The role of sulphur nutrition in yield, quality, nitrogen use efficiency and nitrogen losses in temperate grasslands

Sulphur (S) is an essential plant nutrient with important roles in crop productivity, quality, and nitrogen use efficiency. Sulphur is of great importance for the primary structure of proteins and the functioning of enzymes, Sulphur is a constituent of the amino acids cysteine (Cys) and methionine (Met) and S limitation impacts nitrogen (N) use efficiency. Although numerous roles of S are well acknowledged and improving N use efficiency is a major global challenge, S-focused research is relatively scarce compared to N and phosphorus (P) research, particularly for grasslands. Balancing S – N fertilisation, holds potential for improving nutrient use efficiency, minimising nutrient loss to the environment, and increasing plant productivity. The thesis work presented here focuses specifically on S effects in grassland examining the soil, plant and environment loss response to S fertilisation on a range of temperate grassland soils using a number of lysimeter facilities. Yield response to S fertilisation was found to be related to soil texture, with yield responses of 31-51% in the sandy loam soils compared to as little as 4% in the finest textured soil. Large reductions in nitrate-N (NO3--N) leaching of 45% - 51% were observed in the sandy loams in response to S fertilisation with N fertilisation. The combined effect of soil pH and S fertilisation were tested with a significant yield response of 46% to increasing soil pH from 4.9 to 6.8 combined with S and N fertilisation, and significant reduction in nitrous oxide (N2O) emissions (of 47%) and significant reduction in NO3--N leaching (38%). The combination of increased soil pH and S application significantly decreased NO3--N leaching (major threat to water quality) while improving N use efficiency and grass productivity. Different strategies for S application were assessed showing that S availability from slurry (22 t ha-1) was insufficient to meet the S requirement on a responsive soil at the rates applied. Applying mineral S with cattle slurry significantly reduced nitrate leaching. Sulphur application brought NO3--N concentrations in leachate that would otherwise have been above the maximum allowable concentration for drinking water standard. Finally the effects of different N and S fertilisation rates on nutrient efficiency, leaching, and yield were examined using a sandy loam lysimeter facility. Higher N application rates required higher S application rates to optimise N use efficiency (NUE) and plant productivity while also reducing NO3--N leaching. Overall, results suggest that, in temperate grassland soils where S and N are co-limiting, S and N behave synergistically in terms of yield, N and S uptake and N leaching responses. This response appears to be most likely on coarser textured soils. From a management perspective, assessing and if required, correcting S deficiency should be a first step before increasing N rate to boost plant performance a strategy which can result in higher NUE and lower N losses to the environment. Overall, results suggest that S should to be considered as an important nutrient for the efficient production of grass in temperate grassland soils with significant potential to reduce environmental losses, particularly through NO3- leaching in coarse-textured well-drained soils. This thesis work provides strong evidence for a focus on S management usage to optimise grassland productivity and to address water quality challenges.
The findings of this thesis point to the high potential optimisation of S fertilisation to boost yields and NUE while reducing NO3- leaching potentially addressing global challenges. In addition, the strong negative correlation of pH with N2O emissions measured provides evidence for redoubling focus on soil pH which this thesis has shown can both boost yields and NUE and reduce N losses to the environment.