Mobilization of soil and fertilizer phosphate by cover crops

Incorporation of cover crops into cropping systems may contribute to a more efficient utilization of soil and fertilizer P by less P-efficient crops t hrough exudation of P-mobilizing compounds by the roots of P-efficient plan t species. The main objective of the present work was to test this hypothes is. First a method has been developed which allows the quantification of or ganic anion exudation from individual cluster roots formed by P-deficient w hite lupin (Lupinus albus L.). Lupin plants were grown in nutrient solution at 1 mu M P and in a low P loess in small rhizotrons. Organic anions exude d from intact plants grown in nutrient solution were collected from individ ual cluster roots and root tips sealed in small compartments by an anion-ex change resin placed in nylon bags (resin-bags). Succinate was the dominant organic anion exuded followed by citrate and malate. The mean of citrate ex udation-rate was 0.06 pmol mm(-1) s(-1) with exudation highly dependent on the citrate concentration and on the age of the cluster roots. Exudates fro m cluster roots and root tips grown at the soil surface (rhizotron-grown pl ants) were collected using overlayered resin-agar (resin mixed with agar). Citrate exudation from cluster roots was 10 times higher than that from roo t tips. Fractionation of P in the cluster root rhizosphere-soil indicates t hat white lupin can mobilize P not only from the available and acid-soluble P, but also from the stable residual soil P fractions. In pot experiments with an acid luvisol derived from loess low in available P, growth of wheat was significantly improved when mixed-cropped with white lupin due to impr oved P uptake. Both in mixed culture and in rotation wheat could benefit fr om the P mobilization capacity of white lupin, supporting the hypothesis ab ove. Nine tropical leguminous cover crops and maize were grown in a pot exp eriment using a luvisol from Northern Nigeria low in available P. All plant species derived most of their P from the resin and bicarbonate-extractable inorganic P. Organic P (P-o) accumulated particularly in the rhizosphere o f all plant species. There was a significant negative correlation between t he species-specific rhizosphere acid phosphatase activity and P-o accumulat ion. Growth and P uptake of maize grown in rotation after legumes were enha nced indicating that improved P nutrition was a contributing factor.