Nitrogen Use Efficiency and Herbage Production of an Established Grass Sward in Relation to Moisture and Nitrogen Fertilization

Herbage growth and nitrogen (N) use efficiency in grazed or mown grasslands are generally low, associated mostly with poor response to fertilizer N. The aim of the present investigation was to examine the short-term response of grass to fertilizer N with respect to herbage yield and nitrogen use efficiency (NUE) in order to provide a better basis for improving the efficient use of fertilizer N in grassland ecosystems. Both NO₃ ^? and NH₄ ⁺ sources of N were applied to an established grass sward with three moisture levels, i.e., natural conditions (63% water-filled pore space, WFPS), near field capacity level (71% WFPS), and slightly wetter than field capacity (84% WFPS). Herbage yield, i.e., dry matter (DM), N uptake, N recovery efficiency, yield efficiency, and physiological efficiency were determined over a 7–28 d period. Addition of N fertilizers significantly increased the herbage yield and N uptake of grass sward over that of the control. In the plots where NO₃ ^??N was added as the N source, DM yield was between 1760–1870 kg ha^?1, N recovery efficiency was between 24%–43%, and yield and physiological efficiency were in the range of 2.1–3.2 and 6.4–8.8 kg DM kg^{? 1} N, respectively. In NH₄ ⁺?N added plots, the DM yield was between 3190–3700 kg ha^{? 1}, N recovery efficiency was between 39%–48% while yield and physiological efficiency were in the range of 3.5–5.6 and 9.0–11.6 kg DM kg^{? 1} N, respectively. Results indicated that total DM yield, N uptake, and NUE depend on the source of N and the level of moisture in the field. Assimilation of N is also affected by the stage of plant development after N fertilization. About 50%–54% of applied N was recovered in the initial 14 and 21 d after fertilizer application and thereafter translocation of N slowed. A fall in herbage production and minimal response to N fertilizer has been observed at 84% WFPS, while the maximum herbage yield and N recovery efficiency was recorded in soil near or below field capacity. The grass sward with added NH₄ ⁺?N produced a larger yield and had higher NUE relative to the sward with NO₃ ^??N. Results confirm that applied N was not utilized efficiently by grass sward and a decrease in N uptake and its utilization seem to be the key factors responsible for the poor herbage productivity often observed in pastoral agriculture. These results suggest that both moisture and N source have a substantial effect on herbage yield and N utilization by plants and therefore should be considered for efficient management of N fertilization and recommendations for grass sward.