Soil erosion and deposition effects on surface characteristics and pearl millet growth in the West African Sahel

It is well known that surface mulched crop residues (CR) lead to large yield increases of pearl millet (Pennisetum glaucum L.) on acid sandy soils of the West African Sahel. This effect is generally attributed to mulch-induced changes in chemical properties of the surface soil and the protection of millet seedlings from erosive sand storms. However, previous research has failed to separate the anti-erosive effects of CR on plant growth from chemical effects due to the release of nutrients during CR decomposition. To this end a mulching trial with surface applied millet stalks at a rate of 2000 kg ha⁻¹, an equivalent 10% surface coverage obtained by inert polyethylene (PE) tubes and a bare control treatment was conducted from 1992 to 1994 on an acid sandy soil in southwest Niger. Across treatments, sand flux at 0.1 m height was more than twice as high in the rainy seasons than in the dry months and mulching reduced sand flux by between 25 and 50% during rainy season storms compared with 67% during the dry season. Over the 21 months measurement period, cumulative erosion by wind and water was almost 270 t ha⁻¹ of soil in unmulched control plots. In mulched plots, in contrast, between 160 and 200 t ha⁻¹ of soil was deposited. Surface soil temperature at 0.01 m depth reached above 40 °C in bare plots but was up to 4 °C lower with CR. Mulch reduced soil penetration resistance at 0-0.02 m and 0-0.05 depth by more than half and decreased runoff leading to higher water contents at flowering and grain filling in the upper 0.3 m soil layers in 1993 and throughout the entire profile in 1994, a year with particularly high rainfall. Both mulch types were similarly effective in increasing final stand density of millet in the first two years between 5 and 23% compared with bare control plots. Relative to the bare control CR mulch effects on total dry matter of millet at harvest increased from 35% in 1992 and 108% in 1993 to 283% in 1994, whereas PE mulch led to respective relative increases in dry matter of only 6, 44 and 13%. In 1992 and 1993, CR mulch increased total nutrient uptake of millet at harvest by between 34 and 86% for nitrogen (N), between 31 and 162% for P and between 56 and 126% for potassium (K). These differences were mostly the result of differences in total dry matter and only to a smaller part due to changed nutrient concentrations in plants.