PARTIONING OF FERTILITY AND COMPETITION EFFECTS OF A Gliricidia sepium X lea maise AGROFORESTRY SYSTEM ON SLOPING HIGHLANDS IN THE MID-COUNTRY WET ZONE OF SRI LANKA

Contour hedgerow intercropping (HI) is an agroforestry system recommended toimprove soil fertility and sustain annual crop yields on the steep lands of humidcentral highlands of Sri Lanka. The objective of the present experiment was toquantify the overall tree-crop interaction (TCI) by partitioning the positive fertilityeffect (F) and the negative competition effect (C) of Gliricidia sepium hedgerowson maize (Zea mays L.) grown in a HI system on a sloping (35%) land atPeradeniya in the rnid-elevational (479 m above sea level), humid (rainfall of2000 mm/yr) zone of Sri Lanka. The experimental treatment structure consistedof two hedgerow intercrops with (Hm) and without (Ho) tree prunings added asmulch and two sale maize crops with (Cm) and without (Co) mulch. The highestmaize yields were obtained in Cm whereas C and H, had the lowest with nosignificant difference between them. Hm had an intermediate yield. The overallTCI was positive (ranging from 26 to 112% depending on the method ofestimation of C) because of substantially positive F (85 to 94%) whichoutweighed the predominantly negative C (-67 to +18). Mulching increased theavailability of P and K to maize and increased soil pH. Mulching also decreasedsoil Nand K and increased soil P during the cropping season. There wassignificant competition for light by hedges, especially near the hedgerows.However, positive effects of mulching ensured greater crop growth and radiationinterception away from hedges. Soil water (SW) depletion from top layers (0-30cm) was highest in Ho. Within mulched treatments, Hm showed lower SWdepletion that Cm during the first 10 weeks showing the shading effect of hedges.However subsequently, Hm had greater SW depletion than Cm indicating greaterSW extraction which was confirmed by SW depletion in deeper (30-110 ern)layers. Based on inter-treatment variation of nutrients, water and radiation, it isconcluded that in the present situation, the fertility effect of hedgerow pruningsexceeded the competition effects of hedges.