LIMING AND NITROGEN EFFICIENCY - SOME EFFECTS OF INCREASED CALCIUM SUPPLY AND INCREASED SOIL-PH ON NITROGEN RECOVERY BY PERENNIAL RYEGRASS

Positive effects of liming on the nitrogen (N) contents of perennial r yegrass (Lolium perenne) shoots might be due in part to the effects of increased exogenous calcium (Ca) level on the rate of N uptake by pla nts. To test this hypothesis, perennial ryegrass was grown in soil cul ture treated with different rates of lime, gypsum, and ammonium nitrat e (NH4NO3), in a factorial combination. The effects of these treatment s on yield, N offtake, and shoot chemical composition were examined at two consecutive harvests. At both harvests, liming significantly incr eased plant yield and N offtake. There was no response to gypsum at cu t 1; but at the second harvest, a negative interaction occurred betwee n the two Ca treatments such that gypsum increased plant yield and N o fftake in the absence of lime but not in its presence. The results sug gest that liming affects N recovery by swards in at least two differen t ways, each associated with a different phase in the soil MIT (Minera lization Immobilization Turnover) cycle. During phases of net N minera lization, liming by raising soil pH stimulates biomass activity and in creases the amount of organic N mineralized. In contrast, during phase s of net N immobilization, liming by increasing Ca availability in the rhizosphere improves the ability of plants to absorb N, and thus help s them to compete more effectively with the biomass for mineral N.