Distribution of15N in the soil-plant system during a four-year field lysimeter study with barley (Hordeum distichum L.) and perennial meadow fescue (Festuca pratensis Huds.)

An annual cereal, barley, and a perennial grass ley, meadow fescue, were grown in field lysimeters in Sweden and fertilized with 12 and 20g Ca(NO₃)₂-N m⁻² yr⁻¹, respectively. Isotope-labeled (¹⁵N) fertilizer was added during year 1 of the study, whereafter similar amounts of unlabeled N were added during years 2 and 3. The grass ley lysimeters were ploughed after the growing season of year 3 and sown with barley during year 4. The barley harvest in year 1 removed 59% of the added fertilizer N, while the fertilizer N export by two meadow fescue harvests in year 1 was 65%. The labeled N export decreased rapidly after year 1, especially in the barley, but increased slightly after ploughing of the grass ley. The microbial biomass, measured with the chloroform fumigation method, incorporated a maximum of 1.4–1.7% of the labeled N during the first seven weeks after application. Later on, the incorporation stabilized at less than 1% in both cropping systems. The susceptibility of the residual labeled N to mineralization was evaluated three years after application by means of long-term laboratory incubations. The curves of cumulative mineralized N were described by a two-component first-order regression model that differentiated between an available and a more recalcitrant fraction of potentially mineralizable N. There was no difference in the amounts of potentially mineralizable N between the cropping systems. The labeled N comprised 5 and 2% of the amounts of potentially mineralizable N in the available and more recalcitrant fraction, respectively. The mineralization rate constants for the labeled N were almost twice as high as for the total potentially mineralizable N. The available fraction of the total potentially mineralizable N was 12%, while twice that proportion of the labeled N was available. It was concluded that the short-term ley did not differ from the annual crop with respect to the early disposition of the fertilizer N and the behaviour of the residual organic N.