Barrel medic (Medicago truncatula Gaertn.) was grown in a glasshouse in pots containing solonized brown soils with total nitrogen contents ranging from 1.2 to 2.7 g kg-1. The dry weight and nitrogen content of the plants were determined on three occasions, and the acetylene reduction technique was used to estimate the rate of nitrogen fixation. All soils had been subjected to rotations of cereal and annual pasture, but half were sampled following pasture, while the others were collected after cereal. Both groups of soils had similar ranges of total nitrogen content, but the pasture soils contained higher concentrations of mineral nitrogen at the time of sowing the experiment and produced more mineral nitrogen on incubation. Correlations among these three indices of nitrogen status were exceedingly low for such a collection of similar soils, and only mineral plus mineralizable nitrogen correlated reasonably well with nitrogen uptake by wheat plants grown on the soils for 9 weeks. Medic on the pasture soils generally had higher dry weights and contained more nitrogen than plants on soils which were cropped with cereal, but rates of acetylene reduction were lower. These differences, which diminished as the season progressed, were closely related to the mineral nitrogen but not the total nitrogen content of the soils. Thus, the rate of accretion of nitrogen in the soils from nitrogen fixation, as estimated in this study, was independent of the existing soil total nitrogen content but was decreased by soil mineral nitrogen when the latter was high. The results have implications for the maintenance of the nitrogen status of soils under cereal-pasture rotations.
On nitrogen fixation and “residual nitrogen content” in cellulosic pulps
