A field study of nitrogen dynamics and spring barley growth as affected by the quality of incorporated residues from white clover and ryegrass

Four different grass swards were grown on a sandy loam for 3 years, and then incorporated into the soil through rotovation in the spring. The treatments differed in the proportion of white clover (Trifolium repens) and ryegrass (Lolium perenne), both through seeding in pure stand or mixture and through N fertilization (0 or 150 kg N ha(-1) yr-l) for the white clover-ryegrass mixtures. A control treatment (fallow), differing from the others in that the grass award had been incorporated one year earlier, was also included. A spring barley (Hordeum vulgare cv. texane) crop was established in half of the experimental site and the other half was left unplanted. Carbon and nitrogen mineralization from the residues was measured as soil surface CO2 flux and soil inorganic N accumulation in unplanted plots under non-leaching conditions. Residue decomposition processes, barley dry matter production and N uptake showed clear differences between the five treatments, due especially to the differences in amount of residue N incorporated. Incorporated residue N was highest in the white clover in pure stand (C150), and lowest in the ryegrass in pure stand (G150) treatments, with non-fertilized and fertilized white clover-ryegrass residues (CG0 and CG150, respectively) intermediate and similar in both amount and quality of the residues. However, in spite of this similarity the treatments differed greatly with respect to both C and N mineralization, indicating that other factors than the measured quality parameters (i.e. C, N, C-to-N ratio, water solubles, cellulose, lignin) influenced their decomposition pattern. The highest crop dry matter production and N uptake was measured in the C150 treatment, followed by the CG0 and the fallow treatment, with considerable lower yields in the CG150 and G150 treatments. There was a significantly higher inorganic N content, 60 kg N ha(-1), in the planted C150 and CG0 treatments during the seedling and tillering barley growth stages, with no significant difference between treatments during the later barley growth stages. Apparent net N mineralization measured in the unplanted CG0 treatment exceeded that of the C150, whereas the other treatments ranked similar to the barley N uptake rates. This indicated that availability of soil inorganic N at the early tillering stage was a key determining factor for the final barley dry matter yield and N uptake, with later N mineralization rates having lesser influence.