Competition in tree row agroforestry systems. 3. Soil water distribution and dynamics

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  • Plant and Soil 264: 129139, 2004. 2004 Kluwer Academic Publishers. Printed in the Netherlands. 129

    Competition in tree row agroforestry systems. 3. Soil water distributionand dynamics

    S.J. Livesley1,4, P.J. Gregory2 & R.J. Buresh31Forest Science Centre (DSE), Water Street, Creswick, Victoria 3363, Australia. 2Soil Science Department, TheUniversity of Reading, Whiteknights, P.O. Box 233, Reading, RG6 6DW, U.K. 3IRRI, DAPO Box 777, MetroManila, The Philippines. 4Corresponding author

    Received 24 September 1999. Accepted in revised form 10 December 2003

    Key words: agroforestry, Grevillea robusta, maize, Senna spectabilis, soil water content, water balance

    Abstract

    The purpose of this study was to test the hypothesis that soil water content would vary spatially with distance froma tree row and that the effect would differ according to tree species. A field study was conducted on a kaoliniticOxisol in the sub-humid highlands of western Kenya to compare soil water distribution and dynamics in a maizemonoculture with that under maize (Zea mays L.) intercropped with a 3-year-old tree row of Grevillea robustaA. Cunn. Ex R. Br. (grevillea) and hedgerow of Senna spectabilis DC. (senna). Soil water content was measuredat weekly intervals during one cropping season using a neutron probe. Measurements were made from 20 cm to adepth of 225 cm at distances of 75, 150, 300 and 525 cm from the tree rows. The amount of water stored was greaterunder the sole maize crop than the agroforestry systems, especially the grevillea-maize system. Stored soil waterin the grevillea-maize system increased with increasing distance from the tree row but in the senna-maize system,it decreased between 75 and 300 cm from the hedgerow. Soil water content increased least and more slowly earlyin the season in the grevillea-maize system, and drying was also evident as the frequency of rain declined. Soilwater content at the end of the cropping season was similar to that at the start of the season in the grevillea-maizesystem, but about 50 and 80 mm greater in the senna-maize and sole maize systems, respectively. The seasonalwater balance showed there was 140 mm of drainage from the sole maize system. A similar amount was lost fromthe agroforestry systems (about 160 mm in the grevillea-maize system and 145 mm in the senna-maize system)through drainage or tree uptake. The possible benefits of reduced soil evaporation and crop transpiration close toa tree row were not evident in the grevillea-maize system, but appeared to greatly compensate for water uptakelosses in the senna-maize system. Grevillea, managed as a tree row, reduced stored soil water to a greater extentthan senna, managed as a hedgerow.

    Introduction

    Annual crops often exploit only a small fraction ofthe available rainfall and stored soil water reserves.The integration of perennial trees within a farmingsystem can increase the amount of water transpiredand increase overall biomass productivity (Ong et al.,1992; Wallace et al., 1995). This may be achieveddirectly when trees exploit the rainfall and stored wa-ter reserves outside the cropping seasons and/or whena greater proportion of the rainfall within a cropping

    FAX No: +61-3-5321-4166.E-mail: stephen.livesley@dse.vic.gov.au

    season is transpired rather than evaporated, run-off ordrained to below the rooting zone (Ong et al., 1992). Itmay also be achieved indirectly when modification ofmicroclimatic conditions by trees increases the tran-spiration efficiency of the crop, the unit production ofbiomass per unit water transpired (Brenner, 1996).

    In semi-arid and dry sub-humid areas, evapora-tion from the soil surface can account for 3060%of the annual rainfall (Cooper et al., 1983; Wallace,1991). Trees can reduce these losses and conserve soilwater by providing shade, reducing wind speed andincreasing infiltration with mulch layers and improvedsoil structure (Torquebiau and Kwesiga, 1996; Young,

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    1997). The loss of water through run-off is importantin areas with sloping land and less permeable soil.The presence of contour hedgerows can increase therate and amount of water infiltration thereby reducingsuch losses (Kiepe, 1995; Agus et al., 1997). Treesand hedgerows, though, may intercept rainfall withtheir foliage so that it fails to reach the soil. Wallaceet al., (1995) found that interception loss was about14% of the rainfall for 34-year-old Grevillea robustatrees grown at Machakos, Kenya.

