Vertical Distribution of Roots and Soil Nitrate: Tree Species and Phosphorus Effects

Abstract We hypothesized that trees can rapidly root into subsoil and capture NO 3 , which can accumulate in the subsoil of agricultural soils with high anion sorption. The vertical distribution of root length and inorganic N (NO 3 and NH 4 ) 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 NO 3 , and soil NH 4 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 ≥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 NO 3 in the top 2 m by about 150 to 200 kg N ha −1 within 11 mo after establishment and effectively captured subsoil NO 3 . Fast‐growing trees with high root length densities can rapidly reduce subsoil NO 3 .