Macropore effects on phosphorus acquisition by wheat roots - a rhizotron study

Macropores may be preferential root pathways into the subsoil. We hypothesised that the presence of macropores promotes P-uptake from subsoil, particularly at limited water supply in surface soil. We tested this hypothesis in a rhizotron experiment with spring wheat (Triticum aestivum cv. Scirocco) under variation of fertilisation and irrigation. Rhizotrons were filled with compacted subsoil (bulk density 1.4 g cm(-3)), underneath a P-depleted topsoil. In half of these rhizotrons the subsoil contained artificial macropores. Spring wheat was grown for 41 days with and without irrigation and P-31-addition. Also, a P-33-tracer was added at the soil surface to trace P-distribution in plants using liquid scintillation counting and radioactive imaging. Fertilisation and irrigation promoted biomass production and plant P-uptake. Improved growing conditions resulted in a higher proportion of subsoil roots, indicating that the topsoil root system additionally promoted subsoil nutrient acquisition. The presence of macropores did not improve plant growth but tended to increase translocation of P-33 into both above- and belowground biomass. P-33-imaging confirmed that this plant-internal transport of topsoil-P extended into subsoil roots. The lack of penetration resistance in macropores did not increase plant growth and nutrient uptake from subsoil here; however, wheat specifically re-allocated topsoil-P for subsoil root growth.