    The loss of water through drainage has rarely beenquantified in tropical agroecosystems, but may rep-resent as much as one third of the water balance insemi-arid areas (Ong et al., 1991). When drainagelosses occur there are also possible leaching lossesof nutrients. The root system of a tree can theoretic-ally intercept and take up mineral nutrients and waterthat percolates below the crop rooting zone, therebyreducing such losses. The ability of a trees root sys-tem to take up resources from depth depends on thedistribution of the tree and crop roots, soil hydraulicproperties and rainfall regime (van Noordwijk et al.,1996). The loss of water through drainage may bethe hydrological parameter most easily modified bythe introduction of trees (Wallace et al., 1995) so theloss of nutrients through leaching may also be reduced(Shepherd et al., 1996).

    The overall productivity of a farming system willonly increase with the introduction of trees if thenegative effect of competition with the crop is com-pensated for by the beneficial effects of trees on theirshared environment and the additional harvestableproducts the trees provide. In agroforestry systemsof semi-arid regions, competition for water betweentrees and crops is the principal determinant of cropproductivity (Singh et al., 1989; Breman and Kessler,1995). In areas with > 1000 mm a1 of rainfall,competition for water is rarely reported or even in-vestigated (Young, 1997). Nonetheless, measuring thewater balance of agroforestry systems in sub-humidand humid areas is important because many processesinvolved in the cycling of nutrients (organic matter de-composition, denitrification and leaching) are greatlyinfluenced by soil hydrological conditions. Further-more, the restricted availability of soil water affectsmany plant functions, including transpiration, photo-synthesis and the movement of nutrient ions and theiruptake by root systems (Schulze, 1991).

    The objective of this study was to test the hy-pothesis that soil water content would vary spatiallywith distance from a tree row, and that the effect

    would differ according to tree species and manage-ment. The aim of the work reported in this paper wasto compare the temporal and spatial distributions ofsoil water below tree row agroforestry systems and amaize monoculture. Soil water content was measuredregularly with a neutron probe and a water balanceestimated. This study complimented other investiga-tions into fine root distribution and dynamics (Livesleyet al., 2000) and soil nitrogen uptake, loss and avail-ability (Livesley et al., 2002) in these agroforestrysystems.

    Materials and methods

    The research was conducted in western Kenya(3434 E, 006 N, altitude 1330 m) on a fine kaol-initic, isohyperthemic Kandiudalfic Eutrudox. In theupper 15 cm, 73% was clay and 8% sand. Furtherdetails of the site, design and management are given inLivesley et al. (2000) and an extensive physical, chem-ical and hydraulic characterisation of the soil at thissite can be found in Livesley (1999). The experimentwas established in April 1993 with nursery-grownseedlings planted as a single row of Grevillea ro-busta (grevillea) with a 1.0 m spacing between treesand a single row of Senna spectabilis (senna) witha 0.5 cm spacing between trees. Maize (hybrid 512)was cropped biannually within 6 m on both sides ofthe trees during the long rains (March to July) andthe short rains (September to January). Grevillea wasmanaged for timber production with lower branchesremoved annually to reduce shading while senna wasmanaged as a hedgerow that was pruned to leave onlya 20 cm high stump before each cropping season. Allprunings and leaf fall were collected and removedfrom the site. Maize (hybrid 512) was cropped bian-nually within 6 m on both sides of the tree rows duringthe long rains (March to July) and the short rains(September to January). A sole maize plot (6 6 m)was established 12 m from the grevillea-maize systemand managed in the same way as the maize in theagroforestry systems.

    This paper reports a study during long rains grow-ing season of 1996, three years after the tree rowswere established. Maize was sown on 11 March andharvested on 8 July, 117 days after planting (DAP).Rainfall was measured every 30 min at an on-site met-eorological station, and daily using ten bucket gaugesrandomly distributed throughout the field site. Poten-tial evapotranspiration from a sole maize crop (Eo)

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    was calculated using the PenmanMonteith equation(Monteith and Unsworth, 1990) with stomatal resist-ance set at zero and allowance for the effect of theheight of the crop on wind speed as it increased from5 cm at 7 DAP to 200 cm at 100 DAP.

    Soil water content

    Soil water content was measured using a neutronprobe. Aluminium access tubes were installed in linesperpendicular to the grevillea tree row and sennahedgerow. Sixteen access tubes were installed in eachagroforestry system, in four lines of four tubes, withtwo lines on either side of the tree row. Each linehad an access tube at 75, 150, 300 and 525 cmfrom a tree row. Four tubes were also installed in thesole maize crop. Measurements were made weekly,between 18 March (7 DAP) and 5 July (117 DAP) at20, 30, 40, 50, 75, 105, 135, 165, 195 and 225 cmdepths. Measurements were taken during the morningand required about 3 hours to complete. On each mea-surement occasion, a mea

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