Subsoil nitrogen dynamics as affected by planted coppicing tree legume fallows in eastern Zambia

Nitrogen (N) is a major nutrient that limits crop production in southern Africa. We hypothesized that coppicing tree legumes, which are integrated in cropping systems, would intercept leaching nutrients and could also increase topsoil N in nutrient-depleted soils. This hypothesis was verified in three ongoing experiments at Msekera (experiments 1 and 2) and Kagoro (experiment 3) in Zambia. Planted tree fallows of Gliricidia sepium, Leucaena leucocephala, Acacia angustisma, and Sesbania sesban were compared with natural fallows and with continuous maize cropping with or without fertilizer (no-tree) controls. Top and subsoil samples were taken in the tree treatments and in the no-tree controls to establish short and long-term tree effects on soil N dynamics. 15N was introduced at various soil depths down to 2 m to determine the vertical root-reach of coppicing trees. Samples taken on two different dates showed that planted trees are capable of capturing subsoil N. The amounts retrieved by trees in experiment 2 did not vary with depth or dates except for A. angustisma which retrieved more N from the top 0.20 m than from the subsoil. L. leucocephala and G. sepium had similar characteristics in terms of coppice biomass production and N content, and both species rooted to at least 2 m. G. sepium in a mixture with S. sesban, retrieved more applied N than when planted alone, implying that mixed fallows may be effective in resource capture. There was more inorganic-N in the topsoil of coppiced fallows was significantly higher than in unfertilized maize plots. Subsoil N accumulation was evident under fertilized maize plots. There was less subsoil nitrate-N beneath planted trees than beneath mono-cropped maize plots indicating that trees probably retrieved subsoil N. Maize yields subsequent to coppicing tree fallows were at least 170% higher than unfertilized controls indicating improved soil fertility status in the tree systems.