EFFECTS OF GYPSUM ON THE UPTAKE, ASSIMILATION AND CYCLING OF N-15-LABELED AMMONIUM AND NITRATE-N BY PERENNIAL RYEGRASS

Solution culture studies have shown that plant uptake of NH4+ and NO3- can be improved by increasing the concentration of Ca2+ in the root environment: the same may be true for grass grown in soil culture. An experiment was set up to see whether gypsum (CaSO4 2H2O) increased the rate at which perennial ryegrass absorbed (NH4+)-N-15 and (NO3-)-N-15 from soil. The results demonstrated that gypsum increases the rates of uptake of both NH4+ and NO3- by perennial ryegrass. However, because there was little potential for mineral-N loss from the experimental system, either by gaseous emission or by N immobilization, long term improvements in fertilizer efficiency were not observed. Nitrogen cycling from shoots to roots commenced once net uptake of N into plants had ceased. Labelled N transferred thus to roots underwent isotopic exchange with unlabelled soil N. It was suggested that this exchange of N might constitute an energy drain from the plant, if plant organic N was exchanged for soil inorganic N. The fact that the exchange occurred at all cast doubt on the suitability of the N-15-isotope dilution technique for assessing fertilizer efficiency in medium to long term experiments. There was evidence that the 'extra' NO3--N taken up by plants on the all-nitrate treatments as a result of gypsum application, was reduced in root tissue rather than in shoots, but to the detriment of subsequent root growth and N uptake.