Effect of crop residues on root growth and phosphorus acquisition of pearl millet in an acid sandy soil in Niger

The effect of long-term (1983–1988) applications of crop residues (millet straw, 2–4 t ha^-1 yr^–1) and/or mineral fertilizer (30 kg N, 13 kg P and 25 kg K ha^-1 yr^-1) on uptake of phosphorus (P) and other nutrients, root growth and mycorrhizal colonization of pearl millet (Pennisetum glaucum L.) was examined for two seasons (1987 and 1988) on an acid sandy soil in Niger. Treatments of the long-term field experiment were: control (–CR–F), mineral fertilizer only (–CR+F), crop residues only (+CR–F), and crop residues plus mineral fertilizer (+CR+F).In both years, total P uptake was similar for +CR–F and –CR+F treatments (1.6–3.5 kg P ha^-1), although available soil P concentration (Bray I P) was considerably lower in +CR–F (3.2 mg P kg^-1 soil) than in –CR+F (7.4) soil. In the treatments with mineral fertilizers (–CR+F; +CR+F), crop residues increased available soil P concentrations (Bray I P) from 7.4 to 8.9 mg kg^-1 soil, while total P uptake increased from 3.6 to 10.6 kg P ha^-1. In 1987 (with 450 mm of rainfall), leaf P concentrations of 30-day-old millet plants were in the deficiency range, but highest in the +CR+F treatment. In 1988 (699 mm), leaf P concentrations were distinctly higher, and again highest in the +CR+F treatment. In the treatments without crop residues (–CR–F; –CR+F), potassium (K) concentrations in the leaves indicated K deficiency, while application of crop residues (+CR–F; +CR+F) substantially raised leaf K concentrations and total K uptake. Leaf concentrations of calcium (Ca) and magnesium (Mg) were hardly affected by the different treatments.In the topsoil (0–30 cm), root length density of millet plants was greater for +CR+F (6.5 cm cm^-3) than for +CR–F (4.5 cm cm^-3) and –CR+F (4.2 cm cm^-3) treatments. Below 30 cm soil depth, root length density of all treatments declined rapidly from about 0.6 cm cm^-3 (30–60 cm soil depth) to 0.2 cm cm^-3 (120–180 cm soil depth). During the period of high uptake rates of P (42–80 DAP), root colonization with vesicular-arbuscular mycorrhizal (VAM) fungi was low in 1987 (15–20%), but distinctly higher in 1988 (55–60%). Higher P uptake of +CR+F plants was related to a greater total root length in 0–30 cm and also to a higher P uptake rate per unit root length (P influx). Beneficial effects of crop residues on P uptake were primarily attributed to higher P mobility in the soil due to decreased concentrations of exchangeable Al, and enhancement of root growth. In contrast, the beneficial effect of crop residues on K uptake was caused by direct K supply with the millet straw.