VERTICAL-DISTRIBUTION OF ROOTS AND SOIL NITRATE - TREE SPECIES AND PHOSPHORUS EFFECTS

We hypothesized that trees can rapidly root into subsoil and capture N O3, which can accumulate in the subsoil of agricultural soils with hig h anion sorption. The vertical distribution of root length and inorgan ic N (NO3 and NH4) to 3.95-m soil depth was compared for 11-mo-old sta nds of eucalyptus (Eucalyptus grandis W. Hill ex Maiden), sesbania [Se sbania sesban (L.) Merr.], calliandra (Calliandra calothyrsus Meissner ), markhamia [Markhamia lutea (Benth.) Schumann], and grevillea (Grevi llea robusta A. Cunn. ex R. Br.) grown at two P levels (no added P and 500 kg added P ha(-1)) on a Kandiudalfic Eutrudox in Kenya. The trees were planted at a 1 by 1 m spacing in a randomized complete block wit h three replications. Added P had no effect on root length, soil NO3, and soil NH4 even though the soil was low in available P. Total root l ength was greater for calliandra (15.5 km m(-2)) than other trees (1.2 -5.6 km m(-2)). The slope for the model of natural logarithm root leng th density (cm cm(-3)) as a function of soil depth was affected by tre e species (P < 0.01), indicating differences among trees in the tenden cy for deep rooting. Root length densities, averaged for the two P lev els, were greater than or equal to 0.1 cm cm(-3) to depths of 2.2 m wi th calliandra, 1.8 m with sesbania, 1.2 m with eucalyptus, 0.45 m with grevillea, and 0.3 m with markhamia. Calliandra and sesbania reduced soil NO3 in the top 2 m by about 150 to 200 kg N ha(-1) within 11 mo a fter establishment and effectively captured subsoil NO3. Fast-growing trees with high root length densities ran rapidly reduce subsoil NO3.