Mobilization of phosphorus contributes to positive rotational effects of leguminous cover crops on maize grown on soils from northern Nigeria

Previous research has demonstrated a positive rotational effect of tropical leguminous cover crops on maize growth on a luvisol from Nigeria. This effect could not be explained by a better N supply. The objective of the present work was to further clarify whether improved P nutrition has been a contributing factor. Nine cover crops and maize were studied in nutrient solution-culture with 1 and 20 muM P and with NO3-N as N source for root physiological parameters that may affect P mobilization . Zea mays, Lablab purpureus, and Centrosema pubescens responded to P deficiency by higher rates of proton excretion. Clitoria ternatea excreted OH- with only small differences due to P nutrition. At low P supply, Chamaecrista rotundifolia, Clitoria, and Centrosema had the highest exudation rates of organic acid anions, especially citrate and malate. A major difference between plant species was found in root surface acid phosphatase activity. Cajanus cajan expressed the highest phosphatase activity. V-max of P uptake increased markedly under P deficiency, particularly for maize. Compared to the other plant species Cajanus, Chamaecrista, and Clitoria were characterised by a greater capability to adsorb P at low external P concentrations. The nine cove crops and maize were also grown in pot experiments using two soils from northern Nigeria low in available P. The utisol from Jos had a high P fixation capacity and was more acidic than the alfisol from Zaria. All plant species were precultured in the first season at 100 ( Zaria) and 250 (Jos) mg P per pot. In the subsequent season, maize was uniformly seeded into all pots containing the remaining roots and the incorporated shoot dry mass ( according to 15 mg per pot ) of the pre-crop. No P and 250 (Zaria) or 500 (Js) mg P per pot were freshly applied. Maize grown and P uptake were enhanced after legumes in Zaria soil. Cajanus showed the highest residual effect, and also Clitoria, Chamaecrista, and Lablab showed effects superior to the mean. In Jos soil, Clitoria, Cajanus, and Lablab enhanced maize growth above average. Also, a highly significant positive correlation between P uptake and biomass production was obtained. These results indicate that enhancement of maize growth after leguminous cover crops, at the low P supply of the soil used, mainly depends on P mobilization capacity of the cover crop.