Parameters of nitrogen use efficiency of Kentucky bluegrass cultivars at different N levels under deficit irrigation

Increasing N use efficiency (NUE) is desirable in turfgrass management. Traditional definition of NUE in turfgrass is essentially the inverse of nitrogen concentration in the clippings and is often evaluated once in a growing season. In this experiment, we followed the Brenderse and Aerts' definition of NUE as the product of mean residence time (MRT) and nitrogen productivity (NP). The objective of this experiment was to identify NP changes over time in Kentucky bluegrass cultivars at different N levels under deficit irrigation. Nine Kentucky bluegrass (Poa pratensis L.) and two hybrid bluegrass cultivars from 10 phenotypic classes were tested under N rates of 10 and 40 g m⁻². Deficit irrigation at 60% of the evapotranspiration was imposed. The results showed interaction effects on NP from N level, irrigation, and cultivar. Low-N treatment resulted in higher NP values as compared to high-N for ‘Merit’ and ‘Martha’. No NP difference existed between N or irrigation levels for ‘Blue ghost’, ‘Geronimo’, ‘Heidi’, ‘Bandera’, ‘Impact’, ‘Fielder’, ‘Jackrabbit’, and ‘Park’. ‘Rhythm’ showed lower NP values in high-N combined with full irrigation compared to other treatments. In general, increasing N levels resulted in lower NP, but higher nitrate reductase activity (NaR) and higher net photosynthesis. No N by irrigation interaction effect was detected for effective quantum yield or NaR. In conclusion, leaf-level NP was shown to be an effective parameter for in-season monitoring of the above-ground NUE of Kentucky bluegrass, which provided more dynamic information than clipping yields or a one-time NUE calculation based on traditional definition.