Subsoil Nitrogen Dynamics as Affected by Planted Coppicing Tree Legume Fallows in Eastern Zambia

Abstract

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.

Agricultural success from Africa: the case of fertilizer tree systems in southern Africa (Malawi, Tanzania, Mozambique, Zambia and Zimbabwe)

“…The largest negative potassium balance was obtained in fully fertilized maize fields due to higher maize and stubble yields, which extract a lot of potassium (Table 4). Similarly, soil pH was lower in continuously cropped, fully fertilized maize compared with maize grown in FTS in eastern Zambia (Chintu et al, 2004) and Malawi . This is in agreement with reports from elsewhere that show that application of organic residues can mitigate soil acidity to some extent (Haynes and Mokolobate, 2001), especially due to nutrient recycling from deeper soil layers by tree roots (Akinnifesi et al, 2004).…”

Carbon pools in tree biomass and the soil in improved fallows in eastern Zambia

“…The sites receive a mean annual rainfall of 960 mm in a single rainy season and 85% of rain falls within four months (December through March). Soils in eastern Zambia area yellowish-red to yellowish-brown loamy sandy or sandy soils-Acrisols (Celis et al 1991) and sitespecific soil classes and properties (Zambia/ICRAF, unpublished data) are summarized in Table 1.…”

“…Thus, mean SOC stocks at 60-100 cm (29.9 t ha -1 ) and 100-150 cm (28.6 t ha -1 ) depths were exceedingly greater than those found at other depths. The subsoil (25-200 cm profile) stored 75% of the stocks held at 0-200 cm depth probably because of the volume of the subsoil rather than soil C density (Sanchez 1995), the deep tree root systems and extra root C associated with tree legumes (Mendham et al 2003;Ghathumbi et al 2003;Chintu et al 2004;Chirwa et al 2007), leaching of SOC, deposition of SOM in the subsoil through cultivation (Chintu et al 2004), and reduced susceptibility of SOC stocks to microbial oxidation at depth due to gradients of biophysical and chemical conditions that impose limitations on mineralization (Sanchez 1995;Nandwa 2001). Uniformity in SOC stocks in the 0-25 cm soil profile could be attributed to redistribution of SOC during cultivation.…”

“…F than other treatments could be attributed to high yields of maize stover (4.1-7.1 t DM ha -1 , Kwesiga et al 1994) and root biomass (1-2.0 t C ha -1 , Tiessen et al 2001) because of application of inorganic fertilizers. Regardless of the type and amount of organic residues added to the soil, addition of N from fertilizer increased accumulation of SOM through increased biomass production (Brady and Weil 1996). Tree fallows had less SOC than M ?…”

The State of Soil Degradation in Sub-Saharan Africa: Baselines, Trajectories, and Solutions

“…Most studies in the literature compare and contrast management practices [31][32][33][34] or examine one farming practice across different regions [5,35]. There are relatively few studies that attempt to examine soil degradation at a scale that can encompass the spatial and temporal heterogeneity of farmed landscapes in SSA.…”