Differences in phosphorus acquisition and critical phosphorus requirements among nine Desmanthus spp. genotypes

Abstract

The extensive grazing systems of northern Australia are dominated by C4 grasses and are established in N- and P-responsive soils that receive minimal nutrient input. Under these conditions, tropical pasture legumes are expected to improve the quality of grazing forage and fix atmospheric N2. However, legume persistence is relatively poor, which may be due to a presumed disparity in P requirements among tropical pasture species. This disparity suggests that P-efficient legumes may improve legume persistence, yet the P requirements of many tropical pasture legumes remain unquantified. Nine Desmanthus spp. genotypes were grown in pots to determine differences in shoot yield and root morphology in response to soil P supply (0–100 mg applied P kg–1 soil; 5–63 mg Colwell P kg–1 soil). The shoot yield of each genotype increased in response to increased P supply. When P supply was adequate for maximum plant growth, the shoot yield of the best genotype (3.5 g DM pot–1; JCU 9) was 1.7-fold larger than that of the next most productive genotype. There were also substantial differences in the critical external P requirements of the genotypes (29.4–64.0 mg P kg–1 soil), although these differences did not always reflect the efficiency of dry matter production per unit of applied P fertiliser. Differences in shoot yield and P acquisition were positively associated with differences in the development of root length. The results indicated that P-efficient genotypes of Desmanthus spp. can be identified for improved growth in the P-responsive pastures of northern Australia. These genotypes may compete more effectively with C4 grasses and form resilient pasture swards as climate patterns change.