Vertical distribution of roots and soil nitrate: Tree species and phosphorus effects

We hypothesized that trees can rapidly root into subsoil and capture NO3, which can accumulate in the subsoil of agricultural soils with high anion sorption. The vertical distribution of root length and inorganic N (NO3 and NH4) to 3.95-m soil depth was compared for 11-mo-old stands of eucalyptus (Eucalyptus grandis W. Hill ex Maiden), sesbania [Sesbania sesban (L.) Merr.], calliandra (Calliandra calothyrsus Meissner), markhamia [Markhamia lutea (Benth.) Schumann], and grevillea (Grevillea 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 with 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 length 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 length density (cm cm(-3)) as a function of soil depth was affected by tree species (P < 0.01), indicating differences among trees in the tendency for deep rooting. Root length densities, averaged for the two P levels, were greater than or equal to 0.1 cm cm(-3) to depths of 2.2 m with 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 after establishment and effectively captured subsoil NO3. Fast-growing trees with high root length densities ran rapidly reduce subsoil NO3.