Cycling of nutrients from dying roots to living plants,including the role of mycorrhizas

This paper presents information about the release of nitrogen and phosphorus from dying grass roots and the capture of phosphorus by other, living plants. We have paid particular attention to the part played by mycorrhizas in this phosphorus capture, and the possible importance of mycorrhizal links between dying and living roots.WhenLolium perenne plants were grown with ample nutrients and their roots then detached and buried in soil, about half the nitrogen and two-thirds of the phosphorus was lost in three weeks, but only one-fifth of the dry weight. The C:N and C:P ratios suggest that microbial growth in the roots would at first be C-limited but would become N- and P-limited within three weeks.Rapid transfer of³²P can occur from dying roots to those of a living plant if the two root systems are intermingled. The amount transferred was substantially increased in two species-combinations that are known to form mycorrhizal links between their root systems. In contrast, in a species-combination where only the living (receiver) plant could become mycorrhizal no significant increase of³²P transfer occurred. This evidence, although far from conclusive, suggests that mycorrhizal links between dying and living roots can contribute to nutrient cycling. This research indicates a major difference in nutrient cycling processes between perennial and annual crops.     

Effect of phosphorus source and rate of application on VAM fungal infection and growth of maize (Zea mays L.)

The effects of two phosphorus (P) sources (triple superphosphate and Ghafsa phosphate rock), applied at rates equivalent to 44kg ha^-1 and 22 kg ha^-1, on vesicular-arbuscular mycorrhizal (VAM) fungal infection in roots, dry matter yield and nutrient content of maize grown in an oxisol and an alfisol, were investigated in a growth cabinet. The application of 44 kg P ha ^-1 resulted in root infection by VAM fungi not was significantly different (P<-0.01) from when no P was applied. Root infection was significantly greater when P was applied as triple superphosphate at the rate of 22 kg ha^-1 the higher rate. Phosphate rock treatments at both rates of application resulted in significantly greater root infection than in controls with no P or when triple superphosphate was applied at 44 kg ha^-1. Plant P uptake increased in all soils with the different P treatments compared with the control. No direct effects of the treatments on the aluminium and zinc contents of maize plants were observed. In the gleyic alfisol, reduced Mn uptake as a result of increased infection of plants with the superphosphate treatments was observed. Higher Mn was also found in plants with the higher rate of superphosphate treatment than with the phosphate rock treatments in the haplustox, although infection rates in plants with the latter treatments were higher. With the exception of plants with the phosphate rock treatment applied at 22kg ha^-1, dry matter yields of plants with all P sources were significantly greater than the controls.