POPLAR LEAF BIOMASS DISTRIBUTION AND NITROGEN DYNAMICS IN A POPLAR-BARLEY INTERCROPPED SYSTEM IN SOUTHERN ONTARIO, CANADA

The effect of hybrid poplar (Populus spp. clone DN 177) leaf biomass d istribution on soil nitrification was investigated in two experiments during the 1993, 1994 and 1995 growing seasons in a poplar-barley (Hor deum vulgare cv. OAC Kippen) intercropping experiment established at G uelph, Ontario, Canada. In experiment 1, poplar was intercropped with barley during all three years and the poplar leaves shed during the fa ll season were removed from the soil surface during 1993 and 1994. In experiment 2, poplar was intercropped with barley in 1993 and with cor n (Zea mays cv. Pioneer 3917) in 1994 an 1995, respectively, and the s hed poplar leaves were not removed. In experiment 1, the nitrification rates were lower during 1994 and 1995 when the dropped leaves were re moved from the field. The total above-ground biomass of barley within 2.5 m of the tree row was 517, 500 and 450 g.m(-2), respectively durin g the three years, whereas in the middle of the crop row (4-11 m), the corresponding figures were 491, 484 and 464 g.m(-2). Mean nitrificati on rates, N availability and carbon content were higher in soils close to the poplar tree rows (2.5 m) compared to the corresponding values in the middle of the crop alley (4-11 m from the tree row). In experim ent 2, where poplar leaves were not removed from the field, nitrificat ion rates in soils within 2.5 m distance from the poplar row were fair ly constant (range 100 to 128 mu g 100 g(-1) dry soil day(-1)) during the three years. Results suggest that soil nitrification rates, soil c arbon content and plant N uptake adjacent to the poplar tree rows are influenced by poplar leaf biomass input in the preceding year.