ABSTRACT Background Reducing nitrogen (N) losses to water, lessening reliance on inorganic fertiliser and maintaining agricultural productivity are key goals on farms in many regions. The use of perennial ryegrass–white clover swards presents an opportunity to reduce the need for inorganic N fertiliser on farms. Optimising sulphur (S) nutrition in ryegrass–white clover swards holds the potential to optimise N fixation and yields and potentially minimise nitrate‐N (NO 3 − ‐N) leaching; however, this potential has received minimal research focus to date. Aims To test the hypotheses that increasing S fertiliser rates on a ryegrass–white clover sward will increase herbage yield, N off‐take and the N:S ratio at two levels of inorganic N supply and to test the impact of S application on NO 3 − ‐N leaching and apparent biological nitrogen fixation (BNF). Methods A soil lysimeter study was undertaken. All treatments except the control received 150 kg N ha −1 year −1 (organic N) in the form of cattle slurry (CS) split over two applications during the spring. Two rates of inorganic fertiliser N were applied, 0 and 150 kg ha −1 year −1 . Sulphur rates of 0, 15, 30 and 45 kg ha −1 were applied at both inorganic N rates. The 150 kg N ha −1 year −1 inorganic N rate also received a 60 kg S ha −1 rate. Results When zero inorganic fertiliser N was applied, the application of 15 kg S ha −1 along with CS maximised yield at 12,425 kg DM ha −1 and reduced NO 3 − ‐N leaching by 28% compared to the zero S treatment. However, 30 kg S ha −1 maximised herbage N off‐take at 435 kg N ha −1 . With 150 kg inorganic N ha −1 input, 45 kg S ha −1 along with CS maximised both herbage yield at 14,041 kg DM ha −1 and herbage N off‐take at 442 kg N ha −1 while reducing NO 3 − ‐N leaching by 34% compared to the zero S rate. The N:S ratio of the ryegrass component of the sward was consistently lower than that of the white clover component of the sward, irrespective of the inorganic fertiliser N or S rate. Conclusion Optimised S application increased herbage yield, N off‐take and apparent BNF while reducing N loss to water. However, excessive S application (> 45 kg ha −1 ) suppressed yield and did not reduce leached N losses, demonstrating the need for S nutrition to be tailored to productivity potential.
Kinsella et al. (Tue,) studied this question.