed that the shadow effect was bigger than the root competition for water (Willey and Reddy 1981). When light– pleased crop, such as wheat and peanut intercropped with trees, the yield and biomass of the crop can be enhanced through rearranging the distance, trunk pruning and replacing the arbor with shrub. For instance, trunk pruning of trees in our tested experiment has been shown to significantly reduce the sap flow and certainly indicated a weak shadow effect (Zhao et al. 2005). However, other observations also showed that root competition for water is more

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258

important for tree shade (e.g., Singh et al. 1989; Jose et al. 2000). The installed ‘barrier’ treatments has been shown to successfully eliminated the belowground root competition for water between tree and crop (Singh et al. 1989; Wanvestraut et al. 2004) and resulted in greater leaf area and higher grain yield than the ‘no barrier’ treatment (Ong et al. 1991; Jose et al. 2000).

Conclusion The application of the water balance method in the alley cropping system has been shown to be a practical methodology to allow for the quantification of evapotranspiration, drainage and the other components and for the optimization of water use by crops used in the alley cropping system. The results indicated that the erosion control differences were remarkably large and the competition for water between C. axillaris and peanut was alleviated in the tested alley cropping systems owing to the tree roots using soil water in deeper soil layers. Analysis of the relative net tree effect on water availability to the peanut further illustrated little water competitiveness existed between trees and crop, but may have resulted in decreased peanut yield. Water competition may mainly occur during the seasonal drought period, when deep soil water was scarce, water availability to crop was restricted as visualized by the water flow directions towards the tree row when evapotranspiration was much higher. In comparison with monoculture systems, the alley cropping system changed the characteristics of water usage, indicating that water competition was related to arrangement of tree, root structure, and their ability to absorb water resources deep in soil profile. This study suggested that the spacing arrangement of trees and crop in the alley cropping systems could reduce the competition for natural resources. However, more detailed work in root distribution, water usage mechanisms by roots as well as the interactions of water, soil fertility and light should be done to insure that the system could benefit from high water efficiency, thus, resulting in higher productivity. Acknowledgments The grants have been provided by the International Foundation of Sciences (IFS) (Grant No. D2872–1), the International Atomic Energy Association

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Agroforest Syst (2012) 84:243–259 (IAEA) (Grant No. CPR-10407) and the Natural Science Foundation of China (Grant No. 49701008). Profs. Mingzhu Wang, Xingxiang Wang and Mr. Huachun Zhao are acknowledged for their involvement in this research.

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